Some Real World Battery Life Data, by Cactus Jim
Battery technology has come a long way in the last 10 years since Y2K. Back in the late 1990s, I stocked various types and brands of batteries for long term storage or use. Batteries ranged from store purchased alkaline, rechargeable alkalines, NiCd, generic deep cycle marine
batteries, gel-cell sealed lead acid, lithium and even the ubiquitous flooded lead acid Trojan T-105 floor scrubber batteries. I wrote dates on all the batteries and rechargeable batteries had logs kept of use and maintenance.
In most cases enough batteries were purchased to allow for a reasonable statistical sampling, thus providing a real level of confidence in the results. Note that the word battery and cell are often used below in singular, even though the same test was repeated multiple times on
different units. All voltages and times are given as composite averages of the tests, removing clear outlier data, such as an obviously failed cells that leaked electrolyte during storage.
10 years later, most of those batteries were still in my possession, untouched (with a some exceptions) and I decided to run controlled experiments on them to see how they fared. Each battery type is discussed by type and brand if applicable. Finally, as technology has
provided for improvements, some additional battery types are discussed that have only received short term testing due to being recently brought to market.
All batteries were stored in 60-to-75 degree F conditions with <50% relative humidity.
Generic Alkaline
These are what you find at most stores on the shelf, having virtually eliminated the old carbon-zinc batteries that were still sold in the 1990s. An extensive selection of all standard sizes was tested, including Energizer, Energizer commercial use (not sold via retail) and
Duracell. The cells offered 2-4 year lifetimes based upon their expiration dates. All were stored for 10 years, with the exception of the commercial Energizer D cells, which were 12 years old at the time of testing.
Several of the Energizer cells (2 out of a lot of 50) had developed leakage failures during storage, in one case contaminating a co-packaged battery. This matches my anecdotal experience with this brand, with several case leak failures damaging equipment that had Energizer brand
batteries left in them for longer time periods (1-2 years). I expect these are design related failures since even newer batteries of this brand leaked, spanning a sample period of five years.
Interestingly, the commercial Energizer batteries, of which I had over 50, did not have a single failure. They also performed slightly better even though they were older. No failures were seen with the Duracell alkaline batteries, but there was a smaller sample available (20 of each
type).
The aged batteries were tested on a constant resistance tester that tracked battery voltage until the cells were completely depleted, to a voltage of 0.2V, which would not provide even the smallest amount of usable light in a flashlight. Initial current drain of approximately
1/20th of manufacturer recommended maximum was used. (12 Ohms for AA cells, 2.75 Ohms for D cells)
The output voltage of the 10 year old batteries started out at approximately 0.1V different from a brand new battery and maintained this difference until the battery chemistry failed, leading to a rapid decline in voltage. For AA batteries, the usable lifetime (to the 0.9V mark) was 18 hrs for the 10 year old battery vs. 22 hrs for a brand new cell. The voltage discharge curves tracked each other with the noted 0.1V difference. At the 18 hr. mark, the old cell dropped to under 0.2V a matter of minutes. The new cell soldiered on, declining slowly from
0.9V at 22 hrs to 0.2V at 27 hrs.
The commercial Energizer cells matched their retail cousins almost identically to the 0.9V cut off. However, they did not exhibit the sharp 20 minute decline to 0.2V once the battery chemistry started to fail. Instead they provided another 5 hours of possibly usable output with a slow decline between the 0.8 and 0.2V marks. This would be indicative of a slightly longer life span in an intermittent on/off usage where the voltage would creep back up to a more usable range during the off cycle.
When batteries were tested at high loads, the 10 year old units showed excessive voltage droop very quickly. This matches with published manufacturer recommendations that alkaline cells should not be used in high current draw applications.
All working cells showed an open cell voltage of 1.4V before being connected to a load.
Conclusions:
Alkaline batteries are usable well beyond their expiration dates.
Alkaline batteries properly stored for 10 years will still provide functional capacity of 75-80 percent with lighter loads such as flashlights and radios.
There will likely be a fallout rate with some percentage of cells showing complete or partial failure during storage. Thus large packs of batteries should be broken up into smaller packs to limit the amount of damage one leaking cell can do and extra batteries should be purchased to take into account such failures.
Batteries sold for commercial use may be built better and will last longer than stuff sold into the general retail market.
If the battery shows a voltage of 1.4V or so after storage, it's still probably usable.
Nickel Cadmium Rechargeable
The entire lot of 1990's era NiCd batteries were found to be unusable, showing shorts or inability to take a charge of any capacity. This technology has drastically improved over the last ten years, although such batteries are still of limited long term storage use due to rapid self discharge and not having a design criteria for long life. There are also many variables that affect the durability of NiCd and NiMH, both from a cycle life and long term storage standpoint. My anecdotal evidence points to cheap batteries not lasting long (as little as 0-3 months for cheap no-name brand packs) and expensive brand name cordless tool packs still going strong after eight years of light use. The well known self-discharge and memory problems are still issues with this chemistry.
Conclusions:
Not suitable for long term storage.
Expensive portable tool packs might have long life spans with periodic use and charging.
Probably acceptable for daily use, but there are better alternatives available in NiMH.
Cheaper than other rechargeables.
Rechargeable Alkaline (no longer made)
A group of Eveready rechargeable alkalines were also tested. This technology was produced for a few years but never really saw commercial success. The batteries had low self discharge, thus being ready to go after longer storage periods but could also be re-charged. The recharge
cycle was unusual in that if the battery was heavily discharged it's recharge cycle life was very short, only 16 cycles or so. With shallow discharges, the battery could be "topped off" hundreds of times. Looked like a perfect fit for long term storage, provided that could be topped
up once a year.
The 10 year old AA and D cells were fully charged before testing. All fell significantly short on both voltage and life, even compared to 12 year old alkaline cells. Starting voltage was only 1.2V and within minutes was 0.2V lower than the 10 year old cells. The cells chemistry failed at the 22 hr mark vs. 28 hours for the 10 year old cells.
Conclusions:
Be careful with new untested technologies.
Nickel Metal Hydride
No Nickel Metal Hydride (NiMH) cells were used in the long term test due to their very high self discharge and the technology being in it's infancy in the 1990s. However, this chemistry deserves mention due to some recent innovations. Although NiMH batteries have higher capacity and most of
the memory effect has been overcome, they continue to suffer from very high self-discharge. A fully charged battery can be at 50% in under a month of sitting idle. In general, the higher the capacity of the cell the faster the self discharge.
Recently a new internal construction was designed that allows NiMH cells to retain up to 80% of their initial charge up to year later .[JWR Adds: These are also sometimes marketed as "Low Self Discharge (LSD)" batteries.] I have been extensively testing these over the last year with very good results. No outright failures to date, good capacity compared to alkaline batteries, very good tolerance for high current drains such as radio transmitters and good shelf life.
These cells are often sold as "pre-charged" or long shelf life NiMH. Duracell Pre-charged and Eneloop are the two most commonly available brands.
Conclusions:
A technology to watch, may replace alkaline batteries in many applications.
Long term life span is currently unknown or unpublished.
Lithium primary batteries
Non-rechargeable lithium batteries are the king of long term storage. They have been around for decades and are well understood, with devices still working 20 years after installation. There are many different chemistries that are used, with the actual type not disclosed to the consumer, so be aware that not all lithium batteries will have long shelf life.
The CR123A battery size almost always comes in a chemistry that will allow for 10+ year storage without a problem. I'm still using up my 12 year old batteries and even in bulb style Surefire lights they last so close to a new cell that it's hard to tell the difference. No tests were
performed on this stock of batteries since they are so well understood and quantified.
I had a limited stock of AA lithium cells from the 1990s and they too appear to be at 80+ percent capacity. When they reach 15 years I will test a few and see if the group test should be put at the 15 or 20 year mark. Note that the 1.5V batteries use a different chemistry than the 3 volt CR123, thus they may have a shorter life span, but that remains to be seen. At 10+ years, they are still the top choice with the exception of price.
Conclusions:
Low weight.
High capacity and high current.
Best for low temperatures.
Extensively verified 10+ year shelf life.
Available in AA, AAA, CR123A and various non-consumer sizes.
Industrial/commercial availability in 9V but metal body versions are slightly oversized.
Lead acid gel cells
Gel cells are a type of truly sealed lead acid battery. They are commonly used in backup devices such as emergency lights and alarm systems. Typically seen as 6V or 12V batteries with connecting tabs, but available commercially in over a hundred different sizes, shapes and
voltages.
The small batch (5 units) of lead acid gel cells I had from 1999 all died various deaths over the last 10 years. All were 12V 7Amp Hour packs of the commonly available 5.94 X 2.56 X 3.70 size. All showed degraded performance (over 10% capacity loss) after the 5th year, even packs that
were 100% unused and one pack that was under a constant charge. All were trickle charged at least once a year to 13.8V to make up for any self discharge and four of them were used intermittently for various purposes from charging a motorcycle battery to powering GPS in aircraft. None were
ever subject to severe discharge cycles or overcharging.
Each cell was charged and then test discharged to 50% once a year to check remaining capacity. Charging was done by constant voltage to 14.2V and discharge test was done at 1/20 capacity, constant resistance to 50% state of charge, as indicated by voltage.
At the seven year mark the first cell had a complete failure. The last unit, which had been installed in a trickle charging backup application failed this month.
Conclusions:
Realistic safe life span of five years.
After the five year mark, sudden failures may take the battery out of service without warning.
Require yearly charge maintenance due to self discharge.
Very high current capacity, allowing for use to minimally re-charge much larger lead acid batteries.
Often used inside of car self-jumpstart packs and for backup batteries in alarms and lighting.
Flooded lead acid batteries
I'm going to skip right past car starting / dual use batteries as they are 100% unsuitable for any long term application. While I have had certain vehicle starting batteries last eight years, there has never been any consistency between brand, size or use. I consider any car start battery over 2 years old to be suspect. The fact that they can be seriously degraded or destroyed by a single deep discharge makes them worthless in any situation where one must depend upon them. Even the consumer branded "deep-cycle" batteries are suspect from my experience.
The long term test batteries encompassed two large deep cycle "maintenance free" Energizer batteries from Wal-Mart and a bank of 24 Trojan T-105 6V industrial units. All were maintained as they would be in an industrial setting with water level, specific gravity and voltage checks each month.
The Trojans were connected to a grid-tied solar system and kept at peak charge for the first three years of their life. They were more heavily discharged at least once a year during power outages or for testing. In 2002 the system was converted to use the batteries each day for a period
of 6 hours, with cycling to 25-50% depth of discharge each day. Although their capacity is currently at about 60% of rated and there has been one hard cell failure in the bank, they continue to function.
The deep cycle batteries from Wal-Mart didn't make it past two years. They were used a few times a year to power tools and lights through an inverter. Note that "maintenance free" often means that there is just a slightly larger reservoir of water and acid in the battery. If you want
to try and use these, cheap batteries you should pop off the top caps with a screwdriver and re-fill the water just like any flooded lead acid battery. I consider any such off the shelf consumer batteries as a poor choice and false economy compared to a commercial battery such as the
Trojans.
Conclusions:
Buy true commercial/industrial batteries.
They cost more, but even my bottom of the line T-105s lasted five times longer than the cheap "deep-cycle".
Flooded batteries require maintenance (water & charging) or they will fail.
Note: Flooded batteries make hydrogen gas and a fine mist of sulphuric acid when being charged. These must be vented to prevent explosions and corrosion of battery terminals other any nearby items.
AGM
Absorbed glass mat (AGM) batteries are a type of true maintenance free lead acid battery. They have no ports to add liquid and will re-combine any generated gas internally. The military and aircraft industry use this technology due to low self discharge (1-3% per month) and no liquid to
spill.
They have only recently become widely available, both in starting applications and for deep cycle use. My actual test time with them has been limited to only two years.
I have three units in starting applications. All are in vehicles that sit for extended time periods (6-12 months), but then get used frequently, thus creating a cycle of many starts followed by long periods of inactivity. I have had one internal cell failure on the most used
battery in it's first year. The two others have worked perfectly, allowing me to start a car that had sat idle for six months as if I had been driven the previous day.
One unit was subject to a severe discharge, showing less than 3V when disconnected. The unit was charged overnight on a commercial bulk charger and then load/capacity tested back down to 10V. All indications were that the battery suffered no damage and it was returned to starting
service.
Current specifications for heavy industrial AGM batteries and accelerated life tests indicate life spans of 20+ years even under heavy use. This would not seem unrealistic given that old industrial telecom backup batteries are often sold after 20 years of service with buyers reporting acceptable capacity of these 20 year old batteries.
There are many cheap imports being labeled as AGM. As it's difficult to tell the difference between a gel-cell and AGM battery from the outside, stick with brands that have been making AGM for commercial use.
Conclusions:
Expensive.
May be the best longer term / large capacity battery technology if weight, space and price are not an issue.
Stick to name brand and industrial battery makers.
Heavy industrial AGM batteries are very expensive but will offer a real 20+ year life.
Contact Corrosion
When batteries are placed inside and object that is subject to motion, and left there for extended periods of time, there is the strong possibility that atmospheric oxidation various types of corrosion will occur. Basically the contacts will become dirty and poor overtime,
leading to the dreaded weak or intermittent flashlight output that magically restores itself when you bang the light a few times. Even sealed flashlights will develop this problem, especially if subject to temperature cycles or vibration, such as storage in a car.
This can be addressed in several ways. The batteries can simply be replaced every year. The contacts can be gently cleaned once a year or whenever low output is noticed. Never use an abrasive to clean contacts, as you may scrape away any protective coating that has been
plated on. Coatings such as gold, silver or nickel are often very thin. The contacts can be safely cleaned by rubbing with with a pencil eraser or clean sheet of paper. The batteries contact areas can also be cleaned in this fashion. Finally, you can place fabric or paper barriers between the batteries and the contacts to prevent metal to metal contact until you want to use the device. Note that this can be useful if you have devices such as radios that slowly drain the battery even when powered off. Some newer electronics use solid state ON/OFF switches or run a clock or memory retention device from the battery, thus slowly draining it. You will want to verify that any any stored settings on the device are saved even without a battery present before disconnecting the battery in this way. If the settings are stored for two weeks, it should be okay to leave the battery out indefinitely.
batteries, gel-cell sealed lead acid, lithium and even the ubiquitous flooded lead acid Trojan T-105 floor scrubber batteries. I wrote dates on all the batteries and rechargeable batteries had logs kept of use and maintenance.
In most cases enough batteries were purchased to allow for a reasonable statistical sampling, thus providing a real level of confidence in the results. Note that the word battery and cell are often used below in singular, even though the same test was repeated multiple times on
different units. All voltages and times are given as composite averages of the tests, removing clear outlier data, such as an obviously failed cells that leaked electrolyte during storage.
10 years later, most of those batteries were still in my possession, untouched (with a some exceptions) and I decided to run controlled experiments on them to see how they fared. Each battery type is discussed by type and brand if applicable. Finally, as technology has
provided for improvements, some additional battery types are discussed that have only received short term testing due to being recently brought to market.
All batteries were stored in 60-to-75 degree F conditions with <50% relative humidity.
Generic Alkaline
These are what you find at most stores on the shelf, having virtually eliminated the old carbon-zinc batteries that were still sold in the 1990s. An extensive selection of all standard sizes was tested, including Energizer, Energizer commercial use (not sold via retail) and
Duracell. The cells offered 2-4 year lifetimes based upon their expiration dates. All were stored for 10 years, with the exception of the commercial Energizer D cells, which were 12 years old at the time of testing.
Several of the Energizer cells (2 out of a lot of 50) had developed leakage failures during storage, in one case contaminating a co-packaged battery. This matches my anecdotal experience with this brand, with several case leak failures damaging equipment that had Energizer brand
batteries left in them for longer time periods (1-2 years). I expect these are design related failures since even newer batteries of this brand leaked, spanning a sample period of five years.
Interestingly, the commercial Energizer batteries, of which I had over 50, did not have a single failure. They also performed slightly better even though they were older. No failures were seen with the Duracell alkaline batteries, but there was a smaller sample available (20 of each
type).
The aged batteries were tested on a constant resistance tester that tracked battery voltage until the cells were completely depleted, to a voltage of 0.2V, which would not provide even the smallest amount of usable light in a flashlight. Initial current drain of approximately
1/20th of manufacturer recommended maximum was used. (12 Ohms for AA cells, 2.75 Ohms for D cells)
The output voltage of the 10 year old batteries started out at approximately 0.1V different from a brand new battery and maintained this difference until the battery chemistry failed, leading to a rapid decline in voltage. For AA batteries, the usable lifetime (to the 0.9V mark) was 18 hrs for the 10 year old battery vs. 22 hrs for a brand new cell. The voltage discharge curves tracked each other with the noted 0.1V difference. At the 18 hr. mark, the old cell dropped to under 0.2V a matter of minutes. The new cell soldiered on, declining slowly from
0.9V at 22 hrs to 0.2V at 27 hrs.
The commercial Energizer cells matched their retail cousins almost identically to the 0.9V cut off. However, they did not exhibit the sharp 20 minute decline to 0.2V once the battery chemistry started to fail. Instead they provided another 5 hours of possibly usable output with a slow decline between the 0.8 and 0.2V marks. This would be indicative of a slightly longer life span in an intermittent on/off usage where the voltage would creep back up to a more usable range during the off cycle.
When batteries were tested at high loads, the 10 year old units showed excessive voltage droop very quickly. This matches with published manufacturer recommendations that alkaline cells should not be used in high current draw applications.
All working cells showed an open cell voltage of 1.4V before being connected to a load.
Conclusions:
Alkaline batteries are usable well beyond their expiration dates.
Alkaline batteries properly stored for 10 years will still provide functional capacity of 75-80 percent with lighter loads such as flashlights and radios.
There will likely be a fallout rate with some percentage of cells showing complete or partial failure during storage. Thus large packs of batteries should be broken up into smaller packs to limit the amount of damage one leaking cell can do and extra batteries should be purchased to take into account such failures.
Batteries sold for commercial use may be built better and will last longer than stuff sold into the general retail market.
If the battery shows a voltage of 1.4V or so after storage, it's still probably usable.
Nickel Cadmium Rechargeable
The entire lot of 1990's era NiCd batteries were found to be unusable, showing shorts or inability to take a charge of any capacity. This technology has drastically improved over the last ten years, although such batteries are still of limited long term storage use due to rapid self discharge and not having a design criteria for long life. There are also many variables that affect the durability of NiCd and NiMH, both from a cycle life and long term storage standpoint. My anecdotal evidence points to cheap batteries not lasting long (as little as 0-3 months for cheap no-name brand packs) and expensive brand name cordless tool packs still going strong after eight years of light use. The well known self-discharge and memory problems are still issues with this chemistry.
Conclusions:
Not suitable for long term storage.
Expensive portable tool packs might have long life spans with periodic use and charging.
Probably acceptable for daily use, but there are better alternatives available in NiMH.
Cheaper than other rechargeables.
Rechargeable Alkaline (no longer made)
A group of Eveready rechargeable alkalines were also tested. This technology was produced for a few years but never really saw commercial success. The batteries had low self discharge, thus being ready to go after longer storage periods but could also be re-charged. The recharge
cycle was unusual in that if the battery was heavily discharged it's recharge cycle life was very short, only 16 cycles or so. With shallow discharges, the battery could be "topped off" hundreds of times. Looked like a perfect fit for long term storage, provided that could be topped
up once a year.
The 10 year old AA and D cells were fully charged before testing. All fell significantly short on both voltage and life, even compared to 12 year old alkaline cells. Starting voltage was only 1.2V and within minutes was 0.2V lower than the 10 year old cells. The cells chemistry failed at the 22 hr mark vs. 28 hours for the 10 year old cells.
Conclusions:
Be careful with new untested technologies.
Nickel Metal Hydride
No Nickel Metal Hydride (NiMH) cells were used in the long term test due to their very high self discharge and the technology being in it's infancy in the 1990s. However, this chemistry deserves mention due to some recent innovations. Although NiMH batteries have higher capacity and most of
the memory effect has been overcome, they continue to suffer from very high self-discharge. A fully charged battery can be at 50% in under a month of sitting idle. In general, the higher the capacity of the cell the faster the self discharge.
Recently a new internal construction was designed that allows NiMH cells to retain up to 80% of their initial charge up to year later .[JWR Adds: These are also sometimes marketed as "Low Self Discharge (LSD)" batteries.] I have been extensively testing these over the last year with very good results. No outright failures to date, good capacity compared to alkaline batteries, very good tolerance for high current drains such as radio transmitters and good shelf life.
These cells are often sold as "pre-charged" or long shelf life NiMH. Duracell Pre-charged and Eneloop are the two most commonly available brands.
Conclusions:
A technology to watch, may replace alkaline batteries in many applications.
Long term life span is currently unknown or unpublished.
Lithium primary batteries
Non-rechargeable lithium batteries are the king of long term storage. They have been around for decades and are well understood, with devices still working 20 years after installation. There are many different chemistries that are used, with the actual type not disclosed to the consumer, so be aware that not all lithium batteries will have long shelf life.
The CR123A battery size almost always comes in a chemistry that will allow for 10+ year storage without a problem. I'm still using up my 12 year old batteries and even in bulb style Surefire lights they last so close to a new cell that it's hard to tell the difference. No tests were
performed on this stock of batteries since they are so well understood and quantified.
I had a limited stock of AA lithium cells from the 1990s and they too appear to be at 80+ percent capacity. When they reach 15 years I will test a few and see if the group test should be put at the 15 or 20 year mark. Note that the 1.5V batteries use a different chemistry than the 3 volt CR123, thus they may have a shorter life span, but that remains to be seen. At 10+ years, they are still the top choice with the exception of price.
Conclusions:
Low weight.
High capacity and high current.
Best for low temperatures.
Extensively verified 10+ year shelf life.
Available in AA, AAA, CR123A and various non-consumer sizes.
Industrial/commercial availability in 9V but metal body versions are slightly oversized.
Lead acid gel cells
Gel cells are a type of truly sealed lead acid battery. They are commonly used in backup devices such as emergency lights and alarm systems. Typically seen as 6V or 12V batteries with connecting tabs, but available commercially in over a hundred different sizes, shapes and
voltages.
The small batch (5 units) of lead acid gel cells I had from 1999 all died various deaths over the last 10 years. All were 12V 7Amp Hour packs of the commonly available 5.94 X 2.56 X 3.70 size. All showed degraded performance (over 10% capacity loss) after the 5th year, even packs that
were 100% unused and one pack that was under a constant charge. All were trickle charged at least once a year to 13.8V to make up for any self discharge and four of them were used intermittently for various purposes from charging a motorcycle battery to powering GPS in aircraft. None were
ever subject to severe discharge cycles or overcharging.
Each cell was charged and then test discharged to 50% once a year to check remaining capacity. Charging was done by constant voltage to 14.2V and discharge test was done at 1/20 capacity, constant resistance to 50% state of charge, as indicated by voltage.
At the seven year mark the first cell had a complete failure. The last unit, which had been installed in a trickle charging backup application failed this month.
Conclusions:
Realistic safe life span of five years.
After the five year mark, sudden failures may take the battery out of service without warning.
Require yearly charge maintenance due to self discharge.
Very high current capacity, allowing for use to minimally re-charge much larger lead acid batteries.
Often used inside of car self-jumpstart packs and for backup batteries in alarms and lighting.
Flooded lead acid batteries
I'm going to skip right past car starting / dual use batteries as they are 100% unsuitable for any long term application. While I have had certain vehicle starting batteries last eight years, there has never been any consistency between brand, size or use. I consider any car start battery over 2 years old to be suspect. The fact that they can be seriously degraded or destroyed by a single deep discharge makes them worthless in any situation where one must depend upon them. Even the consumer branded "deep-cycle" batteries are suspect from my experience.
The long term test batteries encompassed two large deep cycle "maintenance free" Energizer batteries from Wal-Mart and a bank of 24 Trojan T-105 6V industrial units. All were maintained as they would be in an industrial setting with water level, specific gravity and voltage checks each month.
The Trojans were connected to a grid-tied solar system and kept at peak charge for the first three years of their life. They were more heavily discharged at least once a year during power outages or for testing. In 2002 the system was converted to use the batteries each day for a period
of 6 hours, with cycling to 25-50% depth of discharge each day. Although their capacity is currently at about 60% of rated and there has been one hard cell failure in the bank, they continue to function.
The deep cycle batteries from Wal-Mart didn't make it past two years. They were used a few times a year to power tools and lights through an inverter. Note that "maintenance free" often means that there is just a slightly larger reservoir of water and acid in the battery. If you want
to try and use these, cheap batteries you should pop off the top caps with a screwdriver and re-fill the water just like any flooded lead acid battery. I consider any such off the shelf consumer batteries as a poor choice and false economy compared to a commercial battery such as the
Trojans.
Conclusions:
Buy true commercial/industrial batteries.
They cost more, but even my bottom of the line T-105s lasted five times longer than the cheap "deep-cycle".
Flooded batteries require maintenance (water & charging) or they will fail.
Note: Flooded batteries make hydrogen gas and a fine mist of sulphuric acid when being charged. These must be vented to prevent explosions and corrosion of battery terminals other any nearby items.
AGM
Absorbed glass mat (AGM) batteries are a type of true maintenance free lead acid battery. They have no ports to add liquid and will re-combine any generated gas internally. The military and aircraft industry use this technology due to low self discharge (1-3% per month) and no liquid to
spill.
They have only recently become widely available, both in starting applications and for deep cycle use. My actual test time with them has been limited to only two years.
I have three units in starting applications. All are in vehicles that sit for extended time periods (6-12 months), but then get used frequently, thus creating a cycle of many starts followed by long periods of inactivity. I have had one internal cell failure on the most used
battery in it's first year. The two others have worked perfectly, allowing me to start a car that had sat idle for six months as if I had been driven the previous day.
One unit was subject to a severe discharge, showing less than 3V when disconnected. The unit was charged overnight on a commercial bulk charger and then load/capacity tested back down to 10V. All indications were that the battery suffered no damage and it was returned to starting
service.
Current specifications for heavy industrial AGM batteries and accelerated life tests indicate life spans of 20+ years even under heavy use. This would not seem unrealistic given that old industrial telecom backup batteries are often sold after 20 years of service with buyers reporting acceptable capacity of these 20 year old batteries.
There are many cheap imports being labeled as AGM. As it's difficult to tell the difference between a gel-cell and AGM battery from the outside, stick with brands that have been making AGM for commercial use.
Conclusions:
Expensive.
May be the best longer term / large capacity battery technology if weight, space and price are not an issue.
Stick to name brand and industrial battery makers.
Heavy industrial AGM batteries are very expensive but will offer a real 20+ year life.
Contact Corrosion
When batteries are placed inside and object that is subject to motion, and left there for extended periods of time, there is the strong possibility that atmospheric oxidation various types of corrosion will occur. Basically the contacts will become dirty and poor overtime,
leading to the dreaded weak or intermittent flashlight output that magically restores itself when you bang the light a few times. Even sealed flashlights will develop this problem, especially if subject to temperature cycles or vibration, such as storage in a car.
This can be addressed in several ways. The batteries can simply be replaced every year. The contacts can be gently cleaned once a year or whenever low output is noticed. Never use an abrasive to clean contacts, as you may scrape away any protective coating that has been
plated on. Coatings such as gold, silver or nickel are often very thin. The contacts can be safely cleaned by rubbing with with a pencil eraser or clean sheet of paper. The batteries contact areas can also be cleaned in this fashion. Finally, you can place fabric or paper barriers between the batteries and the contacts to prevent metal to metal contact until you want to use the device. Note that this can be useful if you have devices such as radios that slowly drain the battery even when powered off. Some newer electronics use solid state ON/OFF switches or run a clock or memory retention device from the battery, thus slowly draining it. You will want to verify that any any stored settings on the device are saved even without a battery present before disconnecting the battery in this way. If the settings are stored for two weeks, it should be okay to leave the battery out indefinitely.
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