Understanding AGM Batteries.

Considered obscure & expensive technology less than a decade ago, lithium batteries are are all grown up now and can be found in pretty much everything. Even the cheap $29.00 off brand handheld radios come standard with lithium batteries. Often forgotten in the mix is absorbed glass mat (AGM) batteries. They’re very adaptable for off grid use and have a lot of overlooked advantages. Amateurs who thought the choices were expensive lithium batteries or cumbersome flooded wet cells should look into AGM batteries as a worthy alternative.

Filling in the battery gap.
Lithium batteries are common only in smaller applications such as portable radios. For off grid radio amateurs who need larger/higher capacity batteries for general purpose electric needs, lithiums are still a very expensive proposal. A 100 amp-hour lithium battery can cost nearly $1000.00 not including specialized charging systems and controllers. Comparable traditional flooded batteries can be found for less than $100.00. AGM batteries fall in between, checking in around $225 for a 100 amp-hour unit.

Flooded batteries are cheap but come with their own list of disadvantages: They’re really heavy, produce explosive hydrogen gas, can leak caustic acid, need to be replaced more often than most other batteries, and require ongoing maintenance that can be a real hassle.

AGM batteries are a practical compromise between these two points. They’re more expensive than flooded batteries, but nowhere near what lithiums cost. AGM batteries do not produce dangerous gasses nor will they leak acid if broken or turned at an odd angle. They are slightly lighter and do not require a lot of upkeep.

The architecture of an AGM battery.
The major feature of AGM batteries is that a mat or pad, often made of Fiberglas but other materials are sometimes used, is placed between the plates. This is the “M” in “AGM”. The purpose of the mat is to act as a sponge to hold the electrolyte in place and keep it from sloshing around.

agm battery

GRAPHIC COURTESY OF SAE INTERNATIONAL

One of the more unique properties of AGM batteries is that their chemical process is recombinant. In conventional flooded batteries, hydrogen is pulled out of the electrolyte (which is mostly water). This breakdown is why flooded batteries occasionally need to be topped off with water and why the explosive hydrogen must be safely vented away. The AGM battery will recombine the hydrogen and oxygen to put water molecules back together inside the sealed system. The hydrogen that would otherwise be gassed out as an unwanted byproduct is effectively “recycled”. It’s a highly efficient process; over 99% of the hydrogen is recovered. This is how AGM batteries get away with never needing outside water added after they are sealed at the factory. They can literally make their own water.

AGM batteries have a safety valve that will release pressure if the battery becomes dangerously overheated but you may never get that far. They will generally show clear signs of their imminent death long before the valve gives way. If your battery feels excessively warm, develops a bulge on any side, or is making crackling/bubbling/ticking sounds, immediately disconnect it, let it stand for thirty minutes or so to stabilize, and discard it in an environmentally ethical manner. After you have cleared the trouble that caused the battery to fail (usually overcharging), go shopping for a new battery!

AGM Battery’s older brother.
Sealed lead acid (SLA) and valve regulated lead acid batteries (VRLA) are older versions of modern AGM batteries. The terms SLA and VRLA are interchangeable.

Are AGM and SLA/VRLA the same thing?
The short answer is yes. The long answer is that AGM is a variant of SLA with a slightly different internal structure and charge requirements. We could have a whole separate discussion about SLA batteries but it would mean getting pretty deep and nit-picky, so we won’t go there. Think of AGM batteries as a upgraded version of SLA.

Gel cell batteries: In gel cells, the electrolyte is mixed with a powdered silicon media so that it has a paste or gel consistency. Gel cells have many of the same outward physical characteristics we’ve already discussed (sealed, maintenance-free, can be used in any position, etc.) but they are not the same as SLA/VRLA/AGM batteries. Gel cells are often mistakenly classified as SLA/VRLA/AGM.

Charging AGM batteries.
AGM batteries will want a slightly higher charge voltage than traditional flooded cells. If you have a AC-powered battery smart charger with an AGM setting, all you need to do is push the appropriate button and let the charger figure out the rest. Most of the better solar charge controllers also have a setting for AGM.

If you’re doing it manually, a deeply discharged AGM battery should initially be charged at 14.6-14.8 volts, then reduced 13.6-13.8 volts for a float charge. A 100% charged, unloaded AGM battery should have an open circuit voltage of 12.8-13.0 volts, descending down per the chart below. AGM batteries are very intolerant of overcharging, so monitor your voltages carefully.

agm batteries

GRAPHIC COURTESY OF TOYOTA MOTORS USA

It is strongly recommended that an AGM-compliant smart charger be used. If you don’t already have one, it would be a very wise investment for your off grid capabilities. I have a Black & Decker VEC1093DBD that I picked up about six years ago. It’s a great unit that I use regularly. Unfortunately, it looks like it’s been discontinued. All I can suggest is to read the reviews on other models and make an informed purchase.

Going deep on “deep cycle”.
The issue of what exactly is a “deep cycle” battery came to my attention recently when Off Grid Ham reader David sent in an email asking about it. This seems like a good moment to clear the air.

“Cycle” refers to how far a battery can be discharged, usually expressed as a percentage. This is important to radio amateurs because we typically want a battery that will release a small amount of energy over a long period of time, as opposed to, for example, the battery in your car that releases very large but very short burst of electricity. It’s two totally different purposes therefore two totally different batteries.

“Deep cycle” simply means a battery can be run down very far numerous times and still be fundamentally undamaged. There is no formal definition of what is considered “deep,” but in general a battery that can be chronically discharged at least 50% would satisfy the requirement.

Deep cycle is an adjective used mostly on flooded batteries to distinguish them from engine start batteries. If you regularly discharged your car battery 50% or more, there is a very good chance you’d need to replace it a lot sooner than you’d like. It’s just not made to be drained that far that often. Many deep cycle batteries can be safely discharged down to 20%.

Keep in mind that no battery lasts forever and every charge-discharge cycle puts you closer to the end. Although you can run AGM batteries down to a 20% charge, it’s better that you don’t. Adding additional batteries to spread the load out will help avoid this issue. Even though it may cost more on the front end, over time you’ll recover a lot of that investment in the form of longer battery service life. No matter what type of battery you choose, never go with just the bare minimum capacity.

agm batteries

GRAPHIC COURTESY OF AUSSIE BATTERY & SOLAR.

AGM battery advantages:

  • Do not need water added.
  • Can be placed into any position.
  • Are physically sturdy-can take abuse.
  • Are less sensitive to low temperatures.
  • Have a low self discharge- 1-3% per month.
  • No hydrogen gas byproduct
  • Will not spill or leak if damaged.
  • Less expensive than lithium batteries.
  • Higher cost over flooded batteries partially offset by longer life cycle.

AGM battery disadvantages:

  • More expensive than flooded batteries.
  • Very sensitive to overcharging.
  • Less energy density.

What you need to know.
Very few radio amateurs can afford to use lithium batteries exclusively, nor do they want to be limited only to flooded batteries. Absorbed gas mat AGM batteries are an excellent compromise. Although the they are more expensive, they offer a lot of advantages and are still within a price range most amateurs can accept. Maintaining AGM batteries for successful long term service is not that hard: Just mind your charge voltages and have enough battery capacity so that you do not regularly discharge them too deeply: Two batteries routinely discharged 40% each will last longer than one battery discharged 80%. More often than not, the middle of the road option is the best choice.

18 thoughts on “Understanding AGM Batteries.

  1. Sean

    Great article. Since many AGM batteries come in drop in replacement sizes, I don’t think many people realize the different charging requirements. In most vehicles you can’t control that anyway so most don’t worry about it. I didn’t think about it much until it came time to replace a battery in a BMW. The manual calls for an update to the charge controller to adjust to avoid overcharging the battery. Pretty straight forward if you have the programming setup for it. Kind of expensive if you just take it to BMW and let them overcharge your wallet instead of your battery.

    1. Chris Warren Post author

      Hi Sean, although you are referencing an automotive application, you bring up a point that is relevant to off grid amateurs: If you simply swap an AGM for a flooded battery and don’t make corresponding changes to your charging system, you may run into trouble. Good solar charge controllers have settings for the different battery types; usually it’s just a DIP switch or a jumper position. You should not need special tools or technical knowledge. Thanks for stopping by Off Grid Ham.

  2. Gordon Gibby

    Great article! Especially the graphics. I use small gell cells for some VHF deployable go boxes. 7-10 Ahr, $10-20. Home solar system uses 6v Rolls flooded lead acid batteries. (38kWhr total) To achieve maximum life for these huge back up batteries, they are only discharged once a month, by 25-50%. Should still be going when I’m gone home. Distilled Water needs to be added in my case every 3 to 6 months. Although end of life for storage batteries is often defined as the decline to only 80% of rated storage capacity, it’s not real clear to me what happens after that, and batteries might still be somewhat usable.

    1. Chris Warren Post author

      Hi Gordon, I have a small 7 amp-hour SLA battery in my portable power pack and they do work well in these applications. And the price is right! I think I paid twelve or fourteen bucks for mine.

      Your Rolls batteries are among the very best. I’m sure you spent some serious cash on those! I’m kind of jealous! With good care they will last many, many years, perhaps decades.

      “End of Life” depends on what you are doing with the battery. For example, a 100 amp-hour battery that can only be charged 80% might be considered “dead” if you’re pushing a 100 watt hf rig or some other high current device. But if you all you need is to power a QRP radio or charge a few handhelds, you can probably keep flying on that “dead” battery for a very long time. If you’ve ever been in a situation where your car battery was too weak to start the car but the lights and radio still function, you understand how this principle works.

  3. randall krippner

    Nice article. I thought I knew about this but it turns out I didn’t know as much as I thought I did. I’ve had 3 vehicles that came with AGM batteries, BMW motorcycle, a Honda Goldwing and a Jeep Wrangler modded by the dealer for off road. Only the BMW dealer told me I needed to get a special battery charger for AGM use when I bought the bike, telling me that a standard automotive type charger could damage the battery because it could overcharge it. Nice to know the differences, advantages and disadvantages of each time spelled out like this.

    1. Chris Warren Post author

      Hi Randall, your dealer was correct, and the motorcycle advice applies to off grid amateurs as well: If you convert to AGM, make sure your charge voltage is set accordingly! This advice especially applies to hams who make homebrew charging devices out of car alternators. A car alternator can go well over 14 volts, so plan your systems with this in mind.

  4. Robbie

    This is a subject that a lot of hams and people in general that are doing solar back up systems for their homes. I’ve got a lot of preppers friends that put a lot of thought and money into their solar panels, wind turbines, and they’re charging systems. For some reason they don’t put much thought into their batteries. I will be forwarding your link to the info to a lot of them.

    1. Chris Warren Post author

      Hi Robbie, Batteries are the weak point of any off grid system. They can also be very difficult to understand, so a lot of hams kind of put it out of their mind not realizing their willful ignorance is costing them money and possible valuable power capacity when they really need it. I hope I’ve helped clarify some of the confusion about AGM batteries.

  5. John Adamec, NV4L

    There is another alternative for off grid storage. The Edison battery. It uses a Nickel Iron chemistry. with a sodium hydroxide electrolyte. It’s heavy as the two metals used aren’t exactly in the lightweight class but is extremely durable and capable of lots more recharge cycles than the Lithium or Lead Acid batteries.

    Unfortunately they’re not made in large quantities in the US and if you go to China for a source you have to buy a large quantity. Their pricing right now is between the AGM and Lithium chemistries.

    1. Chris Warren Post author

      John is right…the “Edison battery” has been around for over 100 years but largely fell into disfavor as better technologies were developed. They are still available and there are even YouTube videos on how to make your own.

  6. Ron Pressley

    Chris,
    I’m thinking of getting at least a couple of AGM batteries for running my shack during emergencies. However, will it make a difference if I space out my purchase of batteries a few months apart as I build up my bank. Since these will not be for everyday use I would have them on an appropriate charger (as per our article) in float mode.

    Thanks,
    Ron

    1. Chris Warren Post author

      Hi Ron, and thanks for your comment. Batteries should be placed into and removed from service as a group. If you want to spread your purchases out over time, that’s fine. Just keep your new batteries unloaded on a float charge until you have acquired them all and are ready to start using them, then place them into service at the same time. Never mix & match batteries that are at different points in their life cycle. I hope this helps, and thanks again for stopping by Off Grid Ham.

  7. Bill Phelan, N2KPE

    Excellent article. However, you don’t mention the latest upgraded LiPO4 batteries. Like the AGM’s, they do not off-gas, are extremely lightweight and small in size.
    Biennia Power in California manufacturers these batteries and is very ham friendly. The owner will give a discount IF you ask for it (he did for me, LOL).
    Look them up.

    1. Chris Warren Post author

      Hi Bill, glad to have you along. This article was specifically about AGM batteries so I did not want to drift too far off that trail. I did discuss LiFePO4 in this article from about a year ago and it does incidentally mention Bioenno Power. I appreciate your comment and hope you’ll stop by Off Grid Ham again soon.

    2. Robert Stessel, K1WXY

      We tried using LiPO4 bats in one of our ocean research buoys @ U.Maine, and found that the battery capacity per cubic foot for LiPO4’s was about 1/2 that of AGM’s. Also, the amp-hour rating of the Lith’s we bought was in “cranking amp-hours”. For low current drain applications the useful amp-hour capacity was much less than the rated “cranking hours”, perhaps because starters will still turn at a much lower voltage than dataloggers will log at, or transmitters transmit at.

      1. Chris Warren Post author

        Hi Robert, I’m not sure what you mean. For example, a 10 amp-hour AGM battery is roughly the same physical size as a 10 amp-hour lithium battery, but the lithium will weigh much, much less. So capacity as a function of volume/space should be about the same for both but the weight/power ratio will favor (by a large margin) the lithium.

        As for amp-hour ratings, it’s important to read the fine print because manufactures will manipulate these numbers to market their products. I know, that’s shocking, right? It reminds me of car engine horsepower ratings. A manufacturer may brag about how much power a car has but they don’t bother to mention it does not hit that output level until the motor is pushed well beyond normal everyday operating conditions. When in doubt, consult the data sheet for the battery you are considering and go by the real numbers it is actually producing. Compare that to what you need for your application and that will help you choose the best battery.

        Thanks for stopping by Off Grid Ham. We hope you’ll come back soon!

        1. Robert Stessel, K1WXY

          Cheery Hi Chris,

          We built a research buoy, did some rough calculations as to battery AHr requirements and filled the well of the buoy with lith bats. We set it up on a test run with all the bells and whistles running. The system ran about 4 days without any charge on the bats. That was not enough, we needed the buoy to operate without solar charge for at least a week. (These buoys are a small rapid response buoy that we deploy in the Gulf of Maine a day before a hurricane or other major storm event to monitor storm surge, and may not get any significant solar input during their deployment). Of course this didn’t come about until close to a deployment window, so we took the Lith’s out and filled the well with the same volume of AGM bats, and it ran for 10 days with out charging the bats. Not a high precision test, but the same volume of Lith’s had about half the capacity than the same volume AGM’s. We didn’t look into it further because the AGM’s were doing the job, and now I am retired. I could check back with the folks working on the project for details, but the takeaway is that the AGM’s had more capacity that the Lith’s for a specific volume of battery, and take the manufacturer’s “specifications:” with a grain of salt.

          1. Chris Warren Post author

            Hi Robert, as your story illustrates, manufacturer specs cannot be completely trusted. Lots of stuff looks good on paper, then in the real world it’s a whole different deal. I can’t argue with success! I’m glad you found a solution that works. Thanks for your comments; I hope you’ll stop by Off Grid Ham again soon.

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