Industry Article

Do I need a special charger for an AGM battery? 

Special: noun: one that is used for a special service or occasion. Adjective: designed for a particular purpose or occasion.

Going by Meriam Webster’s definition, a special charger is called for when charging any battery type. In the early days of AGM batteries, when almost all the existing chargers ran on hand wound timers, and could go on charging indefinitely, you very much needed a special charger. When you plugged a battery into one of those dinosaurs, it was constant current, constant voltage, until the electrolyte had a good boil!

Fortunately, there’s a huge offering of various types of chargers in the marketplace today, some better, some worse. We’ll do a quick review of what to look for when choosing the right charger.

AGM Charge Profile (Algorithm)

First, look for a charger that has a specific setting for AGM.  It’s best to avoid chargers that claim to charge any battery with one algorithm.  Most “one size fits all” products typically only do a mediocre job at best when they try to account for everything under one umbrella.

Amp Output

Second, you need to right-size the charger to go with the capacity of your battery bank. The rule here is a minimum of 10%, but Fullriver recommends 25% of rated capacity in amps for circuits in parallel.  For example, if you’re using our DC55-12, this battery has a capacity of 55Ah.  You will want to find a charger that puts out at least 5.5 amps, but ideally 13-14 amps. If you have 2, DC55-12’s in a parallel string (12V, 110Ah) you’ll need at least 11 amps, but better if you have 22 amps. Conversely, you don’t want to throw too many amps at the battery, or you’ll run the risk of an early failure. While 25% of rated capacity is ideal, we don’t recommend more than 30% if you need to charge a bit faster. Why? It has a lot to do with surface area.  Imagine trying to fill a tea pot with a fire hose.      

Phased Charging

Third, if it’s a reputable charger, it will charge the battery in phases to ensure the battery is fully and efficiently charged every time.  A prominent example of a single-phase charger would be the alternator in car or truck.  An alternator was primarily designed to top-off the vehicle battery after it fulfilled its primary task of starting the vehicle.  A car’s alternator lacks the distinctive phases of absorb and finish/float that ensure a battery is balanced and fully charged, especially important in charging a deep-cycle battery.  The charger should have at least 3 phases of charging, Bulk, Absorb, and Finish/Float.  Some chargers will have a recovery phase in case the battery is inadvertently over-discharged and showing very low voltage. 

Voltage Parameter

Finally, attention should be given to the voltage of a given charger at each phase of the charging cycle.  AGM in general, and Fullriver AGM in particular takes a bit higher voltage range in the first two phases of charging (Bulk and Absorb) than a gel type battery but needs a lower float/finish voltage.  Most charger makers readily share their voltage for each phase of charging.  If you don’t see it anywhere obvious, a simple email to the manufacturer will usually do the trick. 

The table below shows Fullriver’s acceptable voltage range, with the numbers in bold being our recommended voltage:

In summary, we have seen that pairing the right sized charger and battery can make a great difference in charging a battery as

The impact of these voltages will have less of an impact in lighter applications like RV and will be more critical in heavy use applications like golf/ev.

Fullriver Can Help

Fullriver offers a wide range of chargers with verified algorithms to ensure great charging results.  If you’re having trouble finding the right charger for your application, please contact us and we will point you in the right direction.

We want to hear from you.  Tell us your charging story.  Did you know so much went into charging a battery?

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21 comments on “Do I need a special charger for an AGM battery? ”

  1. So, in other words, what setting on the battery charger do you use for AGM batteries?

  2. Please be more specific as many chargers have multiples of settings available.

  3. Great article Aaron! Very informative and I agree! The best way to optimize battery life is by taking the time upfront to determine the best algorithm for the environment being used in. Many people rush in for any solution and wind up paying for a hasty choice later down the road…

  4. Thank you, Scott. Your anecdote happens all too often, and then a hard lesson has to be learned.

  5. Ok, I still need some understanding. So I plan on purchasing the full throttle battery(ft965-27) for my 2011 Infiniti QX56. Are you suggesting I buy a charger also? I got an Interstate battery now which I’ve had for 4-5 yrs, never put on charger and it’s slowly diminishing. If a full throttle and charger gets double the life, I’ll gladly do it! Just explain how often your agm needs to be charged on top of vehicle charging.

  6. Wendell,

    Thank you for your comment.

    To answer your question, I’d need to know more about your vehicle usage. Is this a daily driver or a vehicle set up for overland, off-road, and weekend camping with additional accessories such as winch, light-bars, and mini-fridge?

    If it’s a daily driver used on mainly on-highway, then you do not need to use an auxiliary charger. If, however, it’s the latter example, then a good auxiliary charger, used post excursion would probably be beneficial.

  7. Hi i run 4 , 48 volt 135 agm batterys in my ezgo golf cart what charger would you recomend

  8. I need an AGM charger that can automatically begin charging, without having to push any buttons to start the charging process. I have a travel trailer and often dry camp with no electricity, so I use a pure sine wave inverter if we want to watch some TV. We then are allowed to run our generator two separate times, each being 2 hours to put some energy back into the battery.

  9. Richard,

    Please check with an RV/Travel Trailer dealer or perhaps a forum related to your make and model. We don’t have a lot of specific insight into the nuts and bolts of recreation vehicles and their equipment.


  10. I’m putting together a battery system to run the equipment in a pet grooming van that requires approximately the watts necessary to use about 50 amps. What kind of batteries are best to use and obviously I need an inverter to charge them, please advise.

  11. Orlando,

    Thanks for your comment.

    We would either the need the wattage or the voltage at 50A.


  12. Can my NAPA Commercial 105Ah AGM 2019 battery be charged with an 8 amp battery charger?

  13. You’d need to check with Napa for specifics related to the batteries they sell.

    For a Fullriver batteries, 8A would be too low, need at least 10A for our products.


  14. What charger should I buy to Charge my two power chairs (Go-PRO) which contain the Duracell 12 volt linked for 24 volt. I just installed the new – Duracell 2 ea. WKDC12-20NB 12 volt. Thanks for your help!

  15. What charger should I buy to Charge my two power chairs (Go-PRO) which contain the Duracell 12 volt linked for 24 volt.
    I just installed two new – Duracell 2 ea. WKDC12-20NB 12 volt. Thanks for your help!

  16. Could you discuss “opportunity charging” and its effect on the bay longevity. I run 4fullriver Agm with 830ah storage. I charge at 100 amps dc. Bulk takes 2 hours to reach 14.3v and after 1 hour of absorption @ 14.3 to 14.5, charging drops to 55amps, and I need to get to shore but then…lol. I usually need a second gen set run by night or I will find 12.15v in the morning. I only charge to float and full charge at the dock every month or so.
    Most cruisers do about the same. Your thoughts appreciated in advance

  17. While it’s not ideal for the batteries, if it’s what you have to do, then it’s what you have to do. The primary concern with long-term opportunity charging is a voltage imbalance between cells and batteries in the pack. The more often you can apply a full charge, the better it will be for the pack in the long-term. Our recommendation, best case scenario, is a full recharge once a week.

  18. Hi. Just a quick question.
    I hav a 38 foot class A diesel pusher motor home that uses 2 group D bus batterys with botj terminals at the same end one battery is hooked to an inverter I believe it is somethong like 3kw whick will share the amp load with the 50 amp eletrical plug that runs the inverter/charger when pluged in,even plugged into a smaller 30amp plug the two 12 volt bus bateries are active in the generation of the 110 volt power needed to keep everthing in the motorhome going only the interior light are 12 dc powered the previous owner was thekind of guy that would only take the best most expensive up grade he wanted for this motorhome Im not taking shots at the man dont get me wrong he firmly believer in the old saying that you get what you pay for .after buying this used motorhome from him i mist admit that his philosophy does prove to be correct I have spent 13 years driving this MH around the country both batteries took a dive I treated them like all deep cells need to be. Recharged at 50 %used. My question is this do I replace them with AGM type or bite the bullet and go for broke going for the newer Lipo batteries of the same Ah out put. BTW both batteries ran the moterhome when parked both recharged while driving from a battery splitter. The old owner installed a switch on the dash that brings both batteries together which allows both batteries together wired in series to start the 8.6 l, Detroit turboed diesel motor if the starter battery is to low to start the detroit. Im a fair hand at mechanics and know something about motor vehical electrical wireing but this parrell/sieries charge/start going through a single power inverter that thar aows the electricak cord fo and electerial box and then sharing the load of the motorHome I havent really sat down and figured just exactlly what it takes to do that and still add a propain powered 10kw generator into the mix
    Not sure an AGM OR LIPO set of Batteries will work with all the switching and combing the guy added into the unit I CAN ONLY TELL YOU he made itwork for over the 13 years that I have had it and never blew A fuse or poped a breaker and I would like to keep it that way. Sordy for being so damned worder bet kept touching on things that I thought you ought to know about the system to help you give me your best answer Im old shhool at 72 bont know very much about the advances that the battery industry has .ade i. The last 20 or 30 years .I am sure the oldbattery with the blak cat on the laber saying 9 lives batteries are the longest life batteries on the market aren’t making that claim any more havent seen those batteries on sale for many, many moons
    Thanks for letting me rambel on so long. That just brought another thought to mind.. . … just kidding, thanks again

  19. I have a Ranger 520c bass boat wanting to buy a 31M-AGM-A battery for cranking and electronics not sure my battery charger is compatible. It is a four bank charger, how do you determine if it will properly charge the arm.

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