AGM battery chargers vs normal battery charger?

LloydyLloydy Registered Users Posts: 15
Do AGM batteries need special chargers and why?

I have a small solar project where I will use a 12V 33ah (AGM) battery + 20w panel & 6A charge controller. I need to maintain the battery for a couple of months before putting it into use. I have a 2700mA automatic battery charger and am wondering why it states on the box ......'may be used to occasionally charge a deep cycle battery'....

My question is can I use this to maintain my battery without damage? How often?



  • BB.BB. Super Moderators Posts: 26,758 admin
    Re: AGM battery chargers vs normal battery charger?

    A good place to start is reading the Battery FAQ.

    Basically, AGM batteries are very sensitive to overcharging vs flooded cell batteries.

    When you overcharge, the battery generates hydrogen gas by breaking down the water into hydrogen and oxygen. In a flooded cell battery, when this happens you simply add distilled water to bring the electrolyte level back up.

    With an AGM battery, several things happen differently. There is still hydrogen and oxygen being generated--but there is a small platinum (or similar?) catalyst in the battery cap. This recombines the hyrodgen and oxygen back into water. This both "wears out" the catalyst and generates heat. Do this often and with enough current, it will cause the battery to overheat and "vent"--letting gas and water out of the sealed battery. And you have no way to refill the battery or replace the catalyst (not quite true--there are some people that have successfully replaced the water and put on new caps--the caps are not cheap, and you don't know how much water to replace without special equipment).

    About charging a deep cycle battery (flooded cell)--the batteries will charge on a battery charger--but they are usually not designed to be left on the charger. Either the charger will over charge the battery (too much current into the battery will "boil" the electrolyte--hydrogen and oxygen--causing the battery to fail over time and/or overheat)--Or, not charge with high enough voltage (for a couple hours) to properly equalize the battery (get all cells to 100% charge and mix the electrolyte).

    Chargers designed for long term charging of flooded cell (or AGM) batteries will frequently have a remote temperature sensor (battery charging voltage is temperature sensitive) and 2-3+ charging stages--with the last stage a "float" voltage setting (backs charging voltage back to around 12.7-13.6 volts to prevent boiling a battery dry) and/or an automatic or manual equalize setting (which can be turned off/adjusted for AGM/sealed batteries).

    If you put the battery on charge until the current is reduced/voltage peaks--and don't leave the battery on charge for days on end--you should be OK.

    Also, if you are using AGM's--they have low self discharge and should be able to sit for a few months between charges (see vendor specs.). You can use an accurate volt meter to estimate the battery state of charge (or measure the specific gravity of the electrolyte for a flooded cell) and recharge it when it gets down near 80% state of charge (20% discharged). Bad things happen to batteries when they are stored below ~75% state of charge for days or longer (sulfates harden and battery storage capacity is permanently reduced).


    PS: You can check out this thread about small/good quality maintenance chargers.
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • LloydyLloydy Registered Users Posts: 15
    Re: AGM battery chargers vs normal battery charger?

    Thanks heaps Bill.

    I did read Battery FAQ which was very useful. Your additional information however was just what I was looking for.

    Did I read somewhere that a charge controller could go between charger and battery? Sounds odd to me? Are there any benefits from this?
  • BB.BB. Super Moderators Posts: 26,758 admin
    Re: AGM battery chargers vs normal battery charger?


    I am not exactly sure what you are asking about...

    Generally -- you have a power source (120 VAC, solar panel, etc.), goes to a Battery Charger (or Charge Controller) which may shift the power (from 120 VAC to 15 VDC) and provides current to a battery bank in an appropriate manner.

    At least, from a definitional point of view--A Battery Charger and a Charge Controller are sort of the same thing...

    However, there are many types of Battery Chargers/Charge Controllers out there that are designed for specific uses...

    You have the normal Car Charger (120 VAC to 15 VDC).

    You have simple Solar Charge Controllers (manage the solar panel current/voltage to the battery bank).

    And you have sophisticated Solar Charge controllers (MPPT Type) which optimize the solar panel's I*V curve to the battery bank's I*V curve (maximize power transfer)...

    There are even charge controllers that take energy from a 24/48 VDC battery bank and drop it down to charge a 12 volt battery bank (high voltage truck or something and 12 VDC accessory like a HAM/CB radio).

    And then you have other specialized controllers (one common type known as a shunt controller) which "bleeds" power from the battery into a load (air or water electric heater typically) because the charging source (wind or water turbine) cannot allow to be "unloaded" or they will overspeed and self destruct. The turbine simply outputs as much power as it can into the battery bank + load--and the shunt controller turns on when the battery is fully charged and dumps the current into the load/resistor bank as waste heat.

    And some of the Solar RE controller have multiple function and can actually be setup to operate either as a "pass through" or "shunt" controller--depending on the need of the installation.

    So--if you have a particular application, describe your needs rather than trying to "fit" your need to a particular controller--I think we can help better that way.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: AGM battery chargers vs normal battery charger?

    I think he's referring to the powering of an MPPT controller with a 'fixed' voltage source such as a battery charger or higher voltage DC system. There were two discussion of this, one involving a 24 VDC system on a boat needing to charge a separate 12 VDC system for radios. Here the caveat was to be sure a 12V battery was in place. The other was about using a charge controller to regulate output from an otherwise unregulated battery charger. The problem in that case was the automotive type battery chargers' poor rectification to DC and the need for a substantial filter capacitor.

    If you have a good battery charger that can be set for the proper AGM levels there's no need for adding a controller. If you have to make do with some other type, this is one way around it. Although probably not as cost effective as buying a good charger:

    Morningstar 15 A MPPT $233.75

    Iota 15 A battery charger $122.86

    From the battery FAQS (AGM):

    The charging voltages are the same as for any standard battery - no need for any special adjustments or problems with incompatible chargers or charge controls. And, since the internal resistance is extremely low, there is almost no heating of the battery even under heavy charge and discharge currents. The Concorde (and most AGM) batteries have no charge or discharge current limits.
  • LloydyLloydy Registered Users Posts: 15
    Re: AGM battery chargers vs normal battery charger?

    Thanks Coot and Bill.

    You were right Coot, I was talking about battery charger to solar charge controller to battery, but as suspected wont be doing this.

    I read the specs on my battery and it's discharge rate is 3%/month @ 20C (average) so it will be fine for a couple of months.

    It also states:
    ...Charge Methods: Constant Voltage Charge 77oF(25oC)
    Cycle use 14.4-14.7V
    Maximum charging current 9.9A
    Temperature compensation -30mV/oC
    Standby use 13.6-13.8V
    Temperature compensation -20mV/oC ....

    My Automatic Battery Charger states in it's specifications that the charge controller cut out is 14.2V and cut in 13.4V.

    So it doesn't quite make it to recommended charging V. Is this an issue?
  • LloydyLloydy Registered Users Posts: 15
    Re: AGM battery chargers vs normal battery charger?

    All good. I went back to Battery FAQ's and re-read a couple of sections. All should be fine.

  • DougBennett0511DougBennett0511 Registered Users Posts: 1
    I have a 48 volt electric trike. The battery is four 12 volt dry battery's. It has a charger and I recharge it daily because I ride a  lot. The time comes when the battery does not take me as far as it did. When that distance finally gets down to a few miles, I replace the battery. Replacing is expensive. Is it possible to get these 4 batteries to recharge to their full (new) charge?
  • BB.BB. Super Moderators Posts: 26,758 admin
    Welcome to the forum Doug.

    In general, once the batteries lose capacity--There is nothing you can do. There are certain battery types (like NiFe, possibly some NiCads) that can be "serviced" to restore capacity.

    You say you have a "dry battery"... Need more information to give you specific help (lead acid, agm, gel, vrla, Li Ion of some sort, etc.). "Dry" does not tell us much.

    Mostly, batteries live and die by charging and discharging cycles. Do something "wrong" will shorten the life (to months or even weeks/days of life). But all common rechargeable batteries have limited "aging" (years of standby life) and "cycling" life (500-1,500 near full discharge/charge cycles).

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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