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Charging AGM batteries

Discussion in 'Power Electronics' started by bobdxcool, Jul 31, 2015.

  1. bobdxcool


    Mar 9, 2012
    I have a 24V, 120 Volt/60 Hz inverter. I have two 200AH, 12V AGM batteries connected in series. I want to charge these batteries completely in 5 hours or lesser. What voltage and current should my battery charger have in order to charge these batteries in this time ? Also, is it safe to charge the batteries at a higher current ? Will it affect the life cycles of the battery ?

    Manufacturer recommends a maximum charging current of 50 amps. Is it advisable to go beyond this in order to charge in 5 hours, if at all more than 50 amps charger is required ?
  2. Colin Mitchell

    Colin Mitchell

    Aug 31, 2014
    Anything faster than charging a battery over a period of 14 hours involves a lot of precautions.
    You will have problems of heating, generating hydrogen gas, evaporation of electrolyte, reducing cell life, buckling of plates, and more.
    To charge 2 x 12v batteries in series you will need to deliver very close to 15.5v x 2 = 31v to get any sort of current into the battery when it is nearing fully charged.
    When you are talking about high current charging, the charger has to have a very low impedance and you need to monitor the batteries and or the cells very closely to avoid over-charging.
    You really need to know what you are doing and the "charger" has to be a battery charger and not just a 24v or 30v "supply" or inverter.
    You really need an ammeter and a hydrometer when charging at 5 hour-rate.
    Arouse1973 likes this.
  3. BGB


    Nov 30, 2014
    errm... 14.4v per battery, with a limit of 50A, should be sufficient, and should get them "mostly" charged in 5 hours.

    this would mean 28.8v total, then maybe have a circuit to drop the voltage to 27v when the current drops below a certain point (say, 2A or so), at which point it will trickle-charge the battery.

    going over about 30v at any point is probably not a good idea.

    charging at 27v will be a bit safer, but it will take a lot longer to charge (this would be more sane for a constant voltage charger, and a crude guess could be several days or more to charge them).
    below 27v, the batteries are unlikely to fully charge.

    likewise, exceeding 50A is also a bad idea. they state these limits for a reason.
  4. Alec_t


    Jul 7, 2015
    Is 120V the input voltage or the output voltage?
    If output, then that is too high for a charging voltage. If input, then a 24V output is not enough.
    You have not stated the Wattage of your inverter?
  5. BGB


    Nov 30, 2014
    I understand this as being:
    he has an inverter that converts 24V to 120VAC (such as for running wall-powered electronics), which is run off of a pair of AGM batteries, and needs a charger for those batteries.

    basically, like a big UPS or something...
  6. JMW


    Jan 30, 2012
    Check with the manufacturer, in 5 hours you can dump 250 amps (125 amps each) maximum into these batteries without damaging them. If you discharge more than 250 amps, you won't be able to recover. At any rate, once you reach X%, you must reduce the charge rate. Leaving the batteries at X%,subjects the battery to sulfation, early demise). You sir seem to running up against the problem with storing electricity, so far it's very expensive.Yes, you can charge at a higher rate as long as you factor in such possibilities as decreased battery life, fire and and a possible explosion. These batteries contain a lot of energy, it is available instantly.
    If you have space and no weight issues, you may want to try Edison Nickel Iron cells, they have a much lower density, but are immune to overcharge and have a virtually unlimited life. Just purchase as many as needed and build a 5 hr charger for what ever capacity you need.
    There are no hard and fast rules, other than thermodynamics. Recall that beach explosion a month or so ago? Turns out it was a buried copper pipe that came in contact with acid, the result was an H2 explosion that tossed a woman 40 ft. Don't know the amount or the ignition source, but it can keep you on your toes.
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