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Charging an AGM battery in 5 hours

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Member level 5
Mar 9, 2012
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I have a 12V, 200AH AGM battery. The maximum charging current recommended by the manufacturer is 50 amps. Now, I need to connect two AGM 12v batteries in series to connect to the input of my 24 volt inverter with battery charger.

Is it possible to charge these AGM batteries in 5-6 hours ?

What is the current and voltage required from battery charger in the inverter to charge the AGM batteries in 5 hours ?

Is it safe to charge the battery in 5-6 hours ? If not, what is the recommended charging time ?

Any special precautions to be taken with this kind of battery charger ?

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The challenge with two series batteries is mismatch and accelerated aging from pushing the charge limits as one battery always reaches max boost voltage before the other and over voltage boils out the electrolyte.

You can boost to 80% charge in 5~6 HRS but to obtain the normal life expectancy, you must follow with individual regulated float voltage per battery which is normally done after the boost current has dropped to 10% of initial boost current.

Design for overvoltage protection is required by series and/or shunt regulation, to achieve normal life.
As one battery ages faster than the other ESR increases and cell overvoltage occurs.

Reducing boost current requirements and increasing charge time, will improve life of the battery.
Maintaining accurate float voltage per battery is essential and often temperature compensated.

The usual recommended charge rate is C/10, or one-tenth of the Amp-Hr capacity. Thus it takes 10 hours to fully charge them from a 10.5V level which is considered to be fully discharged.

So this means 20 A is a good rate for your batteries, although the Absorbed Glass Mat (as you have) can take a little rougher treatment than regular lead-acid type.

The 50 A rate is okay if you are in a hurry but it taxes the battery's ability to absorb the charge that quickly.

For the first few charges, it is a good idea to check each cell periodically with a hydrometer, to make sure none gets mismatched.

If you don't have a hydrometer, at least check battery #1 against battery #2 periodically with a voltmeter, to make sure neither rises to a volt level signifying overcharge (a hazard referred to in post #2).

I agree with Brad.

As Specific gravity correlates with battery ESR, the best time to compare mismatch is during high current discharge or while boost charging

Normally 1% ~2% difference is acceptable when new.

Can it be charged in 5 hours? The short answer is yes.
But the long answer is: if you do this heavy charging routinely, your battery life can be severely shortened.

Play it safe and charge them at C/10.


this is not the complete datasheet.
You shoud find a section about charging.

max, current is given with 25% of the Ah rating. This is for quick charge of a single battery.
It is also recommended to charge it for 24 hours.


A 24hr boost charge is recommended once every 3 months to help equalize the electrolyte, ESR and s.g.
But if it is needed to Boost in 5 hrs every day, the float charge must be used.

Continued daily charge & discharge has limitations in life expectancy, requirement for 24 float charging and 24 hr periodic boost charge.

What is you application?
Emergency standby power or
Solar storage power

Aggressive charging causes gassing in the latter parts of the charge, if the internal chemical systems cannot recombine the H2 & O2 to water quickly enough, a crude valve arrangement will operate to release the pressure and you will lose the constituents of water, over time you will de-hydrate your battery and it will lose capacity and get warm when left on a float charge, so a charger that reduces current as the volts come up is the best for any type of sealed battery (Pb-acid), you can get ~80% recharge in 5-6 hours, but overnight is always better at a reduced rate if you app allows it...

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