# 12V -18Ah AGM charging question

Discussion in 'The Projects Forum' started by russpatterson, Nov 11, 2011.

1. ### russpatterson Thread Starter Member

Feb 1, 2010
351
16
I need to charge a 12V, 18Ah, AGM battery (WKA12-18NB). I have a copy of the data sheet but cannot find one online. All the data sheet says about charging is:

Recommended Charging:
Float use 2.28V/Cell, <3.0A
Cycle use 2.45V/Cell, <5.0A

I plan to use it to run some LED lights that draw about 2.1 Amps @ 12V. So I figure at 6 cells the cycle use charge at 2.45 x 6 cells = 14.7V.

My understanding is that a 3 stage charge scheme is best for SLA/AGM batteries. Where in the bulk charge stage you bring the battery up to the 14.7V for a certain amount of time (preferably until charge current drops below a certain amount if you know the charge current). Next you hold the battery at a lower float voltage until it's ready for use.

I'm using a 60 watt solar panel to charge it. I don't have a current sensing circuit available for this project and planned on just holding the battery at the high voltage for a certain amount of time (5-10 minutes?).

My question is, why isn't the charge current provided? How long should I keep the battery at the 14.7 voltage? If I knew the charge current to look for I could test using the expected charge amperage from the panel and get a pretty good idea of how many minutes that might take. Thanks!

Aug 7, 2008
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3. ### bountyhunter Well-Known Member

Sep 7, 2009
2,498
507
This looks like a motorcycle battery. I believe the specs shown by the maker:

Mean you use a 14.7V set point for the constant voltage charger of the electrical system. For float charging you set the charger to 13.7V.

The "cycle" setting is to restore charge faster that was lost starting the engine under "normal" operation cycling the battery. Float charge is for long term charging in standby mode.

4. ### #12 Expert

Nov 30, 2010
16,705
7,358
The way I read this is: Give the battery 5 amps until it gets to 14.7 volts, then immediately change to 13.68 volts and leave it there as long as you want.

Under optimum conditions, the solar cells can deliver 4.08 amps at 14.7 volts. The battery will not be at 14.7 volts when it is discharged, so a 5 amp current limiter would be appropriate. Give the battery any current from 3 to 5 amps until it gets to 14.7 V then change to a constant voltage (13.68V) float charge.

Another aspect is that if you can't get above 3 amps (the sun isn't always at full brightness), you should use the float voltage all the time. That is because the rate of charge changes the terminal voltage target. Higher charge current will arrive at a higher terminal voltage when the battery is charged. If you try to give the battery 2 amps until it gets to 14.7 volts, it might be full long before it gets to 14.7 volts.

5. ### iONic AAC Fanatic!

Nov 16, 2007
1,422
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4A on the equator, but don't expect much more than 3A and even lower with cloud cover or non-optimized light direction. The solar panel will be unregulated and could reach as high as 19V. A solar charge controller would be the best way to go.

6. ### russpatterson Thread Starter Member

Feb 1, 2010
351
16
Yes, I have a solar charge controller that I've built. However I don't have a current sensing circuit yet so I just hold the battery at 14.7V for 10 minutes then hold the voltage at 13.8 until I need to run the lights.