# Charging a 12 V lead acid battery

Discussion in 'The Projects Forum' started by donaldm444, Feb 8, 2012.

1. ### donaldm444 Thread Starter New Member

Jul 16, 2010
23
0
Hi,
I am charging a 12 V lead acid battery (Enersys 12 V 7 A h) with a current limited voltage regulator circuit (based on an LM317). I am monitoring the battery current and voltage with a microprocessor. I want to know what would be the best indicator as to when the battery is fully charged.

I am counting amp hours into/out of the battery (using the microprocessor) to get an estimate of the SOC (In the battery application manual the efficiency varies as a function of SOC). I didn't think this would be very accurate at determining when the battery is truly 100% charged.

The current drops as the battery charges, but I'm not sure what threshold to use in deciding when to stop charging (in the data sheet it says the current drops to as low as .002 C, or 14 mA during the final stage of charging). The other idea I am considering is looking for the charging voltage to reach 14.4 V (the open circuit voltage for the output of the charging circuit).

Any advice would be greatly appreciated.

2. ### R!f@@ AAC Fanatic!

Apr 2, 2009
8,785
771
So get the battery manufacture's specs and read it.

3. ### MrChips Moderator

Oct 2, 2009
12,622
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4. ### CoolBeer Active Member

Mar 29, 2010
40
6
This is what I'm using to charge my SLA(12v/7Ah):

When the current goes down to about 50mA and stays there(that's the lowest it goes) I call the charging finished.
The battery is supplying a clock circuit I built up, so most of the current at the end is for that(driving 7segment leds).

-
Kolbjørn

5. ### wayneh Expert

Sep 9, 2010
12,361
3,220
If you care about details, you'll want to learn about temperature compensation. Basically you can charge a 12V lead battery to a constant voltage, but the target voltage varies a little with temperature.

Somewhere here SgtWookie supplied a spreadsheet for making such calculations. Sorry I can't recall the link.

6. ### SgtWookie Expert

Jul 17, 2007
22,183
1,728
You might want to use this one instead:

If you thermally couple Q2 to the battery's positive terminal, the charger will be temperature compensated for a wide range of temperatures.

I'd posted it awhile back in this thread:

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7. ### R!f@@ AAC Fanatic!

Apr 2, 2009
8,785
771
Hey Sgt.
Is that circuit cool for a standard SLA. I am thinking of proto typing it.
Hope there aren't problems in it.

What do you say.

8. ### donaldm444 Thread Starter New Member

Jul 16, 2010
23
0
I did read the specs, that is why I put a post on here. As I said, they said they current could drop to as low as 0.002 CA.

9. ### bountyhunter Well-Known Member

Sep 7, 2009
2,498
507
That's how to tell it's fully charged, assuming the applied voltage is about 13.6V @ 25C temp.

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10. ### bountyhunter Well-Known Member

Sep 7, 2009
2,498
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It's about -3mV/C for each cell, I used about -20mV/C for my 12V battery charger. Set point for 25C value is about 13.8V for "cyclic" charging (restore charge quickly) and about 13.2V for "float" charge.

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11. ### SgtWookie Expert

Jul 17, 2007
22,183
1,728
Yes, it'll work fine. Set the float voltage @ 25 deg. C according to your mfgr's recommended float voltage.
If you build it as shown, it will work just fine.

12. ### R!f@@ AAC Fanatic!

Apr 2, 2009
8,785
771
Like to ask, what is the maximum AH this circuit can handle ?

13. ### bountyhunter Well-Known Member

Sep 7, 2009
2,498
507
As shown, the current limiting is set to about 100 mA.