# Charging a 12 V lead acid battery

#### donaldm444

Joined Jul 16, 2010
23
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.

#### R!f@@

Joined Apr 2, 2009
9,894
So get the battery manufacture's specs and read it.

#### MrChips

Joined Oct 2, 2009
29,858

#### CoolBeer

Joined Mar 29, 2010
40
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

#### wayneh

Joined Sep 9, 2010
17,475
Any advice would be greatly appreciated.
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.

#### SgtWookie

Joined Jul 17, 2007
22,230
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:

#### R!f@@

Joined Apr 2, 2009
9,894
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.

#### donaldm444

Joined Jul 16, 2010
23
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.

#### bountyhunter

Joined Sep 7, 2009
2,512
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).

Any advice would be greatly appreciated.
That's how to tell it's fully charged, assuming the applied voltage is about 13.6V @ 25C temp.

#### bountyhunter

Joined Sep 7, 2009
2,512
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.
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.

#### SgtWookie

Joined Jul 17, 2007
22,230
Is that circuit cool for a standard SLA. I am thinking of proto typing it.
Yes, it'll work fine. Set the float voltage @ 25 deg. C according to your mfgr's recommended float voltage.
Hope there aren't problems in it.
If you build it as shown, it will work just fine.

#### R!f@@

Joined Apr 2, 2009
9,894
Like to ask, what is the maximum AH this circuit can handle ?

#### bountyhunter

Joined Sep 7, 2009
2,512
Like to ask, what is the maximum AH this circuit can handle ?
As shown, the current limiting is set to about 100 mA.