I m doing a project which is to charge a 12V battery.

What is the maximum voltage of the battery should i stop charging?
And at what voltage should i charge again?

It is a AGm battery with 18AH.
As for the ambient temperature I m not very sure.

Sorry for the lack of information

 

"A 12v battery" doesn't tell us very much.

What kind of battery is it?
Lead-acid automotive?
Lead-acid deep-cycle?
AGM?
Sealed lead-acid?
Gel cell?
NiCD?
LiPo?
NiMH?

What is it's amp-hour rating, or mAh rating?

What is your ambient temperature, and the internal temperature of the battery? You can usually measure the internal temp of the battery by the positive terminal.

The charging rate, current, and termination voltage are dependent on all of these things.

If you tried to charge a 12v car battery with a NiCD charge rate, it would take a month.
If you tried to charge a NiCD with a car battery charge rate, it would explode.

If you charged a lead-acid battery with specifications for the wrong temperature, you would overcharge or undercharge it.

 

Edited first post*

 

Very generic guidlines for a 12V lead-acid battery on permanent 'float charge' system (eg. an alarm backup battery) are to just use a voltage regulator to give 13.8V at the battery, and limit the maximum charge current to no more than 1/10 the capacity, ie. 1.8A for an 18AH battery.

For a cyclic charge, a similar scheme with voltage control and current limiting but up to about 14.8V, then once the current drops below a certain threshold (indicating near-full charge) the voltage is reduced to 13.8 to top off and maintain fully charged state without risk of damage.

You must check the manufacturors data for the specific battery to get the actual voltage and current steps, plus temperature limits if there is any chance of the battery getting hot or being used in low temperature conditions.

 

I m doing a project which is to charge a 12V battery.

What is the maximum voltage of the battery should i stop charging?
And at what voltage should i charge again?

It is a AGm battery with 18AH.
As for the ambient temperature I m not very sure.

Download and read the battery manufacturer's data sheet for your specific battery. The charge voltages for your specific battery will listed there. All you will get here is general guidance.

 

The battery manufacturers have normally very detailed charging directions. Go there!

 

For typical wet cell lead acid, I use 13.2 - 13.4 V (25C) for sustained float charge. I also use temp compensation of about -20mV/C.

Battery makers usually supply charging specs.

 

Best bet is to look at the FAQ's on your battery manufacturer's site, as has already been said.

For AGM batteries, many manufacturers recommend a charge rate not to exceed C/3, in your case that would be a maximum of 6A for a period of 3 hours. You really don't want to exceed about 15.5v during charging either, as you might cause gassing.