# What amount of current should I use to charge a 12V car battery?

I'm planning to use two 12V car batteries in series for running 24V motors. Presently, I'm planning to charge them individually or parallel with a 12V DC adapter. I was going to buy the one with the highest power rating (400W) to charge quickly, but I heard it hurts battery life to run that much current (33A) at a time.

Doesn't an alternator push even more current (~50A at idle engine revs) when it charges the battery? Either way, if 33A is too high, how much current should I be aiming for? My battery capacity will be about 80-90 Ah and I plan to use discharge 20-30 Ah per cycle. I'd ideally like 1 battery recharged (30 Ah) in 2 hours max.

Any help would be appreciated. Thank you.

• Instead of using an off the shelf car battery charger? Commented Nov 11, 2017 at 7:28
• Typically it's 2 to 10 amps, or c / 5. Commented Nov 11, 2017 at 7:31
• I think the question you should be asking is "how do I charge a lead battery?" I would just buy a battery charger, personally. If you want to do it with a power supply, I would say 15 A should be OK, but set the voltage limit to something reasonable (maybe 14.5V), and make sure you disconnect it after two hours. Don't hold it at 14.5 volts for extended periods. Don't forget to check the water level. Commented Nov 11, 2017 at 8:09
• @Passerby I was considering them but they are trickle chargers and it will take me way too long to charge them at a rate of 1-2 A. If I can safely charge the battery with 10A of current, I'd rather do so. Commented Nov 11, 2017 at 8:10
• Any good charger is not a trickle charger. 2 to 10 amp is nominal for a normal charge. Commented Nov 11, 2017 at 8:20

The normally recommended maximum charge rate is C/4 to C/5, ie. 1/4 to 1/5 of the battery capacity in Ah.

If your battery capacity is 90Ah then 30A is C/3. The battery should handle this OK but the voltage will rise faster. Above ~13.8-14.4V (2.3-2.4V per cell) the battery will 'gas' as the water breaks down into hydrogen and oxygen. Apart from the explosion risk, this is bad because it wastes power and the battery will have to be topped up with deionised water.

At C/3 the battery will probably reach gassing voltage at around 50-70% of full charge. To get a full charge the current must then be gradually reduced to keep the voltage below gassing level. Good chargers do this automatically, but your 12V power supply won't.

A 12V power regulated supply will hardly charge a 12V lead-acid battery at all because it doesn't put out enough voltage. An unregulated supply will continue to charge the battery at gradually reducing current until it reaches its unloaded peak voltage, which could be 40% higher than its rating and is dependent on the mains voltage. Another problem is that without current limiting a discharged battery may initially draw too much current out of the power supply, causing it to either shut down or blow up!

For all these reasons and more, you should use a proper charger designed for lead-acid batteries. A regulated power supply can be used only if it can be adjusted to put out 13.8-14.4V and is designed to work in continuous current limiting mode.

• It gets worse for "maintenance free" batteries, they can't be topped up with water Commented Nov 11, 2017 at 10:26
• Maintenance free batteries should be used only with smart chargers, manual chargers are a risk in this case. Commented Nov 11, 2017 at 19:38

If you use 'car' batteries for this purpose they will not last long as they are not designed to be discharged. Frequent discharge will result in material falling off the plates and will result in cell then battery failure. Car batteries are designed to provide a very high current to start the engine for a very short time so have small plate clearances. What you need is a semi-traction or traction battery. These are what are often sold as leisure or caravan batteries. In terms of charging have a look at the information on the manufacturers web site. A lot of modern batteries will be damaged by overcharging. If the battery does not have removable caps to allow water to be added be very careful with the charging regime so the battery does not get into the gassing regime. If a sealed battery does the safety overpressure vents will open and water will be lost and it cannot be replaced - the end of the battery is neigh.

There is a rumor unspoken rule : the slower charge the better battery, it seems charging current is around C/10 and <= 10A is more favourable to prolong lead acid battery. However, better read the battery specs and datasheet to find out.

Example:

Your battery capacity is 80Ah, C/10=8A <= 10A, then maximum charging current is 8A.

If capacity is 150Ah, C/10=15A > 10A, then stick with maximum 10A for charging current.