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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.

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  • \$\begingroup\$ Instead of using an off the shelf car battery charger? \$\endgroup\$ – Passerby Nov 11 '17 at 7:28
  • \$\begingroup\$ Typically it's 2 to 10 amps, or c / 5. \$\endgroup\$ – Passerby Nov 11 '17 at 7:31
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    \$\begingroup\$ 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. \$\endgroup\$ – mkeith Nov 11 '17 at 8:09
  • \$\begingroup\$ @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. \$\endgroup\$ – user2999870 Nov 11 '17 at 8:10
  • \$\begingroup\$ Any good charger is not a trickle charger. 2 to 10 amp is nominal for a normal charge. \$\endgroup\$ – Passerby Nov 11 '17 at 8:20
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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.

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  • \$\begingroup\$ It gets worse for "maintenance free" batteries, they can't be topped up with water \$\endgroup\$ – PlasmaHH Nov 11 '17 at 10:26
  • \$\begingroup\$ Maintenance free batteries should be used only with smart chargers, manual chargers are a risk in this case. \$\endgroup\$ – The_Vintage_Collector Nov 11 '17 at 19:38

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