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My car, which has a 12v silver calcium battery, is flat and I would like to get on the road as soon as possible. I got two car battery chargers that are not identical, but both can operate at 6 an 12 volts. Is it possible (and safe) to charge the car battery using the two chargers at the same time?

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  • \$\begingroup\$ You're much better off just warming up the engine while charging for 30 minutes, so that it turns over first go and then have it fully recharge while driving. An hour of running will be more than enough to guarantee a couple of restarts at least. \$\endgroup\$ – Asmyldof Mar 12 '16 at 13:54
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This is actually hard to give any answer without knowing what exactly inside of your chargers.

But I'd say that any charger should have a diode on its positive output (cathode to output) so it should be safe. However it is pointless to connect two chargers simultaneously for the same (diode) reason (the charger giving lower voltage will not open its output diode and there will be no current at all).

I'd suggest to pick only one charger which is more powerful (basing on its description or on the chargers amperemeter). This is safe and 99% fastest way.

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  • \$\begingroup\$ One of the chargers, the strongest, is probably 20 years old and I doubt it has any fancy electronics inside. The other was purchased this year and has automatic shutoff feature (which means, I think, that it detect the voltage and regulates current). Anyway, only the old charger can charge a dead battery while the new charger won't. I guess the if I connected both chargers that the new charger would detect the voltage from the old charger and shut down. \$\endgroup\$ – tsorn Mar 12 '16 at 13:36
  • \$\begingroup\$ Actually, the output current path is not the only thing that could be in there. The "lower voltage one" may break anyway, if it has output voltage and/or current control. Also a charging battery will dictate the voltage, not the chargers, so turning on will depend on its exact innards, not their open-circuit voltage. \$\endgroup\$ – Asmyldof Mar 12 '16 at 13:52
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Silver calcium batteries (just like regular lead acid batteries) need a constant voltage charge regime. The charger must be able to supply sufficient current and this goes without saying or it is unsuitable.

So, if you have a charger that provides the 14.4 to 14.8 volts and has the required current delivery for the battery then what does another charger bring to the party?

Nothing.

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