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I am asking specifically about automotive 12V lead acid batteries but I suppose the principles may be universal.

If I were to connect a fully charged 15V Li-ion battery to a discharged 12V lead acid battery (at around 11.5V), would the Li-ion battery charge the lead acid battery?

My theory is that since the potential at the battery terminals is about 14.7V when the car's alternator is running, attaching a 15V battery will have the same effect. Is this correct or will it result in some horrible acid flinging hydrogen explosion?

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    \$\begingroup\$ The car battery would be fine, and would be charged - but the current draw on the Li-ion might be harmful to it. \$\endgroup\$ – pjc50 Jan 28 '16 at 13:27
  • \$\begingroup\$ So by adding a current limiting circuit between the two, would that protect the Li battery while still charging the Pb? \$\endgroup\$ – Sam Jan 28 '16 at 13:30
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    \$\begingroup\$ Yes! As long as you have a circuit that limits current to a level safe for the Li-ion battery. \$\endgroup\$ – Tahmid Jan 28 '16 at 13:35
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Yes you could charge a 12V battery with a 15V battery. Since you can not control any parameters when charging this way (arguably you control voltage) it is not optimal, but a constant voltage charger is probably good enough for a lead acid battery but possibly harm your lithium ion battery. With other technologies you probably would like to control the current and possibly the charge cycle.

There are many modes of charging. I include this link after a quick google search where you can read the section "Basic Charging Methods" on this page http://www.mpoweruk.com/chargers.htm.

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