A second battery is not automatically a safe charger. A jump-start can provide power to start a vehicle, but it does not provide controlled charging or prove that the discharged battery is full.
Can one 12V battery charge another?
For a lead-acid battery, use a compatible charger or DC-to-DC charging device with a profile and current limit approved for the exact battery. A nominal 12V source may not provide the voltage and control needed for charging, and an uncontrolled connection can draw excessive current. Follow both manufacturers’ instructions and verify the battery chemistry, voltage and capacity before connecting equipment.
What about lithium batteries?
Do not use a lead-acid charging profile for a lithium battery unless its manufacturer explicitly approves it. Battery-to-battery charging depends on the chemistry and the devices involved; when their manuals do not specify a compatible setup, use a charger designed for the battery or ask the manufacturer.
Stop charging if a battery becomes unusually hot, swells, leaks or vents. Do not charge a damaged battery.
Sources: U.S. Department of Energy: Handbook 1084-95 — Lead-Acid Storage Batteries
