The following circuit is pretty simple. I just finished building it, and it seems to work great.
There is basically a DPDT toggle switch that selects the power source from Main to Reserve battery. Center position is off. The secondary toggle switch enables bank to bank trickle charging.
The max current is 6A for charging, and due to the diodes, the secondary bank should never reach the full charge level of the source bank. (Example... source bank = 13.3V, destination bank = 12.4V)
Assuming the source bank is large, and stays at 13.3 despite the outgoing current... The destination will likely stop charging when the voltage reaches 12.95 Volts. (I believe my diodes to have a .35v drop. I need to actually verify the "lab" voltage in the real world.)
The max voltage difference between 2 "good" battery banks that I can see would be 12.0v vs 14.1v. (I've never had my battery above 13.3Volts...)
This is 2.1V, but the voltage difference would decrease very quickly as current begins to flow. The current is limited by the battery internal resistance, and the resistance of the power lines as well. Obviously this leads to power loss during the transfer, but it is small in the grand scheme of things.
Important note - Make sure both battery sources are fused near the battery! Shorts can cause fires or explosions. These lead to acid burns, loss of property, and suffering.
What would you use this for?
I am using it to put a small 12v battery bank in my office to run laptops, AA battery chargers, cell phone chargers, radio equipment... ETC.
This allows me to use the power line coming from my solar power setup, or to use the local bank.
The local bank will be connected to a trickle charger, and can be maintained at full. If the solar bank gets low, I can boost it from my local bank. Likewise, in the Summer... If my solar bank is full, I can leave the float charger off, and charge the local bank via the solar power bank.
See the schematic above. Note that SWA is 2 sides to the same switch.
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