Running Two Batteries

[Al Skierkiewicz]

I should have stated this earlier but there is a reason the manufacturers do not recommend using the batteries in parallel. While several people have done it in the past, this is not good practice. Even if the batteries are from the same lot, made on the same day, and using the same materials, there is a difference from cell to cell within the battery that will make no two batteries exactly alike. As such, it is inevitable for one battery to have a slightly higher voltage which when placed in parallel will force some current into the lower battery. As that battery voltage diminishes, the other battery will try to back feed the first. This seesaw action will continue until both batteries are fully discharged. If the batteries are closely matched and charged and the same age (charge/discharge cycles included) this back and forth will take place very slowly in most cases. However, certain variables may change that, rapidly drawing down both batteries in a destrcutive cycle. Applications that place multiple batteries in parallel, constantly monitor battery health using specific gravity tests and other procedures to prevent damage. It is possible to use Schottky diodes to isolate the batteries from each other and from the load. These are available from RV centers and are designed to parallel batteries in RV applications.