Increase the capacity of a 12v battery bank by adding new batteries

Question

We have two 12v marine batteries connected in parallel – they power some LED lights and a small inverter used to charge a laptop and the odd camera battery. The marine starter batteries are about 2 months old and are charged from a couple of solar panels.

If we purchase a new deep cycle battery (or two new ones) what is the recommended way of connecting these to the existing batteries? Is there a name of a device that handles the different batteries (ie: charge / discharge separately)?

Is it possible to use the new and old battery bank at the same time (charge one, use the other, auto switch between)?

My understanding is connecting new different batteries in parallel would not be efficient (crappy ones drain from better ones).

Currently travelling and coming up to a bigger town and looking to buy something there – need to know what I'm asking for :)

Thanks ! Dave

Answer 1

For a quick off the shelf solution:

1) buy a diode based "isolator block". That will route your solar charging current to the two banks of batteries plus avoid discharge between the two banks. Basically, a couple high current isolation diodes with a heatsink. Both anodes connected to the solar panel. One cathode connects to each battery. This diode arrangement will send the charging current from the solar panel to the battery with the lowest voltage (lowest charge). The other battery's higher voltage will cause it's diode to block it from discharging. If both batteries are charged, hopefully you already have a trickle charge circuit on the solar panel to prevent over charging the batteries.

2) buy a "marine battery selector switch". That will allow you to select between the two banks to your load. Basically a high current, single pole, double throw switch, which also allows "both on" and "both off". A double throw switch with high enough current would work. Preferably "break before make" type. Center to the load, each pole goes to one battery.

This is not an unusual arrangement on boats, but with the motor's alternator more typically supplying the charging current instead of a solar panel.