Thread: Mixing Battery Types

I have 2 Optima Yellow Tops as my dual battery system along with the D4 main cranking battery. One of the Yellow Tops has started to warp & I am considering replacing it with a SSB Dry Cell AGM. SSB DryCell HVT-50D AGM VRLA 12V 60Ah 580 CCA - Cyclic Use

The Dual Battery System is from Traxide. The cranking battery was replaced about 6 months ago.
Has anyone got any thoughts about mixing battery manufacturers in a single parallel system?

Originally Posted by DiscoMick

Are those batteries the same size? If they are different sizes and connected together, won't they only charge to the capacity of the smaller battery? Or is that wrong?

Hi Mick, when charging with an alternator, batteries connected in PARALLEL, can be any type and Ah ( size ) and the will discharge and charge at the same rate.

For example, if you have a 100Ah cranking battery and a 50Ah auxiliary battery, and you discharge down to 12.0v ( 50% SoC ) you will have drawn 50Ah from the cranking battery and 25Ah from the auxiliary battery.

When recharging using your alternator, it does not matter which battery charges faster than the other, both batteries, if you drive long enough, will reach a fully charged state. The alternator just sees the draw from each battery as a single current load without knowing it is from a battery or two, and continues to provide a current output until the load reduces as the batteries charge.

Now when batteries are wired in a SERIES setup, they must be as close as possible to being the same. They MUST be the capacity and MUST be the same type, and it helps if they are the same age.

Discharging in a SERIES setup will mean the same capacity is used from both batteries, but if one is larger than the other, when recharging, you can very quickly reach a fully charged state in the smaller battery and that battery will then restrict the current flow through both batteries, causing the larger battery to be continually undercharged.

The larger battery will eventually fail.