Interconnecting Batteries for Battery Bank

For any off grid renewable energy system the battery bank is probably the most important component. It doesn't matter how much power you generate - if it is not stored safely and efficiently then you will have no electricity when you need it. Batteries are also one of the most expensive parts of wind, solar and hydro power generation systems so they need to be well cared for.

Unless you have a very small system you will need more than one battery - therefore you will need to connect the batteries to one another to form a battery bank.

Below is an illustration showing how this is often done:

The Problem

Because of the small amount of resistance in the cable used to interconnect the batteries, and from the connection between the cable and the battery posts, the battery closest to the installation is charged the most, discharged the most, and worked harder, whereas the battery furthest from the installation is charged the least, discharged the least, and worked the least.


The Reason

The power from the bottom battery has to pass through the main connection leads whereas the power from the top battery has to pass through the main connection leads and another four sets of interconnecting leads. Although the resistances are tiny - it is the fact that they are so small that makes them have such a big effect on the current flowing to each battery.

An electronics company used a computer simulation in 1990 to calculate the following assuming a battery internal resistance of 0.02 Ohms, interconnecting lead resistance of 0.0015 Ohms per link, and a total load on the batteries of 100 amps:

The bottom battery provides 35.9 amps.
The next battery up provides 26.2 amps.
The next battery up provides 20.4 amps.
The top battery provides 17.8 amps.

...which means the battery closest to the installation is worked twice as hard as the battery at the top of the battery bank! These surprising findings have since been reproduced in real world situations.


Connecting Batteries in a Battery Bank

So it the example given above shows you how NOT to connect batteries to make a battery bank, how should you do it? It is actually very simple - instead of taking the negative AND positive feeds from the same battery (in the example above it was from the bottom battery) , you should take one feed from each end of the interconnected battery bank - e.g. +ve from the top battery and -ve from the bottom battery....