How to connect batteries to your solar installation
At the heart of any solar PV system sits the battery bank; the battery bank can be either a single or multiple batteries connected to each other.
Batteries are connected to each other in order to increase:
- the battery voltage (in Volts, V), or
- the battery capacity (in Ampere hours, Ah), or
- both capacity and voltage.
(Always remember power = Volts x Ampere, or W = V x A).
You can connect batteries either in series or in parallel. The following happens when batteries are connected in certain ways:
- in series: the voltage increases.
- in parallel: the capacity increases.
- A combination of in series and in parallel: both voltage and capacity increase.
A number of batteries connected in series is called a string.
2 batteries in series
2 batteries in parallel
4 batteries in series/parallel
4 batteries in series (a string of 4)
If you need a large battery bank, and you use 12V deep cycle batteries, it is recommended to use a maximum of around 3 to 4 strings of 4 batteries, with the strings connected parallel. If you use more, it will become difficult to balance the battery bank (e.g. keep them at the same voltage). Smart Lithium batteries like SunStore’s I-G3N batteries, have cell balancing and a smart Battery Management System (BMS). This means they can communicate to each other, as well as to any Victron GX device.
Connecting batteries in parallel
One of the common mistakes in connecting batteries in parallel is to connect all the batteries together and then connect one side of the battery bank to the installation, as in the image.
When a load is now connected, the power coming from the bottom battery will only travel through the connection to the installation. The power from the second battery has to travel through the installation connection, as well as through the connecting leads to the bottom battery, etc. The power of the top battery will have to through the installation connection, as well as all the cables that connect all the batteries.
Each lead has its own resistance. This means that the top battery will encounter a lot of resistance and will therefore provide less current than all the other batteries.
If this battery bank is charge, the bottom battery gets charged with a higher current and voltage than the top battery. Current will always choose the path of least resistance. Most of the current will therefor travel through the bottom battery. And only a small amount of current will travel through the top battery. The result is that the bottom battery is worked harder, discharged harder and charged harder. This means that the bottom battery will fail before all the other batteries.
The correct way of connecting multiple batteries in parallel is to ensure that the total path of the current in and out of each battery is equal. There are 4 way to do this:
- Connect diagonally.
- Use a positive and negative post. The cable lengths from post to each battery need to be equal.
- Connect halfway. Make sure all cables have the same thickness.
- Use busbars.
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Source: Adapted from: Victron Energy, Wiring Unlimited - Rev 06 47