Wiring two (or more) leisure batteries in parallel.
Adding in an extra battery can be a simple way to ensure you have enough power to run your electrical system for a longer duration in between charges. It can also be more cost effective than buying one larger battery.
There is, however a correct way to wire them together for optimal performance and battery life.
To keep the system at 12V, we would wire them in parallel, meaning, positive to positive and negative to negative. This will keep the same system voltage, and double the amperage of the battery bank.
When connecting chargers and loads to the battery bank, it is best for the connections to be made as per the diagram above. This will ensure the batteries act together and are drained and charged evenly, This should allow your batteries to do the same amount of work and last the same length of time.
Wiring both positive and negative connections off of the same battery will lead to an imbalance and will damage one of the batteries. we DON'T recommend doing this.
When connecting two or more batteries together (a battery bank) the bank is only as good as it's weakest battery.
Do not pair different battery technologies (e.g. AGM & wet), sizes or voltages together. It's also not worth connecting an old battery with a new one unless you've tested the deterioration of the older battery is minimal as it will deteriorate the new battery until it's performance is the same as the older one.
We would recommend if you're going for a battery bank of two or more batteries, to get them both new at the same time, not only is this better electrically, but it will ensure that your chargers, etc. can be sized accordingly.
To join 3 or more batteries together in parallel, simply add the extra batteries in the middle as per the diagram below. Again, we would recommend using the same battery as the others for best performance.
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Have a great day.