Thread: "Both batteries must be the same type"?

Originally Posted by DiscoMick

I was told any two batteries could be connected by an isolator, but if two batteries were being connected together on the same side of the isolator (ie. a 2nd and 3rd battery connected together) they should be a similar size or they might only charge to the level of the smaller battery. Is that right or wrong?

Hi DiscoMick, there is no need to match batteries.

Again using my systems as an example.

Because of the unique way my isolators work, it is very common for the cranking battery and auxiliary battery to be connected in parallel for years at a time and no problems have ever arisen when this has been done.

I have never come across a single event, not just relating to my systems but with any paralleled set up using dissimilar batteries, where a problem has occurred that would not have also occurred with identical batteries wire in parallel.

As posted, it’s nothing but an urban myth BUT, I think you will find that many people get the requirements of batteries wired in series as being the same for batteries wired in parallel.

With a series set up, batteries must be as similar as possible or you will damage one or both in a series set up.

In a parallel set up, there are no similarity requirements required and you can parallel any size, type or age 12v lead acid battery with any other lead acid battery.

In the 300TDI I am running a Bond marine lead adid battery as the starter and an 80ah AGM deep cycle battery now, and have for some years been using the Traxide USI 160 set up, no issues. The AGM is a lot more forgiving than the spare lead acid I used to use - its wired to a 1000W inverter and use it for charging, fridge etc. The lead acid was flattened to nothing (and was unrecoverable) a couple of years back when I left the fridge on overnight, but to the Traxide system's credit, my starting battery was unaffected and worked as normal (good to know the vehicle will start 6000km from home even if you screw up - yes I know expensive fridges have protection from this idiocy, I don't have one).

Originally Posted by drivesafe

Hi DiscoMick, there is no need to match batteries.

Again using my systems as an example.

Because of the unique way my isolators work, it is very common for the cranking battery and auxiliary battery to be connected in parallel for years at a time and no problems have ever arisen when this has been done.

I have never come across a single event, not just relating to my systems but with any paralleled set up using dissimilar batteries, where a problem has occurred that would not have also occurred with identical batteries wire in parallel.

As posted, it’s nothing but an urban myth BUT, I think you will find that many people get the requirements of batteries wired in series as being the same for batteries wired in parallel.

With a series set up, batteries must be as similar as possible or you will damage one or both in a series set up.

In a parallel set up, there are no similarity requirements required and you can parallel any size, type or age 12v lead acid battery with any other lead acid battery.

Thanks, I think that confirms what I thought. For example, I have a starting battery and a second deep cycle battery of similar size connected together by a Matson isolator and there are no problems - both batteries are very happy. I have run a wire to the rear and intend to plug a battery pack with a third battery into it when we run a fridge in the back. I will make the 3rd battery identical to the second battery i.e. both wet cell deep cycles of the same size. I believe using the same type and size of second and third batteries connected in series like that should mean they will both fully charge. I think that's correct?


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