3 battery setup in campervan

Question

I currently have a 120 Ah AGM LP120 leisure battery in my camper van to power my 12 V fridge, LED lights etc. I have recently bought two further batteries to allow me to stay 'off grid' longer whilst fishing. However the new batteries are slightly different. They are 120 Ah deep cycle leisure battery. There is no mention of them being AGM. Somewhat cheaper also so I'm guessing, not quite the same.

The question is, am I able to parallel connect the 3 to give me the 360 Ah I'm aiming for? II currently have solar connected and a VSR charger from the van.

Answer 1

You didn't mention any brand names or model numbers, and you really aren't sure if the deep cycle batteries are AGM or not. Suggest you dive a little deeper into what you have and find out for sure. If there are filler caps or hidden filler caps, that would be a clue. The manufacturer's website may have more info.

It's not recommended to wire in parallel two or more batteries that have different chemistries because their charging and discharge characteristics and therefore their voltages at any given state of charge are slightly different. You'd end up with one battery preventing the other from providing its full capacity. By "chemistries" (all lead-acid) I mean traditional wet cell, maintenance free, AGM and gel cell.

It's also not recommended to parallel connect batteries of a different age / wear for the same reason.

Ideally, parallel connected batteries are all identical, factory-new, and remain connected together for life. I followed that practice with a bank of eight 220Ah wet cell golf cart batteries in series-parallel and got 11 years out of them.

I'm not saying that a parallel connection won't work, I'm saying it's better to find out what you have before you go any further. When you find out, you can come back with more info to edit into your question.

Answer 2

A "VSR charger from the van" is not a sophisticated charger. It doesn't do equalization or absorption. It doesn't have different settings for different AGM and Wet batteries. It doesn't adjust the voltage for temperature.

If you want to maximize the life of your batteries, buy a 5-state charger and matched batteries, and put the batteries in series (24V or 48V)

It's not the case that the VSR can't correctly do two different kinds of battery connected together: it can't correctly do even one kind of battery on it's own.

Normally we say that you have to know your battery characteristics, and should not connect batteries with different characteristics, and should prefer series connection to parallel. When you don't know your battery characteristics, you don't know which will discharge first and fastest. Normally what happens is that one battery goes flat faster, and wears out quicker, and then one of the batteries doesn't fully charge, and wears out quicker.

But you haven't even got to the starting gate: you don't know the characteristics of the charger, and you've got no way of adjusting it to match any battery. Wrong charge and discharge voltages, poor equalization and poor absorption, aren't something that is going to happen because you've paralleled the batteries: it's happening already.

Sure, mismatched batteries will charge and discharge unequally. But that won't change your charge and discharge levels, which are fixed by your van for a matched starter battery. Mismatched batteries are a bad idea because of mismatched charge, discharge, bulk, float, absorption and equalization levels. Using a VSR charger from the van, mismatched parallel batteries won't make those problems any worse than they already are: your charge system isn't good enough for that to make it any worse.