12V Guru – Add more caravan power for new compressor fridge

Question to 12V Guru

Hi Dave,

We have recently changed our 90lt 3 way fridge to a 110lt compressor fridge and have realised we need more caravan energy so am looking to add a second battery in the caravan.

The current battery in the caravan is 105 amp. Can the second battery be a higher capacity, say 120 amp, or do they have to be evenly matched – 105 amp and 105 amp?


Eric and Robyn Hull, NSW

12V Guru answers:

Hi Eric and Robyn,

Your dilemma with requiring more energy storage to keep your caravan fully powered is not a new one and one we have been coming across more regularly with the new compressor style fridges that are being installed.

Age of deep cycle batteries

Covering age of batteries firstly, if you were to simply add a new 105Ahr battery to your current 105Ahr battery, unless they are both brand new you will be putting two batteries in parallel which are not of equal internal capacity.

The current battery in your van is probably a couple of years old at least and as such the internal capacity of the battery will have degraded slightly as the battery is used.  Depending upon the usage, a level of degradation in the lead plates inside the battery will most likely have occurred and as such the Stage of Health of the battery will be reduced from its’ original capacity of 105Ahr.

With AGM batteries it is best not to use them below 50% State of Charge but seeing as you have two batteries in parallel you will be limited by the capacity of the weaker, in this case, older, battery.  If you simply calculate from the combined estimated capacity the weaker battery will be forced to a lower State of Charge than 50% and thus break down faster, requiring replacement every couple of years.

Another issue is that whilst you will be able to equalise the voltage of the two batteries to the same level by giving a full charge of the batteries you will find that the older battery will self-discharge faster and thus put the voltage across the two batteries out of equilibrium and then you will have the stronger battery charging the weaker one when just sitting idle.

The newer battery is therefore being worked harder and will not last as expected. As there becomes a greater differential in the capacity it will take more effort to equalise the two batteries and will need to essentially “over charge” the good battery to force current through to the weaker battery and this causes even more erosion of the positive plates. 

Use the same brand of batteries

The second issue is using different brands or models can potentially cause issues due the fact that not all batteries are created equal. Different manufacturers utilise their own proprietary technology in the construction of their batteries and also differing materials used in construction. Our extensive testing at BMPRO headquarters has seen batteries of equal capacity (brand new) straight “out of the box” provide total lifetime capacity variances of 27%.

Use batteries of equal capacity

The last point to cover is “paralleling” two batteries of differing capacity (even if brand new). When connected for a period of time the battery with the lower capacity discharges first and near the end of discharge the internal resistance rises and therefore the battery of higher capacity with supply the load and not charge the lesser battery. The battery with the higher capacity is then under more pressure than intended. The same would apply in the reverse if you were fast charging where you could potentially overcharge the lower capacity battery.

Never mix battery chemistries

And, of course, it is definitely not advisable to mix different chemistry batteries at all.  Recently there has been some conjecture as to whether you can parallel Lithium (LiFEPO4) batteries and at BMPRO we have tested these and confirmed that this setup works well with for example BMPRO Invicta Lithium batteries. However – WARNING – be aware  that many of the cheaper Lithium solutions will not have suitable internal electronics (BMS) which can handle this. Always check with manufacturer’s specifications.

So, in short – if you are increasing your caravan energy by building a larger battery bank, use batteries of same capacity (Ahr), brand and same age and this will then deliver you the system with capacity and longevity that you are trying to achieve.

Happy travels – 12V Guru

The article first appeared in Caravan World Magazine

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