12V Guru – adding battery to caravan 12V system

Question to 12V Guru: adding battery to caravan

Hi 12V Guru, I’m going to upgrade my 12V system on my van. I currently have 2 x 100 amp hour batteries, so when I am adding battery to caravan, do I have to use a battery of same capacity or can it be different? Also does it need to be same brand or different? My current batteries are only two years old so I’d like to keep them, or should I start with a completely new system?

Cheers, hope you can help, Ron.

12V Guru answers:

Hi Ron,

Thanks for your question and it is certainly a good one as many RVers are in the same situation where they consider adding battery to caravan as they wish to increase their energy capacity to remain off-grid free camping for longer. Or maybe to ensure they have sufficient energy in reserve if needed.

The absolute best practice is to always only parallel batteries that are of the same age, capacity and brand to achieve the best performance.  Obviously if you are looking at increasing your capacity you want to be able to maximise this investment!

Can I add newer batteries to older batteries?

In your case if you add batteries which are of a different age, they will therefore have a different remaining capacity to any new battery which is added.  All batteries will degrade over time as a combination of sulphation and lead corrosion occurs. The older batteries will have been through a level degradation with the result being that this causes a rise in internal resistance.

The basic maths is that Current (A) equals Voltage (V) / Resistance (r).  Therefore, the higher the resistance internal in the battery the lower the ability to produce output current. Resistance is sometimes referred to as the “gatekeeper”. When the resistance is lower, it is enabled to better deliver the load requirements. 

If you add newer batteries to older batteries, there will therefore be different level of resistance between the old and new batteries.  This causes differences in the way the batteries will be able to product output energy and how they charge. The older batteries with higher resistance will not charge as effectively and will therefore to continue to draw energy from the charger and this lost energy will be converted to heat. This being why batteries warm up during charging.

The two different ages of batteries will not be working closely together.  Whilst they “can’t talk to each other” – they will not be performing equally and therefore the new battery bank will be weaker than planned. 

When a load is placed on the battery bank with mixed age cells the newer battery will be expected to provide more of the energy initially until it reaches the level of the older battery, bringing it down to the lower capacity of the weakest battery in the bank.  This effectively will diminish the capacity of the newer battery faster. Effectively the older battery will pull the newer battery down to its level.

Can I use different battery chemistries?

It is especially important to only place the same type of batteries (such as AGM) in a parallel battery bank.  Deep cycle batteries such as AGM have approximately 5 times less the internal resistance of a standard battery and can cope with higher temperature. Temperature impacts resistance.

Adding battery of different brand

Different brands/manufacturers of batteries utilise different manufacturing methods and have a variety of suppliers of their inputs that go into manufacturing batteries therefore they are not all created equal and again this will lead to different levels of internal resistance and therefore performance.

In summary, whilst technically you can add another battery of same voltage to build a bank of batteries of greater Ahr capacity, your investment will be at risk of poor performance and premature aging. Of course, it will also increase risk of a dying battery (within the bank) creating thermal runaway and being permanently damaged from swelling and venting.

Not recommended from Team BMPRO.

Safe travels, 12V Guru

The article was written for Caravan World Magazine.

See other AGM battery related stories by 12V Guru: Lead Acid Battery Storage and Safety, Is AGM battery maintenance free? Maintaining Deep Cycle Batteries in Storage