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How To Increase Lithium Battery Capacity?

This month we received a question from Brett about the pros and cons of a single v multiple batteries to increase the capacity in your van.

Question: I seek your assistance in the number of batteries connected vs a single battery please.

I am looking at replacing my existing AGM batteries with lithium and, as electrical matters are a mystery to me, would like your advice.

Is it better or more efficient to (a) charge one 300Ahr battery, against (b) charging one 100Ahr battery connected to one 200Ahr battery, thus giving 300Ahrs availability?

I am aware that the more cable is used, the greater the voltage loss, connecting two batteries as against one. Projecta is releasing a 35A charger later this month which is compatible with lithium batteries.
Any help would be most appreciated. (from Brett Keane)

Answer (David Bayliss): Hi Brett. Unfortunately, there is not a direct answer to provide in offering you a solution as it will depend upon your circumstances. There is however a strong preference towards a single larger battery.

Firstly, however let’s address the configuration you are proposing – 100Ahr + 200Ahr to provide 300Ahr. Not a preferred solution as we would recommend either a single 300Ahr or a 3 x 100Ahr solution. Lithium batteries have their own built-in BMS (internal management electronics) systems and, as your proposed battery setup has different sized batteries, they would require different voltage and amperage at different stages of their charging. From a discharge perspective it would function, however when recharging the collective BMS’ would not be operating optimally as each battery BMS acts in its own interests and not that of the total system. While the same logic applies for the 3 x 100Ahr system setup the batteries are at least of the same size and capacity and thus have the same requirements and not compete. Utilising an existing BMPRO 35A lithium compatible charger will see the paralleled batteries as one bank. 

Next, let’s tackle weight and size. We hear many make the case that a larger capacity battery with take up less space – true but it will be a heavier solution. The first thing to note is that 3 x 100Ahr multi battery setup will take up 0.0333m3 in standard battery configuration whereas a 300Ahr takes up 0.0311m3 – a negligible seven per cent difference, however if you were then to include the connections, mounting brackets etc it would be closer to a 15-20 per cent space saving difference in favour of the larger 300Ahr.  Weight though is a consideration - a larger 300Ahr battery weighs in at 35kg whereas 3 x 15kg = 45kg for the multi battery setup – plus the additional wiring, mounting and fusing. So, from a size and weight perspective the larger battery wins.

Reliability would be the next consideration. No power can turn a holiday into a disaster when there is spoilt food, warm beer, no lights and kids iPads can’t be charged. There are two trains of though here. Firstly, with multiple batteries there are now multiple points the system can break down, the BMS, wiring, connections etc. However, if there was a problem in an off-grid scenario and not in reach of a service centre, a fault battery can be removed from the battery bank and reduce the capacity to say 100 or 200Ahr and still provide power whereas if a 300Ahr battery has an issue than there is now power at all.  In addition, if a replacement was required than a multi battery bank it would be a cheaper option to replace a single 100Ahr battery.  Most have not worried about this over time as a single battery has normally been sufficiently reliable. Remember also that if a single battery in a battery bank starts to degrade prematurely it will draw the effectiveness of the other two batteries down with it.

From a maintenance perspective a 300Ahr battery is going to come in slightly ahead as there are less things to maintain. All battery setups require the checking of cables and connections as they can vibrate loose, cables degrade, foreign objects can foul the area or terminate build up if subjected to dirt or corrosion.  With multi battery setups we would recommend recharging each of the batteries individually “balancing” after long periods of storage or every 24 months if used regularly.  This ensures all batteries are maintained at the same collective voltage for optimal performance.

Overall, larger battery is lighter, smaller, less points of potential failure, less maintenance but you need to ensure that it is of proven quality and reliability.

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