Voltage Equalisation of Battery Packs Connected in Parallel

Abstract

In order to have a scalable battery system that can store a lot of charge and output a large amount of power, it is possible to connect multiple battery packs in parallel. This does not come without its own caveats though. During a charge and discharge cycle of a battery pack, the voltage that is present at the output varies. In a scenario where two or more battery packs are connected but have different levels of charge, it is possible that the voltage difference between them is so high that the control system will not allow them to discharge together due to the possibility of large currents flowing between the batteries, which can lead to short and/ or long term damage to the battery. This presents a problem since by only using a percentage of the packs you have connected in parallel, you don’t get the maximum power output or capacity that you could have. The solution to this is to introduce some balancing system. This system will act- ively move charge from one pack to another and allow the voltages to equalise meaning the packs can discharge into a load together again. In this thesis paper, a method for performing this balancing is described, modelled, implemented and verified. It was found that for this specific imple- mentation the time it takes to balance a full battery pack with an empty battery pack is 13 hours and 39 minutes. This full-balance was done by using a PWM signal to limit the current between two packs. Another method was also im- plemented making use of parasitic characteristics of a MOSFET. However, this only served as a method to balance when the battery packs voltages are already somewhat close. Lastly, this thesis provides several fronts for improving the balancing systems and how the implementation can be further fleshed out to shorten the time it takes to balance two packs and how to make it more robust.