Hybrid battery

It has been a long winter here in Canada and while my electric bike was parked and unused during those cold months, I started to test laptop cells to “expand” my current battery.

The battery I have been using is a 20s4p, 74v 6Ah nominal made from makita cells. Those cells are great and can be discharged up to 10C, and can be discharged at 3C continuous giving good performance. Laptop cells instead are capable of only 1C continuous maximum.

I don’t need 10C on my electric bike, but I would like more range, so I decided to increase the capacity of the battery using laptop cells.

I tested a lot of laptop cells (all 2.2Ah nominal) using the CBA 4 Battery Analyzer, and selected only the ones that had more than 75% of the original capacity left.

All the cells tested were “coupled” 1s2p:

As you can see from the graph, there is a lot of variance in capacity between cells. To reduce this gap in capacity I separated the cells and “re-coupled” them to obtain a smaller gap in capacity:

It’s still not perfect but much better than having 15% or more difference in capacity between cells.

Then, after this operation I soldered the laptop cells to the existing battery the battery:

On the left the battery before the addition of the laptop cells.

Then I did a discharge test on the “new” battery on 3 cells connected in series at 1C (expecting a capacity of 9.6Ah) to confirm the capacity and to see the behaviour of the cells and make sure their gap in capacity is not too wide during discharge.

The capacity is slightly less than expected but the discharging curve is still good. The temperature of the cells at the end of the test was about 35-37°C for the Makitas and 53-56°C for the laptop cells.

Here the voltage log during discharge:

The discharge graph denotes a pretty uniform discharge until the very end of the test. This graph suggests that the LVC should be set at 3.0v or higher for the cells.

All the test were performed at an environment temperature of 21-24°C.