For context this is not my first ebike battery build.
I am in the process of building two additional battery packs for my new ebike.
I will be using 18650 cells with the appropriate 16s BMS for each new pack.
I will build two additional packs so that on long journeys I can connect them all in parallel using XT90 connectors.
See my simple schematic below:
The schematic outlines my current problem.
In the "Sunny Day" scenario all three batteries are at the same voltage prior to connection. This means that there will be no issues connecting them in parallel.
In the "Rainy Day" scenario one of the batteries is not at the same voltage as the other two. If this were to happen, when connecting the batteries together the two packs at the higher voltage will try start charging the lower voltage pack, each pack will be able to provide a high current which may damage either the lower voltage pack BMS or the cabling or the cells. I will be using cables with an appropriate gauge for the intended charger, but not for charging via two batteries.
I am aware that this could all be partially mitigated by a couple of methods, such as:
- Always having all three connected and charging all three at the same time
- Having a voltage gauge on each battery
I don't want to use any of these methods as I want some flexibility (sometimes I might want to run with just one additional pack) but I also want a safety element build in.
Points to note before anyone suggests a way forward with this:
- The ebike can draw up to 6000 W, at 6 kW and 67.2 V that's ~90 A. If the batteries are nearly flat 51.2 V thats ~ 117 A. Therefore any suggestions need to be able to cope with up to 117 A!
A possible solution I have come up with (see schematic below) is to use a high current relay powered by a lower current relay (and power source) controlled by an Arduino Nano. How this would work, is that the Nano would be connected to the Vout of each battery pack and if the connected battery pack was outside of a tolerance (+/- 2V) then the high power relay wouldn't be triggered and the battery would remain isolated.
I keep thinking, there has got to be a simpler way, however I can't think of it.
I hope you now understand my problem, any help/suggestions will be very much appreciated.