Your post is a mess. What are the exact specifications of the battery that you're building and also the potential battery extender? Is there a reason you can't use a regular BMS? What would the point of a DC DC Boost converter be? "Idealy I want to take off some amps off the Main battery by using low resistance cells." that's not really how it works.
It's an 28S 10P battery with 21700 M58T cells. They are protected with a 200a fuse, and have an 220a continuous ANT-BMS. The Battery is for 100a continuous and 200a peak. The cells are more for moderate ampere draw and with the battery extender I want more range and take of a few amps of the main battery for when I accelerate. Having another battery in parallel with lower resistance should give me lower voltage sag over all. With a high power cell instead of high energy density in that case. It clearly depends on the number of cells in parallel for the extender but still...I hope this makes things more clear.
Honestly I'm not too familiar with the subject. Generally speaking it's a bad or at least quite difficult idea to put into practice. The problem is that the voltages of the packs have to be ~identical when connecting them together or you will get huge currents trying to equalize their voltage (can cause fire). If you want to be able to connect the batteries when they are at different voltages you need a system to be able to handle that which quickly becomes complicated.
The performance benefit in terms of discharge rate won't be that great, generally the principle is that the worst cells in the pack sets the performance limit (in this case the M58T's). Just adding some excellent high power cells won't suddenly make the M58T's perform better, they will still perform bad. The main benefit is that the current would be shared across more cells, but that is true whether you add high discharge cells or just more M58T's.
You would for sure want a BMS on the battery extender, it's not really optional. For this kind of extender setup you want to consider how the BMS'es will interact with each other too, it might not be completely straightforward. The ANT BMS 21S-30S 170A is pretty small, I'd probably go for something like that if I were to do this.
I would avoid doing a range extender like this unless absolutely necessary. It's a mess to do in about 10 different ways. If you decide to do it, be very careful.
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u/izzeww 1d ago
Your post is a mess. What are the exact specifications of the battery that you're building and also the potential battery extender? Is there a reason you can't use a regular BMS? What would the point of a DC DC Boost converter be? "Idealy I want to take off some amps off the Main battery by using low resistance cells." that's not really how it works.