Helper packs

[E-HP]

I've been thinking about this idea for a bit. Typically high capacity cells have lower discharge limits, so there's a trade off between high current output and capacity. e.g. 25R high discharge lower capacity; 35E higher capacity, lower discharge; 30Q in between.

My thought is around how to get both high capacity and discharge without adding more parallel groups to a pack using high capacity cells, in the form of helper packs.

My current battery setup is 28Ah or 35E cells in a 52V pack (40A discharge, 60A peak), in series with 3x8Ah 6S lipos, making 20S. My general observation is that if I use the pack gently, the 28Ah pack will discharge at even rate as the lipos, with the larger pack having a higher resting voltage per cell after a long ride (expected given the higher capacity). If I thrash the bike with lots of bursts of acceleration, the lipo packs often have a higher resting voltage per cell after the ride. This makes sense, since a higher rate of discharge affects capacity. Also, under a high load, the 6S lipos will sag about 0.2V (out of 24V), while the 18650 pack will make up almost all of the sag.

My thought is to add some low capacity lipo packs (2x6S + 1x2S) in parallel with the 18650 pack to increase the discharge rate and reduce voltage sag. They would function like a large capacitor bank, and keep the 18650 pack from getting anywhere close to it's discharge rating, and hopefully get better us of the stored capacity by keeping the cells happier. It would probably take a few additional monitoring inputs to determine how effective they might be, but I'll probably just use general observations to see if it actually helps. I'll be able to monitor sag and the resting voltages afterwards, so that may be enough. If I can add another 50A of peak discharge with a few little packs, I have a feeling it will make a noticeable difference when using the packs hard.

Anyone running a setup with helper packs like this?

 

[DogDipstick]

YOu can build a molicell 42 battery with 15p in the first place and reach 1mOh / group.

You can buy EV cells and have 1mOh/ group.

You can buy lipos and have higher resistance 2mOh per group. Lol.

A typical ebike pack might be alot more reguardless than any of those three...

chain em up on switches and see if the lipos give ya a boost.

. Point being it is more effective to build with good power dense cells. In the first place. I think the argument is.


If you really had to get fancy, yes this is a viable, complicated idea, mixing the power dense and the energy dense, for application.. Hell even AMC did this with a whole electric car in the '70s. two chemistry, one power dense and one energy dense.

 

[E-HP]

I cobbled together some lipo packs, 3 x 5Ah4S + 1 x 5.5Ah2S and charged them up about 4V per cell to match my 14S pack and paralleled them. I then topped off everything by charging my main pack to 4.1V per cell (57.4V) and went out to test.

Without the helper packs, I'd see 7V-8V of sag at 50A, depending on the state of charge. Out of that, the 6S lipos only contribute about 0.3V - 0.5V of sag. I tested riding up several hills of 15%-17% grades, throttle only, maintaining around 20 mph, and a lot of hard acceleration on rolling hills and flat ground. With the helper packs in place, I was getting around 2-3V sag. I only tested for about 16 miles of pretty hard riding, but with so much less sag, the bike felt like it does fresh off the charger.

Still testing, and monitoring each pack separately when charging and discharging, but the setup seems promising. Monitoring all the packs by feel, and don't detect any heat. I'm going to do some more riding today offroad. If everything goes well, I'll replace the test packs with 3.0Ah 30C or 40C packs, 2 x 3Ah3S + 2 x 3Ah4S. The dimensions are small enough that I can mount them on both sides of my seat post, so they won't take up much room. I'll make a mounting cover of some kind and probably velcro the packs in place. I'll get decent enough packs so hopefully I can mostly bulk charge, and balance occasionally.