I know this topic has been beat to death, but I’m having trouble wrapping my head around the whole battery thing. My goal is to build a board capable of hauling my extremely heavy frame around, so in order to do that, I’m going to try to avoid bottlenecks wherever possible. This means I want to be able to push my motors, vesc, and battery to their theoretical limits without damaging any component.
So here’s my dilemma. Lipos offer high current so I’ll never be bottlenecked when using them with the potential to push well over 100 amps continuous. But, I prefer the idea of a prebuilt 18650 pack LI-ION w/ built in BMS for convenience of charging. But, every prebuilt LI-ION pack I’ve seen has a stupid low max amp limit. For example:
This battery back is limited to 60A continuous and 150A peak. Meaning, I could only pull 30A to each motor.
Let me know if I’m completely off here. I’m still learning and in the research stage of a build. Also, drop in some links if you know of higher discharge LI-ION packs.
Well, what kind of board are you building? For MTBs or similar you probably want to increase that “p”. 12s6p to 12s8p. 4p just doesn’t allow for high current.
So if you want a samsung 30q pack that pushs 100+ amps of current without sagging horribly, then you will need a 12s8p pack. With a bms, that would cost around 500$ if you built it yourself, or 700-800 dollars if you buy it from a vendor.
If you are looking for pure power with only 5 to 10 miles of range, lipos are a good choice. I would recommend the Turnigy Graphine brand. These packs only weigh about 2 pounds each, compared to a 12s8p pack that weighs 11 pounds. You can find those cells at https://hobbyking.com/en_us/catalogsearch/result/?q=graphine%206s.
There is a cheap alternative for lipos called SPIM08HP. Each cell can discharge 200a continuously and they only cost 6$. The only downside is that they are quite large and the cells were previously used, so you must buy a decent balance charger and lipo alarms.
Oh yeah you’re definitely right. With a 4p build I’d only be able to have a continuous discharge of 60A if I went with 30q’s. I’ve been looking at @psychotiller battery packs as well but I did see that the BMS was charge only. Would any discharge protection come from an inline fuse i’d have to place myself or would the VESC settings be sufficient?
I love the idea of these flat pack cells, but man, I’d be super out of my depth with diy battery building. I know the basics of configuration but buying a spot welder and safely routing wires is just a lot for my second build. I suppose if there was a reputable builder for these, they would fit my needs pretty well.
The VESC will only limit the charge current and do a cut off on too low voltage. No balancing or anything is happening there of course.
A fuse triggers very late, it is a “very analog” process. A 80A fuse can take 100+A and more for a significant time until it blows. That won’t protect your cells, it is just a safety measurement to prevent fire.
I also forgot to mention that those flat cells are very easy to assemble. Each cell comes with holes on each terminal where you can clamp each terminal with screws and locknuts. I didn’t even have to solder the balance wires or the main discharge wires because you can just clamp them down with washers. I’m an incompetent 14 year old and I was able to put these cells together without issues.
Oh, sorry to confuse. Sure you can set a (separate) limit for the battery and motor current. It’s just that this is different from a discharge BMS as it does not care about the individual cells, just the whole pack. But in general this is fine, as long as you always balance charge the pack and don’t set the voltage cut off too low.
Edit: but note that longevity is better if you balance discharge as well.
Yes, that’s what you normally do with LiPos, the ESC draws from the whole pack without balancing, then you charge it with a balance charger like a regular hobby RC Lipo charger. Then the next time you ride all cells are perfectly balanced again. After the ride there might be some discrepancies which you then just balance away while charging.
The only person I’ve seen building these packs so far has been eboosted out of Peru. Do you know anyone else who is building these packs with a charge only bms?
I just built a 10S3P flat pack with 18650 cells for a friend and I’m starting one for me with 21700 cells for a MTB (both with charge only BMS).
Drop me a message if you’re interested in one.