will_newton said:
I am interested in the BMS's, but have no experience populating a board. i can solder wiring/connectors fairly reliably. I do know some basic electronics. What's the chance of me getting one of these boards together? I'd like to try it out. Are there pre-populated boards by a third party or a link to some instructions? Help encourage my e-bike habit! It would be good step to helping encourage some safety in charging. If it's doable then i can post up some of my build in this thread for future reference.
thanks!
If you can solder, you can probably handle this, but it does take a fair amount of time. Check out the instructions
here, to get an idea of what's required. On the website, which is
here, you can find bill-of-material (BOM) parts list text files which can be used to automate the parts ordering from www.mouser.com, using their BOM tool. These are for the LiFePO4-based version of the BMS, but if you want to do a LiPo version, there are a few of part changes for each channel, but I can help with that.
The BMS really has two independent functions, low-voltage protection for each cell, and a way to balance the cells during the charge process. For my own use, I prefer to separate these functions into two boards, an LVC protection board that gets mounted on every pack I have, and then one "Charge Balancer" board that is in a separate box that basically goes between the pack and the charger/supply. The reason is that I have lots of different packs, and it simply is too much work to solder that many parts on a board for every pack.
I don't charge them all at once, and I certainly don't have separate chargers for every pack, so I really don't need the charge balance circuitry on every pack. During discharge, the only thing that is really required, in my opinion, is LVC protection on each cell.
One of the big strengths of Lithium-based chemistries is that they will hold their voltage up high, all the way to the end, and then the voltage drops very quick. If you don't catch it, and it goes too low, the cell will be damaged, or even destroyed. The higher the C-rating on the cell, the better it will hold up its voltage, and the more pronounced this "fell off a cliff" voltage drop effect will be. Some would argue that LiPo packs are made from matched cells, and they always stay balanced, so why can't they simply use the controller's LVC function? This comes from how LiPos are used in the RC community, where they usually get balance-charged every time. If you do this, and/or don't drain your pack down below about 20% SOC, almost any Lithium-based cell/pack will stay fairly well balanced. Drain any of these, including LiPos, down to LVC cutoff a few times,
without using a charger-balancer combo, and the cells will most certainly drift apart in voltage/SOC. That's because it doesn't matter how closely matched the cells were at the time of manufacture of the pack, they can and will drift apart a bit, over time, due to a variety of reasons. With balanced charging every time, these differences aren't really noticeable.
Anyway, as soon as I get this next version of the LVC board done, which will have board-mounted connectors for the pack's balance plugs. I will start making them available on my site. I will probably offer these in three versions, board-only with a parts list (like the BMS boards...), a kit with the board and all the parts, and assembled versions. The Charge Balancer will probably be board-only, but again with a set of instructions and the automated BOM lists.
-- Gary
