We then both agreed that if we could fit the LVC circuits in with the CMS channels, and stll fit within the same 3" x 6" footprint that would still mount on the front of a LiFeBatt pack, we'd end up with a full blown BMS, with features usually only found in designs used in larger EV applications. It was a bit of a chore to get it all to fit, but we finally got it. Here's what the layout looks like:
In normal operation, the LVC circuits for each channel will make sure the voltage for any one cell doesn’t drop below 2.1V. When that happens, a pair of FETs will cutoff power to the negative lead. The charger is now connected directly to the BMS, using two separate wires. This can be any suitable CC/CV charger that can deliver about 3.7V x the number of cells. For a 12-cell 36V pack, that would be about 44.4V, and for 16-cells, the voltage needs to be about 59.2V. Many 36V SLA chargers typically output 44-45V and many 48V models output 59-60V, so this is fine. This can also accept any regulated DC supply that can be set to output the voltage needed (i.e. – 44.4V/59.2V…). because this design doesn’t use the CV portion of the charge profile. Instead, the shunts control the CV portion individually, and lets each cell taper the current off at its own pace.
The way this works is that a depleted pack will start charging at whatever the max current the charger/supply can deliver. Once any one cell’s voltage hits 3.65V, its shunt circuit will start bypassing whatever current the cell doesn’t need. At this point a current limiter circuit kicks in, which uses a FET to throttle back the charge current to about 2-3A (this is adjustable via a resistor, so we will set it so that the heat that needs to be dissipated can be handled by the heatsink/plate...). There is a red LED for each channel, to indicate when that channel’s shunt circuit is operating. Once all the shunts are in full bypass, which typically takes about 10-15 minutes after the first channel starts going into bypass mode, the current limit FET is turned off and the charge process stops. When this happens, there is a separate green LED that lights up, to indicate charging is complete and each cell has received a 100% charge. There is also a temperature-controlled switch that mounts to the heatsink. If the temp gets too high this switch will ground the gate on the FET, cutting off the charge current.
The plate, which mounts on the top side of the board, will serve as the front of the LiFeBatt packs. The heavy-duty shrink wrap goes just over the edge of the plate, so the circuits are protected. Two sets of wires will come through the plate, one heavy set (10-12 gauge) for the main pack leads, and a smaller set (14-16 gauge) for the charger.
Most of the existing SLA CC/CV chargers that are readily available, are limited to 3.5-4A. There are a few others, like the ZiVan NG1, that can output as much as 15-20A, but these are $500 (Patrick has them at http://www.thunderstruck-ev.com). LiFeBatt will also be coming out with some 7-8A "Speedy Chargers", but I'm not sure when those are going to be available. Also coming is a new LiFePO4-specific model from Soneil that can deliver 6A. Another option is to use any one of a number of regulated DC supplies available on ebay for very good prices. Bob found a 0-60V, 0-15A HP supply that normally would cost upwards of about $2k, but I only had to pay $125. For 12-cell/36V setups, you could also use the Mastech 5020, which is sold lots of places online. It outputs 0-50V and 0-20A. In any case, with a 10A charger and this BMS, you could recharge a fully depleted pack in just a little over an hour. With a typical 2A Chinese "duct tape" charger it takes 5 hours to fully recharge an empty 10Ah pack, and then another 10 hours to have the RC-styled balancer circuits used in the companion duct tape BMS board, bring down the voltage of the stronger cells to that of the weakest.
Bob has done a great job of not only designing a feature-rich, "big boy"-styled BMS, he was able to accomplish this using a minimum number of fairly inexpensive, readily available parts. My value-added is limited to doing the PCB layout.
I'm not sure of the exact costs yet for this new BMS, but it will be less than what separate LVC and CMS units together would cost. Well under $100, in any case.We will have the first boards and the parts for these by Wednesday. I'm hoping that we will be able to complete the testing by the end of next week. We will then finally start selling LiFeBatt 12-cell and 16-cell packs with this integrated BMS, by the beginning of the following week. We will also offer this BMS separately, with heatsink/plate, for those that would like to use it with other LiFePO4-based packs, duct taped, or not.
As soon as I get the first on together, I'll post some pics in this thread.
-- Gary
