After at least two years of frustrating, and expensive, R&D efforts, the three of us are finally happy with the "new" BMS design. So much so that we gave it a real name, instead of just BMS-v126.96.36.199.188.8.131.52k.
For continuity's sake, and to continue to keep track of updates, we will keep the version numbers around. This initial "production" version will be v4.4.
This first post will be used as a placeholder to the latest instructions, BOMs, etc., and to list the most current/up-to-date set of features. This initial offering will come in two "flavors", a 16-channel version, and a full 24-channel variant. The board, shown below, is designed to fit in either one standard Hammond extruded aluminum case, or in two cases stuck side-by-side. The 16-channel version will fit in a single 4.06" x 8.63" x 1.20" box, and the full 24-channel version will fit in two side-by-side 4.06" x 6.30" x 1.20" cases. The 1A+ shunt resistors make thermal contact with the case(s), which then acts as a heatsink.
All in all, this is not functionally a lot different than the original v2.x series. There are still shunt circuits on each channel, and a charge controller that throttles the charge current if a cell tries to go over the level the shunts can handle. The similarities pretty much end there, however. There is a big change in the philosophy of how the shunts operate with this new BMS design. With the old versions, the idea was you would set the charge voltage a half-volt, or so, above the sum of the cell voltage that the shunts come on at, and then the HVC signal was used to throttle back the charge current to keep the cell voltage right at the point the shunt would start to go into overload. At the end of the charge, you'd simply wait until all the shunts were fully cooking away, and that would mean none of the cells were taking any more current in, so the pack would be full and balanced. That was okay with the old system, which had max shunt currents of about 400-500mA. The new shunt circuits can have max shunt currents of over 1A, so trying to use the same sort of philosophy was not possible without active cooling.
What we are doing now is to set the charge to voltage equal
to the sum of the desired charge voltage for each cell. The shunts are designed to come on at just above this point. The net effect is that the shunts only come on "by exception". If the cells are perfectly balanced, the shunts don't come on at all. If a cell gets full sooner than the rest, its shunt will come on, and keep it there. The HVC signal is used more like a failsafe, tripping only if the shunt gets swamped. With 1A+ shunt currents, however, the cells have to be pretty far out-of-whack in order to overload the shunts enough to trip the HVC signal.
What this "by exception" scheme allows now is for the current to drop all the way down to 0A, if we want. Before, the current would drop to the level of the shunts, but no lower. That made doing an effective end-of-charge detection pretty much impossible. With the new charge controller we now have an adjustable current sensing circuit that will shut down the charging when current drops below a preset value, which can range from 0 to about 2A.
In most of the v4 variants we've been testing the last couple years, we needed a "Start" or "Reset" button to start or restart the charge process. I was actually okay with this, and I think Andy was as well, but not Richard. finally, he broke his brain for a couple weeks, and came up with a very clever way to eliminate the need for the Start/Reset button. Actually, it would have been much easier if I would've let him get rid of the red-green LED.
I held firm, though, so he had a splitting headache for awhile, until he came up with the fix.
Now, when the charger/supply is connected, the LED comes on orange during the normal CC and CV phases. If the HVC starts tripping, the LED will blink. Once the end-of-charge shutdown happens, the LED will change to green. It will stay green until the charger/supply is disconnected, and then it goes off again.
Initially, we will have the 16 and 24-channel boards, shown above, but multiple cell circuit sections can be combined, to support larger setups. For instance, two 16-channel boards can be used to support a 32-channel setup. In this case, a single charge controller section can be used. As before, both LiPo and LiFePO4 setups are supported.
The PCBs, along with a printed set of detailed assembly and test instructions, will be available at my TPpacks.com website. I'll add the link here when they are available. In addition, Andy, and maybe Richard, will eventually offer pre-built/tested versions.
The first boards have been ordered yesterday, so I should have them back by Monday. In the meantime, Andy, Richard and I will be working on the instructions, and the BOMs. Links to these will also appear below, and will be kept updated. I also add a bit more detail later on.
Latest revision: v4.4.3 (01-Jun-11)
Channel Variants: 16, 24
Order site: http://www.zenid.com/goodrumfechter.htm
Bill of Materials (BOM) file: