Without BMS 0 problems can be. No BMS means no complaints if the voltage drops a little bit below whatever thresold upon startup.
You also didn't needed to be so over-caring with the cells, even if the vacuum cleaner drained them at an average of ≈10 A you would still have close to 27 minutes of runtime until depletion from full state of charge. You could even let it do it, at some point the device would notice a more or less sudden voltage drop and its electronics would likely complain and stop working even before it is really dangerous for the cells. I have recharged momentarily over-discharged cells after certain tests, this is harmless because you don't leave the cells to rest in that condition.
It is always wise to use proper battery management systems, in any case.
That eBay 2S BMS is the same one I linked from AliExpress sellers, non-balance version.
Well, BMS boards require to be “awakened” to operate after they're connected to cells, the controller chips are designed to work this way. I once also bought an 1S board pretty much like the one you link, with 6x 8205s, for my hair clipper mod. I ended up uninstalling and throwing it away, didn't knew :facepalm: I needed to activate it. What times those were my God! The hair clipper works fine without BMS, it has my love and light :-) though.
I see what you mean now. With the BMS installed, I had my voltmeter on the battery outputs and I don’t recall seeing their voltage drop at all when I switched on the vac. So that might imply the cells were not the culprit, but I really will not know for sure until I get the higher current BMS. Bummer that the 1S protection circuits have to be awakened too. I was hoping they would be like a protected cell. Anyhow thanks again for all the info.
As others have said before, no multimeter or standard voltmeter is gonna catch the very fast momentary voltage drop during motor startup, you may see some glimpse after a few tries but no way in practice. I am speaking of the very first fraction of time during motor startup till it gets to cruise speed, probably less than a tenth of a second. That's duty of an oscilloscope, for example.
I understand that the meter won’t be able to see the true transient value, but I would expect to see at least a minor flucation on the display. I didn’t see the value change even slightly. All that said, I think I am going to get some of those 3750mAh EizFan 21700 cells to run this vacuum. The startup current definitely won’t be a problem with them and according to tests I see they should run somewhat cooler than the LG 50T cells. The LGs will be used in my Roomba instead (which I understand doesn’t pull near as much current). I considered some Liitokala 21700 cells too, but I can currently get the EizFan ones cheaper and faster — and tests show that they run a bit cooler than the 4000mAh Liitokala cells too.
If anyone is interested, I did some research on DC motors and found the inrush current that occurs at start-up because when the motor is at rest its resistance is very low. So I got to thinking the BMS circuit might be thinking it is detecting a short — so it might not be a current issue. With some research and testing, I found a couple ways to make my existing BMS circuit work. One way is to put a DC-DC converter between the BMS output and the motor. The converters that I had on hand only handle 3A so I wired two in parallel. Their small LED lights dim significantly when I switch on the vacuum, but they recover quickly and the vac run just fine. The second way was to add a 4ohm power resistor is series between the BMS output and the vac. It limits the vacuum to only pull about 2A but once the motor is running I can short/remove the resistor then the motor gets up to full speed. This vacuum actually has two modes; one runs just the suction and the other also turns on a small motor that turns a beater brush. I always run it in the latter mode, so I could install the resistor on only the first mode then it would become just for start-up. I have also read about using a NTC thermistor that start with some resistance that then goes away as the temperature rises so that might an option too.
I haven’t decided what to go with yet, but I am leaning towards the DC-DC converter route because I can drop the voltage output of the battery down to 7.5V which is the maximum voltage that could be seen from the original NiMH battery. The lower voltage should also yield a lower current pull too — which means less stress on the vacuum and less stress on the battery.
BMS modules don't care about shorts, they care about voltage and current.
None of your above solutions is really good bcm00re, you lose energy and efficiency in every case (if anything, an ultra low resistance NTC). Lowering the voltage to the motor also doesn't really makes sense, it's not that much of a difference so grab the extra performance. My hair clipper once run from 2S Ni-MH cells, now it runs from 2P li-ion cells and it certainly is much better, for example. O:)
Improving the BMS or installing two in parallel is the way to go.
Just to be clear, all this talk was just a back-up plan in case the 20A BMS doesn’t work when it finally arrives. And yeah, I got to thinking that whatever I might be saving with the DC-DC converter I would probably be giving away just a much with its inefficiency (since it isn’t dropping the voltage that much anyway). If I have to go the resistor or NTC route, then it would only apply to the one mode (because it would be install within the vacuum) so once the vacuum is started I’d switch to the next mode and be running directly off the BMS output — so no loss there (except of the suction-only mode of the vac). In the meantime I plan to run the vacuum this weekend using two of my LG 50T cells wired without any BMS — I’ll just be sure to only run the vac for about 20 minutes so as to ensure I don’t over-discharge the cells. I am curious to see how warm they get. If things go well I’ll be ordering the EizFan 21700 cells that I mentioned awhile back. Thanks again for all your input Barkuti!
I did consider that but in the end decided against it. I would have had to spend more to get those cells (needing 5 instead of 2) and they would not lasted as long (fewer recharge cycles with NiCd and NiMH). Even with the added parts needed for li-ion cells the overall cost is lower and they should last considerably longer.
Update, the new EizFan 21700 cells seemed to work great! I ran the vacuum for about 30 minutes (I forgot to start my stopwatch for part of it hence my estimation) over the coarse of about 45 minutes or so, and the cells only got warm — instead of hot. I did fully charge them the night before, and once done vacuuming one of the cells read 3.2V and the other 2.9V. So I did run them down further than I expected as I am still waiting on my larger current BMS circuit to arrive in the mail so I can have them properly protected. I plan to run the test again with the other two Eizfan cells this coming weekend. Hopefully I can keep better track of my run-time — and keep a closer eye on the cell voltages so as not to over discharge them!
To circle back on my Eureka vacuum project, the first larger BMS still didn’t work so I also added a NTC thermistor — but even that did not work for long. On the first use a couple of the transistors got so hot their solder melted and they moved loosing connection (then things stopped). I think the seller mispresented what that BMS could handle! I finally found a better BMS (with more transistors) and that one has been working fine for a couple months. This one also doesn’t require applying some power to the outputs to “wake up” the circuit. To be clear, this vacuum has 2 modes and I added the thermistor to the one I don’t use (except to get the unit started).
bcm00re, would you mind sharing a link to the BMS that’s working? always hard to tell which ones are complete BS when looking for inexpensive boards for battery packs. Thanks for sharing
were my God! The hair clipper works fine without BMS, it has my love and light :-) though.