4/5 sub-C - 4/5SC Ni-MH cells inquiry and battery pack restoration

I am considering using four of these to replace the 12 sub-c cells in my old Roomba batteries. The other option I am considering is eight of these cells . I would pair up a couple of those 18650s in parallel then put each of those sets in series to get to 14.8V. On another forum I found where someone did that with good results; they were even able to still charge the Roomba with the stock setup. They appear to have used this PCB for protection.

@Barkuti
Thanks but they don’t have the form factor for my Roomba. Even if they did the battery packs look rather pricey. Amazon has Tenergyreplacement packs (with the shell/case) for my Roombas for about $18, but from reading the reviews I would only expect them to last a year at the most. Honestly I would be worried about the longivity of those AliExpress packs too!

bcm00re, if price is around the same go for the 4S2P HE2 setup, will perform a little bit better than 4S1P M50s. However, since the Roomba isn't really power hungry you could get some more runtime with decent energy cells like LG MH1s in 4S2P (in case cells need to be LG).

Wellp, after inquiring the seller he/she told me the above cell packs weren't 4/5 sub-C, I guess this means they use sub-C cells but @#$%, while I still haven't seen the GSR 14.4 VE-2 battery packs to my eye they look to have the same thickness than that of PSR 1200 drills, and these use 4/5 sub-Cs for sure.

If sub-C, the Turnigy sub-C 5000mAh (cough!) 4400mAh is the cell to get. Sold at Hobbyking, but whether standalone or in packs @#$% with the stock lately.

@Barkuti
Sorry to digress again, but those HE2 cells sold out right after I made my post. :frowning: I am now looking at this LG F1L cell. They have a max discharge rate of only 4.9A, and doing 4S2P means the battery pack would have a max discharge rate of 9.8A — but I don’t know if that is sufficient for a 400 series Roomba. I know some aftermarket Roomba li-ion batteries (that are no longer available) used cells that had a max discharge rate of 12.0A.

EDIT:
After some deeper inspection I think it would be overly complicated to fit eight 18650 cells inside a 400 series Roomba battery case (along with the protection circuit). So I have now turned my sights to this 21700 cell with the plan to use four of them. It is spec’d at 3750mAh but testing shows it is more like 3500mAh — but I think that is at least a good as the original Roomba battery. These have a max continuous discharge of 30A which means there is no concern about these cells providing enough current. :slight_smile:

i never found 4/5sc cells that were worth a damn unless i bought sanyo/panasonic $$$.
it was always easier to find the full size packs and rebuild with sc.
and even then it was sanyo/panasonic to get an end product that held up.
i watched several friends go through the crap aftermarket dewalt packs over and over.
while the ones i still have were rebuilt with old stock sanyo 1900’s a friend got in huge quantity cheap.
these were about 10 years old at the time.
those are still going while all those cheap aftermarket packs are in my rebuild pile for their cases.

That Eizfan cell is a rewrap of some chinese OEM, probably Lishen. It's an 0K choice but, why these cells? I mean, the Roomba doesn't really needs that much current. You would be better served by, coincidentally, a super affordable Lishen cell: the LS2170SA. The LS2170SA is sold by LiitoKala as its Lii-40A, check it reviewed at lygte-info.dk. Can be bought with tabs, a convenient thing for battery pack assembly. At the liitokala Official Store, for example. Advice: price is right now a bit higher than what it should, there's an AliExpress sale in close to 4 days and some sellers bloat their prices for it to seem they are making discounts; I bought 2 of these cells without tabs a few weeks ago and I just paid €5.32, they're at €7+ now LoL.

Also, if you can fit 4x 26650 cells you can enjoy 5000+ mAh capacity for very little additional money, with the Lii-50A (Power Long Battery INR26650-50A rewrap).

I hear you snakebite. The above Turnigy sub-C cells are performing great, though.

@Barkuti,
I didn’t notice that Efan was a Chinese cell — that does give me some pause. I can get the Efan cells for really cheap on clearance from a store in the US. That said since they are really 3500mAh and not 3750mAh like the wrapper says, that means the bang for buck isn’t what I thought it was. Taking that into account the LG 50T cells are actually a little better (mAh/:money_mouth_face:.

I was also leaning towards the Efans because they’d be more in line with the capacity of the original battery. Part of me wonders if I went to the 5000mAh LG batteries, then I could be running the robots considerably longer than they were designed for. I fear that could lead to wear problems on gears, motors, etc.

Lastly, I am planning on using one of the 21700 battery holders I mentioned in this thread: 21700 battery tray recommendations

Thanks again for all your input!

@Barkuti
So one of my BMS circuits finally arrived. It is the 2S one for my Eureka stick vac. I assembled the new battery pack using two LG 50T 21700 cells to replace five sub-C NiCD (or maybe they are NiMH) cells. I connected everything properly but no power. I then “woke” the BMS circuit by applying 8V briefly then I read power on the input/output terminals, but when I try to turn on the vacuum it only very briefly turns the motor then it shuts down and there is no voltage. When I bypassed the BMS and the vacuum starts and runs just fine on just the two 21700 cells. The BMS is rated to do 8A with 15A cut-off, and I measured the vac pulling around 5A when running and a bit higher during start-up. So I don’t know what is going on. I then hooked the new battery pack (with the BMS) to an old 12V drill and it runs that fine. I even applied some pressure resistance to the drill while using my clamp meter and confirmed the pack could supply peaks of over 7A and 6A sustained (kind of briefly). Anyone have any idea why the new battery pack and BMS won’t run the vacuum?

To be clear, the original Eureka battery contained only five sub-C cells nothing else at all — and it only has two connections/terminals.

bcm00re, all I can say is that the Eureka stick vacuum cleaner must be pulling a very high motor startup surge current, and the BMS shuts off. Get a bigger BMS, or use another one of the same type in parallel with it (and preferably thermally coupled).

On another forum someone pointed out that a DC motor that pulls 4-5A continuous should pull much more than 6A at startup; so it appears my meter wasn’t catching the peak startup current correctly. I confirmed this by attempting to power the vacuum with my variable power supply (that I found after a little research is rated to supply 15A with peaks of 18A) and found it cannot start the vacuum either! I was able to use the new battery pack and my power supply together to get the vacuum started, then confirmed just the new battery pack runs it just fine. So I need a much, much larger BMS circuit — Barkuti suggested.

no meter can catch the peak correctly, it happens in milliseconds and can get up to 10x of running current, with some motors,

Yes bcm00re needs a BMS capable of handling more current. And hope the batteries are not the culprit, because M50T cells aren't high current, and if the vacuum cleaner pulls a very high startup current the voltage from the cells may drop too much when state of charge is low. If you look at HKJ's M50T review, he only tests at up to 15 A which already is (continuous) overboard for the M50T. At 20 or 25 A of inrush with cells at low state of charge, the voltage drops below 2.8 V. Henrik's tests are done at 25 °C, but bear in mind cells don't perform that good if ambient temperature is colder (higher resistance when cold and even more voltage drop). Shouldn't be a problem, but see my point.

The following is the BMS type I once used for a small electric screwdriver with 2S Samsung 25R cells, bought them from sunmall back then but I see it cheaper now at HeltecBMS:

I installed two BMS boards in parallel, that way I made sure I wouldn't encounter any unwanted “inrush” or overload cut-off (the overcurrent protection rating actually is low for 25R cells). And the stuff worked very well.

Also, in case you may want to know the only wires to the BMS which need to be thick are the one from the battery cathode to the BMS (from the upper cell's negative/cathode to the BMS “0V” pad in the picture), and the one which comes from the BMS negative output (the black colored line). Neither the wire from the lower cell's positive/anode to the BMS “8.4V” pad nor the red line need to be thick, this is because the BMS only needs a very tiny little bit of power from the battery. Thenceforth, only a small wire is needed from the battery anode (lower cell's positive pole) to the BMS's “8.4V” pad, and no red line. The thick positive wire to the motor is directly wired to the battery anode (lower cell's positive pole) in this case. But if you do like in the picture, then yes both these wires need to be thick.

As I mentioned earlier in this thread, I tested the LG 50T cells wired directly to the vacuum and they had no problem starting it (or keeping it running of course). I fully charged them and didn’t let it run very long so as to be sure I didn’t over discharge them since I was running them unprotected.

Thanks for the other info! I ordered one of these BMS boards last night — it looks very similar to the standard one from sunmall that you linked (which I hope will be fine since I won’t be using it for charging). I also ordered a pair of these too because one thing I don’t like about the BMS board that I have (and I suspect the others might might be the same way) is that you have to apply some power to “wake” it, but I plan to removed these cells and charge them on my XTAR VC4 so that means I’ll have to wake them each time I re-install them. I am thinking those 1S protection circuits might not have that requirement. Both items won’t be in until May or June so it’ll be awhile before I get to experiment.

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!