Bike works while plugged in

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JaroMato   1 µW

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Bike works while plugged in

Post by JaroMato » Jan 13 2022 10:34pm

So new here.

I have had an electric scooter for about 3-4 years now. Custom Made.
Here is what i know.
72V 30ah LIpo Newhop Battery Co.
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the bike can go 40kmh with me on it.
bike was built on a 1981 honda trail 90 frame.
The BMS has been replaced once already.

Now i am having an issue where the bike will not work when connected directly to the battery. When the battery is connected and i plug the charger in than the wheel will spin.

When i turn the bike on when it is only on the battery power ...immediately the voltage drops to zero and than slowly climbs up. It does not work. The wheel does not spin, no lights. Battery also smells, and i noticed a brown spot on the BMS.

When i plug the charger in the bike wheel spins and the lights work.

I am a Battery newby. I will appreciate all help i can get.
I have a fluke volt meter.

Hopefully this is enough info to help.
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amberwolf   100 GW

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Re: Bike works while plugged in

Post by amberwolf » Jan 14 2022 3:50am

What is the smell like?

Burned plastics?

Hot metal?

Acetone, or some similar chemical odor? Perhaps kind of sweet?

The last usually means leaking cells.

What was wrong with the previous BMS?

What was the specific failure and problem that led to that BMS being replaced?


If the BMS has separate charge and discharge ports (the BMS has a C- with the charge negative wire, a P- with the discahrge negative wire, and a B- to the cell main negative wire), then the specific problem you're having means that the BMS is "working", because it is allowing current flow all the way thru it if the charger is attached, but is disabling the output FETs when it's not (because of whatever problem it is detecting), so all the FETs work.

If it has a single charge/discharge port (C- and P- are the same port), then the BMS could be failed (discharge FETs failed) or it could simply be protecting the cells from overdischarge.

That could be the cells being badly out of balance, and one set is very low, or it could be a broken sense wire to a cell.

If cells are not balanced, especially if the smell is acetone-like, then one or more cells has probably leaked, and that could be from age, corrosion, physical damage, etc.


You can test for cell balance (and broken leads at the same time) with the Fluke. Set it to DCVolts. If it isn't autoranging, set it to 20VDC.

Black lead to B- of BMS. Red lead to B+ of battery; if it has this on the BMS use that, otherwise measure at the cell main thick positive (red) wire, or the main discharge port positive (red) wire. NOte this down as the main pack voltage.

Black lead to B- of BMS. Red lead to first balance wire closest to that. If the balance connector has a red wire and all the others are a single different same color, it's probably the wire at the opposite end from the red wire.

Note the voltage down as "cell 1".

Move the black wire to the next balance lead, and note down cell 2's voltage. It will read negative, don't worry about that.

Move the red wire to the next balance lead, and note down cell 3's voltage. It will read positive this time (just like the first cell).

Repeat this until you've measured all the balance leads.

List all the voltages you measured here.

The pack is probably 24s, meaning 24 groups of cells in series. If so, there will be either 24 or 25 balance wires. If there are 24, then you should read a voltage on every test done above. If there are 25, then the first test probably reads 0v (because the bottom balance lead and the B- are the same point in that case).

If the pack is fully charged (having been left on the charger long enough for the charger to indicate charging is finished, you should read about 3.6v on all the cells, if they are balanced. You should get around 86v for the main pack voltage.

If the pack doesn't appear to be charging either, then it's probably in the same state it was when it last worked (meaning if it was partly discharged, it's still like that).

If you get 0v on any balance leads, it probably means that lead is broken between the BMS and the cells. But it could mean the cell group is so damaged it has no charge at all.

If the pack is fairly empty, you'll probably read around 2.8v on each balance wire.

If there are differences between balance wire readings, especialy more than a tenth of a volt, the pack is probably pretty imbalanced. If there is more than a volt between readings, there are likely damaged groups.


Because the pack is made of 26650 cells, it has 24 sets of these that are probably connected together on positive and negative ends by nickel strips spotwelded to them, and then those sets connected in series to each other by more nickel strips.

For 30Ah, there's probably a minimum of 8 to 10 cells in each of those paralleled groups. If any spotwelds are broken or corroded, then those cels aren't connected to the pack any more, and that group is no longer 30Ah, and so the entire pack is now limited to the capacity of the smallest group. So it will not only run out of juice faster, it will also be harder for those groups to provide the current needed to run the scooter, and the scooter probably won't perform as well as it did before this type of problem happened.

The same thing happens if some cells are lower in capacity or otherwise damaged.
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JaroMato   1 µW

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Re: Bike works while plugged in

Post by JaroMato » Jan 14 2022 10:13pm

wow fantastic response!

I would call it a sweet smell.

I will have to digest the rest of your message before i respond to the rest of it.

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amberwolf   100 GW

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Re: Bike works while plugged in

Post by amberwolf » Jan 14 2022 11:50pm

JaroMato wrote:
Jan 14 2022 10:13pm
I would call it a sweet smell.
Most likely a ruptured leaking cell. You'd have to open the pack itself up to check.

Can cells like 26650s don't typically leak easily; it usually takes significant mechanical stress to cause case failure enough to do that.

Corrosion could do it, which wouldn't be surprising given the rear wheel/shock condition, since it is tough to truly waterproof anything--usually when it's done insufficiently, it ends up doing more to seal the water that does get in in there so it can't get back out, than they do to seal the water out in the first place. :(

Overcharge or overdischarge (potentially sufficient overheating) could cause cell damage that eventually ruptures a cell.

Pouch cells rupture much more easily than can cells, especially since most "box" batteries dont' have any containment for them, or cell holders to protect them, etc.
If you found this advice helpful, supporting contributions are accepted here. Presently on unpaid medical leave of indeterminate length.

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JaroMato   1 µW

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Re: Bike works while plugged in

Post by JaroMato » Jan 17 2022 2:32am

so this is what i got.
voltage out of the main +/- wire 78.4V
voltage read from the red positive voltage wire.

3.267
6.535
9.80
13.07
16.34
19.61
22.88
26.14
29.41
32.68
35.95
39.21
42.48
45.75
49.02
52.29
55.56
58.83
62.09
65.36
68.6
71.9
75.2
78.4

I tried to get lower voltage numbers. They were all in this orientation. How can i determine what they are in the 3voltage range?

So when i add 3.267 + 3.267 = 6.534 and than continuing up adding 3.267 to each preceding number none were really far out of spec.

I could try plugging the battery in and charging it to full. However i suspect the bike will still not work. I really cannot remember why the last BMS was replaced.

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