Tear down of a Hoverboard battery

[PV1]

True, voltage will drop after a charge, I just wasn't expecting it to be .1 volts. The most I've seen before on healthy cells was less than .05 volts.

I haven't done a discharge test yet, I may try to run the cells down to 3.3 volts and see what the voltage does. Given how the voltage drop has acted over the last couple of weeks, it must have been the surface charge settling in. The cells only lost .01 volts from the 10th to the 17th.

If my resistance readings were correct for the BMS, my guess is that the cells were over-discharged before the pack was put together. The fact that the person who had the board (found out this isn't the battery from my friend's demo board, but one he sold) had tried charging and using it shows that the voltage protection does work. With the one pair at <.2 volts, the nominal full charge voltage for the pack (42 volts) would take the healthy cells to over 4.5 volts. This pack had the healthy cells sitting at 4.15 volts average (which falls in spec with the balancing cut-off voltage). Any time they tried to use the hoverboard, it would shut off. So, the BMS successfully prevented over-charging the good cells, and it stopped discharging the pack more than it was when I got it.

 

[jsantala]

LiPo and "Li-Ion" cells, like 18650s, or the cells in the i-MiEV, have a very small voltage drop usually. COmpare to LiFePO4, which when charged to say 3.5 volts will drop to 3.3 volts, always. The drop will of course also depend on how fast you charged the cell and did you taper the charge at the end. Kind of shows what the problems with top balancing are - it's nigh impossible in reality to get all cells exactly as full as the next.

 

[kiev]

...
If my resistance readings were correct for the BMS, my guess is that the cells were over-discharged before the pack was put together. The fact that the person who had the board (found out this isn't the battery from my friend's demo board, but one he sold) had tried charging and using it shows that the voltage protection does work. With the one pair at <.2 volts, the nominal full charge voltage for the pack (42 volts) would take the healthy cells to over 4.5 volts. This pack had the healthy cells sitting at 4.15 volts average (which falls in spec with the balancing cut-off voltage). Any time they tried to use the hoverboard, it would shut off. So, the BMS successfully prevented over-charging the good cells, and it stopped discharging the pack more than it was when I got it.

You might take readings using the 20K or 2K scale, if available, just to double-check, but you are likely reading the ~50kOhms correctly. The meter would probably indicate an error or overload message when attempting a higher value on the lower ranges.

And it does sound like the bms card is operating correctly with a defective cell pair in the pack. So maybe something else caused the over-discharge condition of pair #5. Maybe a visual inspection could show if a cell vented?

Without a schematic it is hard to determine if any bleed-off type of balancing is done by that bms board.

Would you be able to measure the resistance of those resistors such as R52-53 to determine if they are 901 (900 Ohm) or 106 (10 MOhm)? probably safer to do with the battery connector plug removed from the board first... If they are 900 Ohms then there may be some level of balancing discharged thru those resistors but it would be slow.

 

[PV1]

Battery space has a very similar board in their Hoverboard battery, and the white paper describes the balancing function.

I got the cells up to 4.1 volts an did a test with the Hoverboard. I didn't ride around on it, but I did spin the one motor. At full speed with no load, cell pairs 5 and 6 showed the same .05 volts drop, and when I stopped the motor, both pairs went back to the starting voltage. I put a clamp on the pressure sensor and just let the motor run for a few minutes. Very little voltage drop was observed, and cell pair 5 stayed in line with cell pair 6. So, internal resistance didn't suffer any major damage, yet.

My friend ordered a Cycle Analyst, so we can get some realtime data.

And so it continues.

 

[sudenaz06]

Tesla owners, at least they ones I've met, are keen to watching the input voltage and current.

Yes, some boards have burnt down homes, but how many cars burn up in the garage? I'm not trying to make the hoverboard issue sound minor. It is a major problem that needs fixed, the problem being how flawed merchandise can be sold without undergoing some kind of testing. True, there is the UL listing, but it doesn't catch everything (as evidenced by the knock-off hoverboards).

On that note, it's even surprising that more smartphones aren't going thermal. My Droid Turbo will take its lithium ion battery to 4.4 volts during turbo charging. Standard charging takes it to 4.3+ volts. By comparison, our i-MiEV charges to 4.105 volts per cell. A fully charged lithium ion battery's voltage is 4.2 volts.

After seeing .17 volts on a pair of the cells in this battery, I'm now tempted to take the battery out of my board and check its balance. I feel confident that it is balanced, being that I have torture tested it twice (very hard riding for 4 hours straight both times) and it hasn't given me any signs of a problem yet.

On the next day that we have decent weather and I have time, I'm going to try to remove these two cells from the rest of the pack and try to recover them. .1 amp charge and far away from the house, just to be safe.

Down the road, I want to test the BMS for balancing, low and high-voltage cutoff, and see if there is over-current protection.

Battery Space sells a replacement hoverboard battery with UN 38.3 certification pending. It contains their BMS and 20 Panasonic cells, and actually has almost twice the capacity of the battery I'm working on.

How many hours does the haver battery last, where can I buy the battery at the best price?