newbie - need help with Segway 73.6V 5600 mah battery

[DanDaMan]

I just bought a Segway for cheap - cheap cause the PO said it needed batteries. It has two lith-ion batteries, each being 73.6V 6000 mah, each made up of 92 LifePO4 (3.2V @ 1500 mah 18650 each) cells in a 23S-4P setup.

When I first got the Segway (which wasn't charged in a long time), each battery had ~75V so I plugged them into the Segway's charging port - after 10 hours I saw one of the batteries flashing green (the manual says that means it was charged and in good shape) but the other was red (again in manual suggesting the Segway or BMS found an issue and stopped charging it). I tested them again with my multimeter and now both were ~84V (which confirms the built-in charger works, and that the cells are charging to their full range - but again odd that one battery was flashing red). So I tried the Segway and it does show full charge on the display but after a minute of average to high speeds it auto shuts off with a battery warning - though if turn unit back on I get the full battery display and again works for a minute and then shuts off.

So being they both charge up to their full voltage, I suspect some of the cells are dead or not accepting enough capacity - hence why the Segway gives me the full battery display (cause the voltage is 100%) but dies cause of low amps (capacity). And being the batteries are grouped into 4 bundles (in parralel) with 23 of them connected in series, being I get the full voltage this would suggest at least 1 battery in each bundle is good (though my guess would be most bundles are good, or most have only 1 bad cell and 3 good ones).

So I want to try and tackle this myself. I am a newb, but very handy in general, and have tinkered with electronic repairs but never direct with battery packs. I have spent the last day on google and youtube and think I can handle this, though at same time realize these can be dangerous. That is why I'd like some advice.

I'd like to know the best way to diagnose the pack. I really don't want to take each battery apart into 92 cells and test each one separate. Looking online it can take a few hours per cell to test the mah, so I don't want to do it this way. Is there a way to test each bundle of 4 - each bundle should be 3.2V @ 6000. I have a imax B6 that a friend lent me - can this be used to test the capacity? If so, can I use this device to test more than 1 bundle at a time? Can I somehow test two or three bundles together. Here is a link to the same battery I have for reference.

View: https://www.youtube.com/watch?v=B7T9SISua8s


Thanks

 

[ajw22]

Whatever you do, do not charge the battery until you know exactly what the problem is.

Measuring the voltage of the whole battery is not particularly useful for diagnosis. Measure each of the 23S stages individually. I suspect most will show 3.7V, but one or more is going to show somewhat ~ significantly lower voltage, likely decreasing over time. Locating those cells would be the first thing to do.

 

[DanDaMan]

Whatever you do, do not charge the battery until you know exactly what the problem is.

Measuring the voltage of the whole battery is not particularly useful for diagnosis. Measure each of the 23S stages individually. I suspect most will show 3.7V, but one or more is going to show somewhat ~ significantly lower voltage, likely decreasing over time. Locating those cells would be the first thing to do.

Thanks. I can't really charge it cause i can only do it through the segway when plugged into it and the segway has a charging system which talks to the BMS and it turns of charging to that battery. It does start to charge and gets up to full volts but shortly after shuts off.

I tried to measure each bundle together. Being they are in series, with my multimeter, i put the black onto the main ground and then put the red onto each bundles red (the wire that ties them together). On the first bundle i got 3.7 and then when I put red onto the 2nd bundle red I got 7.4, etc until i got to the end which was 83v total. So doing it this way didnt really find which is bad or why the BMS is kicking in to turn off charging

 

[ajw22]

On the first bundle i got 3.7 and then when I put red onto the 2nd bundle red I got 7.4, etc

Try measuring between each of the red wires, as that will give the same higher accuracy (3 or 4 digits?) for all measurements. Accuracy down to at least 0.01V is essential here.

Maybe it's just a rounding / calibration accuracy issue, but: 83V / 23 = 3.609V. So no cell should be at 3.7V, which would be notably higher than the LiFePO4 full charge voltage of 3.65V, and probably enough cause for the BMS to raise a fault.

 

[paddy72]

It could be a bad BMS 7 BMS error or cell problem (low capacity, self-discharge, e.g.)
If the cells packs a individually fully charged and the Segway does still not work, the BMS has probably recognized a problem saved as an error parameter in a Microcontroller and so refuses to work.
If the individual cell voltages are all nearly the same (up to 10mV difference is o.k. i guess) than the BMS is either defective or has registered a potentially fatal error so it refuses to charge or discharge. In that case there should be a diagnosis tool available to read the error via Segway service. I would guess it uses CANBus communication as most of the modern ebike batteries do, but not sure here.

The most easiest would be if you could find a reset button on the BMS or try to press the ON-switch for a prolongued time period (5 - 30 sec)?
Are there any instructions in the manual?

P.S.: Be cautious: 80 VDC can already be deadly voltage if touched with both hands unintended! And the battery can produce severe light arcs if shorted and deliver easily up to 100 A - which is enough for welding!