Rebuilt battery and now its worse?

[Zambam]

I've since R&R'ed the 2 cells in group 9, which had AC IR's of 6 & 14 milli ohms, much higher than the 2 milli ohms when the pack was built 900 miles ago.

Correction, the AC IR's I reported earlier are wrong, due to not making good contacts to the copper tabs with my IR tester due to melted plastic insulators on the cells. After cleaning up the contacts, the AC IR's of the 2 bad cells now measures 59 and 152 milli ohms respectively. I either over heated these 2 cells during the build or during unsoldering. Most likely the latter because I inspected every cell after I finished soldering the pack and all insulators looked fine back then.

 

[amberwolf]

I have quite a few loose BMS from scooter packs, what do I look for to determine if the BMS can balance? Someone mentioned vacant PCB pads for cheap non balancing models?

If you poke around thru my posts on BMS balancers, there's at least one thread that has pictures to compare a few random BMSes that have or don't have balancers, to give an idea of how to tell with certain types.

Some don't have individual channel balancers as it's a charge pump system in the BMS chip itself, so you have to look up the ICs on the board itself...but even if the chip has the function, it doens't mean it's enabled in the firmware for those that are controllable that way.

So there is also some description in one or more of those posts of how to test whether balancing is going on or not.

 

[BatteryMooch]

I wonder how you measure DC IR?

Pulse the cell at two different discharge current levels and measure the voltage change. Then use Ohm’s Law to calculate the equivalent resistance that would cause that voltage drop for that current change.

SOC and cell temp affects this (as it does for AC IR too).

The are three different DC IR’s though, just as there are a huge number of different AC IR’s depending on frequency, so you need to know which you want….ohmic, diffusion, and polarization…each occurring at a different time after the discharge starts.

 

[BatteryMooch]

I have quite a few loose BMS from scooter packs, what do I look for to determine if the BMS can balance? Someone mentioned vacant PCB pads for cheap non balancing models?

You can check for an array of larger resistors (and maybe tiny chips, MOSFETs) that equal the number of cells. These are what’s used to drain the cells down to balance them. The resistors are larger since they create more heat than any other resistors on the BMS except the ones for main current sensing.

Many BMS controller chips can do low current balancing using internal resistors though, perhaps up to 25mA average, so a lack of resistors doesn’t prove no balancing.

 

[pishta]

I have quite a few silverfish 36-48v batteries and I charged 4 to full capacity (per green light on stock charger). I can measure each P group once I get the case off the battery and I find 2-3 groups in each (bad per the vendor) battery are under 4v (3.6v) so Im assuming the BMS are not balancing the pack, but are just cheaping out and keeping the pack safe. The rest were consistant 4.19-4.20V. Ill segregate those low V groups and test each cell and find wtf is happening.

 

[99t4]

I have quite a few silverfish 36-48v batteries and I charged 4 to full capacity (per green light on stock charger). I can measure each P group once I get the case off the battery and I find 2-3 groups in each (bad per the vendor) battery are under 4v (3.6v) so Im assuming the BMS are not balancing the pack...

It may take quite a long time keeping the charger connected at full charge to balance the cells. Sometimes 12 hours, to days or even weeks in extreme cases.

What do you mean by

(bad per the vendor)

?

 

[pishta]

It may take quite a long time keeping the charger connected at full charge to balance the cells. Sometimes 12 hours, to days or even weeks in extreme cases.

What do you mean by

?

All these batteries were "in the back" and I asked if I could take them off his hands after seeing them all lined up in a corner. He said take them all! It saved him the hassle of disposing of them "properly".

 

[BATTERIES4YA]

Silverfish style 48v13Ah drop in battery had a range of about 4 miles. Just low capacity? Took every cell out and tested it on a Liitokala M4 charger that has a capacity test, I'm assuming a timed discharge into a 5ohm 1w load down to ? I reused all the cells that tested higher than 2200mah and had a internal.resistance under 30mOhm just as 70% of the cells showed. The other 30% were shot: 0V, under 2000mah or >30mohm internal resistance. I replaced these with 2500mah <25mOhm cells and welded them all back into the frame with new nickle strips made for this staggered frame. Reused original BMS. Battery shows full on meter and on display. Now the bike goes 2 blocks and shuts down. It will start up again but the controller shows full battery but the battery meter shows 1 bar. I literally tested every cell in this 65 cell battery over the course of getting the strips mailed to me and the welds are good enough to leave nickle on the battery if you try and peel off. What gives? I'm wondering what the cutoff voltage is on the capacity charge cycle. I can use another tester that is more manual and will set the cutoff voltage to 3.4 v and see if that capacity matches the liitokala charger. Only thing I can theorized that the liitokala tests down to some ridiculous 3v or something before stopping the "clock" that shows the capacity? Bummed that this didn't work as it took me 2 weeks to get the staggered nickle strips

Sounds like the BMS is tripping. That's usually A) due to hitting max discharge limit, which can be set lower in the controller or B) voltage sag seen in a weak battery when putting out more amps than it can handle and hitting the BMS low voltage cut off or C) an individual cell group reaching minimum voltage cutoff.

C can happen due to loose BMS cabling. It should have a connection to every p-group. It can also happen due to the battery being out of balance. If the BMS supports balancing, typically you can leave it on the charger for a really long time and it will drain the p-groups that are at max while it fills up the lower ones. Some cheap UPP BMS don't have the balance resistors populated to accomplish this, however. You could check the individual voltage of each p-group after it powers off, I suppose.

Voltage sag is tougher to catch. Might be able to measure voltage while dumping the battery into a large dummy load like a resistive heater. If a certain p-group can't handle the amps, might be possible to replace just that one. Cells can be damaged by heat, so maybe some were damaged during the repair

Silverfish style 48v13Ah drop in battery had a range of about 4 miles. Just low capacity? Took every cell out and tested it on a Liitokala M4 charger that has a capacity test, I'm assuming a timed discharge into a 5ohm 1w load down to ? I reused all the cells that tested higher than 2200mah and had a internal.resistance under 30mOhm just as 70% of the cells showed. The other 30% were shot: 0V, under 2000mah or >30mohm internal resistance. I replaced these with 2500mah <25mOhm cells and welded them all back into the frame with new nickle strips made for this staggered frame. Reused original BMS. Battery shows full on meter and on display. Now the bike goes 2 blocks and shuts down. It will start up again but the controller shows full battery but the battery meter shows 1 bar. I literally tested every cell in this 65 cell battery over the course of getting the strips mailed to me and the welds are good enough to leave nickle on the battery if you try and peel off. What gives? I'm wondering what the cutoff voltage is on the capacity charge cycle. I can use another tester that is more manual and will set the cutoff voltage to 3.4 v and see if that capacity matches the liitokala charger. Only thing I can theorized that the liitokala tests down to some ridiculous 3v or something before stopping the "clock" that shows the capacity? Bummed that this didn't work as it took me 2 weeks to get the staggered nickle strips.

did you add any batteries to your parallel, to increase capacity