Battery unbalanced

Three times in the past 20 months my Tonaro battery has gone out of balance necessitating sending it off for attention.

I doubt that it is anything to do with the way I use it because I have another battery, a 2011 phyllion one that is still working well although reduced in mileage.

Further, I have two other batteries on my Woosh bike and both have remained in balance since about 2013.

Has anyone any idea why I get this problem?

Three times in the past 20 months my Tonaro battery has gone out of balance necessitating sending it off for attention.

I doubt that it is anything to do with the way I use it because I have another battery, a 2011 phyllion one that is still working well although reduced in mileage.

Further, I have two other batteries on my Woosh bike and both have remained in balance since about 2013.

Has anyone any idea why I get this problem?

Guessing only in the possible order of likelihood, list may not be complete either, wait on others here:-

1) Battery builder does not have a clue on how to build a battery.

2) Cells are maybe low quality, rejects, or are from different vendors or batches. Were totally unmatched.

3) BMS is defective or incorrectly installed.

All not good.

Best of luck.

Andy

1 and 2 Extremely unlikely

3 I have suspected this

1 and 2 Extremely unlikely

3 I have suspected this

I hope you are right with 1 and 2, but that is not my general impression from certain shops who are apparently buying up rejects....

You may find this informative:-

https://cyclingindustry.news/comment-know-your-e-bike-batteries-inside-and-out/

Regards

Andy

Some batteries have to be turned on to balance. But I have a small (7ah) cheap battery that has a small BMS (no load resistors) that does not have a balance function and needs to be rebalanced manually every few months, which I do myself. I believe this (no balance function in BMS) is common with small scooter batteries as well.

It did occur to me that the balance wires could be tapped and extended to an external plug to simplify matters.

You have to be very careful

It did occur to me that the balance wires could be tapped and extended to an external plug to simplify matters.

You have to be very careful if doing this as the individual cells are not fused, and the wiring wearing or shorting could overheat a group of cells and cause fire. Its also a tedious job individually balancing the cell groups. (It is generally recommended not to open batteries.)

On all my batterie soft packs I run separate balance taps so I don't need to try and disconnect the BMS, it is possible with hard case packs but not very practical.

 

If cells become unbalanced it may be as Andy has said cells may be unmatched in a P group/some weaker or having a higher IR, some poor cells self discharge a little.

It is more then likely that a cell/s have aged a bit worse then others and only needs one in a P group to pull down the rest in the same P group, this one cell can cause imbalance to the whole cell group.

 

The BMS may be at fault, bleed resistors have been known to fail or not shut off .

I'm having similar issues with the latest pack that I've built, this is a 36V 5P10S. Originally I thought this was down to the cells being purchased from 2 different suppliers so having different states of charge, all cells were balanced charged to 42V and terminated at about 500mA. However twice now in 3 months (1500 miles) several of the 5 paralleled cells have gone out of balance and triggered the BMS to shut down on over Voltage.

I do wonder if this might be due to the charger at work being on a timer and possibly turning off when still in the constant current phase of charging.

I think the bypass resisters on my BMS are 68 Ohm so the packs of cells have to have very similar states of charge for the BMS to balance the cells successfully

Checking the cell voltages hot off the charger after an aborted charge cycle gives 6 of the paralled cells a Voltage of 4.2V and 4 other around 4.05V, however not always the same ones.

I'm having similar issues with the latest pack that I've buil

A well built pack with quality cells should not go out of balance in such a short time. Were the cells all new? What system have you used in it's construction?

Edited March 12, 20206 yr by wheeliepete

Well there’s the thing, 30 out of 50 of the cells were about 2 years old but very lightly used, however by and large these are the cells that are behaving, the 20 newer ones are usually the problem. All cells are decent Panasonic NCR18650PF (I think 2700mAh). All cells in each parallel block of 5 are from the same ‘batch’.

 

The BMS is a regular ebay job but these have worked fine before although this bike gets some serious off road commuting abuse (forrest tracks, fields, cobbled streets).

 

Charging at work is by a regular 2A 42V Lion charger on a timer in case I get called of site.

 

Home charge is an intelligent charger based around an Arduino microcontroller that charges at 2.0A until 42.0V and then when the current tapers to 200mA terminates the charge, this also gives me an Ah figure of what has been put into the battery and the time taken. If this is showing 42.0V and 0.0A I know that the battery BMS has tripped on over voltage, the last time it did this was at 97% charge which off the top of my head is around 220mA.

 

My last battery built the same way and using the same charging regime is still running strong after 1600 charge cycles (20,000 miles), I don’t discharge more than 50% unless there is a detour or particularly savage head wind.

 

I use built-in battery heaters if the temperature is below 0 degrees, I have a temperature read out fitted so I can keep an eye on the cells.

 

The physical construction is by 3D printed cell holders inside a carbon composite battery box mounted under the downtube in a similar way to an oversize crud catcher.

Edited March 12, 20206 yr by SwampRodent

Cell age 2 years a part may well be a factor irrelevant of being the same voltage, they likely have either suffering higher IR & capacity loss.

 

Also double check all buss connections, did you solder or weld them ?

Edited March 12, 20206 yr by Nealh

Unfortunately I only had the option to solder, I built a couple of dual pulse spot welders but wasn't happy with the results and don't believe a cheap commercial welder would be any better, soldering I know is the hight of bad practice but I'm confident that my technique put minimal heat stress on the cells i.e big iron, flux, lead solder, heat for a fraction of a second and cool rapidly, probably less damage than a low quality spot welder punching a hole through the cap.

The bus connections are tinned copper braid. This process does make the pack serviceable so I can easily swap out cells if need be.

 

I do wonder if the 20 new cells I bought might have been seconds or there is an issue with my

charging technique.

 

Apologies if I have hijacked this thread.

Cell heat damage esp from soldering is critical on the neg end, the centre of the cell base /neg end is a no go area. Copper braid although tinned still requires a lot of heat for decent joint.

 

I would carefully check all parallel buss welds on each cell + & - end, use a sharpened wooden lolly stick or pointed kebab stick and pry under each connection for any weak /loose solders or cold solder joints.

 

Try manually balancing each cell group so they are the same, I suggest at nom value of 3.6 or 3.7v. Then once stable and all within 0.02v try a balance charge to see what occurs, it might be the BMS at fault and not the cells. Some BMS are better then others .

 

Battery building can be frustrating occasionally but the insight of how they work and fault finding is an invaluable knowledge to know & have.

Edited March 12, 20206 yr by Nealh

Two thoughts:

 

1. Mixing cells in a battery pack isn't a good idea

 

2. Are you giving the BMS chance to balance the cells for a few hours after the pack is fully charged? This needs to be done every so often, not necessarily on every charge.

Water from condensation can cause a battery to becoming unbalanced through a small short circuit. Not enough to do any noticeable damage but enough for the pack to become unbalanced over time. When batteries are properly assembled gloves are used so human sweat which contains salts does not contact the metal. Salt makes condensation more likely. Leave said battery in an unheated outdoor environment and moisture from the air will get inside and settle on the salt patch and cause a slow short circuit.

 

Electronics need to be well designed and assembled to prevent this happening in an outdoor environment. I remember reading that Justin from ebikes.ca found that most of the damaged china sourced ebike battery returns were caused by corrosion, water had got in somehow, my guess would be from moist air. His solution was potting the batteries in epoxy hence the development of his Ligo packs.

 

A way round the problem of battery or for that matter motor corrosion is to keep your bike in a heated indoor environment where condensation will not occur.

Water from condensation can cause a battery to becoming unbalanced through a small short circuit. Not enough to do any noticeable damage but enough for the pack to become unbalanced over time.

Not sure what you mean by a "small short circuit", a high impedance leakage path perhaps? This can occur but the BMS should rebalance the cells if it's given the chance by every now and then by leaving the charger on for a few hours after it indicates that the pack is fully charged.

Yes, a small current flowing in a high impedance path from from the positive to negative casing which is physically only circa 1mm away at the positive terminal.

 

In theory the BMS should rebalance the cells but how long this takes depends on the balancing current, whether the BMS throttles said current because it overheats, at what voltage the BMS starts to balance and whether the charger voltage is precisely set with respect to the BMS balancing voltage. I've heard stories of batteries only balancing when left on the charger for 2 weeks which may raise safety anxiety if charging indoors and the provenance of the battery is unknown.

If the BMS needs 2 weeks to balance the cells it's faulty.

Charge/balance the cells to 4.1v disconnect the BMS, leave cells to sit for 48hrs and see if any group self discharges.

Not sure what you mean by a "small short circuit", a high impedance leakage path perhaps? This can occur but the BMS should rebalance the cells if it's given the chance by every now and then by leaving the charger on for a few hours after it indicates that the pack is fully charged.

Now this is directly in contradiction of what |Andy Mat has posted

Now this is directly in contradiction of what |Andy Mat has posted

How else is the BMS going to balance the cells?

every now and then by leaving the charger on for a few hours after it indicates that the pack is fully charged.

General opinion seems to be that it is a good idea to do this 'every now and then' (as quoted) , but generally to try to stop charging as soon as the light goes green (or even a little before if you can estimate when that might be and don't need to squeeze every last drop out of the charge).

"2. Are you giving the BMS chance to balance the cells for a few hours after the pack is fully charged? This needs to be done every so often, not necessarily on every charge."

 

 

OK that kind of confirms my suspicion, I'm not leaving the charger connected long enough for the BMS to balance charge the cells, then once the cells are to far out of balance the BMS trips and no further charge can take place.

My home charger terminates the charge at 200mA, this is obviously too early, and the work charger is on a timer which again turns of too early so I think this may well be the root of the problem, hopefully this might be what's happening with Mike who started this thread.

If the BMS needs 2 weeks to balance the cells it's faulty.

 

The BMS could be working exactly as designed but still not be capable of balancing the cells except over a very long period of time. For example

 

Take a 36v 15AH battery pack with this commonly used bms https://vruzend.co.uk/products/10s-36v-15a-lithium-battery-bms-balance-function-18650-li-ion-uk-seller-stock it has a balancing current of just 40ma.

 

Now imagine said battery pack has been left in a damp garage for 6 months and corrosion means there's a higher self discharge rate for one of the parallel groups, or maybe through bad luck one of the cells happens to have a higher self discharge rate from the factory. So one group has only 1ah left whilst the rest has 14ah.

 

The owner then puts it on the charger and it appears to finish quickly because most groups have only 1ah to fully charge. How long would the owner have to leave the battery on charge to allow the BMS to fully balance the pack?

 

The BMS needs to add 13AH and can only balance at 40ma, so 13000/40 =325 hours or 13.5 days to balance the pack.

Edited March 13, 20206 yr by Baboonking