Is there a guide on how to "service" your battery pack?

[alcatraz]

Hi everyone..

After nearly 3000km I'm tempted to open up my battery pack and service it. What I mean is to locate the cells that are behaving poorly and replace them.

How do I best find them? Do i need to remove the connections to measure the cells? (I have an inmotion v8 84v 480mAh pack)

At what internal resistance deviation do you consider a cell being bad? I don't want to throw away ok cells but I don't want so-so cells to kill my good cells either. So what's the compromise?

If I do find a dud cell or two, how do I go about finding a good cell model to replace it with?

Are there any cells I should stay away from that are unsuitable for electric unicycles. I use my unicycle in temperatures from -20 to 30 degrees C.

Do I need to be extra careful with the BMS?

I just had an idea and it's to group similar internal resistance cells together in one group. That way their voltages won't deviate much compared to their neighbor cells and kill them. Does this make any sense or am I just crazy? It would mean that you could increase the lifespan of the pack by just moving cells around.

Thanks! /a

[Chriull]

7 hours ago, alcatraz said:

Hi everyone..

After nearly 3000km I'm tempted to open up my battery pack and service it. What I mean is to locate the cells that are behaving poorly and replace them.

How do I best find them? Do i need to remove the connections to measure the cells? (I have an inmotion v8 84v 480mAh pack)

No. Just measure the cell voltages of the paralleled cells. The ones with the lower voltages are the weak ones.

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At what internal resistance deviation do you consider a cell being bad? I don't want to throw away ok cells but I don't want so-so cells to kill my good cells either. So what's the compromise?

Resistance is an important characteristic, but normally they are imo paired by capacity? More to read on li ion battery packs on batteryuniversity.com

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If I do find a dud cell or two, how do I go about finding a good cell model to replace it with?

With the same as the rest. Matched as good as possible.

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Are there any cells I should stay away from that are unsuitable for electric unicycles. I use my unicycle in temperatures from -20 to 30 degrees C.

Do I need to be extra careful with the BMS?

I just had an idea and it's to group similar internal resistance cells together in one group. That way their voltages won't deviate much compared to their neighbor cells and kill them. Does this make any sense or am I just crazy? It would mean that you could increase the lifespan of the pack by just moving cells around.

Thanks! /a

Moving cells around should imho not help - just replacing bad/weak paralleled pairs should be sufficient.

Edit: Ps: best "service" for euc battery packs would be single (paralleled) cell charging. I'd assume you meant 84V 4800mah battery pack, so 20s2p? 20 chargers for each 2 paralleled cells (or 20 times charging with a single charger, 4 times with a 5 fold charger,...)- so all of them are for each ride perfectly balanced (fully charged). If you once measure bigger differences between the cell pairs one could think about exchanging them.

[alcatraz]

Thank you very much for your reply. I appreciate it. To you or someone else I have some follow up questions.

If the bms constantly balances the voltages how can any voltage difference ever be measured unless I go for a long ride draining like 50% and just as soon as I get home rip open the pack and somehow disable the bms before the balancing has occurred? My question is, do you have an idea how to leave the batteries unbalanced long enough so I can measure them?  (I'm assuming the bms is inside the pack on the v8. Is it?)

Also how do you measure individual cells without disconnecting them? It would be a pain to have to break the spotwelding just to measure the cells.(Are they spotwelded on the V8?)

Any ideas?

I saw a russian guy on youtube that managed to make a huge pack with 50% more cells that fits the same battery compartment, and has 3500mAh cells instead of the original 3200mAh. He effectively doubled his range on the V8.

[Keith]

16 hours ago, alcatraz said:

If I do find a dud cell or two, how do I go about finding a good cell model to replace it with?

Apart from not being trivial to do, you need a spot welder for a start, It is pretty much a complete waste of time to do this, unless you have a relatively new battery that has had one or two bad cells in it. To start putting brand new cells (even if they are exactly the same as the originals) in a pack of one or two year old, many times recharged cells, is just going to tax your BMS trying to balance them. At that point all your existing cells are going to appear to be bad when compared to the new ones.

1 hour ago, alcatraz said:

If the bms constantly balances the voltages how can any voltage difference ever be measured unless I go for a long ride draining like 50% and just as soon as I get home rip open the pack and somehow disable the bms before the balancing has occurred?

I do not know of any BMS that works like that. They are voltage clamp devices that start diverting charge around each cell when it reaches 4.2V whilst charging. As far as I can tell it is only some model LIPo chargers that actually balance by draining current from any cell that is higher than any others at any voltage. If a BMS did this it would probably be continually discharging some cells. So a battery that has been discharged will have faulty cells at a lower voltage as the  BMS will not balance

1 hour ago, alcatraz said:

Also how do you measure individual cells without disconnecting them? It would be a pain to have to break the spotwelding just to measure the cells.(Are they spotwelded on the V8?)

You stick the negative lead of your meter on the negative side of the cell and the positive lead on the positive side of the cell. What you cannot measure is individual cells in a parallel group, only what the group measure, but if one cell in a set was faulty it would pull that parallel set down you would then have to remove that set, and identify the fault one(s).

All batteries are spot welded, the heat of  soldering them would risk serious damage to the cell, the only alternatives would be spot welded tags that are then soldered which would be pointlessly costly on a manufactured pack or mechanical connection which would add weight, bulk and potential connection failure, so they are always spot welded.

In conclusion, this is IMHO a complete waste of time, if the battery is failing replace it. Maybe pull the old battery apart and use any still good cells in torches, or whatever but putting brand new cells with old ones is pointless. Most people consider a pack to be past its best when range has dropped to around 50% of new (not forgetting very cold weather can do that anyway!) if the range is better than that just use it.

it is probably worth remembering that a major cause of Hoverboard fires was believed to be unscrupulous manufacturers building batteries out of old laptop battery cells to save money, the mismatched cells were too much for the BMS to handle allowing some cells to overcharge so much that they would catch fire. BMS can only balance very small capacity differences in a pack of pretty much matched cells (I.e. from the same batch). 

[Chriull]

1 hour ago, alcatraz said:

Thank you very much for your reply. I appreciate it. To you or someone else I have some follow up questions.

If the bms constantly balances the voltages how can any voltage difference ever be measured unless I go for a long ride draining like 50% and just as soon as I get home rip open the pack and somehow disable the bms before the balancing has occurred? My question is, do you have an idea how to leave the batteries unbalanced long enough so I can measure them?  (I'm assuming the bms is inside the pack on the v8. Is it?)

The BMS only balances while charging. It just bypasses the cells which reach a certain threshold. So if not charged long enough, or already too much diffetence the cells will be unbalanced even after charging.

1 hour ago, alcatraz said:

Also how do you measure individual cells without disconnecting them? It would be a pain to have to break the spotwelding just to measure the cells.(Are they spotwelded on the V8?)

??? Just measure them. The paralleled ones are just paralleled and not to be measured as single cells.

1 hour ago, alcatraz said:

 

Be very carefull messing around with the batteries!

[alcatraz]

Thank you guys for these generous replies.

Ok so batteries in parallel are measured together then broken apart and inspected. Luckily the parallel groups are 2-3 at most. Not 10 which would be very tiresome to inspect.

You think it's pointless and maybe in europe/usa it is but I live in China. Cells, equipment and labor is very cheap here. What sometimes is missing is quality so if I can oversee the service it will be done well. I'm just trying to learn about it so I do a better job than those hoverboards did.

Lets say I have in my battery pack of 20s2p 17 good voltage groups and 3 subpar. If I break open and measure a good voltage group capacity with a discharger how many mAh I get out on average. Then wouldn't replacing the largely deviating cells with similar capacity ones the prudent thing to do?

Because I saw a guy on the internet upgrade his pack from 20s2p to 20s3p I'm interested to do the same but only adding new cells with similar capacity to the average one I measure in my existing pack.

Sure I get it that two different cells with same capacity can have different charge/discharge curves and that could complicate things but as long as they arrive at min/max voltages roughly the same time the bms doesn't have to work much, correct?

Also the theory of different capacity groupings of similar capacity cells also intrigues me. You say the BMS doesn't work unless min/max voltages are reached. What if a min voltage is reached in a group. Does the BMS cut power from the entire pack or does it during discharge actually bleed more power away from the other cells (fast enough?) and keep the min voltages from going further down?

Then it could be possible to have lets say 2-3 weaker groups and 17-18 strong ones. When reaching 20% capacity and still going, the bms would then prevent the weak groups at  min voltage to continue to discharge. Does it work this way?

I know your patience is limited. Only answer if you feel like it. I'm very patient and welcome others to chat about this. Thank you very much for reading and writing. Take care and stay safe. /a

[Keith]

2 hours ago, alcatraz said:

Lets say I have in my battery pack of 20s2p 17 good voltage groups and 3 subpar. If I break open and measure a good voltage group capacity with a discharger how many mAh I get out on average. Then wouldn't replacing the largely deviating cells with similar capacity ones the prudent thing to do?

I understand what you are saying about costs but the problem is that one or two cells being weaker is all relative. If the BMS is working correctly and the cells were originally well matched or from a single batch then all of them would be losing capacity with age at pretty much the same rate, some may be (say) 10% worst than others, but all of them would have lost some capacity.

Let us say that you have a battery that is several years old and most cells are performing,  on average, 30% down from new capacity and some cells are as much as 40% down so they show the lowest value and you change them. Now you have a battery where the new cells have 100% capacity and the old ones are 30% down on capacity - that is a damn sight worst situation than you had to start with, and, more to the point the battery is still going to perform 30% down from new - at best you have gained a performance improvement of 10% (from 40% down to 30% down) and that really isn’t worth doing, I.e. you’ve spent money for no real gain.

Indeed, salvage the BMS and replace the entire pack of cells if cells are cheap.THAT is worth doing.

2 hours ago, alcatraz said:

Also the theory of different capacity groupings of similar capacity cells also intrigues me. You say the BMS doesn't work unless min/max voltages are reached. What if a min voltage is reached in a group. Does the BMS cut power from the entire pack or does it during discharge actually bleed more power away from the other cells (fast enough?) and keep the min voltages from going further down?

I and @Chriull have already told you the BMS does no balancing except at max voltage under charge. Any BMS with undervoltage protection in the BMS IS DOWNRIGHT DANGEROUS ON AN EUC. This just should not be happening. The wheels electronics should be monitoring the total voltage from the battery and using tiltback if the voltage is too low (typically 3 to 3.5V/cell)

To put all of the above questions and answers into perspective, a BMS can typically shunt something like 50mA around a cell, this would be like peeing in the rain for all the effect that would have on a 10Amp or so discharge - so it would be pointless it doing it. More to the point, it is a small amount even on a 2 Amp charge so charge current has got to drop VERY significantly I.e. all cells VERY close to 4.2V at the same time for balancing to be effective.

Do not get confused by serial and parallel matching. You can have any number of packs of absolutely any capacity whatsoever joined in parallel together as long as the pack voltage is the same I.e. they are all 20 in series. And they MUST BE EXACTLY THE SAME VOLTAGE WHEN FIRST CONNECTED TOGETHER. What you absolutely cannot do is mix capacities in series. you cannot have six 2000mAh cells and fourteen 3000mAh making up a set of 20 and that is, effectively EXACTLY what you are doing if you mix old and new cells. You can have 20 off 2000mAh cells in series and another pack of 40 off 3000mAh in 20S2P absolutely no problem, more current will be drawn from the larger pack and both pack’s voltages will drop at the same rate (they absolutely have to - they are physically connected together!)

2 hours ago, alcatraz said:

Then it could be possible to have lets say 2-3 weaker groups and 17-18 strong ones. When reaching 20% capacity and still going, the bms would then prevent the weak groups at  min voltage to continue to discharge. Does it work this way?

Absolutely not, that would require the BMS to cut power to the wheel causing you a faceplant and possible serious injury. If the total pack voltage is above the wheels tiltback voltage the cells will all continue to give the same current until the lowest cells are so low in voltage they become seriously damaged. To the best of my knowledge only IPS use a BMS where all 16 series connections feed through to the wheel’s electronics which might tilt back on the lowest series cells - how it actually works is not published.

So, say for example your wheel tilts back at 3.5v/cell in theory, actually that will be 3.5x20 volts = 70V. Now suppose that 18 of your series cells are at 3.7V (which is around 30% capacity left) when the wheel sees 70 volts and starts tilting back that is 18x3.7=66.6V then that means the last two series cells (which we can say are the ones with 30% less capacity) are down to a combined 3.4V or 1.7V/cell - those two cells (or parallel group of cells) are now permanently damaged. Ignore tiltback and those cells will plummet to zero and start being reverse charged by the other cells. At this point even a fire is possible.

[alcatraz]

You've made it very clear, thanks.

Ok so no min-voltage protection, for driving safety. That means it's really a bad idea to run an older pack under say 30% because the increasing voltage deviation will bring some groups under the min voltage = permanent damage.

It's best then to use the pack as long as possible and replace all the cells (recycle bms) when the time comes. Also the new cells need to be good stuff and match eachother.

I wish I had other uses for lithium batteries. I have a Ninebot E that probably still uses 2200-2500 mAh cells (new) that could benefit from some used 3200 cells (Inmotion v8). Other than that..hmm.

Do you think that modern BMS that come originally with wheels have good max voltage protection? Lets say a pack is getting old and some cell groups arrive at 4.2v when others are still at 3.9v. Will the BMS be capable enough to prevent 4.2 from being further charged? You said the balancing current is a mere 50mA. This doesn't sound like it's going to be enough when the charging current from the PSU is 1500mA. The current goes down over 80% but still even with half 750mA going through each group of 2 or 3 cells it won't be enough with 50mA balancing current to avoid overcharging. Can the BMS suspend charging while balancing and continue charging after balancing automatically? I'm guessing the exploding hoverboards couldn't but that the euc's can suspend charging. At least I hope...

/a