Balancing L1 LEAF cells

Peter, something doesn't sound right with those cell voltages, I thought the 60Ah leaf cells were the same voltage as LiPo cells:

Nominal Cell Voltage: 3.8 V
Maximum Cell Charge Voltage: 4.2 V
Minimum Cell Discharge Voltage: 2.5 V

Your highest module voltage of 7.55 is only 3.775V per cell which is still below the 3.8V nominal voltage.
I would expect the individual cells to be 4.1 - 4.2V fully charged, which is 8.2 - 8.4V per module.
Is your charger set for the correct type of cells or have you inadvertently charged them on a much lower voltage LiFePO4 setting?

There's a very simple way of balancing all the modules, simply connect all 18 of the modules in parallel with bits of wire and let them stay that way until you are ready to use them and they should eventually all end up at exactly the same voltage.
Thin wire can be used as the 0.2V difference between the high and low cells should only cause a very low current flow.

If they do need further charging before assembly, you can simply charge the whole lot simultaneously while connected in parallel, but charging a 1080Ah pack is going to take some time (9 days to charge from empty to full at 5 Amps charge current).

If I have misinterpreted your cell voltages and you really do need to discharge individual cells, you can use some 12V headlamp bulbs connected in parallel, but you will have to monitor the voltage to make sure you don't go too far.

Alan

Hi Peter,

I suggest just fully charging all the cells - that way they will already be top balanced when they go into the bike

this is actually fairly important, as you have to go to 4.2v (per cell, so 8.4v per module) to get full charge, but you can't go beyond at all

this chemistry does have a linear SOC vs voltage relationship, so you could get away with getting them all to the same voltage, but you would still have to deal with the variation at full charge

My objective is at this point to get all the cells to the same voltage. is there a tolerance that is acceptable, say =/- .01V?
When the cells are fitted and are them charged by the ESD Charger with new Laird firmware, they should be taken to the voltage regulated by that charger?

ah, are you using a BMS?

the ESD charger can only measure full pack voltage - it won't stop if one cell is smaller or more full than the rest

if you are not using a BMS, it is imperative to fully charge every cell before putting it in the bike, to ensure the battery is top balanced

you can do this individually, or with the cells all wired in parallel, but either way it has to be done

If you are using a BMS, then you can get away with not balancing, but it just means the charger gets stopped when the first cell is full.

I suggest just fully charging all the cells - that way they will already be top balanced when they go into the bike

this is actually fairly important, as you have to go to 4.2v (per cell, so 8.4v per module) to get full charge, but you can't go beyond at all

This is going to take some time with a 3 amp charger!

Peter, if your charger is set for LiPo it should cut off at 4.2V, but if it has a safety timer you will either have to disable it if possible or simply keep resetting the charger when it reaches the end of the safety timer period until the correct voltage is reached.
If your charger has a "Capacity Cut-Off" function you will have to set this to its maximum (unless it actually goes as high as 60,000mAh) and keep resetting the charger until the expected final charge voltage is actually achieved.

At three amps, each module would take over 20 hours to fully charge from empty, and if you have 18 modules that's over 2 weeks of continuous charging!

I presume the centre terminal on those modules is for a balance connection between the two pairs of parallel cells connected in series within each module. If so, you should be able to use a three wire balance lead to connect to your charger so each pair of cells is individually monitored by the charger.

If your LiPo charger does not have a balance facility you will need to check the voltages of each pair of cells and balanced them manually if necessary by connecting the charger between the balance terminal and either the positive or negative terminal depending on which cell is the lowest (or the headlamp bulbs if a cell is slightly too high).

Alan

Peter, you bought a LiPo charger which reckons with 3.7V per Cell. Leaf cells are LiMn2O4 chemistry with a higher full voltage. Maybe therin lies your problem?

As LiMn2O4 is a little more linear in it's cell-voltage-to-SOC-ratio than LiFePO4 for instance you may get away with individually pre-charging them to a lower than full voltage and completing the charge with the onboard Vectrix Li charger. But I can't say for sure. I personally would be more comfortable with having each and every single cell at the same FULL voltage before letting the bulk charger at it.

Peter, you bought a LiPo charger which reckons with 3.7V per Cell. Leaf cells are LiMn2O4 chemistry with a higher full voltage. Maybe therin lies your problem?

Forgive me if this is a stupid response but it strikes me that as long as ALL the cells are EVENLY charged to the same "reasonable " voltage.. lets say 7.7v, what is the issue that cannot be resolved by a suitably modified Li firmware equipped ESD charger to take them to their safe max voltage?

take the example at 7.7v = at 7.7v while charging the cells are 35% full

your leaf modules each have capacity between 62Ah and 66Ah (so the 62AH modules have 21.7Ah in them, and the 66Ah modules have 23.1AH in them, and all modules in between)

You won't know which modules have more capacity, and which less, just that they are all 35% full (and even this will have a give or take 5% variation based on cell voltage alone - although way better than the give or take 40% variation with LiFePO4)

say you install them in your bike when they are all at 7.7v, and then use the ESD charger to charge, with a modified program to charge to 150v (for 18 modules or 8.33v average)

when the smaller modules of 62Ah capacity reach full (100% and 8.4v) the largest modules (the 66Ah ones) will have only 63.4AH

but the ESD charger is going on full pack voltage, which will not yet have reached the 150v target, so it will keep on going.
The smaller capacity modules will get overcharged

that is unless you have a BMS

If you top balance (as in all modules reach full/8.4v simultaneously), then this is not an issue.

it does become an issue if you attempt to use the full battery capacity, but that is among the many reasons to limit discharge depth to 80% (or ~48Ah)

Matt

Good explanation, Matt!

NiMh cells can be "top balanced" at low currents without knowing their exact capacity - heat dissipation in the full cells will keep their voltage constant at the voltage value that corresponds to a "full" cell while "slower" cells in the battery pack catch-up and fill-up too. Then the charger will stop because the total voltage of the pack has been reached. Unlike these, the voltage of the full Leaf cells will continue to raise after they are fully charged, and faster than the not fully charged cells. So the full cells will be overcharged and damaged while the "slow" or emptier cells catchup.

An analogy. Let's say you have bottles of unknown size that are around 1 liter each. Some are a bit bigger, some are a bit smaller. Their necks get narrower the higher you go, compared to the body. Like Champaign bottles. Pour in each bottle exactly 750ml of water. Then add in each bottle 250ml. If you do that, some will be full, some will be partially full, some will overflow. Also, the ones that are nearly full by your 750ml and about to overflow will rapidly overflow the fuller they are, because their necks hold less and less water as the level rises. So even a little bit of extra water will cause the level of water in an almost full bottle to raise rapidly. A NiMh battery is like a bottle with a small hole near the top - as long as you fill the last bits of it slowly, it won't overflow because as the water level raises, the water starts to overflow from that hole, and, supposedly, you are prepared for that, unlike water that comes over the top, which is bad and will damage your carpets;) Leaf batteries don't have those holes, so to speak and their charge level will start to raise very fast the fuller they get.

Now, let's say you have a weight scale to tell you the weight of your 18 Champaign bottles. You can carefully fill each bottle to the brim (or to some precise level below full). You can now measure the total weight of all bottles. While you don't know precisely how much water each bottle holds, you don't really care, as long as you know the minimum amount that each bottle can hold. Now, take equal amounts of water out of each bottle by emptying all of them at the same time at the same rate for a few minutes. Keep pouring water out until a point where the smallest bottle is almost empty. Unfortunately, you don't see them while you are doing it so you don't know when stop exactly. Ideally, you could observe each bottle as you empty all of them little by little and measure how much they weigh at the point when the smallest is nearly empty. But you can't. So you make an educated guess that no bottle is smaller than let's say 900ml and hope the smallest bottle is bigger than that. If that is indeed true, you will never end up with any bottles that are fully empty, as long as you watch the total weight carefully as you empty them. You don't need to know exactly how much you took out, as long as you took out the same amount from each bottle and none gets empty below some minimum level. When you want to refill, you simply start pouring water in each bottle at the same rate, until the total weight is equal to the weight you started with before you partially emptied them. You don't need to know how much water went in each bottle - only need to guarantee you are emptying and filling each bottle at the same rate as any other bottle, that NONE gets too empty, AND that at the end when you fillip they together weigh no more than when you started.

So, given your limitations, the only solution is to:
FIRST, fill them each bottle to the same level below their top. This is your top-balancing, all cells fully charged to the same voltage (all bottles full to the same level below the top of their necks)
SECOND, weigh them together and take a note about the total weight. For batteries, that's just the voltage of each cell when charged to your desired "full" SoC, which is the same for all cells, times the number of cells.
THIRD, make an assumption about the smallest size bottle and calculate how much they'll all weigh when that one bottle gets almost empty. For cells, that's the total minimum voltage of all cells together and assuming no cell is abnormally low Ah capacity (so low that its voltage drop will be masked by the still high voltage of all other cells). This is a VERY important assumption - if you get that wrong, you will over discharge a cell and damage it. Without a BMS - you have no way to know if that is happening from just observing the total voltage. Imagine one bottle breaks while the rest are still mostly full: you will not know that because you only see the total weight, which is still way above your minimum total weight when you assumed all bottles were good.
FOURTH, start pouring water out of all bottles at the about the same rate (hoping they have the same neck sizes)
FIFTH, keep notice of the total weight as you pour water out. I.e., watch your total pack voltage and don't let it drop below your target minimum.
SIXTH, stop emptying NO LATER than the point when your total minimum weight has been reached. You can stop earlier at any point.
SEVNTH, fill again to the original total weight.

That's one full top-balance/charge/discharge/charge cycle. You don't need to top balance ever again! When you take out equal amounts from each bottle, they are still unevenly filled, but you don't care (as long as you don't let any one of them get empty and never put more than they together can hold.

Of course, you can't be 100% precise in emptying and filling them up at exactly the same rate. So over time, if you don't periodically top-balance your Champaign bottles individually, some bottles will start to overflow and some will get completely empty and your total full and empty weights will not tell you that your case of bottles is no longer balanced. So you need to, occasionally, fill them up to the top individually, so that your full weight is a correct measure of when all your bottles are full.

It was suggested earlier in this thread that by link wiring the cells in parallel that this would balance the cell packs to the same value ( Voltage and Ah?) then I would need to just check that the voltages from each side terminal to centre one are the same and balance any individual cell pack variance.

putting all the modules (and middle links) in parallel for a long period of time should them all pretty close in voltage
what you are likely to end up with is a small variation in voltage (unless you are charging or discharging externally to a specific voltage)
which translates into a larger variation in state of charge (% wise)
which translates into an even larger variation in how many AH of charge is in each module/cell

I cannot see how a BMS can balance exactly the 4 cells in each pack using only 3 poles

My representation of what's inside the module might help:

The two 30Ah Cells A and B (and C and D) are connected in parallel ensuring their voltages remain identical and they are considered to be (and therefore monitored as) a single 60Ah cell unit by the BMS.

The centre terminal provides the additional connection needed to allow the voltages of both pairs of parallel cells to be monitored separately by the BMS.

As you can see, the simple circuit shown above allows all of the cells to be accurately monitored without having to resort to divine inspiration.

Alan

3. 18 x 8.4 = 150.2v. Surely the modified ESD charging profile for max voltage will be set to less than this number and similarly the critical minimum value cut out will be higher than it's absolute minimum value : 18 x 5v= 90v.

even at 150v across 18 modules (8.33v average) it is still possible to have one module above 8.4v

the same is true on the low end, at the 108v that the new MC versions stop working at, although the average voltage is 6v per module, it is still possible to have one or more at 0v or even *negative*

it isn't enough to do pack voltage alone

5. As but a simple man, I see Amp/Hrs as tank capacity and Voltage as fuel calorific value. So why not fit 20 cells (assuming they will physically fit) with a max charged voltage of say 146 volts and a minimum of 120v (20 x 6) or even 19 for that matter ( we know they do fit!). Clearly I am accepting a lower level of availability of the actual total installed capacity but would be happy with this compromise given the safety margin afforded at both ends of the safe operating voltage range.

that's right - Ah is capacity and voltage is energy realised from using Ah

more than 18 modules and regen is compromised, as the battery voltage is higher than what the motor controller is expecting

the low voltage limit is largely irrelevant - you can kills cells without hitting it, which is mainly why we Ah count instead of relying on a low voltage limit

The BMS I'm using I can probably get a version of the official lithium MC firmware to work with, to prevent any cell getting overdischarged, but that's a fair way off at this stage

6. I have chosen not to use a BMS on two grounds. Other than by divine inspiration, I cannot see how a BMS can balance exactly the 4 cells in each pack using only 3 poles, but am happy to be enlightened, and secondly on cost grounds.

in this situation, the main job of a BMS is to prevent the charger from overcharging any one cell.
balancing is a bonus extra, but you really should balance the cells before installing

If not using a BMS, it is imperative that the battery is top balanced, so that when the charger is rely on battery voltage alone to know when to stop, it does stop before the first cell gets full

I also haven't yet been able to test what the failure mode of these modules are when they are overcharged (not working afterwards is fine, catching fire is not).
ditto for overdischarge

1. I have 20 new LEAF cell packs and based on the information gleaned here, selected 18 as the optimum number to use.

Hi Peter, where did you get them? I'm despearate to find a place to order,
thanks
Afo

Peter Faulkner wrote:

1. I have 20 new LEAF cell packs and based on the information gleaned here, selected 18 as the optimum number to use.

Hi Peter, where did you get them? I'm despearate to find a place to order,
thanks
Afo

Hi Peter, I'm trying to find the message where you explained where did you got the Leaf cells. But actually the information inside this forum is widely spread and hard to find, I¡ll really appreciate if you (or anybody) can point out your 8any) source of leaf cells, or send a PM. Many thanks in advance, and forgive me for asking you the same question twice.