[DaveBlack]

Slight change...its not holding charge! held 30v for an hour or so but is slowly dropping now.

Probably as I think it will need at least 3.1v per cell to activate the chemistry and thats when ill know.

I hope, no idea really - winging it as ever!

Dave

 

[john61ct]

Do a proper capacity load test on the ones that appear to recover, .2-.5C rate, stop at 2.99V as 0%

Just brace for the fact they may be scrap.

I use 3.65V for load testing 100% and if top balancing.

In production cycling 3.45V as 100% will be easier on longevity, fewer balance issues ongoing.

 

[DaveBlack]

HI John - thanks for that, I was really focusing on the batteries being able to be charged from a Lead Acid charger rather than the fact they are potentially dead.

Is that true?

Thanks

Dave

 

[brian_]

But my testing kinda says thats right - currently I am just pumping 30V into one (max of my Variable voltage Dc output!). I can vary the Amperage and slowly increased this over a couple of hours, I left it at 1Amp. Eventually when the battery Voltage reached 30V then Amperage slowly lowered down until it cut off.

My DC bench output does not have that kind of smart ability - I just set an output voltage and current and off you go. But the battery seems to be manipulating the current it draws and the voltage.

That seems pretty bloody clever and if true AWESOME!

There's no intelligence in the battery, and any battery will do that. If you apply a constant voltage, the current which flows depends on

• the voltage of the cells due to their state of charge, and
• the internal resistance of the battery.

As they charge up, the cell voltage rises. The difference between the sum of the cell voltages and the charging voltage is what pushes current through the battery; the internal resistance is what determines how much current will flow for a given voltage.

So, if each cell is at (for instance) only 2.0 V , and there are 12 in series for a total of 24 V, and you apply 30 V then 6 V is left to push the current. As the cell voltage rises, less voltage is available to push current through the same resistance, so current drops. When the cell voltage equals the charging voltage, no current can flow.

 

[brian_]

My query is that on Valence's website for the 'newer' version of this battery the 'U-Charge' version. They say you can charge it with a standard Lead Acid battery charger.

Really???

... I was really focusing on the batteries being able to be charged from a Lead Acid charger rather than the fact they are potentially dead.

Is that true?

Why not? The charger doesn't know anything about battery chemistry, or what is going on in the battery. It is just programmed in stages (maybe only two stages) for combinations of current and voltage limits. If the voltage suitable for charging a stack of lithium cells happens to match the voltage suitable for charging a stack (of a different number) of lead-acid cells, it can work. Some stages of common lead-acid chargers are not at suitable levels for lithium batteries; one supplier of chargers mostly for recreational vehicles sells a "lithium" version, which is just their regular product with all the extra stages turned off and a higher voltage set for the remaining stage.

43.8 V would correspond to 2.43 volts per cell for a 18-cell lead-acid battery (36 volts nominal), which is a little high but not unreasonable for lead-acid. A charger configured for lead-acid would probably limit to bit lower voltage, which would still work for a dozen LiFePO4 cells.

The Valence modules (or batteries) are configured with a number of cells intended to match them to common nominal voltage of lead-acid batteries - multiples of 12 volts. Stack a different number of cells and a "lead-acid charger" wouldn't be suitable.

Lithium batteries are generally assumed to need more sophisticated management of maximum charging voltage and minimum voltage when discharging than lead-acid, but that's what the BMS is for.

 

[brian_]

Before anyone spots the 'why are you charging below the lowest voltage the battery should hold' It is because ALL the batteries are at zero V - no idea why, perhaps a short when the bus crashed or something that was done to make the pack safe?

It would not make any sense to bring the cell voltage below the rated minimum - or even down to that level - for shipping or storage. If they were fully discharged to make them safe, it would be for disposal... having been assessed as trash. Even if they were perfectly good before "making them safe", they would presumably be destroyed by this process.

Perhaps you're lucky and this isn't what has been done.

 

[DaveBlack]

Thanks Brian

So, my 43.8v dc supply (variable A and V settings) that comes tomorrow will act nicely as a test charger to see if I can resurrect one of these lumps. I felt the dropping off of the current was an aspect of the charger - but from what you are saying it is actually as a result of the lowering difference in charger and battery voltages.

Neat

I will do a discharge test after a charge to see what has actually been stored. even if they are 50% good I will be happy for V1 of my car. For free these are both spending the time on and a little bit of money on.

If they are totally dead I may look at the individual cells... but with over 4000 of them, maybe not!!!

Cheers.

 

[brian_]

If they are totally dead I may look at the individual cells... but with over 4000 of them, maybe not!!!

If a complete 144-cell module (battery) isn't up to the fully-charged voltage, that can mean that some parallel sets of cells within it are dead and others are fine, so the total is less than it would be if they were all good. I don't think there's any way around checking voltages at least at this group-of-cells level to assess condition.

 

[DaveBlack]

If a complete 144-cell module (battery) isn't up to the fully-charged voltage, that can mean that some parallel sets of cells within it are dead and others are fine, so the total is less than it would be if they were all good. I don't think there's any way around checking voltages at least at this group-of-cells level to assess condition.

Yeah, if one battery is not able to get to / hold full voltage then I can strip it down into its 12 groups of 12 cells. If a series of 12 has the same issue then do the same down to the cell level.

Im sure from the 4000+ cells I should have enough to produce 'some' working battery packs...or create a massive battery just from the cells - ooohh thats a nice idea, can be any shape I want then.

I can see inside the unit and there is no evidence of any cells having puffed up. Not that shows everything - but from my quad copter flying, the batteries tended to give up around the time they got fat ;o)

Well that and when you hit a tree and the thing catches fire - but thats another story!

Cheers

Dave

 

[pdove]

If the battery reads 0V then it is dead and will never come back.

 

[john61ct]

And when flat-murdered, no swelling

 

[pdove]

And when flat-murdered, no swelling

LiFePO4 cells usually don't swell.

 

[DaveBlack]

If the battery reads 0V then it is dead and will never come back.

ohhh I sense a challenge - seems a lot of people online disagree but results are mixed. Ill keep you updated!

Dave

 

[john61ct]

LiFePO4 cells usually don't swell.

Yah right.

Seen literally hundreds do it.

Pretty silly not to include straps and plates into your build.

Certainly every systems vendor does.

 

[brian_]

I can see inside the unit and there is no evidence of any cells having puffed up. Not that shows everything - but from my quad copter flying, the batteries tended to give up around the time they got fat ;o)

And when flat-murdered, no swelling

LiFePO4 cells usually don't swell.

The bulging may be more typical of polymer-electrolyte cells (regardless of electrode materials) than of these ones.

 

[brian_]

Yeah, if one battery is not able to get to / hold full voltage then I can strip it down into its 12 groups of 12 cells. If a series of 12 has the same issue then do the same down to the cell level.

Are they really assembled into strings of 12, then 12 strings in parallel? It would be more typical to put 12 cells in parallel, the 12 of those sets in series.

 

[brian_]

Yeah, if one battery is not able to get to / hold full voltage then I can strip it down...

Even if a battery does get to your target voltage, that may only mean that most of the cells are overcharged, and some others are dead. Presumably you can get to the cell level via the BMS wiring, so you could check before pushing the whole battery all the way to the target "fully charged" voltage.

 

[pdove]

ohhh I sense a challenge - seems a lot of people online disagree but results are mixed. Ill keep you updated!

Dave

I don't put much stock in what people online say..... unless they have personal experience and seem credible. I have spent considerable time playing with LiFePO4 cells and every cell that went to zero could not hold a charge. The voltage rises during charging but as soon as it cuts off it drops back to zero.

 

[pdove]

The bulging may be more typical of polymer-electrolyte cells (regardless of electrode materials) than of these ones.

The CALB cells I have didn't swell at all. The Bestco cells I tested did swell when shorted slightly and I also had a Hipower cell that swelled but it wasn't zero but had a reduced capacity. So I guess I shouldn't say my experience is varied on this topic. I never played with a Valence cell they look pretty sturdy.