DIY Electric Car Forums banner
1 - 9 of 9 Posts

·
Registered
Joined
·
4,326 Posts
Discussion Starter · #1 ·
I am attempting to re-balance a pack after a 'low-voltage event' which totally killed one cell in the pack. I have bypassed the completely dead cell, and am attempting manual re-balance before using pack charger.

I have been using a 'revolectrix powerlab' 1000watt single cell charger with luck in the past to bring cells up to a set voltage, and terminate charge automatically... which got the cells all pretty close initially, needing just a little fine tuning with a resistive element when the cells were new.

HOWEVER, this time around, there are a number of cells that just don't seem to hold the charge nearly as well, and even with repeat charges to the set voltage, they 'settle' to a lower voltage than other cells... and when I try a pack charge the cells are obviously out of balance with a number of cells heading up to 4v while a few don't seem to keep up.

I suspect that some cells are damaged internally, perhaps having vastly higher resistance, or 'dendrites', or plating or something....

My question to the group is:
when a number of cells become 'unbalanced' with others in the pack, and do not remain balanced after repeat single cell charging/discharging to a set voltage... is there any way to recover them, or are they dead?
 

·
Registered
Joined
·
4,326 Posts
Discussion Starter · #4 ·
1 - Some cells have lower capacity (due to damage). That means they fill quicker and empty quicker because there's less capacity in them.

2 - Some cells have higher internal resistance (due to damage). This means that they fill slower and discharge slower, and get hotter when they do. When charging, you may be giving it 3.65 volts, and it may be showing 3.65 volts, but it's not adding all that as energy. Chargers usually "terminate" when the charging current drops to a certain amount, so if your resistance is high the charging current will be low especially at the top end, and it will stop before it was done filling.

3 - Some cells may have dendrite damage internally, small shorts that constantly drain the battery. So they self-discharge.
I like your explanations, and differing internal resistance may well explain the behavior I am seeing while I am attempting a manual top-balance before reverting to normal use and pack charging. I have brought all cells up to a 'final voltage' cell by cell, but they just don't seem to stay balanced the way they did 18,000 miles ago.

What I am looking for now is a way to identify each cell's IR, and some guideline on how much the IR can vary cell to cell to 'stay in balance' with repeat charge cycles in series. I'd like to limp along for a while if there are just a few cells out of spec, or consider replacing just a few cells rather than the entire pack.

what equipment would I need to establish a cell IR?
method for measuring?
acceptable range of difference to minimize 'drift' with repeat charge cycles?
 

·
Registered
Joined
·
4,326 Posts
Discussion Starter · #7 ·
internal resistance for batteries, usually you test it under different loads, and the change in voltage divided by the change in current is the battery resistance. Though it moves around a bit due to temp and state of charge, and you want to be quick in taking your measurements, and take them on the flat part of the discharge curve.

capacity, just discharge it fully and measure how many amp hours it takes to charge it fully (or charge it fully and see how many amp hour you get while discharging it fully). A bit time consuming, but that is how it is done.
thanks Steve,that was exactly the type of answer I was looking for.... How to quantify the state of the individual cells so I can decide how many are 'bad' so I can decide whether the replace them.

It sounds like measuring internal resistance may be difficult with typical garage equipment; not having a precise way to apply different loads and measure small difference in current. I may have to resort to comparing voltages after cell charge, after settling overnight, and after use/recharge cycles.
 

·
Registered
Joined
·
4,326 Posts
Discussion Starter · #8 ·
The bad news is that, generally, high internal resistance goes hand in hand with lowered capacity.

I think you just have lowered capacity, and nothing can help that other than boosting the low cells with some smaller add-on cells like 18650s.
I am not as concerned with reduced capacity as I am with inability to maintain a decent top-balance with repeat charge and use cycles. Unbalanced end of charge voltages obviously will damage cells further.


I'm not sure you grasp what's going on, you seem to keep using the same terms for a variety of things. Isolate what you're measuring.
I think I grasp things quite well thank you. Sorry my language is too imprecise for you.


Charge all your cells up to their max voltage independently. Then let them sit, then measure them. If you have self-discharge then that's a separate issue.
obviously.... but I am not seeing self discharge, nor is that what I asked about.

If they maintain their voltage, but some cells drop faster under use, then those cells are lower capacity, "balancing" is not going to fix them.
pretty hard to tell if some cells are dropping faster under use since the middle of the curve is so flat. i.e. at 10%-70% DOD there is hardly any difference in voltage.
 

·
Registered
Joined
·
4,326 Posts
Discussion Starter · #11 ·
6 - Do this as long as you like, recording times as you go. This will give you a ballpark for how much life the cells have. If your first cells hits your minimum voltage and the others are all significantly higher, you know it's lost a lot of capacity. If a few others are close behind, you know those are all weak. If they all arrive fairly close to each other, your pack is decently capacity-matched.
Two problems with capacity testing and matching...

1. time. I have 48x100ah CALB LiFePO4 cells; draining each to 80% DOD and recording the time would take hours per cell unless I come up with a super heavy resistance. And then take another 4 or 5 hours per cell to charge back up with my Revolectrix at 20amps.

2. results and correction. Let's say I go thru all 48 cells (taking 4 -5 hours per cell), and find several that are 10% less capacity than the others. Then I'm faced with the dilemma whether to just bypass them and adjust my charger end-of-charge voltage for a smaller pack.... or, if too many cells are short, trying to mix in some new cells.... or junking the entire pack even though most of the cells would probably be serviceable for a while longer.

This is why I keep coming back to end-of-charge top balancing... to see what ideas anybody has on how to make the best of this pack without the expense of replacing the whole thing. I'm looking for some way to quantify which cells will not be 'top-balancable' anymore, (regardless of what type of internal damage with higher IR or reduced capacity) so I can decide whether I can just bypass a couple, replace a few, or have to replace the whole pack.

My goal is to bypass/replace as few as possible, adjusting charger voltage if need be, so that no cells exceed max voltage at end of charge... and drive the pack as many more miles I can without junking the pack.
 

·
Registered
Joined
·
4,326 Posts
Discussion Starter · #13 ·
No. You're not following the instructions. If you read the procedure I gave you, you will notice that I said to discharge them IN SERIES so you will be discharging them all at the same time.
The only load heavy enough to discharge the series is.... driving. I totally GET this, believe me I am not an idiot. I have been driving EVs for 10 years now, and what I am facing is getting some more miles out of a pack that is showing signs of decay.

I can and have driven around until the pack voltage starts dropping, note the amp-hrs and miles and have a pretty good idea where my floor is for the pack. I try to live within that capacity, and I KNOW there is no way to recover capacity.

I totally understand that balancing will not increase capacity. That's not what I am worried about. What I am worried about is that it appears *some* cells are not STAYING balanced and returning to end-of-charge at the same time after a couple charge/drive cycles... forcing some cells higher than I would prefer before the pack charger finishes. i.e. 'drifting' end of charge voltage is what I have been asking about.

What I was looking for was a way to quantify and test cells to be predictive about which and how many will drift unacceptably after a few charge cycles so I could decide if it would be cost effective to mix in a few new cells, or replace the whole pack.

I think we're going to have to accept that the question I'm asking is not the one you are answering, and leave it at that.
 

·
Registered
Joined
·
4,326 Posts
Discussion Starter · #20 ·
Hi Dan,
Talking about testing for internal resistance, one of the best ways is to use the CellPro Powerlab 8 with measures the Internal Resistance for you. This unit will tell you the Capacity and IR of each cell as it cycle the cells.
Get one or borrow one to check your cells.
I have one, but I am in South Australia.
Eric.
I have a powerlab 6.... and have used it with a custom 'charge-only' setting for initial top-balance to bring cells up to 3.66vpc, and stop.

Thanks for the reminder that is has a discharge-charge cycle, which should give me capacity and IR, I have not read thru how to set that up. I was trying to avoid the many hours per cell it would take to drain and recharge.

At this point, I am going to try to squeeze out some more miles and attempt to deal with the 'unmatched' cells. I have bypassed 2 cells that were really bad, and have my eye on one more that shows much less capacity than others. For now, I have adjusted my end-of-charge voltage down on the pack charger, and will keep an eye on the cells that tend to 'drift' higher and higher at end of charge.

When the manual periodic top-balancing gets to be too much of a pain, I'll have to spring for a new pack, or at least most of one after I do a capacity test on the old ones to determine which are worth keeping.
 

·
Registered
Joined
·
4,326 Posts
Discussion Starter · #21 ·
... if you top balance and drive your vehicle extreme discharge you risk that low imbalance and causing a cell to go empty before the others and kill a cell or two and even damage surrounding cells.
yes, this is exactly what my pack has gone thru... twice in 4 years actually. First time was when I loaned the car to a guy and he drove the pack until the car wasn't even 'limping' any more. Second time I was caught taking an unexpected side trip 'rural' and thought I could make it back to town before pack voltage got too low. Both times forced one cell to reverse polarity under load, and caused an internal short in the cell, rendering it very obviously dead.

my concern is that *some* other cells are damaged as well, but may be usable for a while longer.

I KNOWINGLY accepted the risk of top-balancing because I want to allow my charger to work the way it it supposed to and bring the pack up to a 'nearly-full' state automatically, where hopefully all the cells finish at about the same time and none suffer from over voltage. BUT, now that my pack has suffered two catastrophic under-voltage events, I suspect the capacity and IR of several more cells are going to force me to replace some cells soon. I am trying to prolong that and hesitate to mix in new cells with old, but I'd also like to avoid the cost of an entirely new pack if half the cells are still usable and relatively undamaged.

My main purpose for this thread from the beginning was to try and QUANTIFY how bad is bad, and how to measure that so I can have some confidence in deciding how many cells I have to replace when the time comes. i.e. exactly what difference in IR or capacity will make particular cells 'unusable' with regard to drifting over multiple charge cycles (because of damage resulting in differing IR and/or capacity).

My conclusion at this point is that I need to review the capabilities of my Cellpro, and see if I can get any meaningful measurements matching up to which cells start 'drifting' too high at end-of-charge. In the meantime, I have bypassed dead cells, and lowered my setting for the pack end-of-charge voltage to hopefully prevent any single cells from overcharging in a normal charge cycle.
 

·
Registered
Joined
·
4,326 Posts
Discussion Starter · #23 ·
Please be informed that the ir measurements from the CellPro are not much usefull for 'big' batteries.

I think this is because of relatively low C-rate and the thin wiring.
hhhmmm, that is useful info.... using it on a 1s 130ah cell is probably quite different than the more usual use with 10ah cells.....

I think that when I DO get ready to remove and replace the sketchy cells, I will discharge the pack most of the way by driving so the cellpro doesn't have to do as much work before charging.
 
1 - 9 of 9 Posts
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.
Top