My reasoning is that if the pack is brought to 100% SOC and held there for an hour or so before the first discharge, the cells should be nearly matched at 100% SOC at the start of the discharge cycle.
While there is a slight possibility that a cell could discharge to 1V much more quickly than the others, it is more likely that the full charge had already balanced the cells enough so it should not happen?
This is why the process needs to be tested carefully.
I would agree with the testing part .... but that's just me... I like to test everything.
There are some pieces missing from the 100% SoC thinking.
#1>
It assumes all 120 cells have equal Ah of capacity ... which they might not.
#2>
It assumes all 120 cells have equal internal resistance ... which they might not.
#3>
It assume all 120 cells will experience equal temperatures ... which they might not.
Quote:
Originally Posted by retepsnikrep
Assuming we use the low current grid charger to top off an equalise the pack, then using some sort of discharger without some quite sophisticated monitoring of the pack voltage could easily lead to a reversed cell.
I agree... but ...
The sophisticated monitoring you describe ... isn't extremely complex ... a bit but not crazy complex... as long as the whole process is allowed to complete from beginning to end... this is not a on the fly during operation system ... that is when it gets crazy complex.
After the NiMH pack is 100% SoC topped off with a nice gentle ~300mA or less current ... you let it rest for an hour or so for the cells to stabilize a bit... ~12 hours would be better ... but ~1 hour should be pretty good... then you discharge at a constant current... and at a slow rate ... under ~1 Amp rates ... ~300 mA would be better... don't use voltage alone as a terminating data source ... it isn't reliable enough for a 120 cell series NiMH pack... like you fear ... voltage alone greatly risks putting individual cells into voltage reversal... but if you combine the voltage with a time slope when you are under a set and known CC slow discharge rate ... you can identify when any one cell in the 120 cells series stack does its last 1V dive ... the slope at the last 5% to 10% of the NiMH battery when discharged slowly ... will drop off its last 1V pretty clearly ... if you are looking at a voltage over time plot of when a single cell goes into reversal ... you would see a sudden 1V slope drop and then the voltage would level off again as the other cells which are not discharged yet keep the voltage from dropping any more and then begin to force that first cell into voltage reversal.... the control electronics would just have to see that ~1V slope drop no matter what voltage it happens at for the whole battery pack.
---------------
The alternative is to skip the discharge part ... and just balance the pack with the slow trickle charge top off on occasion.
Quote:
Originally Posted by 3-Wheeler
2) Discharge to a known 50% SOC (using voltage measurements; pick a value)
The only problem with that line of thinking is that there is not a single Voltage that one could pick that would actually = a 50% SoC state for all 120 cells simultaneously.
But other wise ... yes the idea is sound.
---------------
This jpg attached picture of a graph might make it more clear... it is of a 10 cell NiMH battery pack being discharged... Because all 10 cells are not 100% identical one of the cells ran out of Ah before the others did... you can see the ~1V sudden drop as that weakest cell ran to 0% SoC... then the other remaining 9 cells which still had Ah left ... were able to keep the total pack voltage above up... 10V... the total pack voltage continued to drop much slower after the one weakest cell got pulled to 0% SoC... then that one weakest cell begins to go into voltage reversal .. as the other remaining 9 cells force current through it... which ends up charging that weakest cell backwards... which can do permanent damage to the capacity of a NiMH cell.... the only reason cell reversal is not a death sentence for NiMH like it would be for Lead Acid or Li is that NiMH are so tollerant of abuse.... it still does damage ... but it doesn't instantly kill the battery.
This is why it isn't safe to just pick any one particular voltage ... if you had ... for a 10 cell battery pack you might have picked 10.5V ... but you still could have pulled one of the cells into reversal ... as shown.
I suppose one *could* use the two cheap but effective tools to get the job of cycling the pack done efficiently.
Since I am not inclined to remove the pack and cycle it on a test bench with the proper tools in hand (dV/dt cycle testing equipment), I could instead:
1) top of the battery pack with the charger discussed here.
2) drive the car normally to work and home using most the battery charge by *not* letting the BCM charge it back up; effectively using the IMA disable (CalPod) switch that is installed in the car. If the battery gets too low, the BCM will let me know, and I can immediately stop the discharge process (*a).
3) after getting back home, another charge cycle can be implemented.
4) this could be done multiple times to fully exercise a limp battery pack if needed, depending on how far down it goes before the BCM decides the battery has had enough of the discharge process.
*a) - IamIan, I have witnessed this sharp cutoff of the battery pack at low SOC, by purposely discharging the pack on the way to work several times. Just as you describe in your graph, I noticed when my SOC was about 40%, it suddenly dropped to about two or three bars in under one minute. And that happened while sitting at a stop sign!! So this is something that I am at least familiar with. The BCM was smart enough to quickly start a forced regen to get things back up again.
My battery pack is actually working fairly well that removing the battery is not required at this time, and it is still under warranty. But I am attempting to understand all the mechanisms that will help me to keep the battery in good working order when the warranty has lapsed.
And batteries are definitely in our future, thus my interest in properly caring for them now.
#1>
It assumes all 120 cells have equal Ah of capacity ... which they might not.
#2>
It assumes all 120 cells have equal internal resistance ... which they might not.
#3>
It assume all 120 cells will experience equal temperatures ... which they might not.
Just tossing in some data...
# 1 They definitely will not be if the pack has not been recently balanced. You could see a difference of 6500 vs 5500 Mah in what appears to be a healthy pack. An unhealthy pack will show variations like 6500 vs 4000.
# 2 On a good pack, internal resistance will vary by up to 20% between sticks (for example from 50-60 Mohm)
# 3 PTC testing shows that under charge, the normal 1 ohm per stick can get up to anywhere from 2-4 ohms (where the overheat trigger seems to be on the order of 100+ ohms). I'm not sure what temperature the 2-4 ohms equates to or how significant it is. The fan does an amazing job of cooling the pack. I charge Insight batteries at 10amps, but I charge HCH batteries at 5 amps because they have no fan in the pack and trigger 300 or even 500 ohms at 10 amps.
4) this could be done multiple times to fully exercise a limp battery pack if needed, depending on how far down it goes before the BCM decides the battery has had enough of the discharge process.
I agree... one could use such a strategy ... and it will give the battery pack some exercise ... specifically if done on occasion ... it could help with keep the pack balanced... but I would not say 'fully'.
part of the 'fully' process involved bringing individual cells down to as close to 0% SoC as you can without going into voltage reversal... this part is the rub / difficulty that gets worse the larger the number of series cells connected ... if it was on big single cell... it would be pretty easy to do... but with 120 cells there will always be 1 cell that is the weakest ... and that 1 cell will always go into voltage reversal before you get the rest of the 119 cells down to 0% SoC.
This is the distinction between balancing a pack ... which one can do just by topping it off at the top... vs the very different process of re-conditioning a pack... if you aren't trying to re-condition the pack... then the discharge cycle can be skipped.
Besides the BCM tries to avoid the pack going bellow 20% SoC.
Quote:
Originally Posted by 3-Wheeler
*a) - IamIan, I have witnessed this sharp cutoff of the battery pack at low SOC, by purposely discharging the pack on the way to work several times. Just as you describe in your graph, I noticed when my SOC was about 40%, it suddenly dropped to about two or three bars in under one minute. And that happened while sitting at a stop sign!! So this is something that I am at least familiar with. The BCM was smart enough to quickly start a forced regen to get things back up again.
Don't make the mistake of thinking the dash SoC gauge is accurate.
At best... it gives you a vague idea of what the BCM is currently willing to give you from the battery... this is not = to what the battery is for SoC ... it is not even what the BCM thinks the battery is.
Quote:
Originally Posted by 3-Wheeler
My battery pack is actually working fairly well that removing the battery is not required at this time, and it is still under warranty. But I am attempting to understand all the mechanisms that will help me to keep the battery in good working order when the warranty has lapsed.
good plan... and I am in a similar boat.
I got a IMA light and error code about ~7 months ago... when I was at 120,000 miles and ~8 years old ... Honda techs wanted to replace the battery module under warranty ... They didn't even say a thing about my MIMA system ... but others might not be soo lucky.
Instead of using the warranty ... I opened up the battery module... I rebalanced my pack... logged some data along the way ... and have been driving for the last ~7 months without issue... car has returned to normal pre-IMA-error operation.
Fixing the balance is only treating the symptom ... and the root cause that created the imbalance is still there ... so I expect the pack to get out of balance again eventually... but I have been curious how long it will take ... and I also eventually want to re-condition my OEM pack.
Quote:
Originally Posted by 3-Wheeler
And batteries are definitely in our future, thus my interest in properly caring for them now.
eventually ... perhaps ...
I figure I shouldn't have much issue using the OEM original pack out to 15+ years or so... as long as I am willing to do the occasional once or twice a year maintenance on it... by fixing the balance and or reconditioning it.
*a) - IamIan, I have witnessed this sharp cutoff of the battery pack at low SOC, by purposely discharging the pack on the way to work several times. Just as you describe in your graph, I noticed when my SOC was about 40%, it suddenly dropped to about two or three bars in under one minute. And that happened while sitting at a stop sign!! So this is something that I am at least familiar with. The BCM was smart enough to quickly start a forced regen to get things back up again.
This sounds more like a downward recal to me. It sounds more like the BCM detecting that the indicated 40% is actually closer to 0% and then recalibrating the bottom of the SOC gauge. Is it repeatable? Can you do it twice in rapid succession, or does it not do it again for a few days?
It's possible that you force-discharging your pack is triggering these due to different cell performance at different pack temperatures. The way to tell would be to reset your IMA and do a battery relearn. Then see if the car acts any differently (the battery charge seems to last longer before depletion, etc).
Fixing the balance is only treating the symptom ... and the root cause that created the imbalance is still there ... so I expect the pack to get out of balance again eventually... but I have been curious how long it will take ... and I also eventually want to re-condition my OEM pack.
eventually ... perhaps ...
I figure I shouldn't have much issue using the OEM original pack out to 15+ years or so... as long as I am willing to do the occasional once or twice a year maintenance on it... by fixing the balance and or reconditioning it.
Amen to that. Although I think that if you top it up every couple of months, you might be able to forestall the problem indefinitely - depending on what the problem is. Topping up will keep rapid depleting cells from depleting and then producing memory effect in good cells, but it will not reverse the process.
A typical 175,000 mile pack (7 years) has dropped in overall capacity from 6500Mah to about 5400Mah. I don't foresee any problem going out 20 years or more with occasional reconditioning. I think the pack would have to drop down below 3000 or so before the BCM would declare a P1447 (every P1447 I've seen has had sticks below 2500 charge capacity on the first reconditioning cycle).
I started to recondition a profoundly dead pack, and found that my chargers were detecting delta V immediately. They wouldn't charge at all. I had to hit the cells with a 12 amp charge for about 2000Mah before the main chargers were able to function. This is an HCH pack and is yellow. The sticks are coming up to 7300Mah charge / 6500Mah discharge after only a few cycles, so they are good. IR is 55-60 Mohms (normal).
I started to recondition a profoundly dead pack, and found that my chargers were detecting delta V immediately. They wouldn't charge at all. I had to hit the cells with a 12 amp charge for about 2000Mah before the main chargers were able to function. This is an HCH pack and is yellow. The sticks are coming up to 7300Mah charge / 6500Mah discharge after only a few cycles, so they are good. IR is 55-60 Mohms (normal).
It is one of the known and common errors in many 'smart' NiMH chargers.
One of the charge termination methods the 'smart' chargers look for the change in the voltage vs time slope... when you first start charging a Battery there is a sudden rise in voltage under the charging current... this sudden rise in voltage then tapers off ... sometimes a 'smart' charger confuses this change in the voltage vs time slope with the change that happens at the top of the SoC slope when the battery is full and the additional charging energy just gets converted to heat... and so it incorrectly thinks that the battery full , even though it is not.
The frequency of this will vary from 'smart' charger to 'smart' charger... and from battery to battery even on the same 'smart' charger.
The other well known and common error for 'smart' chargers comes from the rate of voltage rise vs time slope ... if you have a 100Ah battery it will rise in voltage slower under 2Amp rates of charging than a 1Ah battery will... the 'smart' charger again incorrectly thinks that the battery has reached its peak voltage condition when it hasn't.
In short ... 'smart' chargers ... are very dumb compared to a human ... just because the charger says it is full does not mean it is full yet ... and it does not mean that the 'smart' charger didn't over charge the battery... but are a simple method to try and automate the process so you as a human do not have to just sit there.
similarly a 'smart' charger that does cycles ... generally 95% of the time would miss the voltage reversal event int he picture I posted above... as 95% of them discharge to a set Voltage level.
I have been fabricating a trailer for the Insight on a stand and the other demos, so I have not been on line and following this thread. I see that our battery experts have added a lot of information to consider.
The stock system:
The stock BCM only charges to 80%, never charges to 100%, and stops discharging at 20%. This has not been confirmed, but has been accepted as gospel, and is the cause of battery pack imbalance.
Charging:
The proposed charger will not stop charging when one or 10 or all cells are full, and will just keep 350MA going into the pack forever, which will fully charge all of the cells, no matter what SOC they started with.This gives us a reliable 100% SOC.
The "smart chargers" that have been used to recondition subpacks try to determine 100% SOC based on the slight dip in voltage that happens when a cell reaches 100% and starts to turn charge current into heat. On the first cycle, this dip will be difficult to detect accurately, since we are looking at 6 or more cells, and some of the cells may be at quite different SOC than the rest, so the dip may be masked by an adjacent cell in the same subpack that is not quite at the 100% SOC, so it may still be rising, while others may be fully charged and into the dip.
That means that the 100% SOC point for all cells may not be reached with the "Smart chargers"
If the proposed 350MA charge is allowed to continue, until the voltage stabilizes for at least 1/2 hour, we will know with a high degree of confidence that all cells are fully charged.
I suspect that some of the AH discrepancies that the "smart chargers" are seeing are due to this premature 100% SOC determinimg effect.
Discharge:
The danger of reversing a cell during discharge is a real issue that we need to avoid. Based on some simple experiments, I think that we are pretty safe for 90% or more of the packs just using a light bulb and voltmeter and stopping the discharge at 120V(1V/Cell), after topping off the charge on the whole pack. The key is that we are bringing all cells to a reliable 100% SOC.
The "Smart Chargers" look for 1V per cell as the zero SOC point, and would likely not see a dead or reversed cell, and could terminate the discharge prematurely, skewing the AH determination.
There may be a pack that has a subpack with a cell that is far below the average AH, and this will damage the cell. To do this so that cell damage possibility is minimized, we would need to monitor the ten subpack taps and look for a rapid drop off of a cell like the car does in normal operation. The car may be the safest discharge method we have, since it already does the monitoring at the 12 cell level.
We can speculate on the best procedures, but at this point, we can get the charge part nailed down, and then work on the discharge, and prove the system with carefully run experiments.
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