I've been reading this thread for awhile now, and I still have a few unanswered questions. In addition to my 2010 I2, we've got an '06 HCH II. After the latest sw update, the IMA battery is worse than ever, rarely charging above half full. The SoC will also wildly swing from 5, to completely full in an instant, to flat again, sometimes within a few miles. Since the pack seems to have many symptoms of unbalancing, would a "pack whack" or full charge with one of the systems described here, help recover the capacity of the pack? It was new in 2/06 and just turned 61K miles. We live in northern Illinois to give an idea of the climate. I've been using shades and cracking the window, etc for the last 2 years or so.
Any suggestions short of dismantling the entire IMA battery assembly? Honda's been of little help at this point since there's been no CEL thrown yet.
Would occasionally fully charging my I2 pack help ward off unbalancing and prolong it's life?
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2010 Insight Ex w/navi Tango Red
2006 Civic hybrid w/navi Alabaster Silver
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The ~0.9V sudden drop in voltage is that one cell giving up it last bit of stored chemical energy... once it is gone and that cell is forced into voltage reversal by the other cells continuing to force amps of current through it ... then voltage slope levels back off based on the average of the remaining cells putting out amps.
There is apparently another part to this story - the 0.9V drop is only giving up the second-last bit of energy.....!
Quote:
The typical voltage profile for a cell carried through a total discharge
involves a dual plateau voltage profile as indicated in Figure 14. The voltage plateaus
are caused by the discharge of first the positive electrode and then the residual capacity
in the negative. At the point both electrodes are reversed, substantial hydrogen gas
evolution occurs, which may result in cell venting as well as irreversible structural
damage to the electrodes. It should be noted that the nickel-metal hydride cell, because
it uses a negative electrode that absorbs hydrogen, might actually be somewhat less
susceptible to long-term damage from cell reversal than the sealed nickel-cadmium cell.
The SoC will also wildly swing from 5, to completely full in an instant, to flat again, sometimes within a few miles.
That does not sound like normal behavior. I suggest videotaping it for possible action later.
Quote:
Originally Posted by nursemike
Since the pack seems to have many symptoms of unbalancing, would a "pack whack" or full charge with one of the systems described here, help recover the capacity of the pack?
I'm not sure that's what is going on.
Quote:
Originally Posted by nursemike
Would occasionally fully charging my I2 pack help ward off unbalancing and prolong it's life?
Yes, but remember that you're going to need a different charger circuit. Your battery is 132 cells, not 120 like the 1st Gen cars.
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2000 MT #4227 175K miles - Citrus Yellow, BetterBattery
Quote:
The typical voltage profile for a cell carried through a total discharge
involves a dual plateau voltage profile as indicated in Figure 14. The voltage plateaus
are caused by the discharge of first the positive electrode and then the residual capacity
in the negative. At the point both electrodes are reversed, substantial hydrogen gas
evolution occurs, which may result in cell venting as well as irreversible structural
damage to the electrodes. It should be noted that the nickel-metal hydride cell, because
it uses a negative electrode that absorbs hydrogen, might actually be somewhat less
susceptible to long-term damage from cell reversal than the sealed nickel-cadmium cell.
Once the cell goes into voltage reversal at the first -0.9V drop, permanent damage is getting done... it should be avoided... and that is the last of the energy that cell has to give ... a individual cell can only give / supply energy as long as both plates and the electrolyte can contribute , as soon as any one of three is removed the chemical chain is broken and you can not get any further energy extracted from it as a battery system, which requires all three to work... you might get other chemical reactions but not as a rechargeable battery type of chemical reactions.
you drop into voltage reversal once the first plate is depleted , you are now charging the battery backwards ... the first plate that got depleted is the first to begin to charge backwards ... and you are spending some of the wh of energy from the other battery cells to reverse charge that one reversed cell ... the second plate is not supplying energy by itself , but it my not yet be depleted of its active material ... once it is depleted it also begins to charge backwards ... eventually one of the plates gets fully charged backwards ... than the other plate will eventually get fully charged backwards... or it is also possible that before the second plate gets depleted of active material it might begin to have a different chemical reaction as you are already having different chemical reactions on the first plate and potentially in the electrolyte as well.
Each of these transitional points changes the type of chemical reaction that is happening ... you will get different amounts of internal impedance , heat, pressure, gas generation, etc... from the cell ... you might also vent the cell long before you finish reverse charging it... and if you loose the electrolyte you also stop the battery from performing correctly ... if you keep pumping enough amps you might be able to keep forcing things to happen , but you will continue to get different , non-reversible, non-desirable , chemical reactions... and it is not functioning as a battery.
Nice find though... useful for someone who wants to plan for the what if case for the batteries... although, my suggestion is to instead plan for a what if case ... is if you see the -0.9 V drop... stop... if you can avoid even reaching the point of the -0.9V drop.
Yes, but remember that you're going to need a different charger circuit. Your battery is 132 cells, not 120 like the 1st Gen cars.
The charger based on 3 Meanwell 48 volt supplies and a 48 volt current limiter can easily be adjusted to ~190 volts. For a post 2005 Civic I believe he only needs 180 - 185 volts.
Each of these transitional points changes the type of chemical reaction that is happening ... you will get different amounts of internal impedance , heat, pressure, gas generation, etc... from the cell ... you might also vent the cell long before you finish reverse charging it... and if you loose the electrolyte you also stop the battery from performing correctly ... if you keep pumping enough amps you might be able to keep forcing things to happen , but you will continue to get different , non-reversible, non-desirable , chemical reactions... and it is not functioning as a battery.
Nice find though... useful for someone who wants to plan for the what if case for the batteries... although, my suggestion is to instead plan for a what if case ... is if you see the -0.9 V drop... stop... if you can avoid even reaching the point of the -0.9V drop.
I agree with all of this - but I want to point out that the first part of the reverse charging event, the drop to near zero volt, is much less damaging than the real reverse charging event when the voltage reverses.
I might misunderstand this, but basically at 0V there is zero energy dispersed in the (?pre?)-reversing cell. That means zero heat and zero gas pressure generation. And probably very little damage! Or am I missing something?
As soon as the 0.9V drop occurs the discharge should stop. But this first part of the cell reversal is not that damaging that it needs to be avoided at all cost. It is a very useful indicator for the empty point of an unbalanced NiMH battery string.
I think it is impossible to reliably avoid the pre-reversal of all cells in a long NiMH string, because of growing differences in self -discharge rate as the battery ages.
Keeping the range of charge between 40% SOC and 60% SOC will avoid such imbalance for prolonged periods - because self discharge rate is proportional to SOC (and temperature).
Like this: Good cells cycle between 40% and 60% SOC; weaker cells gradually fall behind until they are so close to empty that their self discharge rate approaches zero (zero charge = zero self-discharge) during the lower-SOC times. That slows down the imbalance creating process.
The scenario inside a Prius battery is probably this:
Best cells cycle between 60% and 80% SOC; worst cells cycle between 20-40% SOC. The difference in self-discharge rate between best and worst cells is counter-balanced by the reduced self-discharge rate at (average) 30% SOC vs. (average) 70% SOC. That keeps it in good shape for many years and many tens-of-thousands of kilometres. And when the 0.9V drop finally occurs in the first cell, it's time to do something soon, or the cell will die within a few thousand kilometres because of the repeated short pre-reversals.
I don't know what mechanism the Hondas use to detect the pre-reversal.
I might misunderstand this, but basically at 0V there is zero energy dispersed in the (?pre?)-reversing cell. That means zero heat and zero gas pressure generation. And probably very little damage! Or am I missing something?
At 0V the cell becomes energy consuming ... it no longer disperses or provides any further electrical energy output.
There is always some heat generated ... at any SoC ... even 50% SoC ... Internal Impedance and current flow = Heat... the endothermic chemical reaction can absorb some of this produced heat, but only insignificant % at any significant current rates.
There is always potential for gas / pressure generation ... the whole cell is not uniform ... just like a room that has a thermometer that reads 80 F is not necessarily uniform ... there will be fluctuations inside the cell ... just like in the middle of that room could be a fire place that has some very hot parts ... there can be parts inside a cell at any point in the SoC that are much higher or lower than others ... thus Gas & Pressure generation is always possible... but you can reduce the amount of it, the type of it O2 Gas or H2 Gas, and the rate of it , etc.... modern NiMH cells are designed by the electro-chemists to reabsorb some of the gasses that are routinely generated... but they have thier limits in terms of amounts, and rates.
I will agree that not all parts of equally damaging ... And I am inclined to agree that the initial , and most shallow reversal would be likely to be the least damaging ... I do not have any hard numbers to know how much so.
Quote:
Originally Posted by Mr. Mik
I think it is impossible to reliably avoid the pre-reversal of all cells in a long NiMH string, because of growing differences in self -discharge rate as the battery ages.
My opinion is that a great way is to do a celebration test of the pack every year or two ... once you know / quantify what you have you can more easily keep the pack in a healthy / safe range ... I think it is border line silly to expect hundreds of batteries to stay identical for 5+ years with no recalibrating ... but to date I have not seen it done much if at all.
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It does occur to me to make a correction / clarification ... because batteries are chemical reactions which always have time delays involved ... it is possible to artificially reach a 0V condition prior to depleting any of the plates entirely ... this results when the rate of discharge is high enough that parts of a plate get discharged faster than the internal mechanisms can re-distribute the remaining capacity on the plate ... which can cause a small localized area of depletion ... which could then be exposed to potential reversal issues even if the sum of that entire plate is not yet depleted.... but this side of it does get a bit into why we put limits on the charge and discharge rates of batteries in the first place.
during this potential partial plate depletion the other parts of the same plate that are not yet depleted can potentially still be outputting electrical energy ... if the output of the other parts of the plate are more than the consumption of the depleted part the cell can potentially still be outputting electrical energy under the load that pulls it down to 0V.
The part I mentioned previously about no more electrical energy output at 0V ... is when the cell reaches 0V ... even when it is not under a load... because as long as it is under a load 0V is not the cells true voltage but , a change in the true voltage to the 0V condition.
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My opinion is that a great way is to do a celebration test of the pack every year or two ... once you know / quantify what you have you can more easily keep the pack in a healthy / safe range ... I think it is border line silly to expect hundreds of batteries to stay identical for 5+ years with no recalibrating ... but to date I have not seen it done much if at all.
I have used such an approach successfully with my Vectrix 102s 30Ah NiMH pack. It had quite severe battery problems early on. Because service was hopeless and they cancelled my warranty, I took the battery apart before any cells were completely destroyed. The weakest ones had only about 60% of the capacity of the best ones left (@ 20A to 1.1V cutoff).
Because no replacement cells were available, I simply re-arranged the cells according to remaining capacity and monitored the weakest cells frequently through a permanently installed wiring harness.
The weakest cell was always monitored when I had to use all of the available range; apart from very brief pre-reversals it has not had further abuse inflicted and is still alive and kicking 7000km later! I also monitored the other 12 weak cells, but found that this yielded little extra information.
The initial damage had developed over just 6000km. Monitoring of the weakest cell has practically halted the destructive process. Now at 13000km there has not been any obvious further deterioration; but, I have heard of many other Vectrix bikes with similar battery symptoms which then proceeded to complete battery failure in short order.
This gave me an idea (yet to be tested in practice) that might be even better than interval calibration test: To maybe use a sacrificial cell rather than a complex per-cell BMS in a NiMH battery. The sacrificial cell could be either the weakest cell in an inhomogeneous pack, or a deliberately introduced cell of lower specs than the others. Just an older cell of the same type would usually do.
If the capacity of the sacrificial cell is, say, 20% less than the capacity of the best cells - and the self discharge rate the same or a tiny bit increased - then monitoring that one cell for reverse charging events will securely protect the entire rest of the pack from reverse charging! The sacrificial call would of course be located in the easiest location for periodic removal and replacement with the next one.
Quote:
It does occur to me to make a correction / clarification ... because batteries are chemical reactions which always have time delays involved ... it is possible to artificially reach a 0V condition prior to depleting any of the plates entirely ... this results when the rate of discharge is high enough that parts of a plate get discharged faster than the internal mechanisms can re-distribute the remaining capacity on the plate ... which can cause a small localized area of depletion ... which could then be exposed to potential reversal issues even if the sum of that entire plate is not yet depleted.... but this side of it does get a bit into why we put limits on the charge and discharge rates of batteries in the first place.
during this potential partial plate depletion the other parts of the same plate that are not yet depleted can potentially still be outputting electrical energy ... if the output of the other parts of the plate are more than the consumption of the depleted part the cell can potentially still be outputting electrical energy under the load that pulls it down to 0V.
The part I mentioned previously about no more electrical energy output at 0V ... is when the cell reaches 0V ... even when it is not under a load... because as long as it is under a load 0V is not the cells true voltage but , a change in the true voltage to the 0V condition.
Through the monitoring system in my Vectrix I can see this in action under load as well. The point at which the weakest cell begins to reverse is dependent on the current draw. The cell will be reversing under full throttle acceleration (>200A current draw) looong before I cannot continue to drive gently without it reversing. I keep the weak cell's voltage above 0.9V by adjusting the load (less throttle). The stock system is completely "blind" to the plight of the weak cell and will allow full power acceleration if I need it in an emergency.
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