Delta V for Insight cells

[retepsnikrep]

I have now sorted my high voltage 300ma IMA pack charger and have also bought a 'Robitronic Overloader' subpack charger/discharger. It's similar to the 'Triton 2' but with higher current capability.

I intend to cycle my old subpacks but am wondering about the Delta V for our Panasonic cells. I've seen ranges of 5-10mv quoted on line for common Nimh cells. Any thoughts?

[Armin]

Quick answer: I don't know the dV spec for the Panasonic cells.

Medium answer: For what you are trying to do (equalization charging), the usual prescription is to discharge down to <1V/cell and then charge at a low rate for enough time to get about 120% of charge (usually C/20 for 24h or in our case, 325mA for 24h).

Long answer: dV end-of charge detection only works for fast charge. The drop in voltage is caused by the cell heating up once it is full. For it to work well, you need to not only know the right dV, but also the correct current to use (usually between C/4 to 1C) and have the right thermal setting (i.e. it doesn't work well in the Insights battery pack, since the inner cells heat faster than the outer ones, and all of them heat more in the enclosure than they would in free air. Most importantly, if you are charging more than one cell in a string, they have to be very well matched and balanced before doing a dV terminated charge, since if the peaks of the individual cells don't line up, there will be no peak in the total voltage of the string. Also note that the cells change as they age, causing them to warm up earlier during the charge and "smearing" the voltage peak out enough to never hit the threshold. This can be dangerous if you have no backup means of terminating the charge! Since you are working on old cells, I'd not trust any specs, even if you can find them. You could determine the value by experiment, simply recoding the voltage during charging. But since the pack is old, every cell will probably behave differently.


I'd say go with the low rate equalization charge and use a timer to teminate it.

[IamIan]

The Triton had a Ah counter / limiter built into it. This would terminate charging when a preset Ah was counted... If your charger has a similar function set it to limit / terminate the charging at no more than 6,500 mAh.

I recommend only opening the ends of the Battery pack, and not removing them from the Panasonic Battery Enclosure... Charge the subpacks in place.... set the charger to 6 cell subpack, NiMH, etc....

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The dV/dT bump for NiMH is between 3mV and 5mV.... where have you ever seen it over 5mV???

It is more difficult to detect at higher rates of charge... Because the cell will go from 1.0 V to 1.4 V over the charge cycle... the faster you charge the faster the 0.4V rise in cell voltage... this change in voltage or dV can dwarf the small 3 to 5 mV bump if you hit the bump to fast... and thus not be detected at all.

It is safest to discharge the cell or pack first at under 1C rates to 0.95V per cell ... then charge it at under 2C rates ... Then include all of the bellow in your charging termination method ....

#1> Terminate the Charging if the cell / pack goes over Voltage.
#2> Terminate the Charge if the cell / pack goes over Temperature.
#3> If the cell container is flexible like on the prius packs terminate charge if the pack begins to swell / pressure increases.... if the cell is ridged like a "D" cell the pressure will just cause the temperature to rise faster.
#4> Terminate the charge if the dV/dT 3 to 5 mV bump is detected.
#5> Terminate the charge if you Ah counter reaches 110% of the rated capacity of the cell / pack.

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The overcharging of a cell occurs after the positive electrode doesn't have any nickel hydroxide to react with the hydroxide from the electrolyte, and begins to evolve oxygen.

The order is:

1> Positive Electrode runs out of nickel hydroxide to react with the hydroxide from the electrolyte... This causes the beginning of the rise in the dV/dT Bump... but is not yet reliably detectable.

2> The continued electrical energy evolves Oxygen... the Oxygen will increase the pressure in the cell... If restricted the increased pressure will increase temperature in the cell, above the temperature rise caused by the internal resistance of the cell and the charging current.

3> The Oxygen travels through the electrolyte via diffusion to the negative electrode where it recombines, eventually causing the the back side of the dV/dT bump and dV/dT in the range of 3 to 5 mV becomes reliably detectable.


If The charge rate is too fast or goes for too long. You will evolve Oxygen beyond the ability of the cell to deal with it. This will breach the safety seals and vent cell material. This will permanently lower the capacity of the cell.

If the charge rate is EXTREME you can produce electrolysis in the electrolyte splitting it into H2 and O2 combined with the heat and electrical flow you are risking an explosion, or fire.

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Interesting notes about NiMH:

Hydrogen is only released at VERY High over charging rates causing electrolysis in the electrolyte of the cell or during Discharging of the cell. Not during normal Charging which moves Oxygen not Hydrogen in the electrolyte.

the Electrolyte in a NiMH cell contains water... contributing to some of the temperature limits of a NiMH cell.

While Charging is an Exothermic chemical reaction putting out heat....

Discharging a NiMH cell is a Endothermic Chemical reaction... meaning that it absorbs heat.... This is another reason why NiMH can be discharged faster than it can be charged.... as the discharge Endothermic chemical reaction absorbs some of the heat generated by the Ohmic effect of the internal cell resistance and the current passing through it.

NiMH cells usually are equal in energy Wh per unit volume as Li based batteries... So the same amount of space is used for a given amount of Wh with either Chemistry... Li is just often Lighter.

Have fun with that

Be Safe.

[retepsnikrep]

Yes, I was planing on using a multi pronged charging strategy anyway. I'll try a DV of 3-5mv to start with. I will be charging the subpacks in situ in the main pack. The end of the subpack temp strips will only stand removing once or twice I suspect before they fatigue fracture.

My cycling charger is here.

http://www.microtechracing.com/product_ ... t=CHARGERS

I also have a Glassman 0-300v 0-3.5a CC and CV DC supply for whole pack charging. I will only be using upto 300ma for this though via the sockets I added a month or so ago to each car/pack.

I might use this for my Li-ion EV pack

So anyone in UK at present with a failing pack and no warranty that they want testing/fixing? No Guarantee offered or implied!

[DaveW]

I think a .03 to .05 V cutoff should be just fine for these six cell packs. I have been involved with r/c car racing since the mid eighties and have even matched and sold 1000's of cells, in that time we have used many brands of nicads and nickel metal cells and we will most always use .02 to .05 cutoff when charging. The best way to charge any cell nicad or nickel metal is to peak charge (not just by setting the AH to a set limit). This will ensure a complete charge without overcharging.

My guess is that you would want to discharge the IMA subpacks at atleast 30 amps to make sure the packs are strong and "weed" out the weak ones before reassembly. Many times a weak cell that may pass at a low amp setting will fail when discharging at a higher rate.

[IamIan]

I agree with DaveW about the 30+ Amps Discharge test.... If you can do it safely.... Especially as a method of identifying the best sub-packs.

But, I recommend 3 to 5 mV for the dV/dT Bump.... not 0.03 or 0.05 which is actually 30 to 50 mV..... 0.03 or 0.05 is fine as a upper limit voltage termination (1.4V per cell x 6 cells per subpack = ~8.4V per subpack ) but is way too large for dV/dT bump use.

Since they will all be charged and discharged together in the Insight it is best to get them all as close to the same stored capacity as you can once you have chosen your selected sub-packs.... Which means they should all be no more than the weakest of the sub-packs selected.... you do not want all 20 sub-packs full charged if the weakest sub-pack holds 500mAh less than the strongest one.... If you did charge them all 100% then the weakest would be more likely to be forced into voltage reversal during use.

An Ah limit is also a better method than .... the timed cut off limit once you know the charging rate...

But it is always better to use multiple methods together than depending on any one method.

[retepsnikrep]

I can do a safe subpack 20A discharge test with my equipment. That will probably suffice.

[IamIan]

If you don't already have a copy...

You might find the NREL study of the Insight Battery Pack useful.

You can download a pdf copy from my web site here:

http://www.geocities.com/ian_p_george/C ... nsight.pdf

No dV information but they do give the internal resistance of the whole 120 cell series pack at different temperates and such.

[Mike Dabrowski 2000]

Since we really have no way to measure the cell to cell differences in SOC that may be happening within each subpack without disassembly.
I agree with Armin, that the DV method of end of charge could be smeared, and unreliable in an old pack, and will be less reliable as the charge current drops. The best equalization may be simply a safe constant current with a timer.
During the discharge cycle, a close watch of each of the subpacks voltage would be prudent, as a cell reversal will cause permanent damage.

As Peter asked, it would be very useful to try this simple equalization charge on some packs that have already set off the IMA code of death. Anyone with a failed pack out of warranty that wants to give it a shot?

Peter, what did you come up with in your search for a PSU that could do the full pack low current charge?

[retepsnikrep]

Originally Posted by Mike Dabrowski 2000

Peter, what did you come up with in your search for a PSU that could do the full pack low current charge?

I bought this from e-bay £155.00 0-300v 0-3.5A CC & CV for my own use

http://www.glassmaneurope.co.uk/downloads/LVP1kWdc.PDF

I am still waiting for a reply from the PSU man I approached re a PSU bulk purchase for the group!