Is weekly Grid Charging Detrimental to the battery?

[eq1]

My main concern here is that after you grid charge to 100% and then reset the state of charge, any current going to the battery is overcharging. In my car, a reset sets the SoC to 75%, and then I have a 6% window, 75% to 81%, until charging is disabled (some cars don't have that). During that window, regen charge can spike up and it seems to take some time for the car to throttle it back, perhaps a half dozen spikes until current is throttled back significantly... I'm not concerned that it's gonna blow up the pack or wreak massive damage, just that, in trying to increase longevity and performance, it would be counter productive.

Likewise, I guess I'm not too concerned about a low current trickle charge, like 0.3-0.35 amps we use for grid charging, as I've read other things that say it shouldn't be a problem. I just wouldn't over do it if I were trying to increase my pack's longevity and performance; it too would probably be counter productive...

After a grid charge and a reset, it would seem prudent to draw the pack down a little, maybe 5%, before letting loose with regen, especially braking regen. That way you have a little cushion, headroom, at the top...

[eq1]

Originally Posted by 3-Wheeler

Did the author mention anything about the RATE at which the battery is topped off, has an effect on this phenomena?....

No mention of rate of charge. He writes stuff like: "The more the overcharge, the more HNi2O3 that is formed, and the longer the lower plateau...The implication from this mechanism is that the major reason for the 'memory effect', a decrease in the capacity at the normal discharge potential, is related to extensive overcharging, rather than to the use of shallow discharge cycles."

"The more the overcharge," "extensive overcharging" -- my conclusion would be that higher rates are worse than lower rates...not very helpful.

One practical question for Insighters contemplating grid charging, people having battery issues, is: To what extent is my battery problem a result of imbalance versus, say, a 'memory effect'? Grid charging should help with the balancing issue, but it wouldn't help with a memory effect issue... And, I wonder, if or to what extent imbalance could have resulted from overcharging and memory effect? Seems possible that a few cells out of the 120 could get overcharged, become 'memory effected', and drag down the pack...

I should also point out that this author, Robert Huggins, makes a case based on data, models, and well-worn science, but not his own direct experiments; he theorizes that this HNi2O3 is formed from overcharging, he hasn't actually ran experiments and turned up physical proof... BTW, the book is called "Advanced Batteries, Materials Science Aspects," 2009.

[E27006]

When Honda UK charged £3000 for a new pack, I baulked at paying up such a sum of money to replace my unreliable pack. Therefore for 3 years I grid charged at 14 day intervals with much success, the car ran without trouble and gave better mpg than ever due to the near elimination of hidden background charging which consumes fuel and hinders lean-burn engine operation.

My policy was to charge until the pack reached maximum voltage, charge for a further 20 minutes then switch off.

I rarely pulled the IMA fuse to reset, the SofC would typically positive recal to maximum during the next journey

[jerrydelrey]

Originally Posted by eq1

After a grid charge and a reset, it would seem prudent to draw the pack down a little, maybe 5%, before letting loose with regen, especially braking regen. That way you have a little cushion, headroom, at the top...

I will do this. Instead of trying to charge to 95%, I will initially drive the car with alot of assist to discharge the battery slightly. Then there will be capacity as the car is trying to figure out what happen.

[jerrydelrey]

I was out sick this week and do not get to drive my car after grid charging for the second time.

I just drove to work now, and the car Won't assist when you first start it, after pulling the fuse.

So yeah I can't discharge. Now I've learned that its damaging my battery I was SO annoyed. The car was charging for a good mile before doing a positive recal.

[Mike Dabrowski 2000]

Whoops duplicate post

[Mike Dabrowski 2000]

Here are some exerts from a battery manual that should answer some questions about the effect of the charging and discharging.

General Principles NIMH cell level
Recharging is the process of replacing energy that has been discharged from the battery. The
subsequent performance of the battery, as well as its overall life, are dependent on effective
charging. The main criteria for effective charging are to:
• Recharge the battery to its full capacity
• Limit the extent of overcharge
• Avoid high temperatures and excessive temperature fluctuations

Low-Rate Charge. A convenient method to fully charge sealed nickel-metal hydride batteries is to charge at a constant current at about the 0.1C rate with time-limited charge termination. At this current level the generation of gas will not exceed the oxygen recombination rate. The charge should be terminated after 150% capacity input (approximately15 h for a fully discharged battery). Excessive overcharge should be avoided as this can be injurious to the battery. The temperature range for this charge method is 5 to 45_C, with best performance being obtained between 15 and 30_C.


Cycle Life
The cycle life of nickel-metal hydride batteries, as all rechargeable batteries, depends on the many conditions to which the battery has been exposed, such as:
• Temperature during charge and discharge
• Depth of discharge
• Charge and discharge current
• Method of charge control
• Exposure to overcharge and overdischarge
• Storage conditions and length of storage
Typically under a standard charge-discharge cycle at the 0.2C rate and at normal ambient
temperature (20_C), about 500 cycles can be achieved with the battery delivering at least
80% of its rated capacity. The gradual reductionin capacity results from an increase in the
cell’s internal resistance due to minor irreversible changes in the electrode structures and
loss of electrolyte or cell dry-out. The increase in internal resistance is accompanied by
the gradual decrease in midpoint voltage during discharge and the gradual increase in voltage
during charge with cycling.
(if your pack is topping off at high voltages relative to other packs, it is a good indication that the pack is deteriated)
For optimum battery and maximum cycle life, the nickel-metal hydride battery should be
operated near room temperature. Operation at extreme temperatures during charge or discharge
will adversely affect its performance. Operation at high temperatures, particularly in the overcharge
condition, can cause the cell to vent, releasing gas and possibly electrolyte through the safety vent. High temperatures will also hasten the deterioration of the separator and other materials in the cell. At low temperatures the oxygen-recombination reaction slows down, the cell is more sensitive to overcharge, and
gas pressure will build up more rapidly.
The charge rate and the amount of charge input during overcharge also are important
factors affecting the cycle life. If the battery is charged at a rate that exceeds the
oxygen recombination rate, oxygen that is generated during overcharge will not be reacted and gas
pressure and temperature will build up with deleterious effects on battery and cycle life. The
more effective the method to terminate the charge promptly when deleterious overcharge
begins, the less is the effect on cycle life.
Finally cycle life is also affected by the depth of discharge. About 500 cycles can be
obtained, depending on the charge termination method, with the battery being fully discharged
on each cycle (100% depth of discharge). Considerably higher cycle life can be
obtained if the battery is cycled on shallower charges and discharges.If the battery is discharged
at the 0.25C rate to about a 60% depth of discharge. The cycle life is increased to
about 1000 cycles. Shallower discharges will further increase the cycle life.



Multicell pack considerations

During early development, failure modes for EV and HEV batteries can include short
circuiting due to mechanical penetration through the separator. The frequency of such events
is usually small if sound cell and electrode design is employed and if manufacturing quality
control is effective. Another failure mode may be abusive overcharge where venting results
in insufficient electrolyte within the separator. Abusive overcharge may result from charge
imbalances caused by thermal differences from one part of the large battery to another. The
problem may be compounded by the sophistication of the charger, where voltage and temperature
sensing is not necessarily done on an individual module or cell basis. Another form
of abuse is overdischarge, where a cell or module within a high energy EV battery is discharged
below the minimum recommended cell voltage of 0.9 V. Overdischarge is usually
caused by state-of-charge imbalance within a high voltage string brought on by thermal
gradients within a battery. Another source of abusive overcharge and overdischarge is the
‘‘weak cell or weak module’’ concept. This involves the statistical predictability within a
large number of cells as to the decay rate of capacity, power and resistance as a function of
cycle number.
A common feature of the above-cited EV/HEV failure modes is the importance of maintaining
state-of-charge balance within a battery pack that may contain several hundred individual
cells. The method used to maintain equalized state-of-charge within an industrial
NiMH battery is a complete charge, in effect to routinely bring all the cells to the same
state-of-charge. This method of using overcharge to equalize state-of-charge is ineffective if
the cell temperature within a battery pack is extreme or if cell-to-cell temperature gradients
are too large. Consequently, one of the biggest factors in replicating the excellent cycle life
of small portable NiMH batteries in EV and HEV applications is proper thermal management,
which is discussed in Sec. 30.11.
If premature failure due to short circuit and abusive overcharge/overdischarge is prevented,
the principal failure mode in large industrial EV and HEV NiMH batteries is increasing
internal resistance with cycling. To the end EV user, the observation is that acceleration
capability will diminish on long-term use, or that vehicle range will gradually
decrease. To the end HEV user, battery failure due to increasing internal resistance and
resultant power loss will be observed as inability of the battery to assist acceleration and
inability of the battery to utilize regenerative braking energy due to excessive heating caused
by the high currents used.
The NiMH battery primary failure mode of increasing internal resistance and power loss
during cycling is caused by the same failure mechanism as observed in small portable NiMH
batteries: separator dryout as a result of electrolyte redistribution due to swelling of the metal
hydride and nickel hydroxide electrodes; consumption of electrolyte due to oxidation of the
separator metal hydride active material and positive electrode materials, and loss of electrolyte
through venting. These mechanisms may be exaggerated for large NiMH batteries due
to their prismatic construction. Cylindrical cells have one positive electrode, one negative
electrode and one separator. NiMH prismatic EV batteries may have 20 positives, 21 negatives
and a corresponding number of separator sheets. The cylindrical design is more effective
for pressure containment than a rectangular container, both for gas pressure and the force
applied by the can on the electrode stack itself. Therefore, another critical factor for large
NiMH batteries is management of the compressive forces within a module. Typically, restraining
bands are used to secure a 10 or 11 cell module which has an endplate to equalize
lateral forces on the case end. Failure to adequately equalize the compression within each
cell in a module and within the internal cell stack itself will lead to premature failure due
to unequal electrolyte distribution within a cell.

Bottom line, I think that on a used pack, the frequent cycling at the gentle charge and discharge rates as well a the conservative charge and discharge rates that the charger/discharger operate at will do more good than harm, so they should be done at a rate that is found to produce the best cycle data considering discharger run time, minimum discharge V.

On a new pack, Eli’s recommendation of a reconditioning cycle series every 6 months , will assure they the new pack stays in balance, and also gives information as to the aging of the pack based on the information gathered during the cycling event.

[jerrydelrey]

Thanks for the information Mike. I'll grid charge twice a year then.

[Eli]

Originally Posted by eq1

I picked up that book again to refresh my memory on this deep discharge point. Turns out the section was about the "memory effect" with nickel-based batteries. I kind of forgot that memory effect was an issue with these batteries, or at least some people have said it is so. Anyway, I can't understand about 98% of what's written. What I can take away from just re-skimming is this: Overcharging is the cause of the memory effect. Much as Eli pointed out, oxygen production during overcharging plays a role. I think it forms some other compound that creates a secondary "voltage plateau"; basically, you end up with less and less capacity at the higher voltage potential (something like 1.34V with respect to hydrogen), and more and more capacity 'locked' in a low voltage plateau under 0.78V.

The author's passage that I honed-in on during my first reading was this: "It is also widely known that this problem can be 'cured' by subjecting them to a slow, deep discharge." Most of the section builds a case for overcharging being the cause of the so-called memory effect, rather than repeated shallow discharges, discharges without fully draining the battery...

So, to the extent that memory effect is a problem with our batteries, grid charging would likely do nothing to help. The potential impacts of overcharging while grid charging or after grid charging and then driving shouldn't be overlooked. For instance, grid charging weekly, especially longer soak charges, should probably be avoided...

Hmmm...

This would be fascinating if true.

Just to clarify a couple of things though - the "memory effect" is a phenomenon specifically attributed to NiCd batteries.

NiMH batteries suffer from something similar called voltage depression. The mechanisms are completely different from what I understand(and I'm not clear on them), but the end result is essentially the same.

[Mike Dabrowski 2000]

If you carefully read the excerpt from the manual, it seems that the overcharging at higher currents is where the recombination cannot keep up, therefore causing the damaging heat and internal pressure. These effects below .1C, are not an issue as the recombination can keep up.
We are topping at~ .05C

Voltage Depression (Memory Effect)
A reversible drop in voltage and loss of capacity may occur when a sealed nickel-metal
hydride battery is partially discharged and recharged repetitively without the benefit of a full discharge.


After an initial full discharge (cycle 1) and charge,
the battery is partially discharged (in this example to 1.15 V) and recharged for a number
of cycles. During this cycling the discharge voltage and the capacity drop gradually (cycles
2 to 1. On a subsequent full discharge (cycle 19) the discharge voltage is depressed compared
to the original full discharge (cycle 1). The discharge profile may show two steps, and
the cell does not deliver the full capacity to the original cutoff voltage. This phenomenon is
known as voltage depression. At times it is referred to as ‘‘memory effect,’’ as the battery appears to ‘‘remember’’ the lower capacity. The battery can be restored to full capacity with a few full discharge-charge cycles,
The voltage drop occurs because only a portion of the active materials is discharged and
recharged during shallow or partial discharging. The active materials that have not been
cycled change in physical characteristics and increase in resistance. The active materials are restored to their original state by the subsequent full discharge-charge cycling.
The extent of voltage depression and capacity loss depends on the depth of discharge and can be avoided or minimized by discharging the battery to an appropriate end voltage. The effect is most apparent when the discharge is terminated at the higher end voltages, such as 1.2 V per cell. A smaller loss occurs if the discharge is cut off between 1.15 and 1.10 V per cell. Discharging to an end voltage below 1.1 V per cell should not result in a significant voltage depression or capacity loss on the subsequent discharges. Discharging to too low an end voltage, however, should be avoided.
The effect is also dependent on the discharge rate. To a given end voltage, the depth of
discharge will be less on discharges at the higher rates. This will increase the capacity loss
as less of the active material is cycled. While the memory effect may result in reduced battery performance, the actual voltage depression and capacity loss are only a small fraction of the battery’s capacity. Most users may never experience low performance due to this behavior of the sealed nickel-metal hydride cell.
Note this is refgering to a single cell. Our packs are 120 cells, and if the whole pack is in that state, the effect can get quite big.
Often memory effect is used incorrectly to explain a low battery capacity that
should be attributed to other problems, such as inadequate charging, overcharge, or exposure to high temperatures.
better not pirate any more from the book or the copy-write guys will be after me.