Charging Less Than Discharge?

jayflash

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My two new multi-chemistry chargers do something my Maha 9000 never did. With some cells of any chemistry, the new chargers will sometimes display discharge capacities larger than the recharge capacity. Those cells seem to reach a normal charged voltage. I wonder if the discharge capacity is accurate. This happens on only a few cells so I'll have to pay more attention to internal resistance, perhaps. This never happens with the Maha. Ideas?
 

john61ct

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Top current rate spec for discharge is always higher than for charging, if that's what you mean.

For evey battery I've ever looked at
 

jayflash

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No, I meant cell capacity (mAh) not rate. As an example: a cell will show that it discharged 1000mAh before terminating at the (dis) charger's set voltage. However, that same cell received only 700 mAh upon recharging in the same charger. Usually, and always with the Maha, the current (mAh) recharging the cell is always greater than discharge because of efficiency loss with charging.
 

john61ct

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Aha yes, mAh in will always in reality be greater than out, CEF is always less than 100%.

So a fault with the measuring circuitry.

An inexpensive coulometer could do the job if desired.
 

jayflash

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It seems this may only happen with old cells & higher self-discharge. My DMM confirms the charger's voltage. I'll have to pay closer attention to the cells this happens to.
 

john61ct

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getting voltage right is easy

but capacity measurement, counting coulombs accurately can be tricky
 

DIWdiver

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Amp-Hours are Amp-Hours. Quite easy to measure, and no mystery about them. Capacities are measured in Amp-Hours. Under any reasonable set of conditions (seems to me, under ANY conditions), discharge capacity is always less than charge capacity.

Correlating charge capacity to discharge capacity, capacity vs lifetime, capacity vs temperature, yeah these have some mysteries (or at least some complexities) involved.

If a charger reports 700 units charge capacity and 1000 units discharge capacity, one of two things is happening:

1. The battery started at 40-50% SOC. If this is the case, subsequent cycles should show charging takes more than discharging.
2. The charge and/or discharge device is quite inaccurate.

It should be not at all difficult to determine which of these cases is true. Simply run another cycle.
 
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john61ct

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Easy to say "Ah are Ah" and by definition true.

But if precision **measurement** is desired, some instruments are **much** more accurate than others.

A precisely timed CC load is more so than any coulometer.

And no matter what, results vary with temperature and C-rate, see Peukert's Law.

But yes Ah in is always more than Ah out.
 

john61ct

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I guess should not assume:
a cell will show that it discharged 1000mAh before terminating at the (dis) charger's set voltage. However, that same cell received only 700 mAh upon recharging in the same charger.
The starting SoC before charging must be the same as when discharging is complete

in order for the test cycles to be comparable.

2.5V or 3.0V OCV at rest are common zero SoC setpoints.
 

jayflash

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I'm not finished digging for clues or keeping a record of when this occurs. If the chargers are stable and functioning properly, I'm wondering if some of the older cells are at the end of their lives and deteriorating during testing. I'll have to try immediate discharge/recharge sequences and retest after several days later and check for rapid self-discharge, although I'm not sure how that would affect results.
 

jayflash

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Do I correctly understand that charging an NiMH cell at 0.5C & discharging at 0.2C (or less) will maintain or restore maximum capacity? Is it also correct that charging at less than 0.5C may extend cell lifetime, as long as the charger terminates properly?

I'm suspecting some of my old Duraloops are at or near their end, because now I've noticed that charging disparity on the Maha 9000, too. A few cells have shown greater discharge mAh than recharging. I'll have to retest a couple times and keep better notes than my vaporware memory.
 

WalkIntoTheLight

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Do I correctly understand that charging an NiMH cell at 0.5C & discharging at 0.2C (or less) will maintain or restore maximum capacity? Is it also correct that charging at less than 0.5C may extend cell lifetime, as long as the charger terminates properly?

I'm suspecting some of my old Duraloops are at or near their end, because now I've noticed that charging disparity on the Maha 9000, too. A few cells have shown greater discharge mAh than recharging. I'll have to retest a couple times and keep better notes than my vaporware memory.

If a cell is fairly new and in good condition, charging at less than 0.5C is a waste of time. You can do it, but a healthy cell won't get very warm at 0.5C.

Once a cell is old, worn-out, and develops high internal resistance, you simply can't charge it at 0.5C. You can't force current in it that quick, at least not with the charging voltages that normal chargers use. You'll have to drop the charge rate down to something like 0.1C, in order to fill the cell to capacity. That may cause termination difficulties, but who cares if it's an old cell?

As for cycling old cells to restore them... yes, that can work, especially if they're sat idle for a long time. Fully discharge and fully charge it a few times. It can bring the capacity up a bit, but don't expect too much. I'm not aware of any specific rates for charging and discharging, so just do it normally.
 

john61ct

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If a cell is fairly new and in good condition, charging at less than 0.5C is a waste of time
I don't understand.

Do you literally mean just because that slow rate takes longer than, say 3C?

The slow rate is much better for longevity, especially in cool conditions.

> You can do it, but a healthy cell won't get very warm at 0.5C.

Of course, but that sounds like you would want them to get warm? Not me. . .

> Once a cell is old, worn-out, and develops high internal resistance

I should be replaced, right?

My EoL cutoff is 70-75% SoH. . .
 

WalkIntoTheLight

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I don't understand.

Do you literally mean just because that slow rate takes longer than, say 3C?

The slow rate is much better for longevity, especially in cool conditions.

Heat is hard on cells. If the cell isn't getting hot at 0.5C, it's fine to charge it at that current. Sure, you can charge it less, as long as your charger can properly -dV/dt terminate at a slower rate. But there's no need to.

I sometimes charge Eneloops at 1C. The do get quite warm at that, so I only do it if I'm in a hurry. So, 0.5C is a better rate. Lower doesn't harm them, unless your charger misses the termination.


> You can do it, but a healthy cell won't get very warm at 0.5C.

Of course, but that sounds like you would want them to get warm? Not me. . .

I'm not sure why you think I said that. I meant that if a cell isn't getting warm, then it's fine.

> Once a cell is old, worn-out, and develops high internal resistance

I should be replaced, right?

My EoL cutoff is 70-75% SoH. . .

If you intend to use it in high-drain applications, like flashlights, then yes replace it. Otherwise, old cells do very well in low-drain devices, such as remote controls (assuming they are low-self-discharge like Eneloops). High internal resistance doesn't matter if they're only being drained at a few milliamps.

I have old, worn-out Eneloops that have very high internal resistance. I can't use smart chargers to charge them, because they won't recognize them as NiMH. But I can use slow dumb chargers, and they charge up fine. They hold their charge very well, too, almost as well as when they were new. They just suck at anything more than about 100mA.
 

jayflash

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My Duraloops don't get warm at the .5C rate and only a little warm at 1C. The old cells with near original capacity register about .04 ohms IR and higher for those with less than original capacity. So, for highest cell capacity a low discharge rate & a .5C - 1C recharge is recommended when reconditioning? For max lifetime .5C is better than 1 - 2C? Thanks for the help.
 

WalkIntoTheLight

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What you should be aiming for is cool cells during charging. If you can do that with 1C, then fine. Otherwise, 0.5C is the usual recommendation. You can go slower, of course, as long as your charger terminates properly. 2C is almost certainly too fast, and will heat up your cells quite a lot. It's not going to kill them if you only do it a few times, but it's definitely going to be hard on them.
 

NotSoBrightBob

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Great thread, so I get the 1C vs .5C charge but is there an optimum discharge amps? I understand different loads will drain the battery at different rates based on the current draw and I understand not to drain a cell below .9 of it's stated voltage but my charger lets me set discharge amps and i don't know what that number should be compared to the mah size of the battery. Thank you and apologize if this is a hijack.
 

jayflash

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Hijack away. High, I'm Jay. :green:

The suggestion is not to drain an NiMH cell below 0.9v - not "within" 0.9v. The acceptably reviewed charger/dischargers, here, should keep the cell at or above 0.9v regardless of discharge current setting. My understanding is that a lower discharge rate will help to drain the cell more completely, thus maintaining its ability to more fully recharge.
 

NotSoBrightBob

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Pleased to meet you Jay, thanks for the input. I think I understand the minimum voltage. My pack is 8 cells totaling 9.6 volts so I set my charger (Keenstone C1-XR) to a minimum discharge voltage of 8.7V but it also allows me to set the discharge current. 2C I think would be 3.2a for a 1600mah pack. Of course if I set the discharge for 1.0a then cycling takes forever and I don't like leaving it unattended overnight. Is 3.2a too aggressive?

Thanks

Bob
 
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