Testing Nicad/nicd or NiMH cell ESR or SLA Gel Cell Battery Internal Resistance

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Why waste time on Nicads when the NMH batteries are so much better?

Your statements are contradictory. For low current applications, the ESR of the battery should be largely irrelevant...unless the cell is dead, dead, dead. Why do you need to zap cells to lower the ESR unless you're discharging them VERY RAPIDLY?

I built a jig for sorting Lithium Ion cells for ESR at a junk dealer. Just used a DVM to measure the voltage and a switch in series with a light bulb to limit current from a eight-cell NiMH power supply.

Just read the volts, press the switch, read the volts again. I needed the power pack because the cells were not charged. If your cells are charged, you can just load the cell to ground.

I had good luck matching cells for building laptop battery packs.

A Tektronix 576 semiconductor curve tracer makes a great tool for quickly evaluating ESR of cells.

A pulse generator and a scope will measure ESR. Just look at the amplitude of the voltage step when you hit it with the known current pulse. You can automate this to any degree you want with a sample-hold and a microcontroller. If you use a square wave current, you can measure the P-P voltage at the cell and even plot it as a function of charge level.

I tried to do this with a GPIB programmable power supply. Problem is that the battery voltage changes continuously when you turn on the current. To get repeatable measurements, you need to accurately control the time between turning on the current and reading the voltage. I couldn't make this work with all the variable latencies in the windows OS and the GPIB controller and the supply. My light bulb scheme worked about as well in practice.

The guys who are fanatics about this reside in the model car racing groups. But you have to do a lot of sifting. Many people have no idea what they're doing and just post anecdotes of how they got 100A out of a cell with no real verifiable experimental results.

NiMH may have higher current densities, but Nicads are preferred for high discharge rates (power tools), from the more reasonable sources that I've read (not just inexperienced kids on hobby forums).

I will be using high capacity NiMH cells for my portable field gear (that have low current requirements), but having a better indication of cell condition would be ideal (not just a battery test meter for voltage).

Being able to test ESR on new batteries should indicate if any are flakey. Finding numerous flakey new cells would mean I don't buy more of that brand in the future.

Just because a battery cell test shows a full charge with a simple battery test meter, doesn't mean that the full charge can be gotten out of the cell. The voltage level of a slightly loaded cell, or pack, doesn't indicate it's reliability.

-- Cheers, WB .............

Why waste time on Nicads when the NMH batteries are so much better?

"Wild_Bill"

** As you can see form the attribution - that article is one of mine.

Bob's ESR meter will not be much use for high capacity cells that have only a few milliohms of ESR - as the lowest reading on the display is 10mohm

However, a pack of 4 or more cells would likely be testable and reveal poor connections of the existence of a bad cell.

.... Phil

I believe that just because a battery cell test shows a full charge, doesn't mean that the full charge can be gotten out of the cell (imbalance).

Checking ESR at random intervals could indicate if cells are unbalanced, I think. Charging at C/10 is said to balance cells, but how often? Having a means to indicate (actually see) imbalance would give the user an idea of how often.

I prefer to test capacitor ESR and other parameters before I install them, as there are always inconsistencies in manufacturing. No, I don't routinely retest them at regular intervals after installation, but when circuit performance is poor, ESR testing is generally the first step.

The only reason I would want to zap nicad cells is to actually see if the internal resistance is improved/lowered. I believe an ESR test would be a good indication of any change that takes place.

Otherwise, an ESR test after cells have been used a while, may be a better indication of it's condition, and possibly it's reliability. I suspect that an unbalanced cell in a battery pack is bad for all the cells in the pack, as it's likely to generate more heat, and possibly result in an incomplete or overcharged state.

Your test fixture seems like a reasonable test, as a quick accept-or-reject type of test.

I wouldn't want to automate tests, although it can be accomplished with the right equipment and quite a bit of effort, by someone brighter than myself. A dedicated analyzer/tester/reconditioner may be a better solution, but costly. I don't have to deal with so many batteries that a dedicated analyzer would be practical.

I've seen a lot of inconsistent statements regarding battery maintenance and performance. One source that appears to be reliable is

-- Cheers, WB .............

Phil, thanks for the concise, definitive reply, very much appreciated.

I have a milliohm meter around here somewhere, but I don't know if it would work for battery cells, and I can't remember the meter's resolution, either. It's one of the digital display meters that are handy for checking circuit board traces (with a variable audio tone) and other low resistance measurements. The pitch of the tone gets higher as the resistance gets lower.

-- Cheers, WB .............

I don't understand why you believe ESR and imbalance have any relationship whatsoever.

Yes, BUT most charging regimes for NiXX chemistries are current-based and overcharge the cells somewhat. This provides SOC balancing.

Do whatever you feel important. I wouldn't bother.

Whose first step?

"zapping" NiCd cells will *only* (and then temporarily) remove conductive dendrites. If the cells have dendrites then they are already on their last legs, and should be discarded before they cause you grief. If they don't have dendrites, *zapping" will achieve nothing, but hey, whatever floats your boat.

ESR (or more correctly, internal resistance) tests on a healthy cell will indicate maybe the extent of irreversible deterioration. To that extent it may provide some useful information.

Yes, but ESR and imbalance are unrelated.

Pffft!

The medium sized UPS gear we used (3x 300kVA/40minutes) had an integrated real-time battery condition monitoring system. This simply applied a known pulsed load to the battery systems (at multiple tap points) and monitored the corresponding voltage steps, which gave a direct indication of cell resistance. If any segment of a battery string reached a threshold value, that string was taken out of service and cell-by-cell testing was undertaken, follwed by any required remedial action.

You could set up a similar tester for manual operation (similar to mike's description) and gain useful insight into internal resistance.

Please let me know the results of your experiments so I can update my page on Bob's ESR meters and battery ESR.

John :-#)#

--
    (Please post followups or tech inquiries to the newsgroup)
  John's Jukes Ltd. 2343 Main St., Vancouver, BC, Canada V5T 3C9
  Call (604)872-5757 or Fax 872-2010 (Pinballs, Jukes, Video Games)
                     www.flippers.com
       "Old pinballers never die, they just flip out."

I don't presume to know.. internal resistance is a factor, maybe not the most critical, but a parameter that could be useful. As I've said, I believe ESR/IR is more useful than a simple battery test meter. As the cell's IR increases, it's capacity drops, from what I've read.

The info I've found indicates the opposite.. imbalance isn't improved by continuing to recharge in the usual manner (the same charging techniques). I believe that imbalance is related to a battery pack's longevity.

And since most battery users can't improve or modify their existing chargers' behaviors, they're stuck with what they have.

I was referring to capacitors here, so ask anyone that regularly performs service or repair work how important their ESR testers are.

Battery manufacturers and serious users disagree.. they've proven that zapping new, high quality Nicad cells increases output voltage levels and lowers the ESR/IR. The reported capacitor values and zapping voltages vary, so I don't think there is an ultimate combination.

As I mentioned earlier, I'm not interested in trying to revive old, weak cells.

I believe that battery industry professionals consider ESR/IR to be a significant factor in cell construction and later for end-user performance.

Essentially the same info, written by the founder/inventor of the Cadex equipment. I'm not just referring to info found in an afternoon's browsing. I've been examining battery info/data for several years. As I stated earlier, I mostly disregard much of the info in the hobby-type forums that appear to be written by inexperienced kids. I'm able to spot reports that lack scientific perspective (just opinions based upon voodoo techniques). I suspect that much of the older info doesn't relate very well to today's newer cells.

I wonder what action. A series of recharge/discharge cycles and retesting? Reconditioning in-house, or sent out? For UPS duty, problem batteries should probably just be replaced. The potential value of a failure could be enormous.

A light bulb and DVM tester could be a handy gage for initial checks.

My reason for asking about utilizing an ESR tester was because ESR testers display an ESR reading, and readings are easily compared. The ESR tester could likely be used on battery packs regardless of the pack's SOC state of charge.

As Phil A pointed out, the Bob Parker ESR meter could be useful for battery packs, but I may need to find my milliohm meter to try on individual cells, if it's capable.

A poor IR reading on a new cell (compared to the other new cells from the same purchase) would be something that I'd want to be aware of.

-- Cheers, WB .............

Unless you are very sure that your milliohm meter is capable of testing battery cells, I would strongly advice you not to try it.

General rule: don't apply anything 'active' to a device that is supposed to measure resistance.

--
met vriendelijke groet,
Gerard Bok

While that is true for older analog meter movements (always measure resistance with system power off) modern meters such as the Bob Parker ESR meter can read the resistance of a 'hot' circuit up to the breakdown voltage of the input capacitor (roughly 50V). As such a meter like the Bob Parker ESR kit (Blue ESR now produced by Anatek) can easily be used for testing the internal resistance of batteries - the question is if that produces useful information!

John :-#)#

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    (Please post followups or tech inquiries to the newsgroup)
  John's Jukes Ltd. 2343 Main St., Vancouver, BC, Canada V5T 3C9
  Call (604)872-5757 or Fax 872-2010 (Pinballs, Jukes, Video Games)
                     www.flippers.com
       "Old pinballers never die, they just flip out."

Good of you to mention it Gerard, and you're correct for many types of instruments.

That funny odor generally means the circuit wasn't meant to be connected to voltage sources.

I never had a schematic for it (the digital display milliohm meter), so I'd just have to guess. I don't even remember the brand name, just that it was made in the UK, I think, and I couldn't locate any literature about it at the time.

If the circuit operates as a small AC signal source it may be able to tolerate a low voltage.

-- Cheers, WB .............

Isn't the impedance of a voltaic cell normally measured by comparing its loaded and unloaded voltages?

I remember reading the comments regarding the use of the Bob Parker ESR meter for checking battery IR/ESR quite some time ago, John, and also Bob's own descriptions of his circuit.

I don't recall the other ESR meter makers, or users of other brands mentioning that the other products could be used for testing battery IR.

I've had Sencore Z-Meters for years, for testing capacitors and inductors/coils. The ESR test will display values of low value resistors, but there are some power resistors across the input connector initially to discharge capacitors, so I don't think it will work for battery checking.

I just did some searching on eBay, and found a few China/Hong Kong makers/sellers offering numerous small instruments specifically intended for battery IR/ESR testing, from coin cells to car batteries. There were also a couple of pulse load testers for common sizes of small cells/batteries.

I noticed folks mentioning battery IR years ago, but they didn't ever mention how they arrived at a reading. I didn't know if the IR was an instrument reading or a calculation. All I was able to find out then, was that the high-end equipment (Cadex) had the capabilities of measuring cell IR. Maybe some lab grade LCR instruments could measure IR, but I didn't find any battery user comments that stated any of them did. Those comments and the Flippers site comments got me started looking for more info lately, regarding the significance of battery IR.

I was aware that IR and ESR could be determined by using generators and scope, although there would probably be widely differing measurements. If everyone is using different test equipment of various accuracies (and different skill levels), there would not likely be any definitive IR numbers to compare, from one specific brand of cell to a different brand, etc. (my own WAG).

As cell IR is an indication of poor cell quality or degradation, it's also an indication of the general condition of the cell. Obviously, cell IR isn't a full evaluation of a cell, just as ESR isn't a full evaluation of an electrolytic.

Since I'll be using about $300 worth of new cells to build some battery packs, I'd like to have some idea of what's going into the packs. So I'm interested in an *easy* test that would show me something I can't see by looking at the cells (numbers would be good), although it definitely doesn't need to be NIST traceable.

-- Cheers, WB .............

No. Note TWO relationships: (1) Internal resistance will have an effect on the usable capacity, in that it will result in earlier discharge termination at the terminal voltage will droop more under load. (2) As the cell's usable capacity reduces due to "wear and tear", the internal resistance will usually rise for similar reasons. But IR is NOT the cause of reduced capacity.

Well you can't have it both ways. About 8 lines above, you state that "Charging at C/10 is said to balance cells". Pick one.

same charging techniques).

First let's be clear that we are talking about SERIES-connected cells. When exposed to charging current cells will recharge. If cells in a series string have a different SOC at the commencement of recharge, the *normal* (i.e. over-) charging regimes - including the "C/10 for

14 hours" - will ensure that the cells with lower SOC are dragged closer to the SOC of the others. This may or may not take more than one charge cycle, but it will happen.

What this means in terms of cells with different usable capacities (as distinct from different internal resistances) isn't a real issue. Discharge is another story.

It certainly can/does lead to cell reversal under deep discharge, which can damage a pack.

Agreed. We disagree though on what that means.

OK, my error.

I use my ESR meter frequently, but extremely rarely on anything except electrolytic caps..

If battery manufacturers beleived that, they'd all be doing it routinely before shipping their product.

I believe it is just more snake oil, but ....

You *believe* that "they consider"? Hmmm?

Replacement.

Exactly. Without being specific, the fact that we had 3x 300kVA units would suggest that our operation and its continuity were considered more important than the cost of replacment cells. And before you ask, suspect cells were replaced individually based on these tests, and when the number of replacements reached a certain percentage of the string the whole string was replaced.

yup

A "normal" (milli-)ohm meter wont give you any useful indication from a charged cell except maybe smoke signals. The Bob Parker ESR meter (I use one) uses an AC excitation and measures the voltage across the target device that results, effectively removing the DC component which will generally smoke your ohm-meter>

Indeed.

I have used the Anatek Blue ESR Meter with various cells and batteries, including the 12 volt lead-acid battery in my car. The problem is that I don't know how to evaluate the readings.

If you are going to use that technique for battery measurements, I'd suggest building a library of measurements of cells in various states of charge. It would probably work for both NiCd and NiMH cells, unless one of them happens to have a resistance below the meter's limit. In my limited experience, the cells were all within the meter's range.

Fred

"who where" "Wild_Bill"

** Bill's post does not say it was - only that there is a correlation.

There is a strong correlation between measured ESR and state of charge for non-rechargeable cells - but it is only when a NiCd or NiMH cell is almost completely discharged that the ESR rises.

The whole idea is that ESR testing is a COMPARATIVE measurement applied to a *number of cells* that are of the same type and with a similar history to see if any show higher readings than the others.

It is also useful to apply ESR testing to a pack of cells and see if the total ESR is any more than slightly over the typical ESR of each cell multiplied by the number of cells. A higher than expected reading indicates one or more bad cells or possibly high resistance spot welds inside the pack.

It can take hours to do a charge and discharge tests on a packs of cells - but only SECONDS to do an ESR test on a pack and find immediately that no further testing is warranted.

.... Phil

same charging techniques).

It may not be as simple as that. Battery manufacturers are interested in profit, quality, reputation etc. It may be that one parameter can be improved by sacrificing another to an unacceptable degree.

It's possible that zapping might lower esr temporarily but result in overall reduction in performance over the life of the cells.

The zapping info seems to come from model car/plane racers. They're interested in ONE thing...winning the race. They don't give a rats ass whether they get 300 discharge cycles out of the battery.

Preoccupation with one parameter like esr/ir may not result in maximum uptime for your servers or lowest overall operating cost.

That's exactly what I believe to be the case, *if* zapping at birth has any effect at all.

yup