AA battery tests

"Mook Johnson"

** The lowest internal resistance ( Ri) is to still be had with Ni-Cd ells - particularly fast charge types as made by Sanyo and few others like Saft. An 800 mash, AA Ni-Cd can be expected to have as little as 25 milliohms Ri when freshly charged increasing to 35 or 40 milliohms at the end of each cycle.

OTOH, an alkaline AA has circa 120 milliohms Ri when fresh and this increases steadily as the cell discharges. So, with a half discharged one, the terminal voltage is down to 1.3 volts and the drop under 1 amp load is

0.25 volts or more - you are then right on the margin of having 4 volts available.

If reliability is any issue - then use a pack of Sanyo AAs - spot welded or *carefully soldered * together. Get them from a good hobby shop - ask for a " 4.8 volt, 800mAH receiver pack " as used in thousands of radio control model systems.

........ Phil

For high current draw capability from a given volume NiCd is probably best, although NiMH seems to be catching up. Your mileage will, indeed, vary as a function of battery technology, although compared to the model aircraft stuff that I'm familiar with one amp isn't that much of a draw. Of greater concern is the fact that you need to maintain 4V with four batteries -- the rule of thumb for NiCd, NiMH and dry cells are for a

0.9V per cell "discharged" point; you'll be throwing capacity away with your 4V threshold.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Posting from Google?  See http://cfaj.freeshell.org/google/

Do you have the option of using a regulated DC-DC converter so that your device can operate from a wider input range and choice of batteries? Even say 2 x C or D cells?

The max rated pulse discharge current of a Lithium AA is 3A for 2 seconds ON and 8 seconds OFF, or 2A continuos, so that might be suitable. The nominal internal resistance of an Alkaline AA is from 150 to

300mohms for a fresh battery and would probably not cut the mustard here. C or D cells on the other hand would be suitable. As others have said, NiCd would maintain its voltage better in this application.

Dave :)

"Tim Wescott

** If you check the Sanyo site - they quote 12 millohms Ri for their KR-800 AA cell.

Equates to 100 amps short cct current !!!

NiMHs, go weep.

** Most cell makers say (plus my own tests indicate) that a Ni-Cd cell is pretty much discharged at 1.1 volts. But you often see 0.9 volts *per cell* quoted as a voltage not to go below when discharging a pack of Ni-Cd cells.

......... Phil

I've used the Sanyo cells extensively (in another life some years ago). For the pulse the OP needs, do NOT get the high temperature version (it can't handle the load). it has a different electrode construction - the best for high load are sintered cell types, in my experience.

One thing about NiCad is the internal resistance is not the be all and end all of terminal voltage droop under load. That said, under a load of 1C, all of my Sony and Panasonic NiCads stay above 1V terminal voltage for a decent charge state.

A typical NiCad will droop below 1V under a 1C load at ~40% charge or less (I have literally books of notes on this - I did experiments covering months and thousands of batteries almost identical to the OP question).

Cheers

PeteS

I'll correct myself here, lest some flames appear ;)

The *nominal* internal resistance is not the be all and end all of terminal droop under load. Internal self heating starts *very* quickly under a 1C load and the effective Rs varies dramatically.

The tests I did were to see if I could (fairly) non-destructively calculate the remaining charge in a NiCad, incidentally (which all the battery mfrs say is impossible, but they are incorrect if ~3% accuracy across the 5% - 95% range is acceptable).

Cheers

PeteS

"PeteS"

** That is not true of the better ( ie low Ri) ones.

Sanyo sub C cells hold at 1.2 volts for 80 to 90 % of a 1C discharge - so do Saft and many others.

....... Phil

"PeteS"

** There is no significant self heating at the 1C rate with good quality Ni-Cds until discharge is almost over.

Even then, the cells rise only few degrees above ambient.

** And they are right.

** Yawn.

Got a few perpertual motion machines on the go too ?

....... Phil

That's 12 milliohms measured at 1000 Hz, with the cell half discharged. I'm not sure just how that translates into DC current with the cell fully charged, so I decided to measure a couple of cells.

I happen to have 4 GE/Sanyo NiCad high capacity (1000 mAh) cells, and 4 Sanyo NiMh 2500 mAh cells. I connected a couple of 3 inch long pieces of heavy copper strap to a 500 amp 50 mv shunt, positioned so that I could short a AA cell through the shunt. All the cells were freshly charged.

One of the NiCad cells pinched tightly between the copper strips gave an initial current of 550 amps!! The current fell off fairly rapidly and the cell's positive terminal heated up equally rapidly. I was loath to leave it connected for more than about 5 seconds. After 5 seconds the current was down to about 300 amps and still decreasing. Removing the short, the positive terminal of the cell was quite hot. After about 30 seconds the entire cell was quite warm. It took that long for the internal heat to diffuse to the outside case.

Next, one of the NiMh cells was put to the test. The initial current was about 380 amps, and in a couple of seconds decreased to about 350 amps, where it remained constant for 7 seconds, when I disconnected it. Its positive terminal wasn't as hot as the NiCad had been, and after a wait of 30 seconds, the entire cell did not heat up near as much as the NiCad had.

The other 3 cells of each type gave performance similar to the first.

This test convinces me that modern Sanyo high capacity NiMh cells would give better performance than NiCads.

"The Phantom"

** No it did not.

Typical AA Ni-Cds can manage about 30 to 40 amps if shorted.

A good sub C manages 100 amps if the connections are perfect.

Your amp figures are all 10 times too high.

........ Phil

The published curves

for the Sanyo 2500 mAh NiMh indicate that it wouldn't have any problem meeting your requirement.

Never could see the point ;)

As it happens, it is perfectly possible to get the charge state (to within a few percent) of a NiCad, but that's not to say it's universal.

The test has to be set relative to the energy rating of the battery

Basically, apply a load of between 0.3C and 1C for some amount of time (sufficient for the terminal voltage to settle) and then release the load. After releasing the load, measure the recovery curve. For a given battery (and according to my tests across thousands of batteries and hundreds of lots, [and 4 manufacturers] a given specific type of battery), there is a very tight correlation with that curve and the remaining charge.

I noted this was some years ago, and no doubt advances have been made in cell construction, but NiCads are nowhere near as common as they once were, for various reasons.

We are, of course, measuring effective Rs, with the quirk that the measurement is during the time the battery is not loaded.

The recovery curve is very close to a capacitve charge curve, incidentally.

Cheers

PeteS

"PeteS"

** Yawn...

** The fact is it is NOT possible to build a "fuel gauge" type meter that can test the state of charge of an ARBITRARY *pack* of Ni-Cds without carrying out a full discharge.

If you carefully characterise the discharge voltage curve a single cell or pack of cells, know for sure they are all OK and test under controlled conditions - that is another matter ENTIRELY !!

........ Phil

You're quite right. I had started out with a 50 amp shunt, but decided to go to a 500 amp shunt to make sure I had a good enough short. I had turned off the DVM before the second test. and when I turned it back on, it reverted to auto ranging instead of the fixed range I had used with the 50 amp shunt. It has a very tiny decimal point, and I didn't pay enough attention. Silly mistake.

The observations about heating still stand, though. I think the NiMh cells are better able to stand up to this kind of abuse than the NiCad.

The heavy copper strap was a big improvement. With the 50 amp shunt, I had used some stranded 12 gauge wire, and only got 20 amps or so. You have to have a really good connection to get that kind of current from a single cell.

"The Phantom"

** Your amp figures are all 10 times too high. ** Nice of you to re-check and say so.

....... Phil

of

test.

Many technically minded people seem to have strong personal opinions and beliefs regarding battery technology, so I hesitate to address anything related to them lest I get sucked into an ugly flamewar.

Instead, I would like to address a less controversial topic: the battery holder and wiring resistance. These resistances should not be ignored, especially the battery holder's resistance.

Common cheap typical AA battery holders will often use a coiled spring contact for the negative side of the battery. This coiled spring contact has deceptively high resistance. Typically it is constructed of some kind of spring steel, with a wire diameter comparable to that of a 22 AWG copper wire. It looks short, and it looks like when the battery is in place it should squash down and adjacent turns on the spring should contact each other thus producing a very low resistance contact, but this is usually not the case. Adjacent turns usually don't contact, and the spring is surprisingly long when uncoiled, perhaps over 30 cm. If it were made of copper, that would still not be a very large resistance, but the spring steel it is made of seems to have a resistivity of around seven or maybe even ten times that of copper.

The net result is a spring contact that can typically add somewhere between

100 and 200 milliohms of resistance in series with every single cell in your pack (assuming one spring per cell). This resistance will seriously degrade high rate performance. This is often the most ugly offender in low voltage high current battery powered designs, but care should also be taken to minimize the parasitic resistance of other circuit components like wires and switches as well.

"Fritz Schlunder"

** Which is precisely why I recommended a commercially made, *spot welded* battery pack if the OP was at all concerned about *reliability*.

Quote:

" If reliability is any issue - then use a pack of Sanyo AAs - spot welded or *carefully soldered * together. Get them from a good hobby shop - ask for a " 4.8 volt, 800mAH receiver pack " as used in thousands of radio control model systems. "

Not only can a spring contact battery box add significant series resistance, sooner or later it will become a little corroded ( particularly when used with Ni-Cds ) and fail to connect entirely.

....... Phil

I have found the Bulgin BX0035 from Farnell is ok. I think the manufacturer's name may have changed / they may have been bought.

Chris

AA Lithium is the best for your application but I think your 4V terminal voltage will be a problem under 1 Amp load. Your device will be leaving a lot of energy in the batteries when it fails. Note Lithium batteries can produce 1.7Volts unloaded so that can blow up many switching regulators that cant handle more than 6V on thier input.

NiMH are great for high current drain too, but they produce a lower cell voltage, so your 4V limit will be even more of a problem.

both of these AA batteries can handle 1Amp pretty easily.

Alkaline/Ultra or whatever are pretty bad at handling high currents. Dont use the charts that battery manufacturers supply because you will be dissapointed with thier real world perforamce in high current situations.

Make sure your battery holder leads and On/Off switch are very beefy or else you will waste power there too.

-howy