NiMH cell charging

Yes.

No. I wouldn't do this unless the charger were specifically designed for it.

NiMH cells show a voltage drop at full charge -- and require a heavy charge to reach that point. It's unlikely a nicad charger would shut off at the right point.

I don't think NiMH cells are bothered by a trickle charge.

You should get NiMH cells with at least 2500mAh capacity. They're not expensive. I also recommend the Maha/Powerex MH-C9000 charger.

Some of the data sheets I've looked at say that they don't like it. My (limited) understanding is that if you treat a NiMH like a NiCd, it'll work but it's life will be shortened.

Isaac

You're right. Although NiCd cells do have a voltage dropoff at full charge, it's more pronounced than in a NiMH. A charger originally designed for NiCd cells may wait until the voltage drops off significantly (negative-delta-V) before terminating the charge. By this point, a NiMH will have been significantly over-charged, which isn't good for it.

NiMH battery manufacturers tend to use a dual cutoff approach - shut off when the cell temperature starts rising, or at zero-delta-V.

Depends on the charge level. A slow-charge rate (say, C/10 or C/20) in an "overnight" charger may overcharge the NiMH cell if left connected indefinitely.

A very low (e.g. C/100 or so) charge rate seems to be safe, I gather, but it's also rather ineffective for NiMH cells - they have a poor "charge acceptance" at low charge rates, and most of the power just turns into heat.

The battery manuals I've read say that if you do decide to use a maintenance charge (to keep the cells charged up indefinitely), what works best is a pulse-charging technique. Hit 'em with a brief pulse of current at a high rate (C or C/2 or so) every once in a while, and let them rest between pulses. Time the pulses so that the total charge delivered averages out to around 1-2% of their rated capacity per day. This should keep them topped up (countering their tendency to self-discharge) without overcharging them.

For batteries which will be charged up, and then discharged heavily within a few days to a week, that's good advice. Ditto if you're planning to keep them hooked to a properly-designed maintenance charger.

For batteries that you're planning to charge, and then leave on the shelf (or in equipment) for weeks at a time, I think it's better to go with the new style of "ultra-low self discharge" NiMH cells - Sanyo Eneloops, or Powerex Immedions, or Hybrios, or types of that sort. Their rated capacity is lower (2000-2100 mAh), but unlike the older style of NiMH they'll hold most of their charge for months.

I second the recommendation for the MH-C9000. A very neat piece of kit.

Charger: if it is an old < C/10 charger - no problem. (I am doing it for years now, occasionally forgetting to take the cells out etc. Very bad procedure, but does not seem to hurt cells much)

If it is a fast charger - don't! delta V is different.

Most batteries fail in cameras due to Ri being too high, not due to lack of capacity. Failing batteries may work well in e.g. a radio, but not in the camera.

I seem to observe that *lower* capacity batteries perform *better* with respect to Ri and lifecycle.

Maha says roughly the same thing. The recommend charging at at least 0.3C, though it's not clear whether the reason is to push the cell towards a significant delta-V at full charge, or to charge more "efficiently". I'm bothered by the temperature rise, and tend to charge at 0.2C, which (based on many years using nicads), seems downright excessive!

I don't expect any rechargeably battery to hold its charge indefintely. I have a big pile of them, and charge them as I need them.

I might add that 2500mAh NiMH cells pretty much mark the end of the alkaline cell, except for "intermittent", low-drain applications, especially where frequently swapping the cells would be incovenient (eg, remote controls). Such cells can provide at least the same run-time as a throw-away alkaline. And, yes, I've checked and confirmed this.

I wish I'd one 20 years ago.

Of course, lifecycle has to be weighed against how often you have to swap out the cells. If a single high-capacity battery gives you no more than the same total runtime over its life as multiple lower-capacity batteries, you're still ahead of the game.

I think it's the former. The higher charging current leads to a more rapid flattening-out of the voltage-vs.-time curve, and also leads to a more rapid rise in temperature at the full-charge point. Both of these make it easier for a good charger's circuitry to detect the full-charge point and shut down the current flow at the right time.

I believe that Powerex actually *increased* their minimum-charge-rate recommendation for the MH-C9000 - it was originally .2C and now they recommend .3C or above. This change, plus some firmware changes in the more recent versions, seems to have eliminated the problem of "the charger never shuts off, and the batteries get really hot" problem that early revs of this charger were prone to.

I've been charging my various low-discharge AA cells in a C9000 at .5C or so (the charger's 1000 mAh default rate) and they don't seem to much, if any, warmer at charge-shutdown time than if I stick them in an old-style .1C slow-charger and let 'em fill up. The C9000 appears to detect the full-charge state quite reliably... I have yet to run into a single hot-battery problem.

At this point, I prefer to stick with one of two charge regimes - a fairly fast charge (.5C) using voltage-and-temperature cutoff, or a slow one (.1C or below) using a timed cutoff.

I can well believe it.

The major niches I see for alkaline AAs are the ones you suggest (e.g. remote controls) and long-storage-time emergency backup applications (e.g. I keep a partial "brick" of AA alkalines with my amateur-radio "go-kit", so I'd have a couple of days of on-the-air time prior to having to drag out a recharger).

For my wife's digital cameras (and my own) I've settled on a combination of low-discharge-rate NiMH for day-to-day use, and lithium AA disposables for travelling (they're light, really long lasting, and it saves the trouble of taking an overseas-voltage-qualified charger along).

Yah. I've been rather dissatisfied for years with commercial NiCd/NiMH chargers, and toyed with the idea of designing my own with a boatload of flexibility and control, good full-charge detection, reconditioning circuitry, etc. - an ideal application for a small microcontroller. Never got around to doing it... and now there's no need to go to the effort!

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Hi, everyone. My sincere thanks for all replies and the aggregated expertise! It seems that I should get a Maha/Powerex MH-C9000 charger. I'm presently using a Sanyo "Super Quick Charger", model NC-MQH01U - bought with a pack of NiMH's at COSTCO a couple of years ago for around $30. I've no idea what the so-called "C-rate" is but the cells get quite hot before it cuts off (my guess is that it does not have a temperature sensors.) Does anyone know a good source, preferably in Canada, for the Maha/Powerex MH-C9000 charger? Cheers, Roger Thornhill, near Toronto, Ontario.

Apologies for jumping in late but before you buy, hop over to Amazon.com and read the reviews on the MH-C9000.

I was particularly "impressed" with the reviewer who felt that its wealth of features made it too difficult to use.

The point about watching what you're doing when recharging with such a powerful unit is well-taken, though. I've had no problems with mine.

snip...snip

Amazon will not ship this to Canada so says another poster.

Try:

I'd suggest seeing if Thomas Distributing will ship to Canada.

You need to know this!

C is the cell's capacity in mAh. This is usually right there on the cell. For example, I have some decade-old Panasonic nicads marked 500mAh. (That's not very high capacity, compared to recent cells.)

If you charged the cell at 50mA, that would be a 0.1C charge rate.

's

Indeed, yes... a bit arcane! Typical NiMH's today (2008) are 2100 mAH so "0.1C" would be 210 mA. I should check my Sanyo charger - my guess is that it exceeds "0.5C", i.e. an amp, or so. Still looking for a Maha/Powerex MH-C9000 charger. Cheers, Roger

Just Google it.

The price has gone up in the past year, so look around a bit before buying.

Thomas Distributing is a good company, but you can find it cheaper elsewhere.

I've been a Thomas Distributing customer ever since I had an intermittent problem with a Maha battery charger and they sent me a new one (next day), and didn't request I send the faulty one back. You can't beat their customer service. Plus, they usually include batteries, battery cases, and charger travel cases and cigarette lighter plugs with their chargers. Lowest price isn't always the best deal.

customer

Agreed, but the price difference is big enough to consider buying elsewhere.

I buy most of my rechargeables from Thomas. They have good prices and good deals.

If you need lead-acid batteries, try