NiMH Rechargeables

[StephenW]

Has anyone noticed how these seem to self-discharge very quickly? Apparently more quickly than NiCads.

I recharged some AAA batteries in July and they were virtually flat when I came to take them from the boat in October - not very inspiring when they were intended to be used for powering my handheld GPS unit if the main one went down onboard.

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[pvb]

Typically 1% a day...

NiMH batteries will self-discharge about twice as fast as NiCd batteries. And self-discharge is temperature dependent too, so it'll be faster in summer. If you reckon on losing 1% of capacity a day, you wouldn't be far wrong. And so it's not surprising your batteries were flat after 3 months.

On the plus side, NiMH batteries have a substantially greater capacity than NiCd batteries and they can easily be topped up because they don't suffer as badly from the wretched "memory effect" which plagues NiCd batteries.

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[Evadne]

Primary cells (manganese alkaline) have a greater capacity and charged shelf life, so are probably better in the role of emergency backup unless the rechargeables are left on trickle charge permanently.

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[andy_wilson]

Have you actually tried then in your GPS when fully charged?

The reason I ask is that due to the slightly lower cell voltage, rechargeables won't even light up my handheld GPS (Magellan). Zinc carbon don't last 2 minutes when in use but Duracells have a shelf life measured in years and last long enough to be a useful back-up.

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[jon_bailey]

I've used NiMH cells in my Magellan Handheld without problem. The only difficulty comes with the fast drop-off in voltage at end of life when there is little warning that cells have nearly expired. You need to keep a back-up set of (alkaline) cells for this eventuality.

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[StephenW]

Yes, I have a garmin GPS12 and the batteries work without any problems when fully charged.

Typically get around 10-12 hours of continuous use, which is not as long as Duracell but good enough for most occasions.

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[jon_bailey]

Andy

Just remembered that my Magellan suffered from poor contact when using some cells. In particular when the coiled springs mated with the +ve (nippled) end of the cell proper contact was not made due to insulation around the end of the cell. This was solved by modifying the springs with a (small) pair of pliers.

Jon

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[gjgm]

Re: memory effect

This was more a case at least 5 years ago with inferior batteires. Yes, theoretically you can still induce memory effect, but you are talking about 100s of partial charges, and a very nominal drop in capacity. In general, most of us can now dismiss it.
The main reason modern fast chargers also have dischargers is because you need all batteries well matched for fast charge peak detection to work efficiently- of concern say in radio controlled aircraft.
However, some electronic equipment is very voltage sensitive, and the characteristics of recharcheables being 1.2v with sudden dramatic drop off may mean the battery appears "dead" faster than a conventional battery that has a nominal 1.5v and a more steady decline.


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[pvb]

It\'s still a real problem...

NiCd batteries are very sensitive to voltage depression (the "memory effect") as a result of partial discharges. It's still a real problem for sailors with vital equipment powered by NiCd batteries.

Radio controlled model planes may well be of concern where efficient battery charging is vital, but I'd reckon that handheld VHF radios are of more real concern to most of us, and many handhelds are still supplied with NiCd batteries.

The pattern of usage of handhelds makes it difficult to avoid partial recharges - I think all handhelds should be specified with NiMH or Li-ion batteries, rather than compromise performance to save a few quid on the list price. The manufacturers are aware of the need for better batteries; Simrad for example ship the HT50 handheld with NiCd batteries as standard, but offer an extra-cost option of a NiMH battery.

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[tr7v8]

I use rechargables in my Mag 320 GPS and they are fine although the end is sudden due to the discharge characteristics. And the overall time of use is lower. What I do have is a Battery genie which came from Maplins and that recharges Alkalines very well. It was about £ 30 and recharges all dry batteries & NiCd & NiMh, must have saved me a fortune.

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Draco 2500

 

[MainlySteam]

Re: memory effect

I agree with you entirely and add that even with NiCd's early on, a very special set of circumstances were required for them to suffer "memory effect" and those circumstances did not occur in typical casual use. Somehow it captured the public's imagination and it seems to now be used as a marketing ploy for NiMH batteries.

What many do not realise with NiMH batteries is that while they accept a larger charge per battery weight than NiCds they have a shorter lifetime in terms of the number of recharging cycles they can go through (for now anyway). In the end the life of both battery types in terms of AHrs delivered over their whole service life is much the same. NiMH has the advantage of lighter weight for the same capacity at the cost of higher price (well for now anyway).

John

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