HP50g and rechargeables

[Chris Smith]

Ok so I've had my 50g for 2 weeks now and the rather expensive batteries I put in it (lithium AAA) are dead as a dodo. To be fair I haven't put it down, much to the dismay of my other half.

I did some research regarding using rechargeables in it and it appears there is some confusion over battery warning voltages if you put 1.2v cells in instead of usual alkaline 1.5v cells. Basically the discharge curve of your average NiMh cell drops off pretty quick meaning the calc will starting complaining quite early.

Logic said: find some cells that have a better up front voltage than 1.2v. So some research tripped over Nickel Zinc cells which are rechargeable and have an initial voltage of 1.6v.

My local electronics shop (Maplin in UK) sell them as well:

http://www.maplin.co.uk/nickel-zinc-aaa-rechargeable-batteries-4-pack-635958

The initial voltage of 4 cells is 6.4v vs 6v so imagine that's not too much for the regulator on the device to handle.

Has anyone tried these or does anyone think it's a bad idea before I go and buy some? Or are they snake oil?

I'm an EE by trade so I'm up for any tech details.

Edited: 24 Dec 2012, 6:59 a.m.

[Marcus von Cube, Germany]

An unrelated remark: If you are close to a computer with USB you can just operate the calculator while powered through the USB port.

I don't think 6.4V is too much. Fresh alcalines may well have more than 1.5V, too.

[Chris Smith]

Thanks - didn't know that. I've got a USB Nokia charger hanging out if my sofa which is used to charge various things - that works too as I've just found out.

[John B. Smitherman]

Or use a USB charger...

John

[Dave Shaffer (Arizona)]

As Marcus notes - check some brand-new alkalines with your voltmeter (I can't believe an EE doesn't have one, or several, of those on hand).

When I do that, I almost always get 1.59 V, so they are (briefly) the equivalent of your alternative rechargables. Conversely, the Nickel Zinc cells may be (well) over 1.6 when they are fully charged. I find fully-charged NiMH to be around 1.34 V, not the 1.25V or so that they quickly sag to (but then tend to hold). (This is all for AA cells, but the AAA chemistry must be essentially identical.)

[Chris Smith]

Zero on hand unfortunately due to Christmas!

[Lyuka]

How about using low self-discharge Ni-MH batteries?

hp50g LowBat(S) Warning and low self-discharge Ni-MH batteries

Lyuka

[Chris Randle (UK)]

I use NiMH 1.2V rechargeables in my HP50g. I've used "Energizers" in both 700mAH and 1000mAH varieties. In the UK, they're silver with a green flash. I've had no problem at all with the low batt warning when using them.

When I first got the calculator I kept a log of usage, and deliberately carried on using the calculator after the low batt annunciator had lit. With an average use of about 1 to 1.5 hours a day, the 700mAH cells kept going for a further three days before the calculator wouldn't turn on any more, and the 1000mAH went a further five days.

I've used only NiMH rechargeables for about 2.5 years, and I usually swap them out on the same day the annunciator lights. I have never had the calculator die on me.

The runtimes are quite acceptable to me too. Using the 50g all day every day running a lot of programs, the worst I've ever had is two weeeks' use. The best, when using it about an hour a day every other day or so, is nine weeks'.

I've also got some 750mAH Sanyo Eneloops. I've yet to try them in the calculator, but in everything else they've been outstandingly good.

[Chris Smith]

Thanks for the reply. Might try some energisers first then as they are pretty ubiquitous and are on sale at the minute in various places.

I can't seem to find a reliable reseller of eneloops in the UK other than RS but they're extortionists.

[Chris Randle (UK)]

Bought mine from Amazon. Got some AA and some AAA.

[Derek Walker (UK)]

Small Battery Company from s-w London have them at £10 for 4 inc p&p:
link here
No knowledge of the company, just found it when looking for another type.

[Derek Walker (UK)]

Just to round off this thread in case anyone is following it, I ordered both the lithium cells that I'd been searching for, and a set of AA Eneloops, from this supplier.

They performed fine, ordered Weds pm, e-mail received Thursday confirming dispatch, arrived in Friday's post. Only minor concern is that they were in their blister packs, but were posted just in a normal unpadded envelope, however they were OK so maybe this packing is adequate.