Batteries - rechgble NiMH in place of Alkaline/Lithium?

Hi chaps,
A friend has loaned me his infrared binoculars (Nightfox) for a few days, but the batteries (Eight! AA) are dead.
I have a supply of rechargeables, but these are NiMH and the instructions for the device say "please use only alkaline or lithium batteries".
Surely rechargeables would be OK? Or is there something terrible that can happen?
Cheers John
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Another John wrote:

8x 1.5V alkali will give the bins 12V, but NiMHs will be closer to 9.6V

They might complain about flat batteries a lot earlier
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wrote:

Yes.
The internal resistance of NiMh batteries is very low. High current devices, flashguns in particular will be seen as almost a dead short by a battery, An NiMh or a NiCad can deliver a lot of Amps from a fully charged cell.
If the device is not yours, follow the instructions.
AB
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snipped-for-privacy@mail.com says...

It's not as if it is going to cost you a fortune - Poundland now sell 12 AA alkaline battery packs for £2.
--

Terry

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Don't be a cheapskate, your mate has lent you the bins the least you can do in return is put the correct batteries in and think of the brownie points the next time you want to borrow summat off him again!
Richard
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On 09/06/2019 08:48, Archibald Tarquin Blenkinsopp Esq wrote:

This.
I blew up the electronics in an "only use alkaline cells" camera by using NiMh. A little discussion (either here or another newsgroup - I don't remember) and it became obvious that the internal resistance of the cells were critical to the performance and survivability of the electronics. It was a 1980s camera so perhaps that design wouldn't have been seen as so negligent in those days.
Nick
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On Sun, 9 Jun 2019 21:28:11 +0100, Nick Odell wrote:

Could be voltage or internal resistance. I've a BP monitor that musn't have NiMH - it has a little pump, so I expect the current would be too high. Seems to me, in many cases, that the high-draw devices need the higher resistance, e.g. so as not to overload a motor or, in the case of a camera, the flash might be the critical circuit. Oral-B toothbrush runs well on NiMH - one lasted just over 3 years, the same as a rechargeable one that cost 3x as much. The Lidl equivalent needs the higher voltage of alkaline. I've a little Aldidl camera that's good on NiMH, so it's not all cameras.
--
Peter.
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On 09/06/2019 22:24, PeterC wrote:

There's just too much "crying wolf" on this matter to know what to do.
--
Max Demian

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On Sun, 9 Jun 2019 23:02:12 +0100, Max Demian wrote:

Two courses: safe way or head up arse and cross fingers! :->
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Peter.
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On 10/06/2019 08:39, PeterC wrote:

I'm an ignoramus on electronics but I can measure the current drawn by a blood pressure monitor and calculate the likely effect of using batteries with lower internal resistance but also lower voltage. So, despite the manual having "Do not use rechargeable batteries", ours has worked happily on NiMh got 6+ years.
There is perhaps a third course: ask yourself if the manufacturers are just (a) covering their arse and (b) deflecting the prats who send stuff back as faulty when it doesn't work 'cos they've stuck in flat batteries.
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Robin
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On 09/06/2019 22:24, PeterC wrote: <snip>

Oh yes. I agree. One of my other cameras has an option on the set-up to choose alkaline or NiMh cells and adjusts the operation accordingly. I'm sure lots of others do too.
Nick
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On 08:48 9 Jun 2019, Archibald Tarquin Blenkinsopp Esq

Also nickel rechargeables work at lower voltages (approx 1.4V to 1.2V) which may be lower than a particular device needs to work.
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I would not expect any normal electronic device to rely on the batteries having a certain minimum internal resistance, and to disastrously malfunction because it was 'too low'. I would have thought that it's a case of 'the lower the better'.
As has been pointed out, the obvious problem is that rechargeable voltages are lower than their non-rechargeable equivalents. I have a camera and a DAB radio that both need at least 1.3V, and fully charged Nicads and NiMHs (which rapidly drop to 1.2V) don't keep them going for long.
--
Ian

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On 10/06/2019 10:29, Ian Jackson wrote:

If it is something that is regulated to deliver either constant power or constant output voltage when the input voltage being too low combined with the very low internal resistance of rechargeable cells results in frying the device. Old cheap flashguns were very prone to this failure.

The things where damage can result are typically drawing a fairly large current from the batteries and relying on their internal resistance to limit the current into the device. It used to be quite a common trick.
Low current devices fail gracefully on rechargables with insufficient voltage to drive the LCD display to decent contrast.
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Martin Brown
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Another reason for the 'no rechargeables' disclaimer might be the self-discharge on NiMH. It's no good if an infrequently-used widget is always flat when you come to use it - and the manufacturer will probably get the blame for flattening the batteries.
These days we have low self-discharge NiMH so this problem is much less, but perhaps the manufacturers still put the disclaimer in because people will use non-LSD NiMH and then complain.
Theo
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On 10/06/2019 10:29, Ian Jackson wrote:

I've a couple of DAB radios that work fine on rechargeable NiMHs - about the same life as alakali. Despite the manual saying not to use them.
Mind, a DAB radio isn't exactly high current. Nor I'd have thought the OP's binoculars.
--
Cheers, Rob

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I would expect this problem to occur with equipment which needs a very high input current, for instance to charge the capacitor in a flash gun as quickly as possible for a given size of battery, and to get the best possible result from alkaline batteries has no intervening circuitry to limit the input current. It would presumably be fairly easy to design it so that the highest possible current even from an outlier among alkaline cells was ok. But then a new cell type with available short term current 10 or more times higher might fry the input circuitry. I have not seen this myself but it makes sense

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Roger Hayter

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lower voltage so no good ...
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...or am I thinking about ni-cads 1.2v ???/??...
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On Sun, 9 Jun 2019 09:43:24 +0100, "Jim GM4DHJ ..."

Voltage shouldn't be too important within reason. Any device that stopped working at 1.25 V would "waste" a lot of bettery power with Alkaline batteries.
Generally outdoor "toys" have to have a bit of resiliance regarding Volts anyway, simply because low temperatures can drop the applied Voltage anyway.
A low source impedance can mean disaster for equipment as a lethargic oscillator will sink a lot of Amps.
AB
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