Batteries - rechgble NiMH in place of Alkaline/Lithium?

They might complain about flat batteries a lot earlier

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

lower voltage so no good ...

...or am I thinking about ni-cads 1.2v ???/??...

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

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

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

It may well use an inverter to drive the EHT for the image tube. The combination of low internal resistance and lower output voltage of rechargeble batteries could kill it by over current. The same issue used to affect cheap and nasty flashguns back in the old days (and may still do so). They relied on the internal resistance of primary cells.

Cheap 1.5v Kodak cells are about £1 a dozen from Poundland or Wilko. Just don't forget about them and leave them in the device for too long. (that said most of the cells I have had leak recently were Duracell).

OP here: thanks so much for the advice! You've also answered a subsidiary question, which is "are cheapo batts from Poundland etc good enough?"

I guess at a pound a dozen or so, and when as many as 8 are needed, the answer is "at a pound a go? what the heck!".

And thanks also, TrickyDicky, for reminding me of my responsibilities as a mate! :-)

Cheers John

Another John explained on 09/06/2019 :

I tried them and they are quite useless, ended up bining them.

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

I've found Screwfix AA and AAA are OK, especially when on special offer

Also nickel rechargeables work at lower voltages (approx 1.4V to 1.2V) which may be lower than a particular device needs to work.

Lidl's seem to be as good as most and, with luck, less chance of leaking than the cheapos. Toolstation has boxes that are quite cheap per cell.

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.

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

Two courses: safe way or head up arse and cross fingers! :->

They are fine if all you want is a moderate load for relatively short periods of time. They are useless at high currents and I wouldn't trust them not to leak if used for longer term in something like a clock.

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.