Ian Jackson wrote:

In message , Archibald
Tarquin Blenkinsopp Esq writes
On Sun, 09 Jun 2019 08:34:18 +0100, Another John
wrote:

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

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

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'.

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




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.


--

Roger Hayter