"Martin Brown"
"Phil Allison"
** If you are after "worst case" values, then look up the figures for "
internal resistance" for AA size NiMH cells and 9 volt NiMH batteries on
their maker's sites.
A few rechargeable AA cells in series will have no trouble setting plastic
coated wires and PCB component & tracks on fire. Then explode themselves.
A pair is more than enough to source 10+A (initally closer to a
50-100A transient spark) for plenty long enough to reach red heat
burning off wire insulation and PCB tracks. The chemistry starts to
get out of hand pretty quickly when only the cells internal resistance
is the limiting factor. Heat gases, boiling electrolyte, venting and
then explosion releasing cell contents. Sanyo reckon 1.2v and 0.025
ohm at so around 50A 1000Hz. I have seen higher initial instantaneous
short circuit currents than that.
http://www.eneloop.info/uploads/media/Datasheet_-_HR-3UTG_01.pdf
The old Kodak DC-120 camera managed to draw 2A continuously from 4x AA
cells in worst case of macro mode with flash enabled. Modern digital
cameras are a lot more frugal although their power packs still pack
quite a wallop.
Products using them are NOT safe unless there is no possible way the cells
can be shorted and there is a fuse link or other current limiting device (
ie polyswitch) in series with the battery supply.
Or better that a cell is used in a physically encapsulated way such
that it cannot ever source a high current no matter what the external
provovcation. The stuff I recall for flame proof areas also included
armoured impact casing and devious mechanical interlocks to ensure
that no-one could ever open one without the requisite servicing tool.
BTW: NiMH AA cells are sold all over the place now, so any product using
"loose" cells can expect to wind up with them installed.
Yep. If it will fit then one day someone will do it. You also
sometimes find AAA cells wrapped with cardboard to make up the
diameter and extended with folded KitKat foil or even a mixture of
different capacity cells which is another *really* bad practice. But
the guy just wanted to get it working again and bodged around with
what was in his toolbox. It is annoying that field engineers who
should know better do these sort of things as a quick interim fix and
then forget about them until something really bad happens (like the
weakest cell dies horribly).
I find the prospect of the OP designing something that is intended to
be intrinsically safe in an explosive atmospher particularly scary in
the light of his question here.
** The OP is an utter ASS.
There is NO WAY a unit containing AA cells or a 9 volt battery will ever
be allowed inside a potentially explosive environment. It will have to
remain outside and be linked by an intrinsically safe interface to whatever
sensor is involved.
My comments were only about the danger that now plagues all devices that
allow the user to replace the cells or battery with whatever will fit.
Even a common old AA alkaline cell will explode if shorted ( ie by an ill
designed battery compartment ) or if fitted in reverse in a series battery.
...... Phil
