NIMH SAFETY NOTES.

No
doubt you will have read or heard the stories of batteries exploding and that the
Norwegian Federation has temporarily suspended all electric racing. We have been monitoring these ‘incidents’ but obtaining
hard facts is not easy, making it difficult to draw any meaningful conclusions.



Given
that there are thousands of cells in circulation, such incidents have been
exceedingly rare in the past. The only
serious incident that has occurred to date in UK was when a receiver pack blew,
that has permanently damaged a person’s eyesight. Although there will have been more injuries
caused by scalpels or super glue in eyes than caused from exploding cells, you
still need to take care!


The
NiMh cells have a chemistry that allows the cell to 'self-discharge' over a
relatively short period, (days as opposed to weeks). Lately, some of the
4200 cells seem to self-discharge more rapidly than other/previous
versions. More importantly the
amount each individual cell within a pack discharges varies (i.e. cell 1
might be .9v, cell 2 .8v, cell 3 .9v, cell 4 .7v, etc.)


If
charged in this state, some chargers will not know when to stop the charge
process, as some cells are demanding to continue with the charge, whilst others
could be over charged. It is also important to control the delta
peak (voltage drop-off after completed charge) to a low value when
charging. Once the cell has achieved full charge, any further charging will
produce gas. The cells have a pressure vent, but if that is blocked
or not working, the gas pressure could cause an explosion.


High charge
rates will also produce more gas and reduce the cell’s useful life. It is
possible that internal gas pressure may cause
distortion to some internal parts that could result in a short circuit within
the cell. An internal short circuit combined with gas pressure is likely
to be a 'big bang'.


Drivers
need to recognise that the 'matchers' recommendations are intended to
achieve maximum performance. It is the recommendation from the cell
manufacturer that should be used.


Best Practice for use of NiMH cells.

1. 
   
   Equalise cells before charging if they have been stored (with charge) for more
   than 2/3 days. If any of the cells in the pack show that
   they do not need to be equalised (i.e. the light does not
   show or goes out straight away) then remove the pack from the equaliser
   and charge for a
   short period (approx. 5 mins. should be adequate). Then carry out the equalising
   process.
   
2. 
   
   Never exceed the manufacturers recommended fast charge rate. This should
   be 1C max., even if the
   manufacturer states higher. (C being rate of charge based on capacity
   of the cell, e.g. a 4200 mAh cell = 4.2 amps)
   
3. 
   
   Disregard any charge rate recommendations by the 'matcher' if it is more than
   1C.
   
4. 
   
   Use a maximum Delta-Peak
   setting
   of 3mV per cell (6-cell pack = 3mV x 6 = 0.018V).
   
5. 
   
   Never repeak cells after main charge.
   
6. 
   
   
   Use a temperature cut-out as an additional safety feature set at 42 deg. C
   max.,
   located on the hottest cell
   (usually middle cell). If charging on a cold day then consider reducing this to around 35 deg.
   C.
   
7. 
   
   
   Allow cells to fully cool to ambient temperature before further charges. Be aware the centre of the cell
   cools slower than the outer casing. Do not put them in water to cool down; in an emergency
   cover the cells with a damp/wet cloth if you have to quickly cool them.
   
8. 
   
   
   Store cells with some charge (30/50%).