Chuck wrote:
"They (Nimh) get pretty warm when they're charging."
good observation. Unlike Nicd's that have an endothermic reaction during inital
charge (and thus
only heat during the last ~150f charge), NiMh's have an exothermic reaction
(heat up) from
begining of charge. This is partly why the voltage roll over as they go into
overcharge is lower
for Nimh and also why you usually need a smarter algorithm to cut off charge
based on temperature.
An interesting fact is that the Nicd's and I believe NiMh's, charge efficiency
is lower at low
charge rates. This means if you charge fast you don't need to put as many mAh
in as at a slow
rate. For example, for NiCds at a C/10 (low ) rate,a typical recommendation is
to put back ~140%
of C. While for the same cell charged at ~C/3, you may only need something like
1200f C.
Charge efficiency drops to <50% at temperatures higher than 50C+, along with a
reduced gas
recombination. As a result charging at temperatures much higher than 40C is
likely to cause
venting and cell damage as the cells self heat.
In general the Nimh recommendation is not to float charge in any circumstances,
especially not at
the C/10 rates commonly used for simple Nicd chargers. For a simple timed
charger the
recommendation is typically to put back no more than 1200f C, at a C/10 rate
for NimH (e.g.
12hrs at 180mA for a 1.8Ah cell). A common recommendation is to limit charge to
no more than 120%
C, if you hope to achieve any sort of reasonable cycle life. Duracell
suggests that C/300
continuous float is acceptable for its NimH cells. (e.g. 6mA for a 1.8Ah cell)
Tim Hughes
TimHughes@xxxxxxxx
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