Re: [EE] Trickle charge current on NiMh

At 03:49 PM 9/6/2008, you wrote:
>I have a single-item application that requires battery backup. I am
>planning, for sake of simplicity, to use 4 cell 650mhA NiMh and keep them in
>trickle charge permanently. Simple limiter resistor will put the trickle
>charge at 20mA max (<1/30C) which is far below the 1/10C most people comment
>as acceptable for trickle charge. The NiMh are high quality Kan batteries.
>
>I saw different recommendations but in most cases manufacturers do not
>recommend trickle charging batteries permanently, although most admits that
>shouldn't damage the batteries. Therefore I perfectly understand this is not
>a recommended practice. Certainly a pulse trickle charge or a monitored
>dedicated charge is a more elegant solution.
>
>What I am looking for in the list is real-life experiences on permanent
>trickle charging being a real problem for NiMh. I am not thinking on this
>for large volume application which may require much better design (and time
>dedication), but in a single-item application I am ok assuming the NiMh may
>degrade to a point in which it has to be discarded in 2-3 years because of
>trickle charge.
>
>Thanks in advance

Trickle charging is *not* recommended as it will liberate hydrogen and
eventually cause enough of the electrolyte to disappear that the cell
will be useless. It *may* be acceptable at something like 1/50 or 1/40C,
but see the manufacturer's recommendations for the particular cells you
have in mind.

Can't you just turn the charge off after a period of time, like 30 hours,
without use?