NiMH battery recharging

[Ellis Easley]

What's the recommended way to recharge NiMHs? I know they can be recharged in existing equipment just by increasing the time but what is the efficient method? I bought an Eveready package containing 8 AA cells and a recharger that can handle 2 or 4 cells, either NiCd or NiMH. I can't resist opening things up to see how they are made and the recharger has an IC and two transistors and even 4 1% resistors. I thought this meant it was using an intelligent method to determine when charging is complete. I don't know the recommended way to recharge NiMH but I know the two best ways for NiCads are sensing when the temperature starts to rise and sensing when the voltage starts to drop (after rising through most of the charging cycle). I traced out the circuit - saved looking up the IC till the end - and the control function turns out to be just a timer! The IC (4541) is a 16 bit counter with built-in RC oscillator, the 1% resistors are the oscillator timing resistors for NiCd (7 hours) and NiMH (14 hours), and the transistors form an invertor to switch an LED from red to green. The label says it charges at 140 mA, which would be near the .3C fast charge rate for NiCads, then switches to a trickle charge, it doesn't say what the current is but I see a 39 ohm resistor in parallel with the transistor that turns the full charge on. There is no difference in the charging current for NiCd or NiMH. I'm disappointed it isn't any "smarter"! I like the general idea of a timer for a dumb recharger but what bothers me is that if the unit is plugged in and the cells are at full charge and the power fails briefly, the cells will go through another complete recharge cycle.

[Kim]

First - disclaimer I'm NOT an expert.

What you have to look for is the term -dV/dT. You can charge the NiMH until the voltage starts dropping (in mV). The you can turn to trickle charge. Otherwise you would have to know the capacity and use that for the timer.

There are a cople of really smart chargers out there but
expect to pay $100 to $200 for a good one. But you get
discharge, regenerate, flash charge, capacity measure and
such 'good stuff'.

The simple way is to charge them with a 10% of capacity
current, which most good ones should withstand for years on end. This is like a constant trickle charge.

My 'about half of 2 cents'

Kim

[Dia C. Tran]

Ellis,
There is a good article on charging NiMH battery on the Eveready website. It said the best method is to monitoring the temperature rise as you said.

[Ellis Easley]

I guess they are not too much different from NiCads. I was surprised to see the Eveready website only has NiCads in the discontinued section.

This little charger was essentially free - the package was about $20 with the 8 AA cells. And a timer is better than no timer!