Charging NiMHs

That's how you determine the termination of charging. It could easily be monitored by a uC. I use an AVR w/ thermoresistor to check the temperature.

Does the thermoresistor touch the battery?

Right next to it.

The author hasn't done a particularly good job. End-of-charge in nickel-metal hydride batteries means that you get oxygen formed at the anode, rather than nickel oxide, and this diffuses to the cathode, where it reacts with the stored hydrogen to form water, releasing heat.

The heating is what indicates that battery is fully charged. Some intelligent battery chargers measure the temperature of the battery, detect this heat dissipation and switch over to trickle charging.

Clearly, changes in ambient temperature could fool the simplest form of such a system. Two temperature sensors, one close up to the battery and one slightly further away could sort this out, or you could pulse the fast-charging current and monitor the change in temperature between current on and current off.

-- Bill Sloman, Nijmegen

And if you continue to slow cooking (sorry, charging) with a dumb charger, the water will vent out and eventually the cell goes dry. If you remember a few months ago, we had heated arguments about simple minded dumb charger with another poster here.

The one I currently use (bought, not built) appears to have the sensors (1 per battery) built into the contacts. If you're in it for the challenge, build away - if you just want a better thought-out charger, they can be bought.

But standard rate (1C) has better EOC conditions than low rates. They don't rate the batteries in 1C for no reason.

Given a constant charging current, the power dissipation is not constant throughout the charge cycle.

Best regards, Spehro Pefhany

It's normal for NiMH to release heat on normal charge, below rated capacity - they are exothermic. Controlled charging usually employs battery gauge techniques - but end of charge is still indicated by a simultaneous increase in both voltage and temperature gradients.

The only way I know of charging NiMH, without overcharging, is to discharge them first, then recharge using low rates that have better defined temperature and voltage gradients at EOC.

The Moltech NiMH application manual is no longer available on-line, but copies of it's previous incarnation, courtesy of Energizer, is.

RL

Build, you say?

If you have to ask, don't even think of charging anywhere near 1C...

AAs (2.5A) are tough. AAAs (0.8A) are doable. There are sub-AAAs as well. We are considering integrated charger/batteries on a device, which already has a uC anyway.

If it were constant, the temperature would be stable. It is 'fairly' constant over the charge cycle, and the temperature is 'fairly' stable for any given charge rate.

The monitoring thermistor used for these applications is standard and pretty easy to calibrate, from one battery to another and from one packaging format or charging platform to another.

I'm not defending lousy charging platforms or SW control. I'm not happy to see what's out there, but I'm not surprised, given the history of battery development and marketing to date.

RL

It seems most charge IC's are not heavly stocked anymore. The MC33340 / 42 can be found in SOIC at Digikey.

But read up on Charging, the best method is Delta +V and Delta T. Delta -V can cause Gassing.

Cheers

cause Gassing.

That's a bad reference, the section on charging techniques is bad and it does a lot of defaulting to advising the reader to contact a Sanyo service representative. The section 3 summary of charge termination techniques is not to be read literally, it is actually an allusion to the various charge control ICs available to do the job, none of which are suitable for optimum NiMH charge termination. Your suggestion falls too short of satisfying the "reading up on" criterion to be very useful...

say?

As good as the Alpha Power Battery Charger may be I found one thing it can't do. It can't charge old fully discharged NiMH batteries. The Alpha gives a Null reading with old NiMH batteries,. I ended up putting the batteries in a $10 battery charger to build up the voltage to a point where the Alpha could finish charging them. And I hate manuals that require a magnifying glass to read the instructions.

Howard

Charging NiMH batteries is tricky. The most reliable way of detecting the end of the fast charge cycle is when the temperature of the batteries rises a certain amount in a given time. If the charger has a temperature sensor, consider it a good charger. It is normal for NiMH batteries to get a little warm when charged fast.

Panasonic has some very nice application notes on this subject. Charging NiMH batteries using voltage to determine if they are full or not is not going to work.