Trickle charge C-type NiMh?

Q1: Yes you can charge them at 330 mA _IF_ you disconnect when they reach full charge, or lower the charge rate when they reach full charge.

Q2: "Safe" is a variable. C/10 is generally thought of as a "safe" trickle rate for "16 hour chargers". So the 16 hour safe rate for your 2200mAh cells is 2200/10 or 220 mA. You won't instantly kill an NiMh if you go 1 mA too high, or 1 minute too long, but to be safe you want to reduce the charge rate, or disconnect, when full charge voltage is reached. (You can also charge at a far higher rate than C/10, but that charge _MUST_ be terminated or reduced long before 16 hours. See the answer to Q1.)

All batteries will go bad in time. Over charging and over discharging shortens their life. For the over charging issue, I use a lower charge rate after the battery has been charged at C/10 to full charge voltage. I use 1.43 volts per cell as the full charge voltage, and a comparator to switch to a lower rate - say C/50 - when that voltage is reached. That would fit both of your questions in practical terms.

Ed

P.S. I remember your curtain controller from years ago - quite impressive!

Thanks, Ed, much appreciate your very thorough answer.

Poor performance prompted me to buy more batteries, but I couldn't find the original 3300 mAH types. (Others were far more expensive; maybe I'll have to expect replacing these cheaper ones regularly.)

The circuit for my venerable curtain controller (2004 completion) includes a constant current source charger adjustable with a preset. After charging and installing the 2200 mAH batteries I found the trickle current was down at 25 mA instead of the 35 mA I'd originally set. I reckon it may have been inadvertently altered and that contributed to the sluggishness of the action I was seeing. And as the circuit relies on microswitches at the extreme open/closed positions to switch the battery power to the motor, it only takes one unnoticed failure to drain them completely.

Anyway, shortly after my first post I upped the trickle to 40 mA and form a day or so after that I've been getting sweet operation again.

The three series 3300 mAH are now on the shed bench. I'm undecided whether to:

- wait until I need them, when the 2200s fail, and then just fully recharge, meanwhile operating curtains manually.

- or dust off my stuff and make a timer circuit to apply constant current 330 mA for 16 hours (from my variable power supply) via a relay every few weeks.

Downside of the latter is that it risks tempting me back into electronics stuff and away from my current long-standing obsession with video/DVD making ;-)

Terry, East Grinstead, UK

Terry,

At 40 mA you're applying a float charge - not a trickle charge. (I mention that in case you decide to read up on battery charging and encounter those terms.) Anyway, you can charge them more aggressively than 40 mA. 40 mA on a 2200 mAh battery is a charge rate of 40/2200 ~ .018 or roughly 1/50. That's a good float rate. It won't overcharge, but will be slow to charge.

If you want (or need) to charge the cells a little quicker, you can add a simple circuit that would allow you to charge them at over double the 40 mA until they reach full charge, and then automatically reduce the rate back down to ~40 mA. If you want, I'll post an ascii schematic here, or I can send a .jpg to your email.

Ed

Thanks Ed. Yes please, if you have a circuit to hand I?d appreciate seeing it. However I?m optimistic about the, er, current setup usi ng that float charge of around 40 mA. After all, the batteries are typicall y supplying full power for only a few seconds daily.

Terry,

If it's working reliably now there may be no advantage to adding the circuit I was talking about. I'll post it anyway, in case you find you need it at some point in the future.

Ed

(View in courier font)

The circuit is a simple shunt regulator, designed to draw roughly 65 mA when the voltage across the battery rises to a level determined by the ratio of R1 to (R2+R3). Your supply provides a constant current of 110 mA. About

108 mA of it charges your battery, until it charges to about 3.92 volts. Then the shunt regulator begins to conduct. Any current conducted through the shunt regulator is subtracted from the 110 mA from the supply, and only the remainder goes to the battery. So when the battery nears the full charge voltage the regulator conducts and the result is 108mA - 65mA = 43 mA.

• --+-----+----+------+ | | | | | R4 R5 | | | | | R1 +----+ | | | | | __|_K_/ | All resistors 1/4 watt | / / \ | R1 = 1000 ohm | R / \ | + R2 = 1000 ohm +--/TL431\ Battery R3 = 330 ohm | ----- | R4 = 120 ohm | |A | R5 = 120 ohm R2 | | | | | R3 | | | | | - --+-----+-----------+

Set your charger to provide 110 mA current. The battery will charge at that rate until it reaches about 3.92 volts. When the voltage rises to 3.92 volts, the TL431 will conduct, pulling about 65 mA through the 60 ohms of R4 in parallel with R5. That means about 43 mA will flow into the battery. As the battery continues to charge, its voltage rises. At 4.29 volts, the charge current to the battery will drop to about 37 mA.

I picked standard easy to get resistor values. Precision is not important, but use the values I posted as they determine the voltage at which the TL431 will conduct. The parallel combination of R4 and R5 provide 60 ohms resistance in series with the TL431, protecting it. The TL431 is available in a 3 lead TO92 package so it's easy to work with. The maximum allowable current through it is 100 mA, so that 60 ohms protects it.

Ed,

That?s great, thanks a lot. It may be a while before I build it, bu t that will be my first project for a few years!

Terry, East Grinstead, UK