Battery discharging

[RedRed]

Hello Folks

Question:

I have a (large) number of Ni-Mh batteries, mostly for photography.

I want to ensure that they are fully discharged before recharging them again....

I have noticed that even when the batteries dont register in the camera, they can still drive 'thomas the tank engine' (a little 1.5V train my daughter has) for at least another 20 minutes at practically full speed
(it might be longer, I got bored with the train ) ....


any ideas?

I was just going to make up a little paralell circuit with the batteries, and discharge them across a bulb and a resistor, but I wanted to know if that would be a bit harsh on the batteries, and if so, what would be a better thing to do.

What would be the best resistence to use (on open circuit can damage them, I have been told).

I am sure that there are probably proprietry devices, but I am being a cheapskate and I want to BUILD something

RedRed

[The PIT]

Discharging them through a resistor and a bulb will be fine. Alternatively you buy a charger that'll do it. I've got a Vanson speedy box that will discharge the battery plus it also trickle charges so you won't overcharge the little blighters.

[RedRed]

My charger is an hahnel model.... It dosent discharge first......

RedRed

[Wombat]

You don't want to totally discharge NiMH batteries. Each cell in the pack has a slightly different zero point. When the battery is low, you'll hit a point where some of the cells are below their zero point. You're running the battery in reverse, and that's going to damage it permanently.

Spend $25, get a good universal charger.

[az]

Wombat's right. Don't discharge the batteries completely (that does also mean don't use them in a torch), it'll damage them. Sorry

The best you can do to them is to get a good, intelligent speed charger. Mine doesn't discharge them, but it says it doesn't need to, and it's recommended by many Dimage 7 users (that cam EATS batteries!). ~50 EUR is a reasonable price for a good charger that holds 4 AAs. It should monitor each battery separately (not in pairs!), should have -delta u detection (to detect the resistance drop when the battery is full, so it stops charging), MUST be for NiMHs (their resistance curve is different from NiCds. It may of course be equipped for both), should have trickle charge to keep the cells fresh when they're full, and should have some kind of "refresh" feature. Discharging NiMHs every time before charging isn't very good for them, btw. My charger's an AccuCell AP2010, it's very good, and you don't have to think about all the stuff I wrote above, you just pop the cells in, however full or empty, and it makes them full in max. 2 hours It can also "cure" some damaged batteries, or so it says

The reason why you can use the batteries in Thomas the Tank Engine or a torch when the digicam says they're empty is that a digicam needs a high current, which means it needs a high voltage also. But a battery's voltage goes down when its capacity decreases, so there'll be capacity left, but not enough voltage to drive the high-current digicam. Thomas and your torch don't care much about the voltage, they'll just get slower or dimmer and discharge the batteries flat, and damage them. This voltage drop occurs MUCH faster with Alkalines than with NiCds or NiMHs, btw, which is why they're completely unsuitable for digicams (apart from not being reloadable). This is also why Lithium non-rechargeables can be used in digicams (for arctic temperatures, where your rechargeables don't work properly anymore, or for emergency when all your rechargeables are empty), their voltage doesn't nose-dive as their capacity decreases.

Oh my, I've written too much again, haven't I?

[the main lobe]

It may be best to do a constant current discharge on them. This can easily be done by properly biasing a transistor (BJT, MOSFET, or IGBT will all work). Don't discharge Ni-Mh any lower than about 70-75% rated voltage.(~1.1V/cell) I don't know how much current they draw when pictures are being taken, but it may be best to discharge at a similar rate.

If you build it to shut itself off when discharged make sure it is not designed so that it turns itself back on when the voltage goes back up. The voltage will return to 1.5V/cell when the load is removed.

Also there is no reason I can think of to discharge to cells independintly. leave them in series just as they are in the camera.

[gt40]

You worry too much, Nimh batteries have far less memory effect than NiCad.
They are fairly robust, and the price of new ones out-weighs the price of worrying ABOUT them.

[rugger]

I agree with gt40 here.

Nimh batteries are cheap ($5 AU each).

We use a couple of rayovac "cook them hard" 1 hour nimh battery chargers to keep our charged and topped up.

The still work perfectly fine after a year.

[Marshmallowman]

if you still want the cheap one off discharge solution, perhaps you could put a diode in series with the resistor so it can't discharge all the way...look for one with a Vf of about 1.1 volts I guess?