Slightly different battery question (off topic)

[prState]

And yet the same. adding lots of smileys

I have a device that is normally used to detect change in the infra field to detect cars in your driveway (motion detector) and send a signal back to the based plugged into an outlet (up to 190 ft with a clear sight line), which in turn operates a built in bell, or alternatively activated a outlet (for instance turning on a lamp.)

However I'm using it in my living room to reduce energy consumption on a fan. When I come into the room, it activates the device, and turns on the fan for 3 minutes. If I don't move or fall asleep, and don't move the fan stays off. I've believe I've calculated over the long run, that I'm saving power consumption. If I just had the fan operating normally, I would generally leave it on as I get up and go in kitchen for instance for 5 minutes, so thus for example it saves two minutes where it would run blowing on nothing living. Or if I forget and leave it on and leave, it still shuts off after 3 minutes even if I forgot it for hours.

Anyway, the detector runs on 4 AA batteries. I'm using 4 NIMH which operate in it 24 hours, (so some loss there). But I haven't figured out how to decide when to throw the batteries in the charger (8 hour charger). I've let them go as long as 12 days 24/7, and whenever I test them, I get 1.2 volts each. Should I be looking for a voltage drop? Should I risk just letting them go until they won't operate the device, or could I end up damaging them?

Thanks! ( it's not world shaking failure if I kill them, but I figure, somebody probably could save me from doing that)

[flecc]

You can let them go much longer than that prState. In theory, they can be safely discharged down to 1 volt without damage, but it's best not to let them go much below that or there's a risk they might refuse to accept a new charge.

I would run them on, checking from time to time for when they drop to almost 1 volt each cell. You might just be lucky and get one month, and then only have to remember to charge at the start of each month which would be ideally convenient. If not, perhaps settling for roughly two weekly, start and mid month would be fairly convenient.

Hope that's some help.
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[prState]

Thanks Flecc, . I will let them run longer than I have then, and test them.

[Brad]

Also remember that the device that is plugged into the fan and wall is probably drawing in the order of 3 watts all the time! So how much are you really saving after subtracting that?

[john]

Originally Posted by prState

Anyway, the detector runs on 4 AA batteries. I'm using 4 NIMH which operate in it 24 hours, (so some loss there). But I haven't figured out how to decide when to throw the batteries in the charger (8 hour charger). I've let them go as long as 12 days 24/7, and whenever I test them, I get 1.2 volts each. Should I be looking for a voltage drop? Should I risk just letting them go until they won't operate the device, or could I end up damaging them?

I've got loads of this type of battery and have been using them for years in everything. I just recharge them when the device doesn't work any more; never had a problem with this.

Anyway, these batteries are so cheap compared to alkaline batteries that they are not worth fretting over.