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Non LBC Circuit Question

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jsneddon

jsneddon

Jedi Knight
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OK.... This is a basic electronics question that one of you engineer-types (and probably many others) can answer for me.

I've got a device. It runs on 2 AAA batteries. It does not have an external power hookup. I want to attach a 12v to 3v converter to it in a car because the triple-a's don't last more than about 20 hours. (the 'good' rechargables - Alkalines only will last about 6 hours). I want it to be on 2 or 3 days at a time non-stop.

So....

I can easily wire up a converter to an always-on source in the electrical system of the car.

But of course it gets more complicated. When I disconnect the power it looses its memory. So I need the batteries to stay in for portability to retain the memory but I'd like to extend the useful life so I don't have to be feeding it freshly charged batteries and then reentering all the settings.

Is there a problem with wiring the power hookup into the connections where the AAA's pick up AND leave the rechargables in the device? Then I could even put it on a switched circuit (although battery drain on the car would be negligible). When the car is off it isn't doing anything anyway. I don't care if it actually charges the AAA's - I can do that myself in the house - I just want it to stay on without killing the batteries.

Attached is a simple diagram I've sketched of what I'm thinking of doing.... Clear as mud? I can't see why this wouldn't work. Any input???

Thanks
 

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Bad idea. The batteries won't take kindly to being connected directly to the 3 volt source, and are going to be charging at a very high rate, probably damaging them. To do something like what you are doing, you would need a diode between the batteries and the device, so that when the supply is turned on, the batteries are effectively disconnected. The downside to that is the voltage drop across the diode will probably make the device think the batteries are almost dead (a 0.6 volt drop is pretty significant at 3 volts). Without getting more complicated and adding a relay and a capacitor to take care of switching the batteries in and out, I'd say don't do it.

I'm guessing these rechargeables are charged outside of the device, yes? If it charges its own batteries, then doesn't it also run off the external power? If that is the case, then the diode will prevent the batteries from being charged. If they do get charged in a separate charger, then the diode switch might work, but you would probably have to find a Schottky diode, which has a lower voltage drop. Not something you would find at Radio Shack, unfortunately.
 
dang. Thanks for the answer.

OK....

but am I right in assuming that I could do it without the batteries? is there some goofyness with resistance through the batteries it is expecting that will cause it not to work with a direct 3v connection in place of them? I could live with the resetting when I unplug it I guess.
 
It should work okay as long as you take the batteries out. Just make sure the polarity is correct and it should work fine.
 
Dave, you seem to have a better background in electronics than me, so feel free to correct me, but as a thought, wouldn't it be possible to hook it up to a well regulated 2.5 V source? The actual output of those batteries isn't any more than that, so the device should work on the lower voltage. At the same time, with a lower voltage, the batteries shouldn't charge?


One other thought, the device could be hooked up to to larger batteries (C cells) that would give it a much longer operating time without the complication.
 
My thoughts were if you placed a resistor in between the batteries and the external power source you would be able to limit the current flow to some extent. But due to the voltage drop across the resistor you would have to increase the voltage out of the external power source slightly.

If you want to get really fancy, you could connect 3 batteries and build a voltage regulator (with diode on the final output stage) that would reduce the voltage to the required levels for the device, and isolate the external power from the batteries.

OR, build a recharging circuit and have the external source recharge the batteries while it's connected.
 
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