Diode help needed

[hightechredneck]

I am looking for some help with what type of diode I need for a project. I ahve an internal and external battery source in a device, to increase the battery life. The internal source is a rechargeable battery and the external source can either be rechargeable or not. The voltage is less than 6 volts. and these are AA internal and C size external batteries I am talking about. I want to only allow current from the C sizes to go into the AA batteries and not be able to back feed from the AA to the C batteries. I want it to not backfeed at all, and still feed the AA batteries until the device depletes the AA and the C cell batteries. Currently, the device is depleting the AA batteries and when the voltage gets to a certain point, the batteries seem to "fight"

Can anyone tell me what type and size diode I would need, and if one in the positive leg is enough to accomplish this, or would I need a different set up. My goal is to keep the AA's Internally charged as much as possible for the longest possible amount of time.
Thanks

 

[Harald Kapp]

Putting the batteries in parallel is not a good way to charge the internal battery. Neither is a diode suitable due to the voltage drop across it.

With regard to AA or C size: it is not the size that maters in this case, but the battery voltage. If the voltages match, then the current will be split between the batteries and both will be discharged simultaneously. If the voltages don't match, the battery with the higher voltage will inject a current into the battery with the lower voltage. This will drain the high-voltage battery and possibly destroy the low voltage battery.

From your description I take it that you want a charger that uses the energy from the external battery to charge the internal battery. The charger must be suitable for the respective battery chemistry (e.g. NiCd, NiMh, Li-Ion etc.). Here is an overview along with a list of battery charger ICs. You can get equivalent ICs from other manufacturers, too.

 

[hightechredneck]

Putting the batteries in parallel is not a good way to charge the internal battery. Neither is a diode suitable due to the voltage drop across it.

With regard to AA or C size: it is not the size that maters in this case, but the battery voltage. If the voltages match, then the current will be split between the batteries and both will be discharged simultaneously. If the voltages don't match, the battery with the higher voltage will inject a current into the battery with the lower voltage. This will drain the high-voltage battery and possibly destroy the low voltage battery.

From your description I take it that you want a charger that uses the energy from the external battery to charge the internal battery. The charger must be suitable for the respective battery chemistry (e.g. NiCd, NiMh, Li-Ion etc.). Here is an overview along with a list of battery charger ICs. You can get equivalent ICs from other manufacturers, too.

Thanks for the response! that is way above my knowledge of electronics components...
the project is home made trail cameras. The camera has 2 AA inside that is 3v total. We use external cells to increase the time that the camera can be out in the woods. This method works, but for some reason drains the internal bateries fast, and the externals keep them charged. Until the external batteries reach a certain voltage. then they will not charge the internals, but the external batteries still show about 3/4 charge when hooked to a voltmeter.
I was just thinking that it was a possibility that the internals were "backfeeding" and drainign once the externals dropped a bit in voltage.
I was hoping a simple diode would do this, but it looks like it is going to be something more involved than my knowledge.... I wonder if there is something I can have built that would serve this function...

 

[Harald Kapp]

but the external batteries still show about 3/4 charge when hooked to a voltmeter.

You should not judge the charge state of a battery from the voltage. Even an almost empty battery can show full voltage if not loaded (no current drain).

How do you connect the external batteries to the cameras? Do you wire them directly parallel to the internal batteries? Or do you use a built-in charge port of the camera? If so, has the camera a built-in charger function?

If you're worried about the different discharge rates of the external and internal batteries: can you operate the camera with only external batteries?

If not, here's an aunconmventional idea: build adapters.
For each AA cell in the camea cut a piece of round wood the size of an AA battery (length, diameter). Put a thumbtack on each end of the stick. Solder wires to the thumbsticks solder the wires to an external battery holder, for each AA battery one pair to the corresponding battery in the external holder. Put the adaptes into the camera's battery compartment and feed the wires to the external battery holder. Now you can have external C-cells or D-cells supply power to the internal AA port of the camera.