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Interested to learn if this is a bad idea and why.

Specific case...

I have battery powered wireless doorbell receivers. They run on 2 AA batteries (e.g. ~3VDC with alkaline or 2.4VDC with NiMH). The problem is that every time the batteries run out I have to do a little "reprogramming" dance after inserting the new batteries.

Since these receivers are stationary and near outlets I figured I could just take the batteries out and solder in a 3VDC wall-wart supply.

However anytime these are unplugged or there is a power outage, they would need to be reprogrammed.

So I was thinking...if I leave the Alkaline (or NiMH) batteries in there then theoretically the wall supply (3VDC) being higher...the batteries wouldn't discharge unless unplugged...then they would function as a backup so I don't have to reprogram the bell if I unplug it temporarily.

Originally I was thinking of just leaving the NiMH in there since they are rechargeable anyway...but with a constant 3VDC applied to them it might kill them in short order (would require some type of charging management circuit I guess)?

Then I was wondering what the consequence would be if I left alkalines in there (low self-discharge compared to NiMH). Their full voltage should be around 3VDC...but if they discharge a bit then their voltage drops off pretty quickly and I am not sure what happens at that point if they have 3VDC applied across them.

Interested to learn as well as have recommendations about what to do or not do here. Thanks.

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2 Answers 2

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The simplest solution is indeed to keep the batteries. However you do not want to connect the 3V wall wart output directly in parallel with the batteries. The alkaline batteries would be trying to get charged from the supply and they are not made for recharging.

Instead you want to arrange a simple circuit like shown below to allow either source to supply power to the load but not allow either one to back feed to the other.

enter image description here

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    \$\begingroup\$ I would advise for using a wall wart with a bit higher voltage (3.3V is common enough). The diode drop will cancel the excess voltage anyway, and it will avoid the batteries taking over and discharge more than necessary, in case the wall wart voltage is a bit lower than expected. \$\endgroup\$
    – dim
    Nov 7, 2019 at 14:39
  • \$\begingroup\$ Thanks Michael. Appreciate the explanation and clear recommendation. I have some diodes laying around as well. @dim...good point. I will have to check the output on the wall-wort vs the alkalines...worst case the batteries run down a bit and the wall wort takes over? Alkalines seem to have a pretty fast discharge curve (below 1.2VDC when over 50% capacity). \$\endgroup\$
    – mattpr
    Nov 7, 2019 at 16:23
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    \$\begingroup\$ @mattpr - Do not use just any old diode. You want to make sure to use a low drop Schotkey diode. \$\endgroup\$
    – Michael Karas
    Nov 7, 2019 at 17:29
  • \$\begingroup\$ Thanks for the reminder. I have some 1N5817s laying around, so I figured I would use those since you recommended them and this application should be significantly under 1A. \$\endgroup\$
    – mattpr
    Nov 7, 2019 at 17:32
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I would suggest putting a supply control with some MOSFETs (where the gate is controlled by presence/absence of wall plug voltage). A big filtering capacitor after that might be a good idea to cope with the switching time of disconnecting one source to use the another.

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