Backup for battery powered device

I would like to have battery powered device that will run continuously with small (~100uA average) current consumption. When batteries are nearly empty they will be removed and recharged. I would like to have some "backup" so the operation will continue while main batteries are being recharged. I planed to use 2 unregulated NiMH batteries as the main power source and recharging them when they are close to 2V. In such setup I cannot figure out how I could reasonably use a capacitor/another batteries/something else as the backup. Do you have some idea?

What about something simple like a pair of battery holders wired in parallel? Insert the new batteries in Holder B before removing the batteries from Holder A.


It needs two sets of batteries and educated user. I hoped for more "user friendly" solution. But if there is none I can do it this way (or recover from power down).

(or recover from power down)

You need to have that capability in any case.


I decided to use 2 AAs as the main power source and a lithium coin cell as backup when main batteries are remomed to be replaced or depleted. I don't know how to connect the batteries. Connecting it by diodes won't work because the lithium battery will discharge into the main batteries. I have some ideas but I am not sure if it would work. Is there a common solution for this?

One common solution is to use blocking diodes to isolate two DC power sources. Use Schottky diodes for the lowest voltage drop.

MOSFETs are another possibilty but the circuit is more complex.

One common solution is to use blocking diodes to isolate two DC power sources.

It won't work - the backup battery will discharge together with the main batteries (battery with higher voltage will supply current). I want the main batteries to supply current even when they have 2V and the backup battery is still 3V.

Silly requirement.


Silly requirement.

I don't think so - the backup should only be used during changing/charging the main battery
so that the backup state-of-charge is predictable.

Implementing it is tricky though, ie its not going to be easy.

Implementing it is tricky though, ie its not going to be easy.


But maybe a physical solution is the answer - make it impossible to remove the tired batteries without flicking a switch which connects the back-up battery?


The "silly requirement" is that the backup battery must be 3V, when the circuit runs on 2V.
Suggest using a supercap.

I wanted to use 3V lithium coin cell because it has the lowest self discharge from common batteries. On the other hand I would like to use 2 AA batteries because of money/cost, and simplicity.
Since voltage requirements of my project are 2-3.6V (nRF TX) I think 2 AAs are great to power it and I would like to drain the AAs as much as possible.

Just thinking out loud ...

Suppose the power from the AA cells and from the coin cell is switchable using two MOSFETs. Normally the MOSFET for the AA cells is ON and the other one is OFF.

Use the Arduino to measure the voltage of the AA cells. When it falls below some threshold it turns ON the coin cell MOSFET and after that turns off the AA cell MOSFET.

It continues to check the AA cell voltage and when new cells are installed it switches back to using them.

(I won't be surprised if there are 100 reasons why this would not work)


PS I still prefer the simple solution I suggested in Reply #1 :slight_smile: