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I have done research on the internet about connecting batteries of different voltages in parallel and it makes sense that it can be dangerous because current from one battery will be flowing into the one that has the fewer voltage. This video explains it https://www.youtube.com/watch?v=44_Uj6D-cEI

But what if you use diodes? For example what if I have something like this:

schematic

simulate this circuit – Schematic created using CircuitLab

(note the battery on the left is a 9v battery connected to a diode and the one from the right is a AA battery connected to another diode. They are connected in parallel to power some load)

My 9V battery has only 500 mah of capacity. I will like to increase its capacity using a AA battery. Will that be possible?

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  • \$\begingroup\$ Batteries must be equal voltage to share supply \$\endgroup\$ Commented Aug 8, 2021 at 21:57

2 Answers 2

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You can't increase the capacity that way. The 9 volt battery will supply all the power until it is almost completely discharged, while the 1.5 volt cell will effectively be disconnected until the 9 volt battery is discharged to about 1.5 volts.

When using diodes like that, the highest-voltage source will supply all the current to the load, until its voltage drops to the same as the lower-voltage source.

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  • \$\begingroup\$ Will a voltmeter at the cathode always show the higher voltage? \$\endgroup\$
    – johny why
    Commented Jan 1 at 11:27
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In addition to what Peter said, this can be (slightly) dangerous depending on the diode you choose, as some diodes allow a small reverse leakage.

If you connected 2 batteries in this way and left them on the shelf for a long time, the AA battery could become overcharged.

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