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Backup battery convert 3.6v nicad to 3.7v li-ion Answered

Have a slot machine MPU backup battery 3.6v nicad. Would like to replace w/ 3.7v Li-ion NON-RECHARGEABLE and NOT have the battery charge. Was thinking I might be able to insert a diode in line with the battery to prevent the circuit from charging. Am I on the right track?

Thank you for your time

Jim B.


Jack A Lopez

4 weeks ago

You are correct in thinking a diode place in series with a battery, will only allow current to flow one way, and thus prevent the battery from being charged.

The only caveat is, a forward biased diode has a non-zero voltage drop. The magnitude of this drop depends mostly on what kind of diode. For the ubiquitous silicon diode, this forward drop is about 0.6 volts.

So the effect of putting a silicon diode (0.6 V drop) in series with a 3.7 V battery, is that you now have a 3.1 V battery.

However, the difference might not matter, because the MPU backup circuit, or whatever the load is, might not care about the exact battery voltage. It might be the case, that it will be happy with any DC voltage in a wide range, e.g. 2.5 to 5.0 V.

I mean that would be typical, although I do not know the exact range for your device. If you look up the data sheets for the ICs involved, those data sheets will usually state what range of voltages the designer of IC says are acceptable.

Also battery voltages are not exact either. A 3.7 volt Li-ion cell also has a range of output voltage, something like 2.0 to 4.2, depending on its state of charge.

Another thing to consider is, there might NOT be the danger you think there is, in allowing this MPU backup circuit to attempt to charge whatever weird battery you connect to it.

I am kind of guessing whatever charging current this thing can manage is a very weak trickle.

Also if there is worry about hurting the backup battery, why not use something cheap, like disposable alkaline cells.

In my home country, there is place called Dollar Tree (r).

They still (at the time of this writing) sell AA size, and AAA size, batteries, in packs of 4, for only 1 USD.

Coin sized cells might work too. Usually memory backup circuits only want a very tiny amount of current.

Which makes me think: If you actually went to the trouble of measuring that current,


then that number could help inform your plans for a replacement battery.