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S18V20F9 - Battery protection?


I know this is probably a bit out of scope, but I thought this was perfect for a project and so bought one (the S18V20F9, that is).

Reading the description it even says:

However, what that does NOT address is the problem of NiMH maximum cell drainage… From what I’ve ready, you typically don’t want to go below 1V/cell, and since the S18V20F9 seems to be happy all hte way down to 3V, it would kill a pack of 8-AA batteries, or so I think. :slight_smile:

What is the preferred way to leverage the S18V20F9 unit to maximize the capacity of a pack of NiMH cells but at the same time not destroying them? I wouldn’t ask here so much, but since the product page itself does reference using NiMH it doesn’t seem unreasonable to follow-up on it.



Hello, Steve.

The S18V20F9 regulator does not have any special features to monitor battery voltage or cut it off at some specific point, so you would need to add some additional battery monitoring circuit to get that kind of functionality.



Hello @BrandonM, do you have a hint on how such a circuit could look like? For example I’d like to use the U1V11F3 with 2 NiMH batteries.


Hello, MrBean.

Since you are using NiMH batteries, it is probably not important to have a strict cut-off voltage, so you might be able to use something like a voltage divider between VIN and the !SHDN pin. If you want to implement a cut off like this, you should try testing to see where the cut-off voltage is for your particular unit and when it turns back on. If you have a variable voltage power supply, you could do this by applying the voltage to the !SHDN pin and lowering it until the output shuts off, then slowly raising it back up until it comes back on.

Using that information, you could then determine the right resistors to use so the voltage divider output is less than the cutoff voltage you found when the batteries are at the desired shutoff point. You should also check that when the batteries are charged, the output of the voltage divider would be high enough to re-enable the regulator. There is already a 100k pull-up resistor on the !SHDN pin, so you should only need to add a pull-down resistor to it.