Simple, elegant voltage shut off - Lipo protection perhaps?

A rather interesting link from John Main of C for Microcontrollers.

http://www.best-microcontroller-projects.com/rechrgbatprotect

Let us know if you use the circuit principle. I already use a MOSFET in a slightly similar way as an on/off switch (Triggered by magnet and reed switch.)

David

HI David
Problem with lipos is you need to monitor each cell in the pack because one cell will always go down in voltage first, you should not really take a cell down below 2.95V or you risk damageing the cell. The circuit that you showed will just monitor the hole pack and that is not the way to do it.

If you had a 3 cell pack of lipos just say 1 cell was at 3.5v and the next was at 3.5v but the last cell was at 3v the 3v cell should be switching the power off to speed controller or what ever but if you put 3.5 and 3.5v together would make 7v plus 3v for the cell that should be turning the power off add voltage up it comes to 10v but if you carried on discharging pack down to 9volts the cell that was at 3volts when the pack voltage was at 10volts by the time it got to 9volts would now be damaged and that is how you would slowly cream cracker a lipo pack. This is just the basic idea that's why it is the best thing to monitor all cells in the pack, say you went to 10 cells now gets worse and becomes more important to monitor the cells
SUB JOHN

Hi John,

In a perfect world it would be best to monitor each cell but it can make it much more complicated.

I think you are speaking more of aircraft applications - many cells and drawing very high amps.

The lipos I have used in subs for several years are only 3 cells and I only draw about an amp. In these circumstances I suspect you don't get big voltage differences between cells and I always balance charge them. I also don't push my luck and sail for too long (I have never had a low voltage alarm.)

So I think in the absence of anything else this type of circuit would give your Lipo a good level of protection.

David