FONA 808 Charging Circuitry Modifications

[hoolby]

Hello!
I need some guidance regarding a modification I want to make to the charging circuitry on the fona 808 GPS+GPRS shield.

My device should run continuously off the battery(connected to the fona shield) until it reaches a certain threshold (lets say 20% battery left). After that, It should change between CHARGING and RUN with software.

What I'm thinking of is desoldering the switch and connecting 2 MOSFETs in its place, that I can then operate via the Arduino pins.

If you have any input or other solutions I'll be more than glad to hear it.

Thanks for your time!

[adafruit_support_mike]

The general idea is okay, but you'd need P-mosfets with low gate thresholds to make it work.

The value quoted as Vgs in mosfet datasheets is usually the value that lets 100uA flow from the source to the drain. To get really low channel resistance, most power mosfets want a gate voltage at least 4.5v (or -4.5v for P-mosfets). That's okay in a 5v system, but doesn't work well in 3.3v systems.

You can get power mosfets with lower gate thresholds, but you have to read through the datasheets to find them.

The other practical note is that you'd want to make sure to use break-before-make switching when you switch from one mosfet to the other. Having both turned on at the same time can do bad things to the LiPo and FONA, so always turn one off before turning the other one on. A gap of a few microseconds is plenty.

You'll get a small break in the supply power when that happens, so add a large capacitor to the switched connection to keep things running during the both-mosfets-off period.

[brethano]

what if we use a P and a N mosfet, one for charging and one for running. the N-mosfet has Vds=50v, Id=30A, Rds=40mohm and Vgs(th)th= 2 and 4 volt. and P-mosfet has Vds=100v, Id=23A, Rds=117mohm and Vgs(th)=2 and 4volt?
we still need capacitor?