I was wondering if you could help point me in the right direction. I have a Adafruit LiIon charger/manager board for a LiIon battery. I want to put a diode on the rail to prevent reverse polarity from inserting the battery the wrong way, but a typical diode won't work because it not only charges (voltage towards battery), it also is the power source (voltage coming from battery), so it will be blocked one way.
So the question is, what would be an appropriate solution so that it can be charged and be a power source, yet prevent a reverse polarity situation? What should I be looking at besides a simple diode?
For charging single Lithium Ion/Lithium Polymer 3.7/4.2v batteries (not for older 3.6/4.1v cells)
500mA charge current, adjustable from 100mA to 1000mA by soldering in a resistor
If you want polarity protection I would fuse it and place a diode Like a 1N4001 Across it with cathode to Positive and anode to negative.
I would consider a 1.5 amp fuse since normal current should never exceed 1.0 amp. I would use a fast blow fuse. Anyway your reverse protection diode goes across the output not inline with it.
Hi,
even if it were possible to have an output, the use of the diode will affect the battery charge, as there is a voltage drop between the poles of the diode, which will result in a false reading for the chip responsible for charging the battery.
Have you thought about doing something like this:
Is that how you thought of doing it?
Not required. Coin cell inserted backwards (upside-down) will NOT make a connection. No "button" or "coin" cell will, as the neg connects thru the EDGE, not the center. This was solved years ago. ISO9000 requires idiot proofing against polarity reversal.
It's not a coin cell this is charging, it's a 18650 in one of those PCB mount holders. And exactly my concern on the diode. Polarized connectors aren't really possible here, this is actually soldered into the board and I'm using a separate screw terminal for connections.
If the battery is connected correctly, the gate will be at 0V and the source will be at the battery voltage. That means the mosfet will be turned on, and mosfets will actually conduct in either direction almost equally well. So charging and discharging should both work. With a good mosfet and typical 18650 currents, the voltage drop should be very low.
But if the battery is connected backwards, then the gate voltage will be higher than the source voltage, and that will turn off the mosfet. In that situation, if you orient the mosfet correctly, its body diode will prevent reverse current flow.
I think you can use a P-channel mosfet on the battery's positive terminal, or an N-channel mosfet on the negative terminal, with the gate connected to the opposite terminal. An N-channel will probably give you the smallest voltage drop, but the difference is probably not significant.
You won't need the zener diode since you're only dealing with 4.2V. But the mosfet needs to be logic level, with maximum Vgs threshold of 1.5V or less.
You trying to protect what? the battery? Its SOLDERED IN. The charger? It should be protected. The load? A bridge rectifier will protect it from reversal. Just how often do you expect to change the 18650? Every 10 years? 20?
Diode won't work because of charge/discharge. Reverse install might result in enough current flow to blow a fast acting, low current fuse before major damage.
Are you modifying an existing design? Is that why you cannot use a unique orientation holder? A polarized plug would help.
