Battery detection with connected charger

[Lt_Flash]

Hi guys,
I've got a circuit that has a battery connect socket on, let's call it, 'left side' and battery charger IC output connected on 'right side'. The question is how to detect when battery is actually connected taking into account that IC charger is active and periodically ramps up voltage on 'plus' rail to 4.2V trying to detect battery itself? The only solution I was able to come to is to have not a two terminal battery connection, but three contact one and one of the contacts is simply shorted to ground, so when battery is plugged in - we definitely have BGND and I can use resistor divider to indicate that condition. But is there anything else that can be done? Thanks!

[David Hess]

I would probably start by leaving the battery charger output disconnected and applying a microamp current to the output while measuring the voltage.  If the voltage falls from the compliance of the microamp current, then the battery has been connected.

[Lt_Flash]

Thanks, nice idea! Can you help me with schematic? I assume I will have to use BJT and not MOSFET in order to achieve this? Charger is quite easily disconnected with a MOSFET, no worries here. Probably that's what charger is actually doing - trying to apply some voltage and see if there's a drop to detect battery?

[David Hess]

It is not that difficult to do.  A high value resistor to a reference voltage which is slightly higher than the maximum expected battery voltage will work as a current source.

Usually if the output from the charge circuit is disabled, then it will be high impedance and not have any affect on the battery presence test.  Adding out output MOSFET to disconnect the charger may work but should not be necessary.

[Lt_Flash]

Thanks a lot, I'll give it a go!