erm... a diode ?, if there is a diode permanently on the battery it is protected, if you want the battery charger to have "no polarity" or "universal polarity" then a rectifing diode bridge will accomplish this, although it does mean that you will loose up to 1.5 volts in voltage drop which in the case of most car battery charging units means 3 watts or more power wasted. the bridge is basically used in the oposit way you would normally conceive its use to be.
if its a physics question its fairly simple (more bluntly do your own homework and have some imagination) I would think that in physics about all the componenets you've been told about are resistors, capactors and well diodes so it doesn't leave much leeway for snything sophisticated although I would say thats taking physics a bit far into electronics