During testing their electronic products, connecting the charger to an acidic car battery, inadvertently or in a hurry when connecting power, a polarity reversal occurs, which in some cases can adversely affect the tested device, battery. (Hindyk microcontrollers, explosion of capacitors, etc.)
To protect their circuits and memory from destructive polarity reversal, many include a diode in the circuit (conducts electricity in one direction), to prevent the current from flowing in the opposite direction, thereby eliminating the harm to the electronic components of your device. But the problem with this approach is that this circuit will not work until you change the polarity, which is sometimes not possible (during testing).
In order not to be soared with polarity reversal, I offer a simple little circuit for testing and connecting your electronic masterpieces and batteries auto. One has only to include in the chain not a big superstructure. You no longer have to worry about the polarity reversal and search for a DC adapter, which is somewhere there at the right time ...
This circuit uses a rectifier bridge that is built into all power adapters. Just plug in the first power supply you get in the circuit (without worrying about polarity). Connect direct or alternating current, this is not at all critical for this circuit, plus or minus is also not important.
The only drawback of the circuit is that the rectifier bridge eats up some voltage, you will need to take this fact into account when testing and connecting the battery, (add voltage to the memory), if this is critical for you during testing.
You can use a ready-made bridge that meets your needs.
Or make from 4 diodes the power you need
Cheap and cheerful! Good luck