Now to test that you've made a good connection with the solder. Set your Multimeter to 200 Ohms and test the connection between the main battery terminals and the parity battery. For visual reference, do as shown in the two images on this step.

The Ohm reading you get by testing the connections should show a solid number somewhere around 1 Ohm. This indicates that you've made a good solder connection.

You'll see that the reading I got when testing my negative connection (second image) shows 1.2 Ohms, compared to the much better reading from the positive connection (first image) which shows 0.9. Although it's not perfect, it's all good as long as the number does not variate to far from 1 Ohm. If it does, the connection is loose.

Another test to do is to check the voltage now shown when testing the main battery. You should now see the voltage read somewhere higher than 3v rather than when the old battery would show something more along the lines of near 3 or 2.98. Either way, seeing a slightly higher voltage than before means you've successfully created a Parity system.

I don't know what would happen if you accidentally put your batteries in Serial, but I'm pretty sure your cartridge would explode (or something like that) if it suddenly had 6 volts running through it. This is why it's important to make sure you put the positive cable of your parity battery onto the positive end of the PCB battery area.