Battery Questions

[Mr Hardware]

So Big Red decided on tuesday that she didn't want to start

So i jumped her off the commodore and she ran fine for that day
then i tried her again yesterday, started fine
went to drive her today, row-row-click-click-click.

Now the battery is a Comet 520CCA that's only just over 2yrs old

Now I haven't done much troubleshooting, but I want to find out whether it is the battery or some kind of drain coming from the car. It's not the Alt, my multimeter tells me that she's giving the battery a healthy 14.10v whilst the car is running and sitting on 12.15v whilst the car is off.

Obviously the easy way is to check is to charge the battery and simply take the leads off the battery terminals and leave it for a couple of days, and if it starts its a drain and if it doesn't it's the battery.

However i'd like to use the car and resolve this before a couple of days time.

A mate of mine said to me, get your multimeter, take the negative terminal off the battery, change the multimeter positive probe to the other socket on the multimeter, then switch the multimeter to amps and put the positive probe on the negative battery terminal and the negative probe on the negative lead (or vice versa? doesn't matter?). If it registers anything other than 0 then i've got a drain.

Is this correct?

Also, if need be, where's a cheap place to buy batteries? I'm sure the servo or supercheap isn't necessarily the best place, right?