Battery Test - In Parallel considerations?

[CharlesHoward]

I had the Auto Parts store clerk test my Passenger Side Battery that is original to my 05 this morning as its been corroding blooming on the negative post for some time. The Drivers Side Battery was replaced over a year ago as the positive post was broken while removing the terminal...



Anyhow, Clerk just connected their Battery Tester across the Passenger Battery and "it" measured 13. 02 Volts, 1008 CCA, Rated 750CCA, Temp 78 degrees. Good to go!



However, on reflection, shouldn't this Old Passenger side Battery have been disconnected from its newer Driver side in parallel mate?

And if so, which cable to disconnect: the negative or positive?



Thanks for expert advice!

 

[BGlidewell]

I always replace batteries in pairs. Was there a corner case for replacing just the driver side? You can disconnect the passenger side to test it as long as you are careful not to ground the positive side with your tools while you're working with it.

 

[Hoefler]

In a word, YES!! Batteries should be/need to be tested separately. Have it checked again, with the two disconnected from each other. Like said above, always replace them in pairs, the old one will drag down the new one and the alternator can overcharge the new one because the old one will require more current to charge it.

 

[Road Dog]

It's safer to disconnect the negative lead. Disconnect the positive and you'll have a loose wire with a lot of juice from the other battery.