When joining old batteries of exactly the same construction with new batteries, some tests MUST be done before hand to see if the old batteries have aged their way out of the same league of chemistry.
A) Charge and equalize 6 cells of the old batteries until specific gravity reaches absolutely full, brand new 1.280 for instance. If gravity varies by more than .05 this means that marriage is a no-no. The bigger the variance the bigger the complaint.
B)If batteries pass the specific gravity match test, perform a 50% CCA discharge using a carbon pile load tester on 6 cells of the new battery and 6 cells of the old battery. If voltage varies by more than three tenths of a volt at the end of a 15 second discharge, the batteries are not compatible.
You will have to hunt up a genuine battery shop to perform the test. If they own an 800-amp AUTO METER adjustable carbon pile load tester, or SUN VAT 33 or 40 or even a Harbor Fright load tester they can do this test.
