I have an accessory battery for powering devices when the vehicle is off. The Secondary battery is connected through a battery isolator. Each battery is separately connected to chassis ground. When the vehicle is turned on the primary(LA,12v,140AH) battery shows 14v across the terminals, the secondary(AGM,12v,80AH) 12.8. Upon throwing the switch, both batteries show 12.8 across the terminals. A meter in my 12v accessory shows 14v out from the alternator.
Measurements are taken across each battery as you would to check the voltage of a battery. All grounds are separate physical connections to the chassis. A Fluke 289 is used to measure voltage. The accessory is the voltage from the cigarette lighter. This runs through a splitter which has a meter on it, measuring its input. No extra load is placed on either battery during the testing. The cable from the primary battery to the alternator is a 2 ft run of 2/0AWG, the cable from the primary to secondary is 10 ft of 2AWG wire.
Shouldn't both batteries still show 14v across the terminals? In either case why is this drop occuring? Additionally, if I do not see the 14v across the batteries this means that niether battery is charging correct.
Best Answer
When you connect two batteries in parallel such as when your relay turns on, the two batteries will "fight" each other to equalize the voltage. Essentially, the higher voltage battery is trying to charge the lower voltage battery. If the lower voltage battery cannot take any additional charge, the combined voltage will be roughly the average voltage of the two batteries.
The word "roughly" is used because it actually depends on the internal resistance of each battery, the resistance of the wiring between the batteries, the voltage and amperage capacity of the alternator, and the state of charge of both batteries.
In general, I would say your circuit is working as expected given the possible variations of the installation.