# Secondary car battery voltage

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.

• Clarify what points on your schematic are being measured, and with what instruments, under what conditions. Is 'accessory' in the text the same as 'accessory' in your diagram? Use schematic numbers for better clarity. Given the accuracy required of vehcile electrics (like not much) I wouldn't be surprised if one meter read 12.8v and another 14v. With high charging currents, it's possible to have drops on ciductors to get the difference. – Neil_UK Jun 1 '17 at 6:15
• How long and what guage cable? What's the load on the aux battery during measurement? – winny Jun 1 '17 at 7:16