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Battery Circuit

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.

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    \$\begingroup\$ 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. \$\endgroup\$ – Neil_UK Jun 1 '17 at 6:15
  • \$\begingroup\$ How long and what guage cable? What's the load on the aux battery during measurement? \$\endgroup\$ – winny Jun 1 '17 at 7:16
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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.

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  • \$\begingroup\$ For confirmation, the secondary battery should have a full charge. Both batteries are rated for 12V, though they have different capacities (140AH and 80AH respectively). Even if I were to discharge the secondary, I should still expect to see <14v on either? If so, is the alternator still charging the batteries? \$\endgroup\$ – Revots Jun 1 '17 at 21:56
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    \$\begingroup\$ 12.8 is within normal ranges for a charged battery. A vehicle with alternator running would normally be about 13.8 volts. So everything appears within tolerance to me. Try measuring from the left side of your fuse in your diagram to the positive side of the aux baterry with the relay on. Any voltage there is a drop due to the relay/wiring/fuse. \$\endgroup\$ – Glenn W9IQ Jun 2 '17 at 3:13
  • \$\begingroup\$ You should be able to see a normal float charge of 13.8v if charged. If you do in fact have a voltage drop from one current source to the next, the battery that is normally sitting at 14V will be feeding current out to the other battery to 'charge' it, which lowers the voltage you're reading out on that battery. Try checking the current flow from your primary battery to second, as well as the resistance between them. Useful information in this case. \$\endgroup\$ – schizoid04 Jul 5 '17 at 8:45
  • \$\begingroup\$ @Revots , how much current is your alternator putting out? If you've got (140 + 80)Ah of battery capacity, is it possible your alternator is not putting out enough current at idle to raise the voltage of the batteries up to 13.8V? What happens if you rev the engine at higher RPMS for several seconds / a minute or two? Voltage rise much? If your starter battery is 80Ah, adding a 140Ah battery is close to tripling the total capacity of your batteries, may take significantly more to rise to float voltage, right? (Correct me if I'm wrong I'm new) \$\endgroup\$ – schizoid04 Jul 5 '17 at 8:47

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