I monitor my aux battery with two Doc Wattson charge current, voltage and ampere hours meters of this type:
One for charging, one for discharging.
This requires a 1 milli-Ohm shunt between the battery's negative terminal and the chassis ground. The two Doc Wattsons must be rid of their enclosures and the original shunts and mounted into a suitable panel.
Nice project that took some work.
So I have a permanent display of voltage, power, current in Ampere and the accumulated charged and discharged Ampere hours. When charging, the left instrument shows the charge current and the right one shows zero. When discharging, the right instrument shows the discharge current and the left one shows zero. When the motor is stopped, there is always discharge. When the motor runs it depends on the load and rpm whether there is charge or discharge.
In winter, I get about 15 - 30 A of charge current when the battery has been discharged considerably before. In summer, it's about 20 - 40 A. But the current decreases after about 20 minutes to about 10 A. Even with 14.7 V charge system voltage the battery will not get fully charged quickly. This is a problem with all these super duper batteries, they provide lots of current but the charge currents are low. The Yellow Tops that I use are among the best in this respect. I had a Varta AGM 90 Ah that was tired from the beginning.
Music listening with the motor off. The right instrument shows the discharge current:
Motor runs, battery is being charged. The left instrument shows the charge current.
The car has two batteries: Aux battery (front of photo) and starter battery:
For the measurements, the aux battery needs a 1 milli-Ohm shunt:
The instruments in the original state:
Original shunt removed:
Two instruments installed in a panel and wired:
It takes four wires from the battery to the instruments, shielded.
1) Shield, chassis ground.
2) Plus 12 V (with a fuse, consumption only about 5 mA)
3) Shunt 1
4) Shunt 2
I only had a coax cable handy with shield and two inner wires and wired the plus separately. A coax cable with shield and three inner wires would have been nicer.
The third, simple volt meter shows the voltage of the starter battery.
Very well working battery isolator that does not require a signal connection to the alternator:
Last edited by JuniorGeezer; 11-30-2011 at 02:45 PM.