View Full Version : Where does electricity come from?

03-04-2016, 04:17 AM
I have done some modifications to run the Espar/REST, radio, interior lights and dash cigarette lighters from the aux batteries.
I also have the isolation relay triggered by D+, only when the engine runs.
I have one volt meter for the aux batteries and this morning they were at 11.6v before starting. I'd ran the inverter all night.
Using the scan gauge to read Volts coming out of the engine battery. Before start, the Scan Gauge showed 12.5 Volts.
So I noticed today that as I turned the key to the on position, the Scan Gauge voltage dropped to 11.6v to match the Aux batteries.
So I'm wondering where this comes from? Why would the source to the Scan Gauge change?

03-04-2016, 04:31 AM
A couple of ideas:

- if it is the same voltage, then your isolation relay is not triggering from the alternator
- you might have an aux circuit cross connected to your auto system

Here is an experiment:

- disconnect the aux battery and repeat your experiment - see if there is a change

03-04-2016, 05:59 AM
I know that the isolation relay works correctly, i can hear when it kicks in. I'll try the other tomorrow.

03-04-2016, 07:20 AM
As the engine is cranking and starts, the engine battery voltage is at 12.5V. Once the engine is running the relay is energized from the D+ line, which connects the two batteries together. Because the house battery is at a lower voltage than the engine battery, the two batteries will try to equalize, current will flow from the engine battery to the house battery and the engine battery voltage will drop until the alternator can charge both batteries. The voltage displayed on the scan gauge always measures the engine battery.

Check that the you have the relay coil voltage connected to the D+ terminal 1 and not Terminal 3 on the attached drawing. Page 2.



Eric Experience
03-04-2016, 10:43 AM
I think you are jumping at shadows. When you turn the key on your glow plugs pull the voltage down to about 11.5 so the is probably nothing wrong. Eric.

03-04-2016, 07:30 PM
I agree with Eric: turning the key on kicks in 65 to 90 amps of glow plug load. That will drag any battery down quite a bit.

Quick test: disconnect the house battery. The ScanGauge drop-at-start report probably won't change. (as Ben suggested).


03-05-2016, 12:34 AM
I did as suggested and turned off the aux batteries. After I reprogrammed my radio (I've memorized the code by now), I discovered....
I agree with Eric. :professor: :bash:

03-05-2016, 06:15 AM
Yes I forgot about the Glow plugs:bash: