Help with fog light relay/switch install on dual batteries system

[allochris]

I just can't figure out why my setup with dual batteries, solenoid, a relay/switch kit, and fog lights is giving me trouble.

What works:
Fog lights only works well when solenoid is turned on, connecting both batteries together

-Main Fog light power supply is hooked up to the spare battery.
-Solenoid is activated via a ACC powered wire.
-Fog light Switch power supply (labelled as: "---Good---") is hooked to 3rd solenoid terminal, together w/ & powered by the ACC powered wire.

Main Problem:
Fog light cannot light up properly(turns on, but barely/really dim.) when Fog light Switch power supply is also hooked instead to the spare battery (labelled as: "---Weak---"), with the solenoid in "OFF" position disconnecting the connection btwn both batteries.

-Main Fog light power supply is hooked up to the spare battery.
-Solenoid is not activated, cutting off the batteries connectivity.
-Fog light Switch power supply is hooked instead to spare battery, labelled as: "---Weak---".
-Turn Fog light Switch "On", relay clicks, but fog light barely lights up.

Alternative:
I also tried adding a 40A circuit switch coming out of the spare battery. The main Fog Light power supply is then
attached to the "AUX." of the 40A circuit switch.

The problem, is that when I tried attaching the Fog Light Switch power supply also to the "AUX." of the 40A circuit switch, the 40A circuit switch started buzzing. Is this a sign of faulting from short-circuit or current overload?

I just can't figure out the logic behind this, maybe related to the law of the "Path of least resistance"?

Picture of current setup:

Both batteries are charged.
Main battery: Optima redtop
Spare battery: Marine non-sealed conventional type
(Yes, I know i'm not suppose to mix batteries )

 

[Rando]

I am not sure what you mean by "40A circuit Switch"? It looks like a circuit breaker, in which case if it is buzzing I assume that means it is tripped (or in the process of tripping). This would indicate to me that you have a short in you fog light wiring somewhere down stream of the circuit breaker or your fogs are pulling a LOT of current. This would explain why your lights are dim, there is a short (not a dead short but a lower resistance path to ground) which is pulling down your aux battery voltage. What wattage are your lights? Your aux battery is probably fairly low on charge (check it with a voltmeter), but when your main battery (and alternator) are in parallel the circuit voltage is
higher which makes your fogs brighter.

First thing to do would be to measure your aux battery voltage while isolated and then check your wiring carefully for some sort of short.

 

[allochris]

... a circuit breaker, in which case if it is buzzing I assume that means it is tripped (or in the process of tripping).

Yes, circuit breaker is what i meant, & it only trips/buzzs if i manually connect the Switch Power Supply wire (whether on "ON" or "OFF" position) to the positive lead of the spare battery.

Conversely, if I removed the 40A circuit breaker & connect both Switch Power Supply wire & 55W Fog light power supply wire together to the positive lead of the spare battery, the fog lights then barely turn on when relay switch is "ON".

This would indicate to me that you have a short in you fog light wiring somewhere down stream of the circuit breaker or your fogs are pulling a LOT of current.This would explain why your lights are dim, there is a short (not a dead short but a lower resistance path to ground) which is pulling down your aux battery voltage.

If both relay circuit switch & fog lights have the same power supply (i.e single battery) & ground, is it possible that the current tends to prefer to go through the path of least resistance more (I have those mini relay switch circuit board with red & green led indicators), and less through the highly resistance 55w=12v*4.59A lights. (Kirchhoff's current law?)

Both batteries are charged by the way.

 

[Rando]

Okay I miss-understood your issue. It sounds more like a problem with your relay. Do you have any more details on the relay your using? Is it a solid state relay?

Yes, circuit breaker is what i meant, & it only trips/buzzs if i manually connect the Switch Power Supply wire (whether on "ON" or "OFF" position) to the positive lead of the spare battery.

Conversely, if I removed the 40A circuit breaker & connect both Switch Power Supply wire & 55W Fog light power supply wire together to the positive lead of the spare battery, the fog lights then barely turn on when relay switch is "ON".



If both relay circuit switch & fog lights have the same power supply (i.e single battery) & ground, is it possible that the current tends to prefer to go through the path of least resistance more (I have those mini relay switch circuit board with red & green led indicators), and less through the highly resistance 55w=12v*4.59A lights. (Kirchhoff's current law?)

Both batteries are charged by the way.