Battery Light

[battenberg]

I just put in a split charge relay to charge a second battery...

I took the live feed for the relay off the (12v running) live output from the alternator.

now i get a feint battery light on the dash...

alternator seems to be kicking out enough juice to both batteries 13.8v on the meter.

any thoughts please?

 

[The Mad Hat Man]

Relay primary coilis too low a resistance and is not allowing the output to rise to 12volts. Get the relay changed.

 

[battenberg]

Run that past me again please MHM...

Are you saying I need a beefier coiled relay to allow more current to pass through the coil..?

 

[The Mad Hat Man]

no - exactly the opposite.

I reckon the the switching side of the coil is too low a resistance. this would act as too high a load on the lighting circuit and stop that signal going up to 12volts.

As i understand the warning light system....
the bulb is connected between 12volts and the alternator. When the alternator is off (ie engine not running) the alternator earths that side of the circuit and the light lights up. When the alternator starts charging , it outputs 12volts on that wire - the bulb has 12volts either side of it - no potential difference (no volts) and therefore it doesnt light.

If the relay is keeping the voltage low, then there will be a difference across the light and it will glow.
I believe the relay impedance is too low. Contact the supplier and see what they say.

 

[Trewey]

i thought all 12V relays had 100ohm coils?
are you sayin there's different types?

I connected mine (the X Eng 100A one) the same way as battenburg and it works fine. only thing I noticed is that it takes a few more revs to put the charge light out than before.

 

[The Mad Hat Man]

The resistance must be that required to energise the coil. So the impedance might be different. The only reason the light glows is that it must still be seeing a voltage across it. I find that I have to rev a bit more to extinguish the charge warning light, but I understood Batties point to be that the light always glows. I do know of some peeps that put in a higher resistance bulb in to stop a glimmering bulb, but I would contact the relay supplier first and ask their advice. It could be that the other side of the bulb is connected to a higher source than 12volts?

i have just found this...

"5 volt coils normally have a resistance of 500 ohms and 12volt coils are 1000 ohms"
"relay 12 volt coil. Coil resistance is 1050 ohms"

 

[battenberg]

no - exactly the opposite.

I reckon the the switching side of the coil is too low a resistance. this would act as too high a load on the lighting circuit and stop that signal going up to 12volts.

As i understand the warning light system....
the bulb is connected between 12volts and the alternator. When the alternator is off (ie engine not running) the alternator earths that side of the circuit and the light lights up. When the alternator starts charging , it outputs 12volts on that wire - the bulb has 12volts either side of it - no potential difference (no volts) and therefore it doesnt light.

If the relay is keeping the voltage low, then there will be a difference across the light and it will glow.
I believe the relay impedance is too low. Contact the supplier and see what they say.

gotcha... so basically there is a feed to both sides of the warning lamp. and when they balance each other out there is no current flow and therefore no lamp light up. makes sence now cheers...

 

[The Mad Hat Man]

yup - thats it. So if the light lights up , then there must be a flow of current - or more precisely a voltage (potential) difference across the bulb.

 

[battenberg]

so if i measured the resistance of the relay and put that resistance on the other side of the lamp it would extunguish the bulb... thoretically..

could it also mean the alternator could do with a new set of brushes?

 

[The Mad Hat Man]

no.

you just want a higher resistance relay. Contact yo supplier.

A simple check would be to measure the voltage output on the light wire from the alternator with and without the relay fitted - i reckon you will see a voltage drop with the relay fitted. Also measure the voltage the other side of the warning lamp. A "correct" impedance (resistance) value can then be determined.

 

[battenberg]

right.

I fitted a 100A relay in replacement of the 30A one (theory here was the coil would draw more current as it's a bigger unit and possibly have a higher resistance) However, the relay is still protected by 30A fuses both sides which it ok by me.

Now the battery light is on realy bright... it did the reverse of what i understood it was gonna do.

Mmmmnnn... Confused now..

did I totaly misunderstand??

 

[The Mad Hat Man]

V=IR
where V=volts (12)
I = Amps
R= Resistance

as V is fixed (almost) - then to get more current , the Resistance DROPS!

however, having said that, the current switch limit of the relay has no direct relationship to the resistance of the relay coil.

the battery dash light, relies on the resistance of the light filament. It looks like you have to low a resistance on the relay energising coil.

 

[battenberg]

I think now i'm just gonna leave it as it is, but fit a switch in the cab to energise the relay.

that way i can always isolate that circuit on the fly and get the correct feedback from the dash light.

Thanks for your help MHM