dual battery voltage control relay

[shooter]

Gday all,

I am after some info on a voltage control relay for my dual battery system. I've installed one of the supercheap auto $80 kits and it has a manual override for engaging the second battery when the main is flat. This is done through ground switching a wire off the voltage control relay. Now my problem is that the relay won't switch to the second battery when the first is flat. So I propose is it a manufacturing fault or is my voltage control relay gone. Either way it would be good to know what other relays could be used or if they have replacement relays for these kits.

[-Scott-]

I'm not sure I understand what you're doing, or what you expect.

I'm not familiar with your Supercheap kit, but a 4wd dual battery system will typically connect the "auxiliary" battery to the starting battery when the starting battery is charging, and disconnect when it's not charging. In this way, you can discharge your auxiliary battery as much as you like, without risking your starting battery.

I've never heard of a system that will automatically select the auxiliary when the starting battery is flat; I've always understood that to be something that you need to do manually, using an optional switch.

From what you've described, I don't think anything is wrong. Can you tell us a little more about what you're doing, and what you expect to happen?

[Shadow]

I'm not sure I understand what you're doing, or what you expect.

I'm not familiar with your Supercheap kit, but a 4wd dual battery system will typically connect the "auxiliary" battery to the starting battery when the starting battery is charging, and disconnect when it's not charging. In this way, you can discharge your auxiliary battery as much as you like, without risking your starting battery.

I've never heard of a system that will automatically select the auxiliary when the starting battery is flat; I've always understood that to be something that you need to do manually, using an optional switch.

From what you've described, I don't think anything is wrong. Can you tell us a little more about what you're doing, and what you expect to happen?

I think what he means is, main battery is flat, and he presses the manual over-ride switch, and nothing happens because the main battery hasn't got enough power to throw the relay in and connect the two batteries together.

I don't think its a design flaw, I think they are relying on the main battery having enough power to pull the relay in, which is not unrealistic, even from a flat battery.

[drivesafe]

Hi shooter, just how low is your cranking battery?

If you have a multi meter, turn your ignition ON, but don’t try to start the motor.

With the ignition on, measure the voltage at your cranking battery terminals.

If you have less than 8 or 9 volts, as posted above, you probably don’t have enough power to energise the relay.

[shooter]

Thanks drivesafe and yer you guys know what im on about, it is the fact that i can't energise the relay to engage the aux battery when the cranking battery is flat ie 9 volts.

I intentionally ran it flat the other day to test if i can manually connect the aux incase it happened in an emergency, which as you can tell it didnt work.

Now im going to try and see if i can manually overrride it when both are charged, if this works then i know its working and thats the way the kits works BUT I am after info on how to use my aux battery incase the main runs flat from headlights being left on etc.

[DamTriton]

It should only need a brief connection of the two positive posts by a modest sized link to get the power across to the cranking battery enough to energise the relay, at which time the relay would take over.

The whole setup you seem to have confuses me a bit because the relay is there to STOP the aux curcuit items from flattening the main battery. The only way you would be able to really flatten the cranking battery would be if you left your lights or ignition on. If the battery is running itself flat then you need a new cranking battery.

[shooter]

Yeah that is how the set up works, it disconnects the aux battery when the cranking battery gets flat. Now the way it is set up is the aux is just added onto the existing wiring circuit ie had one battery to run everything and then added the dual battery setup .

So everything is still run off one battery and the aux is going to be used for camping fridges etc. Now if i leave my headlights on by accident and it drains my cranking battery i want to use the aux to start the car.

I thought this was done by pushing the manual overrride switch when the cranking battery was flat but it doesnt work. I can put in a manual bypass that links the 2 batteries through an isolator but it'd be a hell of a lot simpler to just push a momentry button to engergise the relay and enage the solenoid so both batteries are linked when the cranking battery goes flat.

Can this be done and how?!

[-Scott-]

OK. Now I think I've caught up.

Without seeing the wiring diagram for the controller it's hard to know what your options are. If you can access the +ve terminal for the relay coil, and wire that through your momentary switch, then you should be able to energise the relay from your auxiliary battery.

Or go with Gary's suggestion of briefly shorting the two batteries to energise the relay and connect the two batteries. If you do this BEFORE you try to crank, the "short" shouldn't carry much current, so you won't need a big wire or have large currents or big sparks. Worst case, stick something like a 1 ohm/10W resistor in your "short" line - it won't be able to carry much current.

[shooter]

thanks, im going to install a voltmeter or LED battery indicator and monitor how both batteries charge up. At the moment it looks the relay isnt doing anything when it falls below 12.5 volts or passes 13.5 which to me says its gone So im going to install a toggle switch to the active of the solenoid and switch the aux manually until i can get another VCR. any tips or advcie on VCR's?

[drivesafe]

Hi shooter, your Calibre solenoid is not that good in the first place and while it should have a short blue wire for bridging the batteries together for emergency jump starting, again, if the voltage of the cranking battery is too low, the solenoid may not work.

This is actually a good feature as it rules out trying to join a good battery to one that is possibly shorted internally.

The same goes for trying to link the auxiliary battery by connecting a wire between the auxiliary battery and the cranking battery terminal of the isolator, especially the suggestions that only a thin wire is needed.

This is a sure fire way to melt cable in a bad situation.

Get you cranking battery charged, then, with the motor off, see if the jump start feature works.

Also, if I read your post correctly, you have posted that the isolator cuts out at 12.5 and cuts in at 13.5. if this is the case, then it’s working as it was designed too.

[shooter]

thanks,
And yeah the relay isnt engaging whatsoever and my aux battery is going flat. The cranking battery is fully charged and the emergency jump start feature isnt working. So im going to run a few more tests and see what happens.

[shooter]

for future reference the VCR relay was fine but the solenoid was faulty replaced with and "ABR sidewinder unit" for $80 and then got the LCD screen and neutral switching battery control unit( which tells you voltage of each battery and indicates if they are charging) for another $80 and haven't had any troubles since and recommend this unit although additional controller isn't needed.

[drivesafe]

Hi shooter and don’t put too much faith in battery voltage monitors, they are more of a waste of money than any real use.

They only tell you what the voltage is at the battery but give you no real indication of whether the battery is actually charging and unless you have a fair idea of what the voltage is actually indicating, and most people don’t know, a high voltage reading at an auxiliary battery can actually mean the battery is stuffed and not taking any charge, while a low voltage reading contrary to looking like something is wrong, can actually indicate the battery is charging well.

To get a proper indication of the correct state of a battery, you need both the voltage and the current to be monitored and monitored over a period of time and then all the data needs to be calculated and then you can see what your batteries are doing and what state they are in.

Nothing short of a full battery monitoring system can do this but at a high cost, and usually a lot higher than is viable for most RVers.

A $10 multi meter from the likes of Dick Smiths, is by far a better investment and can tell you a hell of a lot more about all your batteries and can be used for many other purposes.