Duel battery charging voltage

Ive a projecta duel battery system, I did the after installation check to see if everything was as it said in the instructions and all was good, I noticed yesterday at work that the 35ltr fridge was not working and the my inverter was beeping for low voltage, my question is , when your starting battery is charged and the isolater then changes charging to the aux battery, how many volts should show up on a volt meter on the aux battery terminals? I checked today and its only coming up as 12volts with the engine running. I thought it should be more like 13volts.. I have already checked the alternator and it is putting 13.5 into the starting battery. The battery is in the back of the car which I know is a bit back from the isolator but Ive seen others do this without a problem. Any ideas? thanks.

What size/type of cable did you use?

Ideally you should be getting up towards 14.4 to 14.7V from your car's regulator. You should be able to get ALMOST that to the rear. Things that might interfere:

* The projecta unit may have a fault. At the projecta unit, measure its output voltage - if it's too low, it may be faulty.

* The electrical wiring to the rear may be too low a rating. DC power loses voltage at an alarming rate over distances. You should use 6sqmm (60A) cable to run the length of the vehicle - if you've run smaller, you'll see a decent voltage at the projecta unit and a low voltage at the rear. Fix that by using the larger electrical cabling.

my redarc (and i thought the projector was the same) unit isolates the second battery until the starting battery is fully charged then connects and charges the second battery. you can usually hear the solenoid click over. my second has a volt gauge and charges at 14.4v every time, but if i run it down it may take a few mins to click in. check your starting battery and charge voltage (at the battery & the isolator), that may be preventing the solenoid connecting.

Will's right, the main battery could be the issue, especially if as you say you're only getting 13.5 into the starting battery. It still could be a cable issue to the back battery but if the alternator isn't pumping around 14.4 (as Tony quotes) then the issue could be way back at the start not the end.

I'd start at the beginning and check voltage from the Alternator all the way to the second battery if it was mine.

Thanks for the quick replys, in a bit of a panic as were going away for a long weekend, and with no fridge working i may have a problem, The cable im using is the stuff that came with it, it says its 6B&s (14mm squared).It is 6metres long going from the engine bay to the tray. The negative cable is 3bs(26mm squared) which comes off the aux battery and on to the chassis. I just checked the output of my alternator and its putting out 14 volts. I checked the isolator and that is also putting out 14volts but by the time it gets to the back its down to 12volts. Its really confusing me now. Thanks for any help again.

Where is the isolator, my Projecta had a max cable length of less than 5 meters from the main battery to the isolator. I wanted to put my isolator in the tub, mainly because the D40 has bugger all room under the bonnet but the limit Projector claimed was to close to do it that way. Cable from the isolator to the second battery can be further than from the main to the isolator though.

earth

check your earth as you said it was earthed to the chassis.
I used twin cable and ran the earth all the way to the back as have had earth issues before

would it be showing 14v across the aux battery if the battery was flat (say 12v)?

disconnect +'ve from battery and measure voltage from lead to earth?

KraftyPg said:

Where is the isolator, my Projecta had a max cable length of less than 5 meters from the main battery to the isolator. I wanted to put my isolator in the tub, mainly because the D40 has bugger all room under the bonnet but the limit Projector claimed was to close to do it that way. Cable from the isolator to the second battery can be further than from the main to the isolator though.

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The isolater is in the engine bay. I will also have to check the earth again.

Also the aux battery is new and just had a full charge yesterday, I also got an auto electrican to check the condition of it and it came up as "condition good"

Like d22dave I ran my earth all the way back to the front as well, I'd definitely be checking the earthing next.

"Condition good" doesn't necessarily mean what it implies, it could really mean many things. If the battery is only a small capacity battery running a 35 litre fridge could drain it flat in a few hours, you'd hope any battery in a dual system wasn't that small but it does happen in systems that weren't set up for running fridges.

Also depending how the "condition good" was obtained it may not have taken into account anything more than terminal voltage, a failing battery can still read 12+ volts across terminals with no load but die the second a load is added to it.

It might not be the battery that is the issue but it does pay to remember that what appears to be a fully charged good condition battery may not be exactly what it seems.

d22dave said:

check your earth as you said it was earthed to the chassis.
I used twin cable and ran the earth all the way to the back as have had earth issues before

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As said double check and test your earths. I had a very similar problem and have the battery in the tray earthed to the chassis, turns out i had a dirty earth connection all running sweet now.
Running an earth cable right back to the start battery is recommended but at the moment i haven't had any issues so earthed to the chassis will do for now.

^ second that. The only way to truly test a battery is to monitor its voltage as it is discharged over time. Just checking the voltage at one point in time doesn't really tell you a lot.

KraftyPg said:

"Condition good" doesn't necessarily mean what it implies, it could really mean many things. If the battery is only a small capacity battery running a 35 litre fridge could drain it flat in a few hours, you'd hope any battery in a dual system wasn't that small but it does happen in systems that weren't set up for running fridges.

Also depending how the "condition good" was obtained it may not have taken into account anything more than terminal voltage, a failing battery can still read 12+ volts across terminals with no load but die the second a load is added to it.

It might not be the battery that is the issue but it does pay to remember that what appears to be a fully charged good condition battery may not be exactly what it seems.

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Thanks for the good advice ,Its a 75 amp battery and im fairly sure it is ok, but what you have said to me has made me think again about the starting battery, because i have flattened it a good few times (this is why i got a second battery)! So if this battery had a problem the isolator might keep charging the starting battery and not pass the volts onto the aux battery, Does this sound possible?

Sounds reasonable to me. There might be sulphation in the start battery - check to see if it gets warm on a charger.

On a side note - that 75A battery will only give you about 15Ah before you start damaging it unless you fully charge it (then you get 37.5Ah). Alternators generally only charge batteries to about 70% of their capacity to prevent boiling of the electrolyte.

I'd definitely be checking the starting battery if you've flattened it a few times. I believe most if not all isolators are smart enough to ensure the main battery is charged before allowing any aux batteries to take charge from them. Some allow you to use an aux to start the car if the main goes flat but I believe they nearly all provide some form of protection against the main not getting charged in preference of the aux.

gadgets said:

So if this battery had a problem the isolator might keep charging the starting battery and not pass the volts onto the aux battery, Does this sound possible?

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Absolutely! less than full charge on the starting battery isolates the two unless you manually trip the solenoid. but not all setups have a manual switch, it's an option.

X2 on the earths. Sand back the paint on the chassis to make sure you've got a good connection.

One way to test your voltage drop is to connect your std 240v charger to the cars main battery. Put it on charge and see what voltage you get across the terminals on the main battery. Manually switch the solenoid on, see what voltage you get across both batteries. Main should drop slightly/aux should come up to around the same voltage ~0-0.4V less. If its more than 0.4V you may have a problem fully charging your aux battery.

KraftyPg said:

I believe most if not all isolators are smart enough to ensure the main battery is charged before allowing any aux batteries to take charge from them. Some allow you to use an aux to start the car if the main goes flat but I believe they nearly all provide some form of protection against the main not getting charged in preference of the aux.

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How do they do this?

I'm pretty sure, once conected together, the battery's should equalise, then charge/act as one battery.

The soleniod can only monitor voltage and once the vehicle is started it's almost always above the 13.5V required to parallel the batteries. Might just be a timer based on how low the starting voltage was before you kick over the engine?