Duel battery charging voltage

[gadgets]

Thanks for all the sugestions, after only 5 hrs outside now and reading all yer suggestions my problem is solved!!! It was dark outside by now so instead of trying to get at the aux battery's earth i joined my jumperlead cables together, put one end on the negative of aux battery and the other end out my back window and around the side of the car and onto the negitive of the starting battery and hey presto i got a charge going to the auxillary battery! 13.8volts which I guess is not bad with it having 6mtrs of cable away from the engine bay. I never realised the difference a good earth made, Makes me think now about checking the earths for my lightforce to see if they are getting all the volts they need. Thanks.

 

[Matt76]

Thats great news Gadgets, You have helped me out too now, I was told when I did mine to run the earth back through to the start battery, but I was never quite sure on whether I had wasted my time or not. Seems I didn't.

I hope you get it all sorted for the weekend.

Cheers

 

[Old.Tony]

Yep, good news. Never had that problem myself. The cable I use is 60A, double insulated - so inside, there's a red-insulated cable and a black-insulated cable. I always run that and connect both, I never rely on "chassis ground".

Might be a standard question we can ask next time someone has a problem involving batteries.

 

[KraftyPg]

How do they do this?

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

As far as I am aware that is the difference between a solenoid system and an isolator. My terminology is probably wrong here but for want of something better I'll go with it. A solenoid is a direct pass through allowing both batteries to charge together, an isolator as suggested by the name isolates the two batteries so they can act independently, however most if not all isolators allow for them to act together as well for things like cranking from the aux battery. If the isolator sees that the voltage of the main battery is over x volts it will allow the second battery to be charged where as below x volts the isolator will stop or slow down the charging rate to ensure the main battery is getting as much as possible. A solenoid only system doesn't allow for this it shares the charging voltage with both batteries.

Something which may not be entirely relative to this case that I thought of last night was that a fridge going off and setting off an audible alarm doesn't automatically mean the battery is flat or dead or unusable. Most fridges will have circuitry in them that will tell them to stop drawing current when the voltage it receives is less than a pre set figure (from memory I think we adjusted mine to 11.9). Once the battery stops feeding a the desired voltage to the fridge the fridge will automatically switch off to prevent it from draining the battery further. Rapid discharging of batteries (as when a fridge is running) could easily hurt a good battery so these fail safes are used to make sure that doesn't happen, however just because your fridge turns off doesn't always mean the battery is flat. If you plug a low voltage globe into the battery that no long runs the fridge you can probably get hours of light out of it. The same globe can run a battery down a lot further and a lot more often without fear of ruining the battery than something like a fridge, so most fridge manufacturers put the safety feature into their designs.

Some things to remember is that flat isn't always dead, 12 volts or higher doesn't always mean good, current is quite often more valuable to know than volts, batteries and cables must be chosen for the task you want them to do (there is no one size fits all), your cars system isn't stupid and can variable charge a battery and with an isolator between your batteries you can have the two batteries act independability.

But as I think we've hashed out here over the last 3 pages the most important thing is to ensure that your earth is a good earth. While I subscribe to the earth back to the source theory for aux batteries if this isn't possible always ensure the earth is clean of paint, rust and dirt.

Glad you found the problem gadgets.

 

[nathan96]

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

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

I know the problems been solved but does anyone know the answer to this?

 

[KraftyPg]

Although I probably wouldn't expect it to maintain 14 or higher if it was flat or faulty I'd still have to say yes it could maintain a higher reading than 12V. But volts aren't everything.

You can have 12 volts at 75 amps and you can have 12 volts at 2 amps, if your battery is putting out 75 amps (forget for a moment that it may never theoretically put out its full amperage) and 12 volts then it's working as it's started, however the same battery could be reading 12v or higher across the terminals but only be putting out 2 amps which makes the battery in dire need of charging or faulty.

12 volt or higher on a multimeter rarely tells the full story

 

[will]

the 14 volts from an alternator can turn into 13 or 12 volts pretty quick once a load is applied. i used to have a load tester and when it tested the load and battery condition it said to put the lights on high beam, Air conditioner on full with the heater on full too (engine running). this produces the maximum "continuous" load your charging system is designed to handle.

you can go to any autobarn to have a load test done, that will give you an idea on the alternator output, battery condition etc.

 

[WoodyD40]

I have run the earth right back to the crank battery on my dual battery setup - and i only have a solonoid, not an isolator system.

I've had it installed for nearly a year now, and the only problems i had was running 30amp cable initially, so i upgraded to 50amp's and no more problems.

Now i've put a winch on the car, i've upgraded the cables and solonoid to a very heavy grade (i think 16mm cable) only 2 weeks ago.

touch wood - no problems so far with the cable upgrades.

 

[Peter ST_R]

The tub on the D22 isn't connected directly to the chassis (negative earth potential) so anything earthed to the tub will suffer from voltage drop when electrical charge or load is applied. I have bonded my tray to the chassis so I can now earth accessories to the tray rather than run an earth back to the chassis. With the solonoid isolator the voltage across both batteries should be identical once the solonoid has closed, if not then it is most likely a poor electrical connection not cable size at fault as lead acid deep cycle batteries will only take about 10% of their capacity as charge per hour (75Ah = 7.5A). When measuring voltage in various places around your vehicle be aware that you can easilly be fooled by your volt meter as you will only detect voltage drop while load is applied but where there is no load then the voltage will read normal (i.e fridge blinks low power but when you unplug it and measure at the plug you still get good voltage).