Twin Battery Charging

[paulyg]

Twin battery system with a Redarc isolator, older model but still works well.
I have a volt meter for the second battery in the back, I just noticed that when I put a battery charger on the starting battery after a while it shows 13.5 volts on the volt meter,
does that mean it's charging both battery's? and if I was to connect a solar panel to the start battery would it charge both?

[Cuppa]

With the motor running, once the crank battery comes up to the opening threshold of your isolator the crank & the auxillary battery are connected. Once connected the two batteries will over a short period of time equalise. With your aux battery showing 13.5v, if you were to put a volt meter onto the crank battery I would expect it to read the same. At 13.5v both batteries have a way to go before they reach max charging voltage. Expect that voltage to rise to something like 14.8 or 14.9v.

Yes, if you connected a solar panel to the crank battery it could raise the crank battery's voltage above the connecting threshold of the isolator, & once connected would add charge to the aux battery. It is possible that if the aux battery is heavily discharged or only a very small solar panel is used that the isolator may fluctuate between being connected & disconnected & not work very well. If this were to happen it would be because when the two batteries are joined (isolator open) that the equalising between the two batteries takes the crank battery below the opening threshold of the isolator, causing it to disconnect until the solar brings it's voltage up again.

If both batteries are already well charged, a small solar panel connected to the crank battery would be a good means of keeping both batteries charged if the vehicle is to be laid up for periods of time. Larger panels would require a regulator.

[paulyg]

Thanks cuppa, I have a 255 watt solar panel, If I am camping for a number of days would I be able to connect the panel to the start battery and keep both battery's charged?

[Cuppa]

Thanks cuppa, I have a 255 watt solar panel, If I am camping for a number of days would I be able to connect the panel to the start battery and keep both battery's charged?

Yes, but you would need to use a regulator with a panel of that size.

[paulyg]

Thanks again cuppa

[sil3nt_dr3ams]

Yes, but you would need to use a regulator with a panel of that size.

Not only is your beard impressive but your knowledge of auto electrical has me following your trail of bread crumbs thru post's.

[Cuppa]

Not only is your beard impressive but your knowledge of auto electrical has me following your trail of bread crumbs thru post's.

Thank you, beards are quite useful for collecting breadcrumbs!
I try to be helpful when I can, but there are others on this forum with far more extensive knowledge of auto electrics than me. My knowledge, such as it is, is limited to what i have learned whilst setting up my bus & Patrol & talking to others who have done similar over the past decade or so. My willingness to share what i've learned should not be confused with 'expertise' however.

[Wizard52]

With the motor running, once the crank battery comes up to the opening threshold of your isolator the crank & the auxillary battery are connected. Once connected the two batteries will over a short period of time equalise. With your aux battery showing 13.5v, if you were to put a volt meter onto the crank battery I would expect it to read the same. At 13.5v both batteries have a way to go before they reach max charging voltage. Expect that voltage to rise to something like 14.8 or 14.9v.

Yes, if you connected a solar panel to the crank battery it could raise the crank battery's voltage above the connecting threshold of the isolator, & once connected would add charge to the aux battery. It is possible that if the aux battery is heavily discharged or only a very small solar panel is used that the isolator may fluctuate between being connected & disconnected & not work very well. If this were to happen it would be because when the two batteries are joined (isolator open) that the equalising between the two batteries takes the crank battery below the opening threshold of the isolator, causing it to disconnect until the solar brings it's voltage up again.

If both batteries are already well charged, a small solar panel connected to the crank battery would be a good means of keeping both batteries charged if the vehicle is to be laid up for periods of time. Larger panels would require a regulator.

Hi Cuppa and others

The same result if solar panel is connected to aux battery. While batteries are connected through isolator even after turning engine off,(voltage of starting battery is above threshold) main battery is also charged by solar via aux battery. Obviously as both batteries drop below threshold, isolator disconnects and aux battery powers fridge etc until you run out of power or recharging commences again. This was the unintended result and benefit of my installation.

[sooty_10]

Wizard I believe that is all dependant on the isolator used. Some are dual sensing and others are not. As you've described I think it would require a dual sensing isolator to feed from the Aux to the starter? Also another point to consider is if the isolator requires 'ignition' to be live for it to work, I have a piranha dbe180s and it does require ignition to create the link, although if I was to install a permanent solar panel I would look at putting the 'ignition' feed wire to the aux battery and having the isolator on full time and running the solar through the main battery (as I believe the DBE180s is not dual sensing). Would this work cuppa??

[Cuppa]

Wizard I believe that is all dependant on the isolator used. Some are dual sensing and others are not. As you've described I think it would require a dual sensing isolator to feed from the Aux to the starter? Also another point to consider is if the isolator requires 'ignition' to be live for it to work, I have a piranha dbe180s and it does require ignition to create the link, although if I was to install a permanent solar panel I would look at putting the 'ignition' feed wire to the aux battery and having the isolator on full time and running the solar through the main battery (as I believe the DBE180s is not dual sensing). Would this work cuppa??


Hi Sooty,
What Wizard describes would work with a one way VSR. When he switches of the car's ignition it takes a little while for the crank battery voltage to fall below the cut off voltage. If the solar is connected to the aux before this occurs, it is possible for the solar input to keep the crank battery voltage up & thus to keep the two batteries paralleled ...... until it gets dark. Using a dual sensing vsr achieves the same but without having to think about it.

I,m not familiar with the Pirahna isolator that you have, but i guess if it needs the ignition on to work then it is not a voltage sensitive relay. I imagine that doing as you suggest would work to keep the two batteries paralleled, but would defeat the purpose of the isolator to protect the crank battery from over discharge. I don't advise it, but if you were going to do this on a full time basis, you might just as well remove the isolator altogether.

With a VSR (preferably dual sensing) charging both batteries in parallel from solar will help to extend the life of the crank battery & on poor solar days (when cloudy etc) when only a small charge is coming in, that small charge should be enough to keep the batteries connected allowing use of some of the crank battery's capacity to extend the time the batteries can keep for example, the fridge going. It's not a big deal, but every little bit helps.