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I have a RedArc solenoid and a Victron 75/15 Smart Solar Controller. Can I use the combination of these two units to charge an 12V AGM Deep Cycle auxiliary battery from the car battery/alternator when the car is running ?

The RedArc solenoid is no longer battery sensing and only works via the override lead being pulled to 12V. I'd connect this override lead to the car's ACC or running output of the vehicle.

Second, the Victron unit provides multi-stage charging and is configurable for multiple different batteries and provides some nice feedback via the Victron App.

Third, if this combination does not work, can I add a RCNUN BuckBooster ( Input 8-32V DC, Output 13.8V 8A - Constant Current output from 8-13.8V) between the solenoid and the MPPT Controller to regulate the output?

I have been pondering this for some time. I have the piece hence the interest. Thank you.

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  • \$\begingroup\$ It's difficult to tell from your question how exactly you want to hook this up. Do you want to pull power from your car (which is running), and also from a solar controller to charge an external battery? Or do you want to use the solar controller as a battery charger? Some diagrams would be helpful. \$\endgroup\$ – Drew Mar 13 '19 at 5:20
  • \$\begingroup\$ Correct. No solar panel. I'd like to pull power from the running car through the solar controller to charge an external battery. \$\endgroup\$ – Geoff Mar 13 '19 at 6:10
  • \$\begingroup\$ Sounds feasible. Without buck-boost: If MPPT voltage input range includes whatever voltage the car supply is at. | With buck-boost: If MPPT voltage input accepts 13.8V. \$\endgroup\$ – Russell McMahon Mar 13 '19 at 6:27
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Your alternator is already set up to charge a battery. It won't put out more than 13 - 14 volts and i believe charge controllers are set up to run form much higher solar panel voltages at much lower currents. You could just (Jump) the AGM battery in parallel and the alternator would charge it along with your car battery. You may be over complicating this problem with the charge controller?

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The alternator is a very dirty power source so I wouldn't just connect up your deep cycle AGM battery unless you want to break it! Alternators are designed to keep charge up on a basic SLA battery, not a deep cycle battery. The Victron Smart charger is the only viable solution i have fond for this exact purpose. I believe a solar MPPT charger is not suitable because it relies on excess voltage to produced current, something which doesn't really exist because the alternator and main battery operate at about <=13.8 volts usually.

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