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What is a reasonable approach to charging a 24V battery system from a 12V source such as an alternator? Can I simply use a 12v-to-24V step up converter? Seems I’d actually need this converter to hit 5V or more above the battery rating in order for it to work, so maybe a 12V-to-29V nominal.

If the system was in a trailer with 7pin tow vehicle connector, should I disconnect the power wire coming from that connector from the trailer’s 12V panel and run it directly to the batteries via the step up converter? And likewise, disconnect the negative wire from the chassis and run that to the converter’s negative port?

The rationale for doing this is improved inverter efficiency that comes with 24V to 120V as well as doubling solar wattage for a given capacity MPPT. This trailer needs to have a very large battery capacity (400Ah @ 24V), a very large Solar Array input (1400W) to be able to run 120V appliances/tools. 12V appliance use is minimal so I am not at all concerned about conversion losses going back down to 12V for them. When not in use this trailer is attached to my house grid.

A few more considerations. The grounding system. The existing 12V dependencies all use the chassis as their ground/return so I am guessing I need to connect the chassis to the negative port of the step down converter. Also, what about the trailer brakes and running/signal lights? What do I need to do, if anything, to keep those working correctly?

This is what I have in mind: Victron Energy Orion IP20 12/24-Volt 10 amp DC-DC Converter Non-Isolated, High Power

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    Have you checked the current rating of a 7-pin socket? Have you considered having a second 24v alternator? A solution for many.
    – Solar Mike
    Sep 19, 2021 at 6:22
  • @SolarMike that is a good suggestion. Not sure if my Ram 1500 Ecodiesel has room for a second alternator. Also, that would probably be more expensive than using the Victron step up converter. Also, I believe the 7pin is rated at 20Amps @ 12V. Which matches that converter with 24V 10Amp output.
    – reka18
    Sep 19, 2021 at 6:25

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I found the ideal setup.

Put a 10A output 12V-to-24V step up converter on the black hot lead and white ground coming in from the RV standard 7pin plug to get some charge to the 24V batteries from the truck alternator while driving. This is not a big deal since there is so much solar power on the roof that the batteries don’t even really need power from the truck. Also, since that converter is kind of dumb and can’t sense the charge level of the batteries I can just wire it to the MPPT input terminals and let that smarter device throttle current when batteries are full.

The real relief is that the wiring that goes to the running lights, turn signals, brake lights and the all important trailer brakes are completely independent of the trailer battery system, except that they are grounded to the chassis; this is also not an issue since the chassis ground is attached to the step down converter negative terminal rather than directly to the 24V battery negative.

This is much cheaper than installing a second, 24V alternator alongside the 12V alternator.

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