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Old 01-19-2025, 08:15 PM   #1
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Which DC -DC Charger?

I have a 1994 Ford E350 RV and want a DC to DC charger to keep the house batteries charged while on the road. One of the key features I'm looking for is the ability for it to recognize the RV is no longer running and will stop charging to keep from killing my generator starting battery.

I don't think isolated or non-isolated matters to me, I do have all negative cables tied together if that matters.

Can y'all help me out?

Thanks, Phil
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Old 01-19-2025, 08:44 PM   #2
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Unless your using lithium batteries, you just need a battery combiner, like the Cyrix 120.
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Old 01-20-2025, 08:36 AM   #3
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My Renogy DC/DC has an ignition sense wire so it only charges when that wire is at +12VDC. Turn the ignition off and the charging stops. I would imagine that others have a similar feature but not familiar with them.
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Old 01-20-2025, 10:40 AM   #4
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I have the same thing, but don't want to run the switched 12v from the cab all the way to the rear bedroom and drilling through several layers of flooring...

And there nothing near the converter either, at least I haven't found anything there.

Renogy has that new alternator sensing version, but I don't want anything that new and suffering through the bugs.

I was hoping for one of the Victrons, though more expensive, it will help me out of this situation of having to remember to flip a switch.

IDK maybe I'll put up with the aggravation of running that wire to save a few hundred bucks.

Thanks.
Phil
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Old 01-20-2025, 11:06 AM   #5
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Quote:
Originally Posted by RVers29325 View Post
I have a 1994 Ford E350 RV and want a DC to DC charger to keep the house batteries charged while on the road. One of the key features I'm looking for is the ability for it to recognize the RV is no longer running and will stop charging to keep from killing my generator starting battery.

I don't think isolated or non-isolated matters to me, I do have all negative cables tied together if that matters.

Can y'all help me out?

Thanks, Phil
Are you changing the overall capacity of the battery bank? If not you may be able to do drop in. DC-DC charger should be sized 10 to 20 amps per 100 A/H battery capacity. At 20 amps/100 that puts you at a 5 hour charger rate.
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Old 01-21-2025, 10:42 AM   #6
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Yes, I'm adding more batteries, right now I have 200ah with a jump to 600ah total for a 4kw inverter.

The renogy dc-dc I have is 20amp, so maybe I should bump up that charger.

BTW, as for a DC-DC that recognizes alternator I would have(?) to have an intelligent alternator. I don't think that 1994 e350 would have that, so this thread may be dead.

Thanks for the input!
Phil
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Old 01-21-2025, 10:58 AM   #7
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For the DC-to-DC charger, you want one that actually needs a switch input to turn it on. Don't rely on alternator detect based on voltage. The at rest voltage of lithium (13.6) is too close to alternator output as low as 13.8 to be reliable.
How are you getting to 600 A/H? How many batteries? You want as few as possible while still meeting the BMS delivery capacity to support the inverter and if needed start the generator.
What size is your alternator? Consider an upgrade? Based on existing battery bank you may be limited to 40 A/H DC to DC Charger. What size charger is in your 4,000 watt inverter? Maybe you just use that on generator. Would you like some dedicated engineering assistance?
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Old 01-21-2025, 11:59 AM   #8
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I'll be adding 4x100ah or 1x 400ah or just 2 more 100ah... haven't got that far yet. I know I'll want more that the 2 I have.

I was thinking about putting in a 40a dc-dc and have determined that auto-magic cutoff is not possible with the old alternator. And had an idea in a dream about running the switched 12v from the cab to the dc-dc.

Sometimes when I realize I'm dreaming I try to focus on these projects and see if my minds eye can see a path forward. It did last night.

I'll be using my new converter to charge the batteries when needed on-site. Installed 8 months ago.

Thanks for your offer to shoulder surf this project. I may take you up on that. I'll be discussing this with my RV tech and see what his recommendation may be after we get the scope laid out. Right now I'm learning as much as possible to make sure I ask the right questions.

Thanks, Phil
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Old 01-21-2025, 12:25 PM   #9
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Lithium batteries work best in matched sets. This is due to the interaction of the BMS in each battery. Lower cost 400 A/H batteries only have a 200- or 250-amp BMS so that will not support a 4,000-watt inverter. For a 600 A/H total bank the best option would be 2, 300 A/H and a 3rd could be added later if you need more capacity. The sweet spot is currently a 3,000-watt inverter/charger with a 150-amp charge section. This works out well for retrofits and 10-gauge, 30-amp circuits in and out are easy to install and manage. The 150 amp charger works out well for generator usage as that puts you at a 4-hour charge rate. You will need at least a 2K genny to support that. Goal is to get genny usage to around 2 hours or even less per day. If you don't have, you want a battery meter of some sort to manage power in and out and calculate state of charge. You can get batteries with Bluetooth apps. With multiple batteries you want to manage the complete bank. For now, if you had a 200 amp bank and replaced with same capacity, let the alternator charge as it did before. Adding more capacity will require more controls or upgraded alternator and possibly wiring.
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