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Charging house batteries while driving


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Thanks to the good gents here I have learned a ton about solar the past year and have a question regarding charging house batteries while driving, which we don't hardly do any of at night nowadays.


Okay, in the new to me Motorhome I'm rehabbing, I have 470 watts of flat rooftop solar, four T -105 Golf cart Batteries in series/parallel, and a "Smart" 4 stage temp compensated MPPT Solar Charge Controller. When the engine starts or the emer start button is depressed, an isolation relay closes so the engine battery is connected in parallel with the four house batteries via a 15 ft 4 Gauge cable so the engines alternator charges the house batteries to some extent and quality at least.


HERES WHAT IMA THINKING. I think I will disconnect the wire that closes the isolation relay when the engine starts (leave emer start function wire in place) so the alternator doesn't have to pump current into that big mass of house batteries, but instead rely upon and let the solar panels and controller charge them during daytime driving. It seems it could possibly tax the alternator to have to pump current into that HUGE four house battery bank, especially if they were discharged. The RV came with two 12 volt semi deep cycle RV/Marine batteries in parallel which isn't as much of a load on the alternator and, of course, it didn't have Solar from the factory and that's sort of how they wired them.


So in order to reduce the load on the engines alternator, Ima thinkin just let Solar (or my Smart charger when plugged in) charge my house batteries and let the alternator just keep the engine battery charged.


Good or bad?? Your thoughts please.


PS when I'm plugged in I usually turn off the solar and let my smart charger do all the work, good or bad idea also ????


A never too old to learn John T

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John - 400w solar, 660 ah of agm's in a fifth wheel. Overnight, CPAP (65ah), lights, TV, fans (30ah), breakfast, coffee, tea (50ah).


Usually down 125 -150 ah overnight. Bright sunlight, rolling through Arkansas I-30 I-40 Texarkana to Memphis, early May plenty of sun. Running the inverter, small fridge on line. End of the day, still down Ah.


New plan, inverter in the truck, charge the trailer at 120volt / 5 amp while driving. Trailer inverter/charger pushes 50amps into the batteries.

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Part of the issue is the size alternator that you have. If it were me, I'd keep what you have now but install a higher capacity alternator to give it more capability while driving. We had no solar on our class A but did have a high capacity alternator and found it to work very well for us since we rarely spend more than one night dry camping in a row.

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Bill B,


I agree if you're towing a fifth wheel with a truck use its alternator with an Inverter to operate a 120 VAC powered charger back in the trailer is a good plan for your setup, but its still using your alternators energy (50 + amps) to charge the trailers batteries WELL DUH. IFFFFFFFFFFFF possible I'm trying NOT to use my engine alternator but ONLY solar to charge the house batteries, if its not enough (subject to sun and angle and hours) I can always use the engines alternator for more amps if needed. THANKS FOR YOUR RESPONSE




I know its at least 100, maybe 130 amps, its called a "Heavy Duty" in the VIN search and I found its amps once but cant right now. I'm just hoping my 470 watts of flat rooftop solar alone will maintain my house batteries, so I'm not hardly working my alternator, but if not, its sure easy (its how its already wired now) to stick the little engine running activation wire back on the isolation relay. I guess I will just use it a while and decide once I'm in the field and can see just how it works. I do spend days on end dry camping and when I just had 400 solar watts I harvested enough energy in the day to keep up with night use, but with LED lights and an occasional water pump and vent fan perhaps furnace we just don't use much energy.


PS what's your thoughts on disconnecting the solar when I'm plugged in at an RV park and my PD 9280 Smart Charger is in use??? I'm just unsure how the two interact, I guess if I save utility energy I may as well run the solar if there's no problem with BOTH running??? Ive done it before with no problems but I have a new set up now.




John T

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Hey John!


Your energy requirements are so modest that your new array and battery bank shouldn't have any problem keeping up with your usage. Running juice back to your AGM's off your alternator would be the "dirtiest" and an option of last resort, IMO. Plan B and C being to run an inverter back to your house charger while driving, or... heaven help ya... kick over your lil genny for a couple few hours as needed. Not that it wouldn't be a bad idea to run an in-line switch (or manual plug and play) to your solenoid so that you DO always have that option. It's already wired so the expense of retaining that capability is moot.


Extended shore power stays... that's a personal choice. You know as well as any.. heat=life in electronics. It sure wouldn't hurt to give your controller a break, but if you're one that likes to keep continuous logs (since you have the sensor installed) to compare against your meter, it wouldn't hurt to keep it on, either. If you happen to be on shore charge your controller would also jump in to assist any heavier 12v loads, but again.. the way you folks drink juice I can't imagine that would ever become a "need".


In your case.. if logging isn't an issue.. shut'er'down.


To be gentle.. I would shut off the controller prior to plugging into shore power and reverse that when disconnecting. Knowing me.. I would probably tie a string on my power cord to remind myself to turn my solar back on. :P

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