Batt to Batt charging and Solar Solar Questions

[Capebuild]

Is tow vehicle battery to TT house battery charging a given.... does everyone have that?

Solar question: I'm still doing research on having solar panels but quick question... I'm trying to decide on whether a single 100 watt panel would suffice,
or is having 2 /100 watt panels better? I suppose this might be an obvious answer (having 200 watts would be better than 100) but I guess I'm just looking for
thoughts about it. We could be out in our trailer for a month at a time, so maybe the more solar power the better... The only energy hog we'd have would be
a Dometic fridge. Other than that, the Max Fan, some LED lighting, probably not much else. We'd have 1 / 100 Ah Battelborn lithium battery.

Thanks!

John

[John61CT]

Charging properly from the alternator over distance takes a fair bit of trouble and cost,

really only worthwhile if your usage pattern, ratio of driving vs off-grid staying put justifies it.

A small portable genset gives more flexibility to supplement solar.

As to panel wattage, 100W is marginal to supply a fridge unless the season location weather are giving excellent insolation conditions, as in CA or the SW sunshine.

Say in a cloudy Canadian winter or PNW grey, even 200W may not be enough.

Tilting the panels toward the sun vastly improves efficiency, up to double overall.

[tony.latham]

Solar question

PM sent.

T

[saltydawg]

Okay some real numbers. You can expect 25 to 30 amp hours a day if in full sun from a 100 watt panel. So to charge your battery from dead could be 3 or 4 days.

With a lithium battery you could safely charge 15 to 20 amps an hour thru a dc to dc charger and not risk hurting you alternator. So you would need to drive 5 hours to fully charge your battery.

Do you plan on driving say 2 hours every day to keep the battery full, plus the 100 watts of solar. to be able to charge the possible 50% discharge of the battery.

200 watts of solar could recharge your battery in less than 2 days from dead. 200 watts would recharge your battery from about 60% discharge everyday.

I dont see you using 60 amps hours a day, even with a fridge.

Even with fridge I would bet your no where near 30 amps a day, so a 100 amp panel would keep you going for ever. Until you have a cloudy day, or even worse 2 of them. Now the sun comes out and your only putting in what your taking out, never gaining ground. You would then need to drive to charge up.

All of this is simply stating put 200 watts on it and dont worry about having to charge by shore power or towing.

[troubleScottie]

On DC-to-DC charging: it is a good idea to have a MPPT controller between the battery and your charging source. Many of them allow for two inputs eg solar and the DC source. If you add solar and its controller, you may get access to DC-to-DC charging from the towing vehicle. Can you go direct without the controller? Yes.

To charge at any reasonable amperage, you really cannot use the power connection in a 7pin trailer cable. It is not designed to carry that much power.

That said you still would have to bring the towing vehicle power to the controller so:

• fuse
• isolation solenoid
• 10-20 feet of heavy weight wire
• electrical coupler between towing vehicle and TD
• second fuse
• a hole to get cable in

This is like cabling up a high power amplifier/speaker system in a car. Not particular hard. Just money and lazy weekend. The major negative is the tow vehicle system is not easily transferred between vehicles. If you have multiple tow vehicles, you need repeat this in each vehicle. Granted the TD stays the same.

[TimC]

John, my teardrop has LED lighting throughout, Small case fans for ventilation (very low draw), charging ports for electronic gadgets and a 35L Alpicool 12v fridge.

Obviously the fridge is the demanding appliance. In tests run in 2019 after purchase I monitored it for 58 hours after pre-cooling on 115v. It used 33.9 aHrs in that period with ambient temps at around 70f during the day and mid 50s during the night. That's under 14 aHrs per day. From Cosmo's comments his Dometic 35L has similar power draw.

I have 200 watts of panels (two 100w Renogy) mounted flat on my TD roof, a decent MPPT charge controller and 144 aHr battery bank (AGM). I've never had less than 12.5 v showing on my MPPT for battery level (I'm very conservative in my power use while camping). I went with MPPT as I wanted to mount my panels "permanently" on the roof and the MPPT will generate a smidge more amps that way. Even on a cloudy day I can look at the charge controller and see in the morning that it is beginning to trickle charge the batteries before noon.

Look into a larger, newer PV panel. A new 200w panel is smaller than my two 100w panels. Make sure you pick a charge controller to match its output (I am not an expert to help there). If you go with an inverter for 115v power you will have to ramp up your system. I find no reason to use 115v appliances.

NOCO makes some seriously good chargers for when you are in a pinch and have shore power. I chose a Genius 7200 (7.2a) because I have multiple uses for it at home. A 3500 (3.5a) would be enough for most teardrops I would think.

As far as my Alpicool 35L here is a link to the page where I detail the power usage and one or two minor problems which I easily fixed...
http://tnttt.com/viewtopic.php?f=2&t=71874&start=60

Can't help with the DC to DC charging. Don't know enough about that and have seen too many posts about critical mistakes that can be made wiring it up. I'm satisfied with solar. Battery bank is going on 6 years old (eight 35aHr Mighty Max AGM batteries) and holding a very good charge. I would definitely go with AGMs again if they fail. In the winter my TD sits in a garage. I dismount one 100w PV panel and put it on a saw horse in the yard next to the garage and hook it up to the TD. Five winters doing this and not any problems (N. Wisconsin).

[John61CT]

DCDC is fantastic for someone driving cross-country staying in a different spot every night, or every other night.

Other use cases too, where the miles driven ratio is high compared to stationary overnights in a row off grid.

Weekenders can make do without anything but mains charging given enough Ah capacity.

As stated all depends on the usage patterns.

For actually living full time mostly off grid, then you want to make use of all sources, mains, alternator, portable genset and solar.

Living off solar **only** requires extreme conservation or a big roof area.

[RJ Howell]

I run DC/DC charging and stopped carrying the solar panel. My LiFePo4 get s me 2.5 days without recharge running the DC frig and lighting. I'm doubling up my battery power now, so will be 5 days. This system works because we don't stay longer than a couple days in a spot and typically up tp charge in a couple hours of traveling.

If you tend to stay in a spot for a week (or so), my system will not work for you. I used to carry a 130w panel and it did the deed, but I ran my frig on propane back then.