Solar + DC to DC Chargers

If you drove the truck even weekly, why would your solar system need to charge the truck battery?

Can't you just hook your D+ wire to a switched wire in the truck that is only hot when the key is on? This should work as long as you don't spend much time with the key on but the truck not running. I wouldn't think you would spend much time with it that way.
There are voltage controlled relays. Sometimes also called automatic charge relays. Blue Sea makes one. They are typically used to combine 2 batteries to be charged by an alternator only when the engine is running. They will sense the voltage, make sure it's above 13.6 or whatever the set point is and then combine. They are typically large and made to carry high charge currents which you wouldn't need for your application with just a signal wire but it would still work just fine.

Quick? lol

Look for “voltage sensitive relay”. There are several kinds; sense on one side, sense on both sides, manual override, etc. Blue Sea Systems makes some very good, albeit expensive, ones.

I would connect the DC-DC to the normal switched trailer charging wire. DC-DC charger should be close to the TC battery. This should then work normal and independent.

For the solar... pull a wire to the truck battery direct and again let it run independent.

Lwiddis wrote:
If you drove the truck even weekly, why would your solar system need to charge the truck battery?

X2 beside that there is no sense to put AGM batteries under the hood.
Costco has good batteries for $70 to do that.
I think you overcomplicated the issue.
I had my 7.3l Fords sitting for month without batteries charging.
It is newer truck with bluetooth and exterior lights coming on with each door closure that drains the batteries faster.

There is no easy way to do this while maintaining the solar charge to the truck. The most reliable way would be to run another wire for the solar charger, and have the main wire for the DC-DC charger be switched.

A '14V relay' is not going to work. First, you would want a Voltage Sensing Relay (VSR) not a '14v relay'. Second, when your solar charger is charging your truck battery is will trigger the VSR, which will start your DC-DC charger, which will pull your truck battery down and it will likely end up in a on/off cycle.

One other question - do you really need the DC-DC charger? What sort of charge current do you get with out it?

Kayteg1 wrote:

X2 beside that there is no sense to put AGM batteries under the hood.
Costco has good batteries for $70 to do that.
I think you overcomplicated the issue.

???? This helps how?

I ended up putting a switch on the DC to DC charger inside the camper. That way I can activate it if need to but turn it off (assuming I remember) if I don't. The way things are setup now, the 200w of solar hits the charge controller that tops off of the camper battery first and then will switch to the truck battery second. The DC to DC is only used as a secondary backup if the solar can't keep up in shady areas and we are on the move.

Ordinarilly, our setup with a 100AH Battle Born LiFePo4 battery would be more than suficient. However, we did run into some low power issues during our California Foothill Booze Cruise trip two weeks ago. What caused it was a combination of high use items to include a bottle warmer for the twins (pulls ~23 amps for ~8 minutes through our 2000w inverter). The coffee maker (pulls ~50 amps for ~4 minutes). Then at night we needed to run the furnance to keep the babies warm and that pulled ~10amps for ~3 minutes every 30 minutes. After about the 3rd day of doing this and with the sun lower in the horizon, getting the battery topped off was getting difficult.

Up till this point I wasn't really using the DC to DC charger because I didn't have the switch quite figured out but once the battery got low, I decided to run it while driving. Intially it didn't seem to be charging right as the amount of amps it was pushing to the battery was ~3 amps. Turns out, it was just a really cold morning and the BMS system was limiting the charge. Once things warmed up, it started putting 13 amps consistently into the battery. We also started using the generator more often for the coffee put and bottle warmers just to make sure we had power for the furnance at night.

So a couple of thoughts on the lessons learned.

1. The need to keep babies warm required the furnance use at night where as the wife and I would normally just put more blankets on.

2. 200w of Solar is probably the minimum of effective capacity to maintain the 100AH LiFePo4 battery in the winter. Looking to add another 200w for 400w total when time and money allow.

3. Temperature of battery is a factor if high amperage use is needed at night when it is coldest. I am considering pulling the battery inside the camper completely. I have a compartment that would work well for it I think but I'm not really excited about rewiring all of my power cables at the moment.

4. DC to DC charger comes in handy when solar can't keep up. Just need to be careful because it can draw down truck battery if you don't turn it off when the truck is off.

5. Solar charge controller that I have requires voltage from the battery to work. If the BMS on a LiFePo4 battery cuts the battery off when it is drained, then the solar won't start charging it even if the sun is shining. By activating the DC to DC charger for a few miutes, it put enough charge back into the LiFePo4 battery for the solar to see it and start charging it again.

6. I will look into a more power efficient furnace for the future as a possible upgrade.