Interaction of Solar and Alternator

[Rustofer]

My camper has a single 100W roof-mount panel, which meets my power needs (2 @ 6V batteries) when the sun shines. However, when camping among trees for several days it is easy to consume 70 to 100 Ah with no solar gain.

I have found a 5-hour drive in the sun restores only about 20 Ah. I have measured approx 14V at the camper connection, so there is alternator voltage there; I'm wondering if the solar panel, pushing 4 amps at 13.5 volts is preventing the alternator from doing its job. Does the alternator see the 13.5v from solar and think there's nothing to do? If not for solar, the house battery would be at approx 12v, and I expect the alternator would push more than the 4 amps I'm getting from solar. Intuitively I expect current flow to be proportional to the voltage delta, but is that the case? I'm thinking about adding a disconnect to the solar for this scenario. Thoughts?

[BFL13]

Your problem now is just how to get the batteries up properly after running them down, since driving doesn't do it and you don't have enough solar to do it in one day. (If you decide to go to a full hook-ups for one night to get the batteries back up and maybe hit their laundry.)

You need a decent charger or converter that will get those 6s to 14.8v and hold there till the batts are full. I lost my "learner" set of 6s having only a converter that did fixed 13.8v.

I could never get those 6s into the green on my hydrometer with that converter, no matter how long it was hooked up. I found out about 14.8v and how to do all that after some wasted time before joining this forum. Been able to keep'er going ever since, thanks to the guys on here. 🙂

[Rustofer]

Well, thanks for these responses. My next step will be to get a better handle on actual charging voltages and wiring losses, but as far as dynamics between solar and alternator I'm now satisfied there are none. I am in the fortunate position of having meager consumption, and can manage 4 days without sun, but any more than that makes me want to move on anyway.

[grizzzman]

Rustofer wrote:
Winter camping at Rathtrevor was exactly the scenario! We left and drove through Lake Cowichan down to Jordan River to finally warm up!

Grizzzman: how many amps does your system generate in full shade?

Around 5 amps.

[landyacht318]

The voltage the vehicle itself allows, and for how long it allows it, along with the circuit resistance, and battery resistance will dictate how many amps the battery will accept.

MOst vehicles might start out iun teh 14's, but quickly drop to mid to high 13's/

I regularly see 3x more amps flowing into my AGM at 14.7v compared to 13.7.

Since I can spin a dial and change vehicle voltage, i can change the rate at which my battery charges, but it is pretty much 14.7 anytime it is not full, and 13.6 to 13.8v when it is.

Many people who add secondary batteries use the starting battery as the + feed for the house battery, basically tacking the house battery onto the end of the starter battery charging circuit. The original alternator to battery charging circuit was never intended to carry the extra current of depleted house batteries.

Solution, Hook house battery to alternator dierctly through solenoid, rather than engine battery to house battery.

I have fairly thick copper between alternator and house battery and it was consuming 60+ alternator amps on start up and 59+ amps when I parked some 10 miles later, at 14.7v.

YOu cannot easily change the vehicle's chosen voltage so alternator charging is almost never saturation charging, bringing the battery to 14.5volts nearly instantly and holding it there.

I think a pair of healthy golf cart batteries at 50% can suck up 70 to 80 amps for a while before the voltage mearured at their terminals hits the mid 14s.

If youo vehicle is only asking for 13.7v then those same batteries might only require 25 amps.

So even if you were to wire your alternator directly to house batteries with fat copper + and - cables, the vehicles voltage regulator might decide 13.6v is just fine and dandy, and there is nothing you can do.

But the DC to DC chargers can remedy this, with some limitations and rather ridiculous price.

Some vehicles can have their voltage influenced.

I had to trick my engine computer into thinking my externally regulated alterantor is still connected to it, and I use an external adjustable voltage regulator, and can basically always charge my battery as fast as it is possible to do so.

The Solar just adds its amperage to what the alternator can make as I have its absorption voltage set for 14.7v too.

If I had solar set for 14.6 and vehicle for 14.7, then the solar would not contribute once 14.6+v was reached.

So a thick copper circuit can help only so much, you are still at the mercy of the vehicles voltage regulator. if you have a long thin house battery circuit, and your vehicle has decided to drop to 13.6v, you will be lucky to see 5 amps flowing into the gc-2's

[Rustofer]

Winter camping at Rathtrevor was exactly the scenario! We left and drove through Lake Cowichan down to Jordan River to finally warm up!

Grizzzman: how many amps does your system generate in full shade?

[grizzzman]

I know you are not interested in more solar. The reason I have overpaneled is not when the sun is out. (It's clipped at 31 amps) It's for when it rains and in the shade of trees. And quicker full battery charges daily.
It's very likely a larger wire straight from the alt. Would help.
Goodluck with your project!

[BFL13]

If 14.2 is your normal running after warmed up, then cold start might see 15v or whatever and then taper after warmed up.

In any case you still need to get by in the woods for days with not much solar. You are the only one with enough information on your particular scenarios to solve that.

With the 5er, I had a generator for camping at Rathtrevor in the winter and didn't bother with solar. Left the 5er there and went out in the truck. Now with the Class C and no gen, but lots of solar, I can go around to the various local open spaces during the day and go back to the campground. Nobody steals our spot (yet?) You can even tow a cargo trailer full of extra batteries and a gen with the TC on the truck if you have long tongue on the trailer for the TC overhang at the back.
You can take the truck with the camper and DW can take the car (with the gen in the trunk) and leave the truck in your spot and use the car to get around town in.

[Rustofer]

MEXICOWANDERER wrote:
It's not if a person is fine
It's up to the batteries
Are -they- fine?

I admit my measurements have been coarse, but my little solar panel matches my low consumption most of the time: my Xantrex reads "Full" most afternoons, and the batteries seem fine with that . . .

[Rustofer]

red31 wrote:
I thought the start battery is much closer to the alternator voltage sense and therefore after a short bust of amps, they taper drastically as the starter battery is up to snuff.

I've wondered that too: does the truck's ever-more controlling computer tell the alternator to lay off once the starter battery is recharged? Some time ago I did connect to truck voltage (through the OBD-2) and verified that the output voltage remains in the 14.2 range over a longer haul.

[theoldwizard1]

red31 wrote:
I thought the start battery is much closer to the alternator voltage sense and therefore after a short bust of amps, they taper drastically as the starter battery is up to snuff.

Basically true. Many bars now have some kind of battery "state of charge" sensor that is an input to the "smart charging" algorithm in the PCM that determines the optimal vehicle battery voltage.

This might "maintain" the battery in your RV, but it is NOT high enough to recharge it !/

[red31]

I thought the start battery is much closer to the alternator voltage sense and therefore after a short bust of amps, they taper drastically as the starter battery is up to snuff.

[MEXICOWANDERER]

It's not if a person is fine
It's up to the batteries
Are -they- fine?

[Rustofer]

BFL13 wrote:
With the monitor for your ammeter, it is easy to do some tests parked in the sun.

With the batts near full, just turn everything on in the camper and get as many amps draw as you can, say minus 20 amps. Now with no solar, turn on the engine and see what the amps do. Might be minus 5, so charging is 15 amps. Wait a few minutes and those amps will taper as voltage falls to 13.x. Might then only get 8 amps charging.

Same with solar and no engine on. Now you get your 4 amps solar, so amps go to minus 16.

Now turn on engine and see what happens to the amps.

If the charging amps go above the draw the diff still works with a few positive amps showing.

Funny. I was outside doing just that.
Disconnected solar and with the fridge running on DC I was -21.2 amps. Voltage was 12.0 at that point.
With truck on it came up to -8.2.
Truck plus solar -5.6 (scattered clouds today).
Solar alone -18.5

This tells me the truck is pushing roughly 13 amps back when the battery is around 12 volts. I'm fine with that.

With the fridge off, truck and solar together were showing 7 amps, but the voltage was back up around 13.2.

I'm still perplexed wrt my previous experience while driving, but at that point there was no negative load on the camper battery - just the solar and truck amps flowing in, and I don't know what the battery voltage was. It must have been high enough that it could only make the 4 amps.

[ndrorder]

Amps flow from higher voltage to the lower voltage. If the alternator is producing more voltage at the trailer battery than the solar controller, the controller should turn itself off and all of the alternator amps would go to the trailer battery. If the solar controller is producing more voltage at the trailer battery than the alternator, then there will be no amperage transferred from the alternator to the trailer battery.