Parasitic Draw on RV?

[LipschitzWrath]

I am getting ready to add a small solar system to the top of my 5er to keep the batteries topped off when we leave the camper at the lake for extended periods. My wife and I generally pull the camper out on a weekend and then leave it there for weeks at a time. We pull it to a nearby dump station when we need to dump or add water. We cycle LP tanks to keep the fridge cold.

Last weekend, we returned to our camper after two weeks of leaving it unattended. The batteries were COMPLETELY dead. This seemed odd to me. I mean, assuming we turn everything off before we leave, the only draws are the LP/CO detectors and the little current draw to run the fridge in LP mode, right?

So how much juice do these items consume?

After doing some internet research, I found one site where some dude claimed these items add up to approximately 37Ah in a 24 hour period. I've seen other sites say the fridge pulls 17-20Ah per day by itself in LP mode. The fridge is a Dometic fridge/freezer, I don't have the model number handy but it's from 2004.

Do these numbers sound right?

I have 210Ah of AGM batteries installed. If these numbers are close, then that would certainly explain the dead batteries.

This brings me to the solar panel install. I guess a secondary question would be, do you guys think a single 100W panel would be sufficient for the sole purpose of keeping the batteries topped off when the camper is unattended?

I don't mind occasionally running the generator(s) when we are actually out there using the camper.

Thanks in advance!

[LipschitzWrath]

Okay guys, back from my first weekend with the solar. It performed every bit as well as I'd hoped. Solar harvest did go up while inhabited, but the solar system certainly seems to be capable of more. Our lifestyle is pretty obviously not heavy on 12V usage, haha.

We didn't get the generator out of the compartment at all. That is a first for us. I couldn't be more pleased.

[Harvey51]

This is like our boondocking. We have 100 watts of solar, no generator, never plug in. The batteries never go below 80%. The refrigerator is a Norcold model N510 with no extra features like defrosting. In your situation, I would be thinking it should work with a 100 watt solar panel, and measure the total load current and where it is going. A clampmeter would be very handy.

[pianotuna]

Hi,

The only time the panels will send power is if a heavy load is placed on the battery bank. Then they will help support the load.

[red31]

yes, if the batt is full with no loads then at first light the voltage will climb to abs voltage and it will take very little power to keep it there (lots of off time). During use you may draw the battery down enough so it goes back to bulk charging (no off time) and once it gets back to set pt voltage of 14.?v the off time increases.

Others may suggest that the current tapers as the battery charges but what actually happens is the OFF time increases so that the average current over time tapers.

A recent thread about

Solar wasted watts/volts

[LipschitzWrath]

red31 wrote:

LipschitzWrath wrote:
will it "count" amp-hours harvested even if it can't use them to serve loads or charge the battery?

No, absolutely not. During 'regulation' the controller is basically an on/off switch, when it is on it charges but during regulation it maintains batt voltage, during the OFF time no power is generated.

As the battery can not accept current (cuz it would exceed set pt voltage) the OFF time is increased, this on.off @ high frequency is how the constant voltage is maintained with increasing off time as the battery gets fuller. then it regulates float voltage the same way.

So it's entirely possible that I will get out there tomorrow and it will show little/no increase in harvest compared to at home, assuming the battery is full?

[red31]

LipschitzWrath wrote:
will it "count" amp-hours harvested even if it can't use them to serve loads or charge the battery?

No, absolutely not. During 'regulation' the controller is basically an on/off switch, when it is on it charges but during regulation it maintains batt voltage, during the OFF time no power is generated.

As the battery can not accept current (cuz it would exceed set pt voltage) the OFF time is increased, this on.off @ high frequency is how the constant voltage is maintained with increasing off time as the battery gets fuller. then it regulates float voltage the same way.

[LipschitzWrath]

Thanks for the information, a lot to digest.

When I got home last night, the panel array had harvested more than enough power to run the RV. And that was on the side of my house where the array was partially shaded for several good sun hours due to neighboring houses and trees.

We pulled it out to the lake last night and got it set up with the intent to return tomorrow night to camp over the weekend. There are no shade sources anywhere close so I am hoping to see even greater harvest.

I know this question is probably highly dependent upon the individual controller I have (Renogy Adventurer 30A), but will it "count" amp-hours harvested even if it can't use them to serve loads or charge the battery?

For example, if the load on my unattended camper is only 20Ah per day but the array is capable of harvesting 40Ah, would my controller say that I harvested 20Ah or 40Ah?

The controller has a logging feature on the bluetooth enabled app, so when I get there tomorrow night I should be able to see what it harvested today and tomorrow.

[Almot]

The difference in losses btw #10 and #8 cable from 200W array (with MPPT) would be less than 0.2 AH a day. Not enough to worry about.
I couldn't justify running #8 (as opposed to #10) even with 500W array. Big arrays are so much easier with MPPT.

[BFL13]

With MPPT, One percent drop on output watts of 200w and battery voltage of 13v, is 15.39 amps at 200 vs 15.23 amps at 198w

Five percent drop is 14.62 amps at 190w

That output watts is after panel heat loss of 10%, voltage drop on wire, and controller efficiency loss, so the actual panel rating was more than 200w of course. (In case you wanted to do it for equivalent PWM and chose 200w unfairly 🙂 )

[Almot]

BFL13 wrote:

The voltage on the controller to array wires is battery voltage with PWM, so voltage drop importance is not so clear to me. You do want the amps though.

With MPPT on the controller to array wiring, voltage drop is important because it affects the "watts in" to the controller, which then has "watts out"

1. I understand that as long as the V-drop doesn't cut below your desired voltage on the battery, it's all good. Then at some point it suddenly becomes not good at all. Series PWM installs without roof junction box are typically limited to 300W, with higher wattage people use wire #6 or better and need a J-box. Some pain... 400W with #8 "might" still let him get away without J-box.

2. Voltage drop with MPPT with RV-sized array of 24V panels is more of an academic point, it does exist but of very low importance. I run the numbers when deciding on whether to use #10 or #8 with 500W in series, and the difference on 60-something ft was 0.5% at max output. Literally a couple amp-hours a day, since most of the time it is below the max current.

Higher MPPT losses with higher voltage - that Piano mentioned - are more important.

[BFL13]

pianotuna wrote:
Just remember that if the voltage to the controller is doubled, the voltage drop is reduced by 50%. Be aware that losses in the MPPT controller rise with the input voltage. Worst case is about 96%.

I do harvest 17 amps from a 256 watt flat fixed installation, with panels in series/parallel giving a controller input voltage of 33. My line losses between panels and controller are on the order of 1%. They would be much higher if I had gone all parallel.

Trying to stay 12v here for both PWM and MPPT to not confuse the voltage drop comparison and the whole thing about wire gauge with 24 vs 12.

On the "12v" controller to array wires, with PWM you have battery voltage (12-15v) and with MPPT you would have 17v Vmp during the Bulk stage, so only a little higher voltage with MPPT there.

The amps on that wire pair would be the 25 amps with PWM at Isc value, and Imp with MPPT being a little less in amps.

[pianotuna]

Just remember that if the voltage to the controller is doubled, the voltage drop is reduced by 50%. Be aware that losses in the MPPT controller rise with the input voltage. Worst case is about 96%.

I do harvest 17 amps from a 256 watt flat fixed installation, with panels in series/parallel giving a controller input voltage of 33. My line losses between panels and controller are on the order of 1%. They would be much higher if I had gone all parallel.

[BFL13]

You could get 25 amps to the battery with PWM and the 400w array, so it is all about distance for the wire gauge. (tables for that)

The voltage on the controller to array wires is battery voltage with PWM, so voltage drop importance is not so clear to me. You do want the amps though.

With MPPT on the controller to array wiring, voltage drop is important because it affects the "watts in" to the controller, which then has "watts out" and you divide that by battery voltage to get your amps to the battery.

[LipschitzWrath]

Alright, good news. There was an issue with the charge controller and it is now functioning correctly and showing actual output.

Today will be a fun day to see what the PV system harvests and if that will be enough to offset the normal daily draw of the camper.

I will report back.