# of watts solar system needed to MAINTAIN coach batteries?

[calthage]

Class A with 2 12 volt coach batteries. Want to maintain while not in use.

[Kaz]

Jim@HiTek wrote:
I know that in my Class A rig the chassis battery draw is much, much more than 100mW (0.1 Watt). It's more like 12 Watt even with the salesman's switches 'Off'.

The draws are to the dash, which supplies the radio memory, the CO detector, and the propane leak detector. Then the steps, also stay powered. Some newer class A rigs have both a computerized tranni and engine which have memories that are, again, kept powered even with the salesman's switches 'Off'.

I think you have a unique rig if yours only draws 100mW while just sitting.

Wow! 12 watts seems surprisingly high. Besides my own measurement at 50mW, the 100mW figure comes from this old rvdoctor.com article (http://www.rvdoctor.com/2009/12/which-circuit.html). And drawing at 12 watts (roughly 1 amp!) would drain my batteries in less than 3 days and that sure doesn't happen.

I assume you've measured the draw, but did you wait for equipment to go into sleep mode? I assume you know this, but when you energize the circuit by reconnecting the battery through the ammeter, lots of the onboard stuff goes into start-up mode where it draws much more current, and it takes several minutes for everything to go back to sleep. If your 12-watt measurement is after everything has settled down, I never would have expected that.

P.S. Great blog! I hope to get back to the Northwest sometime next year, and your posts are definitely encouraging me to make the trip.

[Jim]

Kaz wrote:
Unless I'm missing something (very possible), all you need to is to offset the parasitic drain on the batteries, which is normally less than 100mW (mine is only about 50mW), plus the self-discharge (assume it's 5% per month for a lead-acid battery). That assumes everything in the coach is off and the only load is for the CO/propane detector, the clock battery, and things like that. 100mW x 24 hrs/day = 2.4 watt-hours/day. With a 70-amp-hour battery, assume a 5%/month self-discharge rate = 3.5 amp-hours/month, or roughly 40 watts/month, or 1.3 watt-hours per day. The total is around 3.7 watt-hours/day. Even at 50% efficiency, a 10-watt solar panel with an output for a couple hours per day would provide 10 watt-hours/day, which is way more than you need. That's why most commercial solar battery maintainers are all in the range of a few watts and work just fine.

Or, the easiest way is to do like other people suggest and buy something with excess capacity and then you don't have to worry about it.

Skip

I know that in my Class A rig the chassis battery draw is much, much more than 100mW (0.1 Watt). It's more like 12 Watt even with the salesman's switches 'Off'.

The draws are to the dash, which supplies the radio memory, the CO detector, and the propane leak detector. Then the steps, also stay powered. Some newer class A rigs have both a computerized tranni and engine which have memories that are, again, kept powered even with the salesman's switches 'Off'.

I think you have a unique rig if yours only draws 100mW while just sitting.

[CA_Traveler]

Adding a disconnect switch for the battery and storing fully charged will also solve the problem.

[RCtime]

Kaz wrote:
Unless I'm missing something (very possible), all you need to is to offset the parasitic drain on the batteries, which is normally less than 100mW (mine is only about 50mW), plus the self-discharge (assume it's 5% per month for a lead-acid battery). That assumes everything in the coach is off and the only load is for the CO/propane detector, the clock battery, and things like that. 100mW x 24 hrs/day = 2.4 watt-hours/day. With a 70-amp-hour battery, assume a 5%/month self-discharge rate = 3.5 amp-hours/month, or roughly 40 watts/month, or 1.3 watt-hours per day. The total is around 3.7 watt-hours/day. Even at 50% efficiency, a 10-watt solar panel with an output for a couple hours per day would provide 10 watt-hours/day, which is way more than you need. That's why most commercial solar battery maintainers are all in the range of a few watts and work just fine.



Or, the easiest way is to do like other people suggest and buy something with excess capacity and then you don't have to worry about it.

Skip

Right on

[Kaz]

Unless I'm missing something (very possible), all you need to is to offset the parasitic drain on the batteries, which is normally less than 100mW (mine is only about 50mW), plus the self-discharge (assume it's 5% per month for a lead-acid battery). That assumes everything in the coach is off and the only load is for the CO/propane detector, the clock battery, and things like that. 100mW x 24 hrs/day = 2.4 watt-hours/day. With a 70-amp-hour battery, assume a 5%/month self-discharge rate = 3.5 amp-hours/month, or roughly 40 watts/month, or 1.3 watt-hours per day. The total is around 3.7 watt-hours/day. Even at 50% efficiency, a 10-watt solar panel with an output for a couple hours per day would provide 10 watt-hours/day, which is way more than you need. That's why most commercial solar battery maintainers are all in the range of a few watts and work just fine.

Or, the easiest way is to do like other people suggest and buy something with excess capacity and then you don't have to worry about it.

Skip

[time2roll]

Just as easy to mount a 100 to 140 watt panel and know you have the power to both charge (slowly) and maintain. If your RV does not charge the chassis battery from the house I recommend a Trik-L-Start combiner.

Panel 120w $136

Controller $50

[Golden_HVAC]

My solar panels, and how the screws are placed under the panels, so that rain will not directly fall on the screws. Then I coated the roof to make it watertight again (it was about 14 years old when these pictures where taken).

Golden_HVAC wrote:

rickthescot wrote:
Sounds like a great idea. I guess asking for pictures again is not going to persuade you though. We are visual creatures and I don't think 1000 words will accomplish what one image will. You can do it, just one pic.

More pictures that I took a few days ago to post pictures of my solar panels, and their mounts. Some pictures are really close up pictures of the solar panel mounts, and give a clear picture of howrough the surface is.

Golden_HVAC wrote:
Hi,

This is my front pair of panels. 120 watt mounted on a curved roof, 102" wide.



This is the mount it is held on with three rivets into the frame, and then the bottom half is 6" long aluminum, with nutsert in the upper piece, bolt you can not see from the outside edge pointing inward.









This picture shows the bigfoot panel mount sold by RvSolarElectric for $35 a set back in 1994. I wonder what they cost today?


This picture shows the panels tilted while I was working on the roof.

Fred.

[Golden_HVAC]

You can buy a 140 watt panel for less than it would cost to replace the batteries, or buy a full tank of gas, so why not get that size panel, and be able to enjoy the solar system?

SunElec.com

Look for a 12 volt panel and PWM controller. If you wire it directly to the battery, and use the factory installed disconnect switch, then the battery will still stay full anyplace (well except covered storage or a campground full of trees).

My motorhome used 0.8 amps 24/7 to run the CO meter and propane leak detector, and 1.1 amps (total draw) while the refrigerator gas valve is on, so about 0.3 amps per hour the refrigerator is running. . This works out to about 35 AH daily, or what a 120 watt solar panel will produce in one 'average' day, given there will be clouds, hot panels make less power than cold solar panels, ect.

So forget about the 10 or 15 watt solar panels for $30 - $45. They are in the $3 per rated watt cost range, and not worthwhile. The link above has some great aluminum framed 140 watt panels for $229 and many other brands for much less than that. . .

You can get 25' of UV rated grey romex at Home Depot, along with 2 feet of 2" angle aluminum. Cut into 6" lengths, you will have 4 panel mounts, that will keep the panel about 1" above the roof, so air will circulate under the panel, and keep it cooler (thus more power).

Good luck,

Fred.

[spotrot]

2 watt is more than enough if batteries are disconnected.

My 5 batteries (2 coach, 1 chassis, 1 SUV and 1 motorcycle) were fully charged (12.7v) after almost 5 months in low-light Boise.

Nature Power Solar Battery Maintainers and Trickle Chargers- 2-watt Catalog Item # 56004 $28
model is weather-resistant for indoor or outdoor use

Dimensions: 16" x 5" x 1 1/2"
Manufacturer: Rdk Products
Mfg Part #: 40019

CW low-cost Solar Battery Charger worked over winter

In Nov, I removed the batteries, connected them in parallel, and used an externally mounted CW solar charger (I believe it was a Nature Power Solar Battery Maintainers and Trickle Chargers- 2-watt Catalog Item # 56004 $28) to keep them topped off. I mounted the panel pointing to the south and at an angle I guessed would be perpendicular to the sun's rays during winter there.

This solar charger has no regulator, so I feared its reverse current draw (during night and low light) might do more harm than good by draining the batteries and ruining them.

However, it worked well. Newer models say they have a built-in blocking diode to prevent reverse flow of electricity

I would LIKE to think manufacturers would state the reverse draw of their products but such is often not the case. The year before the solar battery maintainer I purchased from CW, Sun Force 1.8W, User's Manual says nothing about reverse draw. When I got to my RV storage location and measured it, the draw was of an amount that I guessed it would ruined the batteries. So I had to leave them without charging over the winter. Next year I had to replace both the coach batteries.


Sunforce 50012 is $17 from Home Depot

[DSDP_Don]

To make life easy and save any guess work, just buy a 100 watt panel. They're the most commonly available panel.

[Jim]

Those 'optional' solar panels on top of many RVs ACs are 8 Watt and I've never heard of them working very well to maintain even small 12V coach or chassis batteries.

I wouldn't get anything less than 50 Watts in the desert areas, and 100 Watts in the northern states.

[RoyB]

If your battery is disconnected with no load then a very small solar panel will do great...

If it is connected to your truck systems or your trailer system than you have to have enough power to overcome the parasitic drains as well as maintaining the battery...

I have a 9WATT solar panel panel laying in the dash area of my truck and it does not do well keeping the truck start battery topped off. I have too much parastic drain for it...

I leave my trailer system batteries plugged inall the time when parked at home during the season and they PD9260C converter/charger keeps them in good shap. During the winter months I just get my batteries charged up good and then completely disconnect them but leave them sitting in the battery box on the trailer tongue.

They do just fine making it through the winter months here. I do occasionally read their charge state and they usually drop down to around 12.3-4VDC by the end of the winter months.

Roy ken

[deandec]

harold1946 wrote:
I would also recommend 100 watts of solar and a dual bank controller. That would maintain both the house and chassis batteries quite easily.

I also have 100 watts of Solar and 440 hour battery bank.

Battery (and linked Chassis Battery) stays full if I switch off the house battery switch.

If I do not use the disconnect switch, the current draws exceed the solar replacement.

This is due, I think, to my storage area providing sun to the panel for only about 6 hours per day.

[pianotuna]

Hi,

Bare minimum 15 watts per 100 amp-hours of storage assuming no snow cover.

Better 30 watts per 100 amp-hours. This would actually recharge the bank between trips.

60 watts per 100 amp-hours if you wish to be able to equalize the bank.