Solar and battery help

kjenckes wrote:
Good Morning. I am hoping someone can provide a simple answer or 4 for me. I just read a post on the TC forum about flexible, light weight 100 watt solar panels and love the look of this, I have a Northstar 850SC pop up that this would go on. I have plenty of room over the bed end for these with out shadows from roof top stuff. I currently have 1 Interstate SRM-27 listed as 160 RC. (I will be adding a 2nd) I would prefer 31s but am not sure about the space available yet.

#1- Does the 160 RC on the battery mean 160 amps?

#2-50% discharge or 80amps (If #1 is yes) or 12.2 volts is the lowest I want to take this battery on a regular basis, correct? Therefore 2 would give me 160 amps?

#3- The listed optimum amps for this 100W panel is 5.7 amps with a max fuse of 10amps. Does this mean that 2 of these on the 20 amp controller is appropriate as the minimum? 30 safer? Could I run 3 on a 30?

#4- What is the rule of thumb for minimums watts per battery? I know sunlight hours and shade, etc affect all of this. I see some with 270 or more watts but no mention of how many batteries this supports.

My goal with all this is to get my wife out more with me. We both use CPAP (12v ). She is not as comfy with out some of the comforts of home as I am. We live in NH and the State Parks in NH and VT as well as the White Mt. and Green Mt. National Forest sites are almost entirely off grid.

I have learned a lot from this forum and want to thank every one for their posts.

1. Maybe 105 amp hours (not "amps" BTW)

2. Correct, except two 52s

3.a. Use the panel's Isc for your "expected amps" in full sun. The usual 100w panel has Isc about 6.3a

3.b. Size the controller to have a 20% margin over the total Isc. So two 100w in parallel gets you expected amps of 12.6 and 20% of that is 2.5 so plan for a controller at least 15a, so a 20amp is plenty.

4. There is no valid rule of thumb since there is no common scenario for each Rver. The weatherman runs the show anyway, so you get what you get on any given day. Just get as much solar as you can fit or can afford and be ready to run the generator if the weatherman is in a bad mood. But in general, for the likely usage of a truck camper, 200w is bags of solar most of the time unless you are in the woods where solar is fairly useless no matter how many watts you have.

5. An inverter will let you run 120v stuff off grid. DW will love having some 120. (as long as she knows not to run the batteries down too much)

6. Find the "Screwy 31" thread. Read it and weep. Don't get 31s

Lots needs to be considered to properly set up solar for your rig. You can base it on how much power you use. How much battery you have. What kind of conditions you are in. Honestly though, it's a combination of all of them.

A properly balanced system starts with you knowing how much power you use daily. You need enough battery to support that use for X amount of days. I say X because it is affected by room and budget. Once you know how much battery to have, get enough solar to get them back up quickly after a few days of clouds. A prudent thing to have is a back up means of charging for when MR sun doesn't play nice for too long.

Now that you know how much solar to get, balanced with amount of room on the roof and budget, you can start thinking about which panel/s fit your needs and conditions the best. The most efficient panel in bright sun sucks in lower light. The panel that can provide in lower light is huge in foot print for the watts it produces. The choice is a balance of room, budget and production. In the end you may have to compromise.

One thing I have learned about batteries from the forum is that some of the cooler ones are just a PITA to keep charged up. I have the same grp 27 as you and it is easy to keep up. I would have to replace my charge controllers and converter to switch to fancy AGM batteries. Again, making sure al of the components work well together is part of the balance.

You asked about the watt vs AH ratio. We have 720w of panel for 345 ah of battery. That gets the batteries up fast when we have limited sun and keeps them in float all day unless we run a big load like the microwave. Even then the recovery is very quick and the batts are always fully charged by night fall. Loads that are smaller than the output of the panels are not seen by the battery at all. The TV/sat runs all day and never has a effect. next year I'll run a 5000 BTU AC unit and if it doesn't run straight off the panel, what comes out of the battery will be very little.

My panels are huge and not 12v, so I'll let others advise you on the proper set up.

Don't get all that excited with 100WATT SOLAR PANELS... A 100WATT Panel will only produce around 4-5AMPS DC CURRENT for the time they are in HIGH SUN. The rule of thumb is around 5-6 hours each day is the normal time you can expect full output of the panels. Out west in wide open country gives you more high sun of course...

Your SRM-27 with a Reserve Capacity of 160 minutes will only produce 12VDC Power@25 AMPS for around 160 minutes (2.7Hrs). Having two in parallel will raise this to around 5.3 HRS... If you do this your batteries will be drained to their 0% charge state or 10.5VDC. You really don't want to ever do this to your batteries and most of us respect the 50% charge state and I really get excited when my batteries drop below 12.0VDC. Most of us think continuing to drop below the 50% charge state will do harm to your batteries.

My thinking is to beef the battery banks and battery associated battery cables etc to run the thing you want to run when camping off the power grid so that you can operate things until 8AM the next morning when you are allowed to run a generator to re-charge your batteries...

To me adding solar panels only enhances this operation and perhaps you won't have to run your generator as long each day to keep up with battery drains.

All of us knows that in order to re-charge a 50% depleted battery it will take around 14.4VDC at a 20AMP DC Source for each battery in the battery bank. This will drop back to 13.6VDC after a two hour run and give you a safe 90% charge state on the batteries in around three hours total time. With a 90% charge state you can do the run off the batteries again for the next day/night battery run.

If by adding solar panels you can expect around 5AMP of charge current going into your battery bank and if you have enough panels you might get this up to around 15AMPS for 6 hours on your TC roof size. In 5-6 hours this will put back around 30AHs into your batteries. If you day/night run off the batteries is less than 30AHs then the solar panels will be great for you to have. You will never get behind the battery drain curve having them.

Of course this is not always guaranteed as some days will be cloudy and this could last several days where you won't get any charge at all that you can use. You would need to have the generator running your on-board converter/charger to replenish your battery charge for these days...

I have been playing this day/night run on my batteries for a few years and we get by pretty good with our routine with around 255AH Battery Bank capacity. This of course have to be re-charged each day starting at 8AM each morning with our 2KW Generator running our trailer and allowing the on-board converter/charger to bring them back up to their 90% charge state so we can do it all over again the next day/night run off the batteries.

My OFF-ROAD POPUP Trailer roof is much the same size as your TC ROOF and I am planning on adding two 120WATT Panels in the front and then a 240large panel on the rear side. This 480 Watts of solar should give me a good 14.4VDC @ 20AMPS for around 5-6 hours a day (160AHs) and really help me out keeping my battery bank drain so that using the generator will be minimal. Since my three batteries normally draw about 53AMPS DC current when first hit with 14.4VDC charge voltage I will have to use the generator for that first 0NE hour charge period. After the battery start tapering off the solar panels can deal with the charge source amps pretty good then. Then I can hopefully switch to the solar panels for the rest of the day to top off my battery bank to the 90% charge before the high sun goes away... This is going to be my basic routine at any rate when camping off the power grid.

This is my take on adding solar panels for our camping off the power grid routine. All of this takes planning... Others have different ideas on what to expect but I am pretty confident based on my few years of running entirely off the batteries this is what will happen for us when we finally get solar panels installed. We watch our BATTERY MONITOR PANEL like a hawk knowing when it hits 12.0VDC we have to shut it all down... This is planned to happen at 8AM the next morning when I can deal with it...

For your situation you may get by just fine having the solar panels and not get into your battery reserve capacity. What happens if it doesn't work like you have planned. Got to have a good PLAN B hehe... Not fun when lights go out at 10PM at night especially for needing the CPAP at night... For us a generator is must have item and as you know we folks that live on the East side of the US have all of the generator run time restrictions to deal with. So bigger battery banks and solar is good way to go...

Just adding another GP27 battery will only give you an additional 96AH of capacity. So you question is can you live on 192AHs of battery capacity especially when you can only use 96AHs of that total so that your batteries will not go below 50% charge state. Add up your DC power drain and how long you expect to use it and see how you want to be battery wise. i.e your CPAP draining 12VDC@3AMPS needing to run for 8 hours will consume 24AHs of capacity... Just considering that only leaves you 72Ahs for other 12VDC drains. Not much to play with. Watching TV for 6 hours will eat up at least 24AHs... Hopefully you have changed over to LED lights already.

Just passing along for food for thought on your end...
Roy Ken

Battery length 10-3/4" Group 24 85 amp hours

12" long group 27 100 amp hours

13" lone group 29-30-31 110 amp hours

RC 160 does not indicate amphours and most group 27 batteries hover around 100 amphours give or take and that is for a 100% fully charged, non-sulphated battery in prime condition.

Adding a second battery would indeed add amphours, but the second battery will be dragged down by the older battery. You would add the amphours if using 12 volt jars giving you around 200 amphours with roughly 100 amphours to burn before reaching 50% discharge. Of course these are estimates and real world performance is usually lower than expected. Pairing batteries of the same age, specs, and manufacturer would yield better results.

As long as amps don't exceed a controllers capabilities you could group as many as the controller is rated to handle. A 100 watt panel will require a 10amp controller, but you could group four 100watt panels on a 30amp controller. The max amps delivered by the 100watt panel will only occur for a few hours a day if conditions are ideal.

That is not an easy question to answer as there are too many variables involved. The main concern is what exactly do you expect from your solar array. Are you needing to get through a few days of camping and just need to get battery bank to roughly 80-90% SOC, or are you planning month long excursions where you want to reach full SOC on a regular basis.

A 100 watt panel for each battery would be a good start without knowing your actual daily amphour consumption, with decent sunshine you would more than likely get into the 80-90% SOC daily which is fine for a week or so of camping. Actual results will vary of course, metering the amount of amphours you consume and amps delivered by your solar array will yield pertinent information for fine tuning your system to achieve the expected results you desire.

Solar has worked well for us, but camping in the White Mountains of NM or AZ in our neck of the woods usually means that solar will be limited due to the tree cover, for those situations we use our generator and Iota converter to quickly restore consumed amphours.

Additionally, having the genny along we can run high amperage items like the microwave/convection oven, AC unit, portable space heaters, and heated hair appliances used by my DW and 3 DDs when camping off the power grid. We prefer boondocking and being 100% self-contained makes everybody happy knowing we can power our entire rig as if we were connected to shore power.

I have the Interstate grp 27 and it's 95 ah new and fully charged.

Very good advice on Solar as stated above. :)

Quote from RoyB "your CPAP draining 12VDC@3AMPS needing to run for 8 hours will consume 24AHs of capacity"

Thing is, you have TWO to run. The draw is also determined by whether you use one or both with or without a humidifier.

How often do you Dry Camp ? For how long of time ? What converter/charger do you have ?

Myself, I cannot go without my CPAP. I use a dedicated sealed battery for it & charge as needed with my portable battery charger & generator. My converter/charger is old & worthless for charging purposes.

I believe one can multiple the hours of discharge at a certain load to get capacity at that load.

86 ah @ a 5A load, 72 ah @ 15A load
http://www.interstatebatteries.com/m/category/marine/deepcycle

red31 wrote:
I believe one can multiple the hours of discharge at a certain load to get capacity at that load.

86 ah @ a 5A load, 72 ah @ 15A load
http://www.interstatebatteries.com/m/category/marine/deepcycle

However, we don't run the load to battery death, we stop short of that and the battery recovers to be able to run more, so the capacity figure for the load in such tables is lower than you get in real life.

RVers run a mix of high and low amp loads all day, so the battery capacity rated at the 20hr rate is about right for a two-battery bank with about 11a as the rate in many cases.

If the RVer averages less than the 20hr rate, then his bank capacity is more than it is rated for thanks to Mr Peukert.

kjenckes,

That battery is not a true deep cycle battery, it is what is sometimes referred to as a hybrid; part starter battery, part deep cycle. They serve partly to provide low draw power on boats, but also provide high cranking amps when needed for starting an engine.

Golf cart batteries, otoh, are closer to being true deep cycle batteries, and although they can be used as a starter battery, they do their best job when handling long sustained low amperage draws. You may want to consider a pair of 6 volt gc-2 batteries, if you plan to boondock a lot.

As for your choice in converter/charger, please note a 'converter', with a battery charging feature, will indeed drop to around 13.6v, once your batteries reach somewhere near the 80% charged state. The problem is, this adds many hours to the charge time required to bring your batteries back up to 90-100%. If on generator, this is far from ideal. In such a case you require a charger that continues at the original higher voltage setting. And no, this will in no way harm your batteries. If on solar, however, this is not so much a concern, seeing as solar is clean, quiet, and runs on free fuel from the sun, not expensive, smelly, petrol. But if for some reason you find yourself relying on gen power more often than not, you will need to use a different charging unit. Remember, there is a good reason why (off grid) boondockers and dry campers go solar, and that is primarily to reduce generator use. So to use a device designed primarily for on-grid camping makes no real sense.


I too am in the process of figuring out my solar needs, and as you yourself and others have stated, it depends on needs, location, weather, and so on. I have concluded I need a minimum of 180-200 watts of poly portables, for instance.
As for these 100w flex panels, they are very pricey! For example, an aluminum framed glass panel will cost $1-$2 / watt, where as a flex panel could cost upwards of $4-$5 / watt. There are some good deals out there, however, so hopefully you can get what you want without breaking the bank.