Need some solar help for our flatbed camper

Skyko · February 2014

Hi,

I would like some opinions on what would be a good setup for our RV truck camper. A little background: We wanted a true flatbed camper with a lot of headroom but they didn't exist. We ended up building our own aluminum skinned camper to go on a Isuzu NRR flatbed truck (rear of the flatbed for toys like motorcycles).

The plan is to put a generous amount of solar up on the roof for remote camping for weeks at a stretch. I am going with a Nova Kool RFU9000 12V compressor fridge, so that will need about 1000 watt-hr per 24 hours. We have a small 500 watt AC unit that we might operate for a few hours at night to get to sleep. All lights are LED, so the power there is minimum. Laptops probably a few hundred watt-hr per day. We will have a little Honda 2000 generator but hope to use it very little.

I am going to put 2 group 31 AGM in the camper and possibly 2 or 4 more under the flatbed. 400 to 600ah total. We are going with a Magnum 3000 watt true sine inverter with generator sharing.

On the solar, we are thinking of going with four 265 watt monocrystaline panels made by solarworld. Each of these panels is 37.5 x 66 inches. In full sun they would provide 1000+ watts. The fridge will probably need around 800 to 1000 watt-hr per 24 hours. Assuming 6 hours of full sun, we would have a surplus of 5000+ watt-hr for other needs. In cloudy weather we could expect about 500 to 1500 watt-hr per day.

I need to figure out what MPPT controller I need and what wire size would be acceptable. A series parallel configuration would be about 60V at 18 amps. I^2R losses in a 25 foot run (50 feet of copper) would be 6.5 watts total for 6AWG cable. In a 30V 36 amp configuration, I^2R losses in the same cable would be 26 watts. I have not looked into what maximum voltage a MPPT controller can take.

This is what the four panels would look like on the top of the camper (still a bit of room for other stuff). What do you think? Any suggestions or opinions on the panels or a suitable controller or how the panels might withstand the rigors of the road?

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Cariboocoot · February 2014

Re: Need some solar help for our flatbed camper

Okay let's look at the basic math.
Four 265 Watt panels is 1060 Watts. They won't provide 1000 Watts. More like 800, depending on how your sun angle is. Panels flat on the roof means the sun angle isn't likely to be direct unless you're at the Equator. They will need to go through an MPPT controller as that will be 68 Amps peak current (on a 12 Volt system) which should be sufficient for even 600 Amp hours @ 12 Volts.

Number one: you will need a big MPPT controller due to the >60 Amp current potential. We're talking FM80 or Classic 150.

Number two: you're going to have trouble keeping those batteries charged with them being all in parallel and located in different spots. There is no way the wiring resistance will be the same on each one, and as such the current sharing will be uneven. This is a particularly bad problem with 12 volt systems. The only solution is to put the batteries all in one place so the wiring between them and the loads/charging is the same. Preferably use larger capacity 6 Volt batteries in series to reduce the parallel connections.

600 Amp hours at 25% DOD on 12 Volts is approximately 1.5kW hours AC. Is that enough for all you want to do? It sure isn't 5kW hours. The over-all system measurement, even with 6 hours of good sun (which you will probably not get) is more like 3.3 kW hours. Roughly half your estimate. That is with having to store all power in batteries. The more you can use 'directly' from the panels (while the sun shines and the batteries are full) the better the efficiency will be.

A 'small 500 Watt A/C' could use your total stored capacity of power over night. If it has a 1/3 duty cycle (runs 1/3 of the time) in 9 hours it will eat up 1500 Watt hours. Boom; dead batteries.

I've got two suggestions: go for a 24 Volt system and skip the inefficient 12 Volt stuff, and get a generator.

vtmaps · February 2014

Re: Need some solar help for our flatbed camper

Cariboocoot wrote: »

Number two: you're going to have trouble keeping those batteries charged with them being all in parallel and located in different spots. There is no way the wiring resistance will be the same on each one, and as such the current sharing will be uneven. This is a particularly bad problem with 12 volt systems.

And the problem is even worse with AGM batteries than other lead-acid batteries. A short discussion of parallel batteries.

--vtMaps

Skyko · February 2014

Re: Need some solar help for our flatbed camper

Good feedback thanks. I wonder about using some other battery chemistry also. 25% DOD doesn't sound really great.

If we went with a 24V system, we would need to have some way of providing 12V to most of the wiring, as a lot of the RV stuff is 12V. Either have some sort of isolator or a 24 to 12 dc-dc. The fridge can run directly on 24VDC.

Cariboocoot · February 2014

Re: Need some solar help for our flatbed camper

Skyko wrote: »

Good feedback thanks. I wonder about using some other battery chemistry also. 25% DOD doesn't sound really great.

That isn't the maximum, just the standard. Taking batteries down to 50% repeatedly has a noticeable effect on their lifespan. And recharging from that depth requires even more panel (because you only have 'so much' time to do the charging in from sunlight).

If we went with a 24V system, we would need to have some way of providing 12V to most of the wiring, as a lot of the RV stuff is 12V. Either have some sort of isolator or a 24 to 12 dc-dc. The fridge can run directly on 24VDC.

I think I'd be eliminating the 12 VDC stuff as much as possible. Where it's not possible a DC to DC converter is viable, especially on lower-powered loads. Hopefully you weren't planning on using the vehicle engine as a supplementary charge source.

Skyko · February 2014

Re: Need some solar help for our flatbed camper

Thanks. The space inside the camper I designed for just two group 31 batteries. It is vented but is underneath the spot for the Magnum inverter and also is vented somewhat to the living space. For this reason I had wanted to use AGM batteries, at least for those two. The batteries I put under the flatbed could be regular wet cell lead acid.

What if you had two 12V AGM batteries in series in the camper, for a total of 100AH 24V and had another bank of four 6 volt golf cart batteries under the flatbed at 200ah. Could you then either build a circuit to bank switch these or perhaps there is some way to equalize the charge and discharge between the two banks? I would need to be able to disconnect the batteries under the flatbed from the camper when it is removed, and it would be nice if the camper still had power when it was removed (ie, the 100ah onboard AGM batteries).

Cariboocoot · February 2014

Re: Need some solar help for our flatbed camper

Skyko wrote: »

Thanks. The space inside the camper I designed for just two group 31 batteries. It is vented but is underneath the spot for the Magnum inverter and also is vented somewhat to the living space. For this reason I had wanted to use AGM batteries, at least for those two. The batteries I put under the flatbed could be regular wet cell lead acid.

On the top of the "don't do it list" is mixing battery types. They don't charge the same, so you get a choice of improperly charging one or the other.

What if you had two 12V AGM batteries in series in the camper, for a total of 200AH 24V and had another bank of four 6 volt golf cart batteries under the flatbed at 200ah each for 800ah. 24V. Could you then either build a circuit to bank switch these or perhaps there is some way to equalize the charge and discharge between the two banks? I would need to be able to disconnect the batteries under the flatbed from the camper when it is removed, and it would be nice if the camper still had power when it was removed (ie, the 200ah onboard AGM batteries).

Having batteries of any type in two different locations used on the same system is going to create charging problems. Using a dual charge controller may alleviate this but for he problem of having to isolate the two separate battery banks which makes it inconvenient for use on one load.

You could have a 12 Volt system for the 12 Volt items and a 24 Volt system for the 120 VAC items. The 12 Volt system could be run a a sub of the 24 Volt by using the latter to charge the former, or have its own solar charge source.

westbranch · February 2014

Re: Need some solar help for our flatbed camper

It looks to me like you have need for the 2 different systems and at least 2 if not 3 voltages.
You have 3 large loads, the fridge, the A/C and the electronics,
and at least 2 voltages 110V AC, 12 V DC and
3 groups of loads (watts), A/C hi watts =~ 1000W A/C surge or possibly more, medium DC wattage (fridge) and low Wattage (?~ 50w) A/C for electronics.

So you might also want to look at having 2 inverters, big one for the Air Cond. and a small one for the electronics, or just run the Honda for cooling.

Lots of options.

vtmaps · February 2014

Re: Need some solar help for our flatbed camper

Skyko wrote: »

The space inside the camper I designed for just two group 31 batteries. It is vented but is underneath the spot for the Magnum inverter and also is vented somewhat to the living space. For this reason I had wanted to use AGM batteries, at least for those two.

fwiw, when AGM batteries are installed to code, they have the same venting requirements as other lead acid batteries. This is because there are a number of ways they can be overcharged and vent (or even burn).

In a perfectly designed system there would be no need to vent an AGM battery... but there would also be no need for fuses.

Another measure of safety that pertains to batteries is temperature compensation. When things go wrong (for example, a cell shorts out), batteries usually heat up before they burn. Temperature compensation will lower the charging voltage to a hot battery. btw, a shorted cell would also likely cause hydrogen venting.

westbranch wrote:

It looks to me like you have need for the 2 different systems and at least 2 if not 3 voltages.
<snip>
just run the Honda for cooling.

Lots of options.

Westbranch is on to something... and there are lots of options. I suggest you look at some of the marine solar electronics such as Victron. Multiple battery banks are very common on boats and there is a lot of gear designed for that purpose.

--vtMaps

Skyko · February 2014

Re: Need some solar help for our flatbed camper

I want the camper to work when it is off the flatbed truck and the battery bank under the flatbed is disconnected. An example is we are boondocking and unload the camper onto the ground. We then take the flatbed on an errand, or go collect firewood, or take it to the shop for repairs (and drive our WR250R back to the camper to spend the night). I still want the fridge, lights, and inverter to work, and it would be nice if the solar on the camper was doing something while the camper is not connected to the flatbed.

I like the idea about 24V as the I^2R losses are 1/4 that of the 12V. The 24V Magnum 4000 watt inverter is about the same price as the 12V 3000 watt inverter and is 94% efficient vs 90%. The umbilical connecting the under the flatbed battery bank to the camper also would have lower losses on 24V vs 12V. Even using 00 cable you are going to have significant drop @ 200 amps on the 4000 watt Magnum...200^2 * 0.001 ohm is still 40 watts, but better than 160 watts!

So to keep the camper independent from the flatbed, and to keep all batteries happy, I am now thinking about some sort of bank switching arrangement. Either a mechanical battery selector switch (honkin 200 amp switch) or an electronic switch that flips banks when a bank has been discharged or fully charged. That doesn't really sound complicated to buy or build.

Two 12V 100AH AGM in the camper in series, 1000 watts of solar on the roof, a Flexmax 60 MPPT, Four 6V 220AH wet cell under the camper and a bank switch mechanism to rule them all.

Skyko · February 2014

Re: Need some solar help for our flatbed camper

Cariboocoot wrote: »

On the top of the "don't do it list" is mixing battery types. They don't charge the same, so you get a choice of improperly charging one or the other.

The only reason I wanted to mix battery types was because I thought AGM would be a teeny bit safer inside the camper. I only have space for two batteries inside the camper, so really for a 24V system it has to be two 12V. I am now really wanting to use the flooded 6V batteries after reading a bunch of other posts on this forum. I like the Trojan T-105 and they are only $150 for 6V 220ah.

Since 90% of the time the camper will be mounted on the truck, I could treat the bigger bank of four 6V 220ah series wired batteries as my main bank and keep the bank selector switch connected to them. The two AGM 12V series wired batteries inside the camper would be reserved as backup and would not be charging or discharging on the same system as the flooded cell batteries. With my 1000 watts of solar (or thinking of it as about 700 watts actual output in the southwest and non-tracking) I should be able to put 25 amps into the 24V 220 amp-hr golf cart battery system for 5 to 7 hours on a sunny day. Thus I could probably bring that system back to a full charge from a 50% DoD. It doesn't sound like I need much more than this battery capacity for my small amount of solar (which is a pretty large amount of solar for a truck camper actually).

On the solar panels, would it be a good idea to go with smaller higher cost panels and use more of them? I do not know if the large 38" x 66" panels will hold up to the stresses of truck camper life. Wind load, vibration, snow, pinecones, small branches. I am flying blind here.

Cariboocoot · February 2014

Re: Need some solar help for our flatbed camper

Here's the thing: if you connect the AGM's to the same charge source as the FLA's and set the Absorb Voltage for FLA the AGM's are likely to vent, meaning you've just lost their "no gassing" advantage - as well as the batteries. I can definitely see what you're aiming for here and the problems it presents. I don't see any obvious solution (in part because I don't know all the design limitations).

One thing you would have to evaluate is whether or not the smaller 'inside' bank could handle the power needs when the truck is not present. If "yes" then you need to question whether the larger 'outside' bank is truly necessary. If "no" then you have a problem which is not going to go away without some redesign.

The only advantage to going to multiple small panels is reduced shadow interference if there are any shadows to be dealt with. Otherwise the panels are all constructed pretty much the same and will withstand the elements alike. One issue you may have with larger panels is flexing (while going down the road and being loaded/unloaded), which would not be a good thing at all. There's no easy way to evaluate that possibility either. Probably would work best if the panel orientation was kept 'parallel' to the camper design: long dimensions aligned lengthwise rather than across the width.

Skyko · February 2014

Re: Need some solar help for our flatbed camper

I would probably connect the Magnum 4000 watt inverter's charging system (and moniter) which is 125 amps to the flooded 6V golf cart battery bank and also connect the MPPT charger to this bank. What I might do for the inside smaller AGM bank is have a much smaller MPPT solar charger that taps into the same solar panel setup. I can't see there being an issue with two MMPT chargers connected to the same solar bank...how would they even know each other is there? I could set the bigger MPPT charger to the correct setting for flooded cells and set the smaller MPPT charger for the correct setting for the inside AGM batteries.

A big advantage here is redundancy. If for some reason one of my chargers went out, I could limp by on the other battery system while waiting for parts.

Aside from needing to buy one more charger and maintain two AGM, I am not seeing a lot of downside to the two battery bank setup. I think we need the larger capacity of the 6V flooded cells but I also think for short periods we could reduce power consumption and rely on the two inside AGM batteries (just for running the fridge, lights, and furnace fan, for example).

This discussion really has me thinking, which is a good thing before I buy stuff. Thank you!

vtmaps · February 2014

Re: Need some solar help for our flatbed camper

Skyko wrote: »

I can't see there being an issue with two MMPT chargers connected to the same solar bank...how would they even know each other is there?

It won't work... each MPPT is trying to load the panels to match the battery's needs. --vtMaps

Skyko · February 2014

Re: Need some solar help for our flatbed camper

vtmaps wrote: »

It won't work... each MPPT is trying to load the panels to match the battery's needs. --vtMaps

From a curiosity technical standpoint, what would happen? Would they go haywire trying to fight each other to steal the most power from the panels? Could it work with one MPPT controller and one PWM controller both connected to the same solar array?

Cariboocoot · February 2014

Re: Need some solar help for our flatbed camper

vtmaps wrote: »

It won't work... each MPPT is trying to load the panels to match the battery's needs. --vtMaps

Yes: as a rule, it won't work. You can connect the outputs of different controllers to the same bank of batteries but no the inputs to the same array of solar panels.

Technically it can be done but ... not worth the hassle.

Besides which there is no point to doing so: if the array is too big for one controller, you would use two anyhow and not need the panels connected together. If you have two different batteries to charge off one array there are dual battery controllers (somewhat limited in capacity) such as this: http://www.solar-electric.com/modubachco25.html

BTW if a charge controller fails (which is actually quite rare) you can bypass it and connect the array directly to the batteries, dumping whatever current is available. Then watch the battery Voltage and disconnect when it gets in the Absorb range. It isn't perfect, needs to be watched, and should not be used long-term but in a pinch it can save your batteries from being left in a low SOC and sulphating.

Skyko · February 2014

Re: Need some solar help for our flatbed camper

Ok, then maybe another way is a mechanical or electrical switch that routes solar panel output to either one charge controller or the other, depending on need.

In this fashion you would not need to reprogram the two MPPT charge controllers for different batteries, but could just route solar power to one or the other. To each according to his need, from each according to his ability.

Or we could try and find one of those dual bank output charge controllers that would work with a 24V system at a 60 volt panel power input.

Cariboocoot · February 2014

Re: Need some solar help for our flatbed camper

Look at it like this:

You have two battery banks. A large one ('A') and a small one ('B'). Together they require 1000 Watts of panel to recharge. But each is a separate bank, so in reality 'A' needs 600 Watts and 'B' needs 300 Watts. As such there's no reason why you can't keep them entirely separate on the charge side, each with its own panels and controller. That eliminates the differences in charging profiles.

Now you want to use them together. Or do you? You could have 'critical' items dedicated to bank 'B' and 'non critical' items dedicated to bank 'A'. You can switch your DC refrigerator between the two, so they could even be different Voltages if need be.

Or at the same Voltage you could, for example, use a Blue Sea battery switch to select 'A', 'B', or even 'A&B' if necessary. It will require some attention on your part because you would not want to connect the two different battery types together for very long (current would flow from the higher Voltage one to the lower under certain circumstances and depending on how much wiring resistance is actually present between the two).

Perhaps you could give some thought to making the larger, external battery bank something that can be 'dropped off' the same as the camper. After all, even though it will be large and heavy it's bound to be smaller and lighter than a portable building.

zoneblue · February 2014

Re: Need some solar help for our flatbed camper

Interesting project. When i fitted out our motorhome, i was like many RV people and tended to overrestimate the performance of solar, and under estimate the loads.

1. solar panels dont make what we 'intuitively" think. Mainland US, 6 sun hours is optimistic, unless you only camp in the desert, in the summer. Check out the actual values where you go, when you go.

2. solar panels flat on a hot roof make even less energy. Theres the off angle of incidence for a start. Then theres the extra heat from not having the recomended 6" underside clearance. And the cruncher is shading. Most places worth camping have trees, and often youll want their shade to help cool the structure which has off necessity minimal insulation.

3. Two group 31s wont power your fridge. The maths dont add up. A DC compressor fridge is a relentless load, and none more so than one running in a hot and inadequately vented confined space. Im not saying this to cast aspersions on your build, im sure youve done a better than average job of it else you wouldnt have done it yourself. Im just speaking from my own experience and mistakes!

Ok, that out of the way. Im not so much worried about finding a way to run two diff banks, with some creativity that could work. But the idea of disconnecting the camper from its service base, sounds nice, but i wory about the complexity of disconnecting the plumbing, the electrics... Batterys need crimped lugs nuts, bolts and 70mm2 cable. While i dont doubt that some form of 500A quick disconnect exists somewhere on the planet, you better check the prices. Edit: A quick check shows anderson powerpoles go up to 350A.

Re the PV they are suprisingly strong. I have 300W panels that if you press the center, the bow is really quite considerable. But we had 25mm hail here last year, a regular gale force winds, and not a scratch. In fact they will probably add bracing to your structure! And they will certainly shade the roof, which has its benifits for internal temperature. However there are anecodotal reports that small stones can break the glass. Theres a thread here about weed eaters. And we once had a snotty neighbour tossing small pebbles at our builders for being noisy that shattered a couple of solar thermal panels which are made of simialr glass. And if the chips on the top front area of our motorhome are anything to go by stone damage might be something to at least think about. Redundancy is always good in any system, and having lots of possible string configurations in the event of trouble is good.

Re batterys i would seriously consider lithium. In some circles boaties are fair flocking to them, becasue they solve almost all of the issues with lead chemistrys in small, mobile, weight constrianted, charge constrained environments. See here for a good start:
http://www.cruisersforum.com/forums/f14/lifepo4-batteries-discussion-thread-for-those-using-them-as-house-banks-65069-215.html#post1408018 . For the weight of those groups 31s will get about 5+ times the energy storage using LiFePo4. And the price (at least the TCO) is now competitive with lead. Be prepared for a big learning curve and understand what youre getting into though.

While there probably isnt much substitute for experience, do as much reading as you can. The topic is bigger then it looks and there are several dangerous elements to solar electric systems, that have the potential to make mistakes less forgiving. But sooner or later youll get to try out your design in the field, and thats half the fun, getting to redo it, better, later

Skyko · February 2014

Re: Need some solar help for our flatbed camper

zoneblue wrote: »

3. Two group 31s wont power your fridge. The maths dont add up. A DC compressor fridge is a relentless load, and none more so than one running in a hot and inadequately vented confined space. Im not saying this to cast aspersions on your build, im sure youve done a better than average job of it else you wouldnt have done it yourself. Im just speaking from my own experience and mistakes!

I was looking at the Nova Kool RFU9000A, which draws around 4.5 amps at 12V. Even if it ran 100% duty cycle for a 24 hour period that would be 108ah, which might just fit into a 50% DoD on two group 31 AGM. More realistic would be a 30% to 40% duty cycle which would be 43ah. Assuming you actually managed even 100 watts from your 1000 watt panels for some 4 hour period during the day would bring it down to maybe 35ah, or about 3 days use. I do own a Honda 2000 portable generator and would keep that as a backup when we did not have the camper connected to the bigger bank of the flatbed truck.

It is all good info to think about and I get your point that people overestimate the usability of solar in mobile applications.

I like the idea of Lithium, except I have seen a few battery fires. I actually made my own electric tadpole trike with a 1000 watt hub motor and 1000 watts of LiPo (goes 45mph top speed...scary) but I keep the batteries in a vented metal box in the garage on a concrete floor :-)

I know about LiFePO4...didn't use it in my bike at the time because it was new and LiPo was cheaper and higher energy density. My 1000 watt-hr pack is about 15 pounds and I can get 800 watt-hr out of it. Imagine how heavy a lead acid would be that you can get 800 watt-hr out of.

Maybe it is time to revisit LiFePO4. 80% DoD and a third the weight/volume of lead acid...

zoneblue · February 2014

Re: Need some solar help for our flatbed camper

Skyko wrote: »

I was looking at the Nova Kool RFU9000A, which draws around 4.5 amps at 12V. Even if it ran 100% duty cycle for a 24 hour period that would be 108ah, which might just fit into a 50% DoD on two group 31 AGM. More realistic would be a 30% to 40% duty cycle which would be 43ah.

30% duty is realistic in a cool environment. But in mobile applications can be much higher. I generally find that building more pessimism into designs means its more likely to be trouble free. Old engineering habit.

Ok so you have a 800Wh fridge, so thats 30% DOD on a 2500Wh bank, right?. When all the (routinely) usable capacity is taken, you still need to consider the rest of your gear. But if this is only for emergencys then yes it will get you by in a pinch..

I know that lifepo4 thread is daunting, but everything you ever need to know about the technology is in there, all 200+ pages of it. Very much still an enthusiast market, but very promising for mobile.

Need some solar help for our flatbed camper

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