2 competing goals - Can I do it?

DirtboatDirtboat Registered Users Posts: 2
I have been interested in putting solar on a small 13' fiberglass trailer for a few years now. I am going to do it this summer, but I have a trip planned in March and I want to extend my battery life on this trip, so I'd like to get a PV for this upcoming trip and then reuse it as a trickle charger for later storage.

The more I read, the more I realize I'm clueless. I have clearly been abusing my battery since I got it. I'm changing my ways.

I would like to get a small PV and maybe a controller to charge or minimize loss on my 10 day trip in March. I would then like to use that same set up as a trickle charger for times when the trailer is in storage.

The loads for the March trip are very low. I know I'm supposed to start with loads first, but I don't have a feel for those numbers yet. Ideally I would charge a phone, run a laptop for a couple hours each day, and run the water pump for a quick shower. That is all. No lights, no fans, no TV or radio. Fridge is on propane and I am probably over estimating the laptop use. Last year, I borrowed a 10-15 watt pannel and wired it directly into the battery and I made it through fine, but I now understand that probably did next to nothing. This year, I'd like to use the laptop more.
The current battery is nothing special - Walmart 115AH.

Future loads with the full solar system installed would more than double. More pump use, lights, occasional fan, etc., but still nothing excessive. I assume my future battery would be about the same size if not slightly larger.

I have read that 1% charge rate is the cut off for needing a controller, but at that level, I am not really getting any charge. I would like to make it through the 10 days of dry camping with no problem, so I am considering a larger PV and a small controller - maybe 55 watts and that Sunguard 4.5Amp.

If I got a 55 Watt PV, that would put me around 2.5% charge rate 115Ah * 14.5V * 1/0.77 * 0.05 = 108Watts. -- Half the recommended 5%Min.

The problem I see and the one I would like comments on is that 50 watts is too small to charge, and maybe too large to trickle. I would be violating the rules of thumb. Since my loads are low, it that a good idea?

I don't want to waste money and that is why I want to essentially buy my trickle charger for my future set up now and use it right away, but I want to buy it large enough to actually provide some charging during the March trip. Are these competing goals compatible? can I do it?

What would you do?

Thanks for your comments,



  • BB.BB. Super Moderators, Administrators Posts: 32,018 admin
    Re: 2 competing goals - Can I do it?

    Welcome to the Forum Keith!

    RV batteries are typically abused and if the last a few years, most people are pretty happy. Small bank, not too expensive to replace batteries, and only used a few weeks out of the year.

    If you were designing a system that was going to be off grid 9+ months of the year and had an expensive battery bank, you would like it to last 5-7 years or so.

    Do you have a small genset already (Honda eu1000i/eu2000i or 900/1,600w watt). You can use these very nicely a few hours every couple of days. They don't suck gas to badly, and can be used for running larger loads (power tools, etc.) when needed.

    Also, look at the price of large solar panels vs small solar panels on a $$$/Watt basis... For a 55 watt panel, you may pay over $10 per watt ($170 per panel). But for 130 to 300+ watt panels, you may pay around $1.90 to $1.10 per watt.

    So, pay 2x as much and you can get 5-6x more wattage per panel (55 watts vs 240 watt or more). There are issues (will panel fit on trailer, cost to ship and insure, cheap PWM vs expensive MPPT charge controllers and such).

    But don't try to create an upgrade path. It is very difficult to, cost effectively, upgrade an off grid power system. Usually, you will end up tossing much of the older equipment when you attempt a 2x or greater upgrade.

    You really need to look at your power needs closer. My suggestion would be a 2 or 4 x6 volt golf cart battery system (220 or 440 AH @ 12 volts). That would be somewhere around 200 to 1,000 watt solar array. Depending on your exact needs/desires.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: 2 competing goals - Can I do it?

    abuse of battery noted and be aware that battery won't last too long anyway as 3yrs is typical of that battery. do be aware that even that 50w pv could boil out the electrolyte in that battery if it is charged from the start and no cc. even those small pvs in the 10-15w range could do this if done long enough connected directly to a fully charged battery so keep this in mind. others would argue this point, but i had 3 of those batteries being fed with a 3.88a us64 pv and it boiled the electrolyte out causing their failure. being in the low % charge area means they would not only charge slower, but the other losses, like self discharge for example, come into play too.

    as to float charging a battery for storage a 1%-2% charge rate area is technically good and i'm sure the 50w pv would float charge your stored battery well. remember 50w is stc rating so after real world efficiency is factored it's more like 50w x .77 = 38.5w. dividing that by 12v gives a 3.2a max current. that represents 3.2a/115ah=2.78%. in fact i'm noting here that this would be using a controller like an mppt as a pwm cc would not take into account any voltages higher than what it needs to charge resulting in a max of the pv imp rating to start off with and would be about 77% of that current max. this higher wattage also affords a tad better overall output when it's a bit cloudy compared to one that may deliver 1% only during full sunshine.

    now would that pv be enough for charging that battery with your loads. i can't answer that as you'd need the total ah used as compared top the total ah generated, plus losses.
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: 2 competing goals - Can I do it?

    In this case the loads aren't really the big issue because they are what they are and the battery size is limited to what will fit in the space. The best you can do as adapt to that by trying to get as much panel as you can so that the battery will at least last longer if not actually be recharged.

    "Too small to charge to large to trickle" isn't really an issue under the circumstances.
    So if all you can fit is a 55 Watt panel like this: http://www.solar-electric.com/so55wamusomo.html it puts out 3 Amps which is 2.8% on your battery. Use a charge controller by all means, but you only need a little one: http://www.solar-electric.com/sg-4.html

    I'd say that's my two cents worth but pennies have been discontinued in Canada so I guess I have to round up to a nickle. :p
  • RybrenRybren Solar Expert Posts: 351 ✭✭
    Re: 2 competing goals - Can I do it?
    I'd say that's my two cents worth but pennies have been discontinued in Canada so I guess I have to round up to a nickle. :p

    According to the Gov't, you should round down from two cents; therefore, your advice is worth nada. 8)

    I, on the other hand, have learned a great deal from you (and others on this site); so, I think that you should round up to way more than a nickle. Maybe even as high as two-bits. :p
  • DirtboatDirtboat Registered Users Posts: 2
    Re: 2 competing goals - Can I do it?

    Thanks everyone for your time and replies. I appreciate it.

    Answering a few questions asked.
    - I have no gen set and I don't really want one. I try to travel light and I think the hassel of a generator is more than I want to deal with.
    - The travel trailer is 13' long from tongue to rear bumper. Roof space is probably around 8-9 feet long. It is very small and has limited room for larger 6Volt batteries and hundreds and hundreds of watts of PV. I was a hiker/ tent camper and I look a the trailer as a more comfortable tent. I boon dock 100% of the time. I could probably squeeze 2 x 140 Watts of PV on the roof max. No more - it's a small area.

    The intent wasn't to create an upgrade path. I want to install a solar system this coming summer. I don't know what that will look like exactly, but I can guess it's be either a single or dual 140 watt PV, a PWM controller, and staying with my 115ah battery until it dies. I want a trickle charger for this new system, but I want to buy that now and use it as my only charging source on a upcoming trip. I want to buy it large enough to help charge given my low loads, but sized right for a future trickle charger. I looked at the 55Watt PV and sungard 4.5amp controller and they seemed fine to me, but I wanted input on whether that would be too big to trickle. I think the focus should be on the trickle side - If that 55w/4.5amp will be good to trickle, then I'll go with that. Maybe another way to ask it is what is the largest PV you would recommend to trickle charge a battery with a 115 to 150ah capacity? I know, not an easy question to answer. The trickle would be done in Wisconsin, so not exactly a high sun climate.

    I agree with Cariboocoot that the loads are what they are and given my limited space, the loads will probably grow to what ever my system will support and I'll stop at that. I am camping after all, not full timing etc.

    As some have said, the 55Watt can trickle. Would you go with the 65Watt instead?

    thanks again, I appreciate your help.

  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: 2 competing goals - Can I do it?

    no, that won't be too big to trickle charge your battery. the battery will basically only take what it wants after it has been fully charged and is sitting at the lower float voltage. if you expect to still use a pwm controller with a future expansion of your system then get the pwm cc that will accommodate all of the pvs you think you will have later. it's cheaper this way then selling off your small pwm cc and then needing to buy the bigger one later anyway.

    they also make smaller mppt controllers if you'd ever consider that route.
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