solar battery tender

The friendliest place on the web for anyone with an RV or an interest in RVing!
If you have answers, please help by responding to the unanswered posts.

blw2

Well-known member
Joined
Aug 9, 2012
Posts
3,360
Location
Saint Johns, FL
I'm thinking of going solar, but small.
I'm not looking for something to recover charge in use.  Only to put a bit back in the battery while it's sitting for weeks in storage to offset the self discharge and add just a bit of charge back over time IF it is a little bit down
I want to keep it cheap and simple
and I don't want something uncontrolled that would boil my batteries while in storage.

I so far have done fine without solar.  As much as I love the idea of a large solar set-up with an inverter, controller, etc.... for off grid supoport, but I just honestly don't need it.

I disconnect my batteries at the post when I store, and the rig will sit in storage typically no more than maybe 4-6 weeks between uses.  Self discharge hasn't really been a problem...
but
I suspect that many times when I put her to bed that the house battery could be a little less than full.

I see that Batter Tender has 5 watt, 10 watt, and 15 watt panels
but even the 15 watt panel would only be like 1.25A at 12 volts, right?  That doesn't seem like it would be enough to offset a 2% self discharge rate.  What am I missing?

Anyone here have experience with these things?  Are they weather resistant enough to roof mount and leave in the weather? 
Are the controllers reliable?
 
X2 on this one only I would like enough so I don't have to detach at the posts.  On mine there is a parasitic drain on the chassis so I might need more than 15Watts.  My house  has a 10 watt tender and it works great.  My year they didn't put a tender on the chassis.  Go figure.
 
blw2 said:
I'm thinking of going solar, but small.
I'm not looking for something to recover charge in use.  Only to put a bit back in the battery while it's sitting for weeks in storage to offset the self discharge and add just a bit of charge back over time IF it is a little bit down
I want to keep it cheap and simple
and I don't want something uncontrolled that would boil my batteries while in storage.

I so far have done fine without solar.  As much as I love the idea of a large solar set-up with an inverter, controller, etc.... for off grid supoport, but I just honestly don't need it.

I disconnect my batteries at the post when I store, and the rig will sit in storage typically no more than maybe 4-6 weeks between uses.  Self discharge hasn't really been a problem...
but
I suspect that many times when I put her to bed that the house battery could be a little less than full.

I see that Batter Tender has 5 watt, 10 watt, and 15 watt panels
but even the 15 watt panel would only be like 1.25A at 12 volts, right?  That doesn't seem like it would be enough to offset a 2% self discharge rate.  What am I missing?

Anyone here have experience with these things?  Are they weather resistant enough to roof mount and leave in the weather? 
Are the controllers reliable?

I would like to do the same thing. Keep it simple, just enough power going in to keep batteries topped up while sitting without being plugged into shore power (like while the MH is sitting at the RV shop unplugged). I am guessing here, but thinking I would need to measure parasitic draw to know how much power input I need, plus I have 4-6 volt batteries to charge.
 
Seems like you have hit on a hot topic. 

The idea of roof mounting and then running the wiring and hooking to the batteries intimidates me.
 
yeah, it gets a lot more substantial if you aren't disconnecting batteries.  Much bigger draws to offset.
What I'm after is just enough to offset self discharge + a tiny amount more to top them off from a nearly charged state to full over a few weeks.
 
Brad, I don't know what your battery's AH capacity is, but those 10 watt panels just don't cut it for most RV uses. Keep in mind that the output of a 10 watt panel is only about 1/2 to 3/4 of an amp per hour, under ideal conditions. Physically they'll hold up fine on your RV's roof, but I can just about guarantee you will not be happy with how well they'll keep your battery banked topped off.

I understand that you're only trying to keep your battery(s) topped off while in storage, so at the very least, I'd recommend nothing less than a 50 watt panel for a single 12 volt battery, and a 100 watt panel for two or more batteries. We can't depend on ideal solar charging conditions, so the larger panels will help compensate for that. A $20.00 PWM controller will prevent overcharging.

99Dart, with four GC batteries, a 10 watt panel wouldn't put a dent in your charging needs. Sorry. Our coach came from the factory with a 10 watt panel, and it's probably the most useless piece of equipment in (on) the coach. It's output didn't even register on the battery monitor, even though the red LED charging indicator said it was charging.

Kev
 
You bring up a great point Kevin.  I should have mentioned my battery set-up.... doh!~

I'm just running a single junk battery for my house, currently a marine "hybrid" type group 31, 105Ah at the 20 hour rate

I was going to replace my smaller group 27 with a true deep cell when it died, but I was on a trip when i replaced it and couldn't find anyone with a 12V golf cart battery in stock.  Oh well, the old group 27 did ok for our needs so I'm a little better off now, till you consider that I'm a little  more likely to put it to bed with less than full charge.....

Based on my experience killing batteries in my boat and old pop-up camper while in storage...gut feel, nothing scientific.... I'd say that the rule-of-thumb 1% to 2% self discharge rate seems about right
so
Am I right to think along these lines,
my battery is 105aH capacity, and
1% of 105aH = 1.05aH
2% of 105aH = 2.10aH
Am I correct that I might expect that I would need something approx. 2A x 24 hours = 50 amps for every 24 hour solar cycle, just to offset self discharge alone?

Looking at the specs for that 15W battery tender panel, MAX output is listed as 830mA.....and since that's MAX, I could only expect that for an hour or two mid day when there aren't clouds.... so on those perfect days I might be getting roughly 3 amps during those peak hours + more during the off peak hours so maybe roughly 6-10 amps total as just a guess....less on cloudy days.

Am I thinking about that correctly?

So I guess I'm looking for a 100W set-up then!
but 100W/12V=8.3A.....reality is likely something less that 8 amps, so if I need 50 amps very day is that even enough?
I have a feeling I'm missing something....
 
I think you're overlooking the fact that the 1 to 2% self discharge is per month, not per day.
 
ChasA said:
I think you're overlooking the fact that the 1 to 2% self discharge is per month, not per day.

There you go, thank you!  I knew I was off by some huge factor..... numbers didn't smell right.
Yes, that makes more sense!!
I got lazy and ignored the cardinal rule learned in school to always write down my units when figuring....  PER MONTH

so 2.1aH loss/30 days = 0.07amps per day self discharge loss.

that smells more like it!!!

so if I'm stocking away say 3-10amps per day from that 15W tender panel on good days, even considering many days are less than good, then I'd be more than offsetting self discharge in an average month in storage.... probably.  maybe
So while I can see it wouldn't be enough for mid trip recovery necessarily, and wouldn't do a great job of recharging a dead battery, maybe it would be good enough for what I'm trying to accomplish....  I hear what Kevin was saying though.  Reality is that many days are less than ideal, panels get dirty, etc.... all variables that make this idea a "maybe"
 
We used to keep our 5th wheel plugged in all winter.  I was tired of worrying about the cord across the driveway with snow.  This solution is going on 3yrs and working perfectly.  When I bought it, someone on the forum commented how I wasted my money on junk... It's turned out to be a great solution and was very inexpensive and simple.  This is an 18W panel that I purchased, came with a 7W charge controller.  Keeps the batteries at 12.6 or 7 without problem.  I have 4 AGM Optima Blue top, 220Ah bank. I have a battery cutoff switch so there is no parasitic draw.  I bought the panel on either Amazon or Harbor Freight, frankly can't remember.  Hope this helps.
 

Attachments

  • 18watt.jpg
    18watt.jpg
    91.6 KB · Views: 29
Kevin Means said:
Brad, I don't know what your battery's AH capacity is, but those 10 watt panels just don't cut it for most RV uses. Keep in mind that the output of a 10 watt panel is only about 1/2 to 3/4 of an amp per hour, under ideal conditions. Physically they'll hold up fine on your RV's roof, but I can just about guarantee you will not be happy with how well they'll keep your battery banked topped off.

I understand that you're only trying to keep your battery(s) topped off while in storage, so at the very least, I'd recommend nothing less than a 50 watt panel for a single 12 volt battery, and a 100 watt panel for two or more batteries. We can't depend on ideal solar charging conditions, so the larger panels will help compensate for that. A $20.00 PWM controller will prevent overcharging.

99Dart, with four GC batteries, a 10 watt panel wouldn't put a dent in your charging needs. Sorry. Our coach came from the factory with a 10 watt panel, and it's probably the most useless piece of equipment in (on) the coach. It's output didn't even register on the battery monitor, even though the red LED charging indicator said it was charging.

Kev

With all appropriate respect, I find that the 10 watt panel keeps the three house batts at 13.1 volts when I put it away full and come back a month later.  100 watts to accomplish this seems like an awful lot.  My RV sits outside and I cut the batt disconnect switch when I leave so it should only have to keep the batts topped off which it does nicely.  It would not provide any additional power. 

Oddly though, nothing is there for the 2 chassis batts which have draws.  that is all I need to improve. 
 
Kevin Means said:
99Dart, with four GC batteries, a 10 watt panel wouldn't put a dent in your charging needs. Sorry. Our coach came from the factory with a 10 watt panel, and it's probably the most useless piece of equipment in (on) the coach. It's output didn't even register on the battery monitor, even though the red LED charging indicator said it was charging.

Kev

My thoughts exactly! I'm thinking at least 1- 100 watt panel would be needed. With all the reading I have been doing, it seems solar power is a huge investment for a system capable of re-charging a system effectively.
P.S. After I install my power cut-off switch, the power required to keep them topped off would be considerably less.
 
Just thought I should post the follow-up....

I had emailed Battery tender several days back, about roof mounting their trickle chargers, how weatherproof are they, etc....

Here is the response I got.

"Yes our solar panels are weather resistant but they are not waterproof. I would not recommend permanently mounting them to an RV because the panel is glass and could break while the RV is moving. The glass could break and void the warranty as well as not work."

And my follow-up question.....
    where is the generally considered best place to get solar panels from?

I'm still on the fence about how much panel I want.  On my boat at home I have a plug-in model battery tender, 850mA I think it is.  That has proven more than enough.  Now I understand that solar would only be working part time where this thing works 24/365, but still.... 850mA has proven to be plenty to keep topped off and even put a little charge back in to top off a battery over time....
But
As I think this through.... If I can get off cheap, a trickle charger would do.  If I need to spend a bit more money then I probably might as well get a bit more capacity to help out when doing an occasional boondock night or two....
  Still, I'm mostly hoping to get off chap and easy and don't really want a lot....
 
I don't have my RV yet, but I do have a solar setup on my sailboat.

There are 3 deep-cycle house batteries and one marine starting battery. All 12-volt.

I have one 60 watt solar panel (and these are getting better and cheaper every year).

The boat sits weeks at a time on a mooring in the summer and sits in the boatyard through the Wisconsin winter.
Even snow-covered, the panel manages to keep the batteries at peak charge.

The charge controller I have is the SunSaver 6, rated at 6A. It has been working well for 3 years so far.

I added a small digital volt-meter to the cabin wall so I can keep an eye on things.

You can go onboard, turn on EVERYTHING (stereo, chartplotter, lighting, auto pilot, water pump, phone charger, and see the voltage drop. Then if you turn everything off, you can see it return to full charge within 15-20 minutes.
 

Attachments

  • Screen Shot 2018-04-24 at 11.00.20 AM.png
    Screen Shot 2018-04-24 at 11.00.20 AM.png
    110.8 KB · Views: 11
Kevin Means said:
Brad, I don't know what your battery's AH capacity is, but those 10 watt panels just don't cut it for most RV uses. Keep in mind that the output of a 10 watt panel is only about 1/2 to 3/4 of an amp per hour, under ideal conditions. Physically they'll hold up fine on your RV's roof, but I can just about guarantee you will not be happy with how well they'll keep your battery banked topped off.

I understand that you're only trying to keep your battery(s) topped off while in storage, so at the very least, I'd recommend nothing less than a 50 watt panel for a single 12 volt battery, and a 100 watt panel for two or more batteries. We can't depend on ideal solar charging conditions, so the larger panels will help compensate for that. A $20.00 PWM controller will prevent overcharging.

99Dart, with four GC batteries, a 10 watt panel wouldn't put a dent in your charging needs. Sorry. Our coach came from the factory with a 10 watt panel, and it's probably the most useless piece of equipment in (on) the coach. It's output didn't even register on the battery monitor, even though the red LED charging indicator said it was charging.

Kev

stirring up my old thread....
I've just about come around to doing this. 

And I've come around to the idea of doing a larger panel to gain the side benefit of charging for more than just trickle in storage...  Maybe a single 100 watt panel (or a bit less).  I figure putting a panel just flat on the roof will be good enough for my needs, and I'm thinking I can run the cable from the panel down the fridge chimney and into the kitchen cabinets where it will be an easy run over to the side of the cabinet where my use/store switch and other stuff is... panel mount if it's recessed or just hidden with the water pump if there's no need to look at it.  That'll put it close to the battery where it lives under the entrance steps.  See any flaws in this approach?

My only big issue is that I have that sprayed on roof (rvroof.com) and I really hate to penetrate that.  Sure do wish I could stick a panel down with adhesive, like 5200 or something...but it seems like they are all substantial with an aluminum frame so doubt if that would be a good idea.  I don't know...I'll either take it back to them to spray in the mounts, or just have one area to re caulk up there from time to time

Anyway....Did a lot of poking around and shopping online and reading a few solar 'blogs'.  Frankly, I'm overwhelmed. 
I can't put a whole lot of money in it (DW will get mad)
but I don't want to get ripped off with cheap garbage either.
If you all were trying to accomplish this, which panel and which controller would you choose?
What size cable will I need to run from the panels to the controller...guessing a 15 ft run?
 
So I finally just jumped in by ordering a panel on Amazon.  100 watts.  I have no idea what I am doing but I wouldn't mind learning by setting up some solar powered nightlights at my sticks and bricks.  They could be motion activated LED's until I learned how much power I get versus what I use.  I figure the amount of money I spend is kinda like a course on solar.
 
Hi again Brad. I think you're wise to go with a larger panel - even for just keeping your batteries topped off. Renogy has a good name, and they sell a 100 watt mono panel, with a basic PWM controller, for $170.00 (or so). You can probably find a combo system that's a bit cheaper, but the Renogy system is pretty tried and true.

Under good conditions, that panel puts out about 21 volts and 5.75 amps. To keep the voltage loss less than 2% on a 15 foot run, you could use 16 gauge wire and you'd be fine. Personally, I'd use 12 gauge wire. It won't be that much more expensive, and every little bit helps.

Kev
 
I bought a Renogy.  If I get this far, is it roof mountable??  Will it come apart at highway speeds??
 
A long as the panel is mounted securely to the roof, it should be fine traveling down the road. We've had solar panels on the roofs of our RVs for amost 20 years, and I've driven in some very strong winds, but our panels never budged.

Kev
 
Back
Top Bottom