18 replies to this topic

[hpcbmw]

I've got a 1997 FWC on a 2000 F250 diesel pick up. I have 2 starter batteries in the truck and 1 deep cycle battery in the camper. All batteries are 12v wet cell batteries. The camper battery is connected to the truck batteries via 10feet of romex solid wire, connected to a battery isolator.

The truck and camper sit unused for months at a time, and are not parked close to a 110 outlet.

I purchased a 7w solar panel from home Depot. It doesn't have a charge controller.

Can I use the 7w panel, hooked up to one of the truck batteries without a charge controller, to keep all 3 batteries fully charged?

My other option is to use a 15w harbor freight solar panel and charge controller hooked to one truck battery with the same goal.

Will either of these work?

Thanks

[smlobx]

Of the two choices mentioned I would go with the 15 watt panel with charge controller.
Theoretically you want a controller to limit the charge from the panel to keep from overcharging them but I’m not sure that a 15 watt panel would do that.

I’m sure that others more knowledgeable will be along shortly to help.

[Vic Harder]

7W?  Why bother?  I suspect that's for charging cell phones, not truck batteries.  You MUST have a controller on the solar panel.

[hpcbmw]

Why the 7w you ask? It's easier because I don't need a charge controller, and it's easier to mount the 7w panel because it's smaller. My thought was that a small panel would be enough to act as a trickle charger, but not so much that it would overcharge the 3 batteries.

 

If I use the HF 15w, that panel is one of a set of three. Using it would then limit my options for using the full HF setup on my 5th wheel or other solar projects.

Thanks for the replies.

[roverjohn]

Assuming you don't have any parasitic drains (which you probably do) the 7 watts should cover self discharge. The resistance of the Romex will probably prevent the deep cycle battery from seeing any charging or there will at least be a voltage drop so effectively you'll have no charging.

So... If it were me...I would disconnect the starting batteries from the truck and the deep cycle from the starting batteries. Then use your 7 watt panel with a cheap PWM controller on the starting pair. A duplicate of the panel and PWM hooked to the deep cycle and you'd be good to go.

 

I know it will take a minute or two to hook everything back up and that your phone won't do it for you via wifi but it will at least give you the opportunity to clean your battery terminals. Batteries are expensive and 7 watt solar systems are cheap so this is pretty much a no brainer.

 

You probably have parasitic loads you are unaware of like a clock or maybe an LED display or a memory in your radio that draw down your batteries so slowly you don't notice it. Maybe an alarm or a corroded switch or the tops of your batteries are filthy. After months you'll have a dead battery so disconnecting and cleaning them is your best bet.

 

In your case you don't need to 'charge' anything, just cover your batteries self discharge rate.

It's definitely not safe to directly connect a 12 volt panel to a battery. It's probably is safe for a day or two or maybe a week or a month. Who knows but once you reach fully charged the panel will start boiling your battery dry.

[cwdtmmrs]

7w? What is the best you can get from that a day in reality? 1-3 amps?

[Old Crow]

IMO, the answer is 'maybe' for either your truck batteries or your camper battery (but not all three).

 

I have several solar maintainers and they don't put out much current so they can't stay ahead of any significant drain.

 

When I first got the one maintainer I connected it my van's single starter battery and two days later still didn't have a charged battery.  But then I thought to fully charge the battery with my regular charger BEFORE putting the solar maintainer on.  That was about a year and a half ago and it has been keeping that battery topped off ever since.

 

I also have one on my ATV and one day left the ATV's key on.  The battery drained completely as there was just too much current going out for the panel to keep up.  Once I turned the key off and charged the battery back up with my regular charger I found the panel still kept the battery up but the LED indicator on it no longer works.  

 

I have another on one of my Tundras-- the '02-- as it sits a lot.  The battery on that one is going bad and the maintainer was staying ahead of it.  But just a few days ago I found the battery at 12.4 even with the maintainer on (and that's a 10-watt maintainer with a good sun angle). 

 

You would also have to think about how your batteries are connected.  You have an isolator between the truck and camper and it probably doesn't connect them together until it sees a voltage above 13-something.  I typically only see voltages in the high 12's or low 13s on the batteries I have the solar maintainers on.

 

If I were in your shoes, I think I'd just give truck batteries a good charge and put one of the maintainers on to see whether it keeps up (note that I'm assuming the truck batteries are connected together all the time).  And I'd charge the camper battery and put the other on that one.  Try to orient the maintainer panels for maximum sun and try to avoid long wire runs.

 

As far as the controller issue, conventional wisdom is a controller should be used.  Some manufacturers claim a controller isn't needed if the panel is putting out less than something like 10 watts.  Others will claim a controller is needed... but they're also in the business of selling controllers.  Not sure what to believe there.

 

.

[roverjohn]

Lets do some rough math which admittedly is pointless as we don't know the detail but here goes:

 

7watt panel probably can average 4 watts in the real world

4 watt remaining probably sees an average of 5 hours of sunlight= 20 watt hours

20 watt hours divided by 12 volts= 1.6 Ah/day

3 batteries so .555 Ah pre battery per day

 

Now, let's speculate wildly;

 

Each battery is the equivalent of a 100 Ah battery (probably false)

Each battery is new (most likely false)

His storage yard stays at a constant 65 degrees F (very unlikely)

All loads are disconnected.

 

Each battery will self discharge at around 5% of capacity per month

 

100Ah x 5%= 5Ah/mo

5Ah/mo divided by 30 days= 0.1667 Ah per day

0.1667Ah/day x 12 volts = 2.0 Wh/day

2.0 Whx 3 batteries= 6Wh/day

 

His 7 watt panel puts out 3x as much power needed to overcome self discharge under Ideal conditions (not likely).

However 7 watts is better than zero watts and will delay flat batteries as least for a while and maybe long enough. (possible) 

 

The OP does not have to charge anything because he will reach full charge just by driving to his storage yard from wherever he was before that.  (wild speculation)

Edited by roverjohn, 31 May 2019 - 06:22 PM.

[Vic Harder]

Lets do some rough math which admittedly is pointless as we don't know the detail but here goes:

 

7watt panel probably can average 4 watts in the real world

 

The OP does not have to charge anything because he will reach full charge just by driving to his storage yard from wherever he was before that.  (wild speculation)

I like how you think!  

I still have questions about the effect of feeding the raw output of the panel directly to the batteries.  .56A isn't much so might not be as bad as hooking up a 300W array without a controller.  That said, what are the onboard electronics in the truck/camper going to think about 17V being applied to them?  

[Beach]

If cost is not a prohibiting factor, why not buy 2 panels use one for the truck batteries and install(wire) the second panel directly to the camper house battery, thus avoiding the long run of romex and the voltage drop. If you don't have one, you might want/need to install a cut off switch between the truck and camper during storage.