7watt solar panel as trickle charger?

[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)

 

[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.

 

[roverjohn]

Vic,
I would never feed the battery directly because I'd have no idea how long you could do it safely. I just looked online and found some 3A pwm solar charge controllers for $4/ea. I'm sure I can find the modules for a buck or something but I'm not sure it's worth the time. I'll order a couple tonight just to satisfy my own curiosity. Like the OP I have a few 5-7 watt panels sitting around.
I'll post a link once I find out if I've wasted my $8.

 

[roverjohn]

Vic,
I missed your 2nd question. Your truck is never going to see 17 volts unless you boil the battery dry. We're talking about 1/3A charging current here so even the slightest load will pull the voltage down if you left the batteries connected which I would not do.

 

[hpcbmw]

Thanks for all the input. I'm thinking that the 7w is pretty much useless for the truck and camper. I'll probably get a cheapo charge controller like Roverjohn suggested and use the 7w as a battery tender for my motorcycle/quad batteries.

Using the math above, it seems like the 15 Harbor Freight panel is still probably not enough to maintain all three batteries- but I've got three of the 15w panels. How about if I hook up one 15w panel to the truck batteries and one 15w panel to the camper? I'll run both through a cheapo charge controller.

Roverjohn- what website did you find the $4 charge controllers? I did a quick search and couldn't find them for under $7.50 on amazon, and not much cheaper on Banggood.

Quick question about charge controllers: I've got an older charge controller that came with a 100w panel I got used (it came with the camper). I had a problem one weekend with the batteries seeming to lose charge overnight. Is there a way to check and see if the charge controller is drawing power FROM the batteries when the sun goes down? It could be that the batteries were old at the time, I've since replaced them, but now I'm afraid to use that controller for fear of damaging my new batteries.

Thanks for the help!

 

[ntsqd]

This company has been selling these kits for a very long time:

https://www.pulsetech.net/store/solar-battery-chargers/12v-solar-chargers.html

Yeah, you could probably build something like it for less. How much is your time worth?

 

[Happyjax]

Buy a 50 watt panel and a cheap solar controller and you can do lots with it for about $100

 

[FoxenTec]

Related to this discussion. I noticed when I was testing with a 5 watt solar panel from Zamp and a medium sized motorcycle battery, that the solar panel would bring up the battery during full sunlight to about 12.9 volts but as soon as the sun went down, I could see the voltage dropping on the battery.

Was the solar panel because of no controller actually drawing energy away from the battery during these "no sunlight" hours?

Sorry, still trying to learn.

 

[ntsqd]

Usually there's a diode function in solar panels, so I doubt that's what you were seeing. More likely is that the battery was achieving it's resting voltage. Can't measure a lead-acid battery's voltage immediately after the charging source is turned off. Need to let it rest for a while before such a measurement is valid.

 

[roverjohn]

HPCBMW,
The charge controllers I found were on ebay. Just search '3w solar charge controller'.

 

[Old Crow]

Related to this discussion. I noticed when I was testing with a 5 watt solar panel from Zamp and a medium sized motorcycle battery, that the solar panel would bring up the battery during full sunlight to about 12.9 volts but as soon as the sun went down, I could see the voltage dropping on the battery.

Was the solar panel because of no controller actually drawing energy away from the battery during these "no sunlight" hours?

Sorry, still trying to learn.

You may just be seeing normal activity. In general, a fully-charged battery has a 'resting' or 'no-load' voltage of something like 12.6 or 12.7 volts. The solar panel applies a higher voltage (when the sun is shining) to counter-act the battery's tendency to self-discharge. When the sun goes down it can no longer apply that voltage and the battery's voltage will drop back to its resting voltage.

You can see this same thing in your car's charging system. If you measure battery voltage after the car sits overnight, you'll see its resting voltage. Measure again with the car running and you'll typically see voltage in the 13s or 14s. That's the alternator applying a charging voltage. Once you turn off the car, the battery's voltage will go back down to its resting voltage.

I used the word 'may' to start this post because we don't quite know enough about your Zamp maintainer setup. According to this Zamp maintainer page, those panels are supposed to be paired with a controller. In some cases, manufacturer's put what are called 'blocking diodes' in panels to prevent the very problem you're asking about and those panels wouldn't drain the battery. But Zamp may not do that, particularly in their panels specifically designed to include a controller. If that's the case, it would indeed drain the battery.

.
PS- I see ntsqd's reply came in while I was writing this up but I decided to post my wordier version anyway.