Tell me if this is good or dangerous...

Antero

Hello!

I have a 12v 7Ah small battery. I will go camping and I bought a solar panel to charge it or at least to keep it charged.

The solar panel is: Pm:5W Vmp:18V Imp: 0.278A Voc: 21.90 Isc: 0.30A.

Can I plug it directly to that battery?
Can I plug it directly to a car battery? (it has a lighter plug for that)

Or this is dangerous and I have to buy a charge controller?

or this is not useful because this solar panel would take ages to charge that battery?

Thank you for your help and patience.

Kind regards from Azores.

Antero.

Cariboocoot

Re: Tell me if this is good or dangerous...

Welcome to the forum.

It should not be a problem as it will not be connected permanently and it won't put out a lot of current.
You are correct that it won't do much for actually charging the battery; it would only be able to keep ahead of self-discharge.
If you had two such panels you'd probably do all right, depending on how much you discharge the battery.

niel

Re: Tell me if this is good or dangerous...

you could have a problem if you put the wires backwards. put a fuse in line just in case. use 1/2a.

Cariboocoot

Re: Tell me if this is good or dangerous...

Good point, Niel. I got to thinking it was a polarized connection due to the mention of the lighter socket hook-up but he's going to use it on a "free standing" battery so that's an issue.

Now I wonder if it has a blocking diode built in because that would be another issue: leave it connected all night and the battery can discharge through the panel.

I've got one of these little panels to keep the van battery up while not in use and it works fine: plugs into the lighter and produces enough to stay ahead of self-discharge without over charging or needing a controller. Mine has a blocking diode because it is designed just for this use. It's an older version of this: http://www.canadiantire.ca/AST/browse/Green/EnergyConservation/RenewableEnergy/PRD~0111884P/Blue+Planet+Solar+Panel%2C+5.5W.jsp?locale=en

I see they have a 1.8 Watt "battery maintainer". Can't imagine that will do much for a full size car battery (about 0.15 Amps).

Antero

Re: Tell me if this is good or dangerous...

Hi!

Thank you for prompt answers.

It apeares to have no diod.

This is the one I bought: http://www.ebay.es/itm/221053511520?ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1497.l2649#ht_4930wt_805

It says it can be plugged to a battery like mine. But one never knows...

Niel, what do you mean with putting the wires backwards? Invert polaritys?

Thank you!

Cariboocoot

Re: Tell me if this is good or dangerous...

Okay, so remember to disconnect it when not in use.

Most of the connectors for that are polarized. The only trouble comes when you connect to your free standing battery: get the (+) and (-) mixed and you have a problem (the panel will create a direct path for the battery to discharge through and it will do so quickly at full power, hence Niel's suggestion of a fuse to prevent the current from igniting the panel or wires).

Much of the success of this project hinges on how you connect the panel to your battery.

BB.

Re: Tell me if this is good or dangerous...

yep, connect a solar panel backwards to a battery, and instead of charging the battery, you will have a short circuit through the panel and destroy it.

So,.any plug connection should be polarized so that you connect plus to plus every time. All it takes is one oops.

-Bill

Antero

Re: Tell me if this is good or dangerous...

I see...

The only mistake I can make is with the clamps to the battery. All other plugs are polarized.

Where do i put the fuse? Positive? I Think I can't do it alone.

Cariboocoot

Re: Tell me if this is good or dangerous...

Antero wrote: »

I see...

The only mistake I can make is with the clamps to the battery. All other plugs are polarized.

Where do i put the fuse? Positive? I Think I can't do it alone.

Yes; it's standard practice to fuse positive conductors on DC circuits. The fuse should be small, like 1 Amp; enough to handle the panel's output but will blow quickly if things go wrong.

If you get a polarized connector hardwired to your battery first it should greatly reduce the potential for any such problem. Once you have that wired right, it's plug-and-play.

Although we all know how well that works with Windows OS on computers, eh?

BB.

Re: Tell me if this is good or dangerous...

For such a system, it really does not matter where you put a fuse--Positive is the default location.

If you are clamping to the battery--Mark the positive post/area bright red so that it is obvious. Solar panels are actually a giant set of diodes, and if you connect them "backwards" (positive clip to negative battery terminal, etc.), they will conduct the full current of the battery. A fuse may not even help (although, a 1/2 amp fuse would not hurt).

Many small batteries have a nut and bolt. You could take any small polarized connector/plug from an electronics parts shop or possibly auto parts store (rated for 1/2 amp or more) and hardwire that to the battery and solar panel. That will reduce the chance of miss-connection.

-Bill

PNjunction

Re: Tell me if this is good or dangerous...

In regards to the 7ah battery, I would definitely use a small charge controller from the like of say Morningstar.

That panel seems to just meet the minimum 5% requirement, or C/20 rate: 7/20 = .350 amps. If you take it down to a 50% DOD, it might take about 2 solar days to fully recharge. I'd have to look at the solar insolation values of the Azores to be sure.

Be sure to doublecheck with your own multimeter the listed ratings - sometimes I have come across small utility panels like those that are pretty optimistic. Test for VOC and ISC.

Also, if that panel is made from crystalline chips (most likely), then you can test each module by shading them one at a time with your finger or something else, and there should be a noticeable drop in voltage. If you cover a module and you don't detect any voltage drop, then that module itself may be bad.

inetdog

Re: Tell me if this is good or dangerous...

BB. wrote: »

Many small batteries have a nut and bolt. You could take any small polarized connector/plug from an electronics parts shop or possibly auto parts store (rated for 1/2 amp or more) and hardwire that to the battery and solar panel. That will reduce the chance of miss-connection.

-Bill

Other really small batteries such as alarm and emergency lighting have a tab which goes into a crimp-on female connector on the wiring harness. The better ones have different sized tabs on + and - so that you cannot hook it up wrong. But as BB said, make sure that any point at which you will routinely disconnect the system uses a polarized connector. Get the others wired up properly the first time and then never disconnect them.

PNjunction

Re: Tell me if this is good or dangerous...

Speaking of polarized connectors, be sure to doublecheck the polarity with a multimeter. An example is Harbor Freight panels quick disconnects are not the same polarity as Battery Tender charger jumpers that use the same style of quick disconnect. I found this out the hard way and was scratching my head for awhile. So when mixing and matching different vendors connectors, doublecheck them with a meter even if they seem to physically connect up just fine.