solar panel calculations

I'm thinking about putting a solar panel on my kayak to help extend the battery life of my fish finder. I think the longest my FF battery has lasted was about 6 hrs when new (first of the year), but now it only seems to last about 4 hours, probably because I've let it run down too much on the water and also probably overcharged it at home. Anyway, 6 hrs is not always enough (can't catch fish at a fast enough rate yet --I'd like it to last 8 hours if desired, and I would also like to have a way to charge it on multi day camping trips.
The panel I'm interested in, mostly due to reputation of the company and dimensions of the panel, is this one:
http://www.powerfilmsolar.com/products/?r7&show=product&productID=271515&productCategoryIDs=6578,6580
Here are the panel stats:
Operating Voltage: 15.4
Wattage: 7
Current: .45 amps
Width (mm): 368.3
Length (mm): 584.2 unrolled
Width (in): 14.5
Length (in): 23 unrolled
Weight (kg): 0.269 kg
Weight (lb): 0.594 lbs

My fish finder is this one:
http://store.humminbird.com/products/604309/386ci_Combo
Power draw on the finder is listed as 380 mA with "lights off".

The solar panel company has a calculations page, but I've never been that great at physics, and my eyes kind of glaze over as I try to understand the calculations. Still, I did try them out, and if my math is correct, I think this panel would satisfy my needs, but just minimally.
Any solar experts care to weigh in?
I would appreciate it, thanks!

Matt

What battery are yoy using?

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One thing to keep in mind is a charge controller.   A charge controller will ensure you do not overcharge the battery.

Amps X volts=watts is a basic formula to use in the calculation.  FishingForTheCure will figure everything out I'm sure.

One thing to keep in mind is a charge controller.   A charge controller will ensure you do not overcharge the battery.

Amps X volts=watts is a basic formula to use in the calculation.  FishingForTheCure will figure everything out I'm sure.

+1.  Running a solar panel unregulated into a battery isn't a good idea.

-Allen

Thanks for the suggestion to add a charge controller.

The battery I have now is 12 V, 7.2 Ah.

The finder power draw is 0.38 A at 12 volts. 0.38 x 12 = 4.56 Watts. The panel is 7 Watts, so should be adequate to replace the power used by the finder, right?

Matt

Getting a spare battery is simpler I think.

This is one of the reasons I switched from a Humminbird 385 to an Elite4. For some reason the Humminbird's use a lot more power. I can run the Elite for probably 20 hours on that 7ah battery but instead use a 5ah to save on weight & that still lasts over 12 hours.

 If your 7ah is only lasting 4 hours now then that battery is likely bad & you need a new battery. I had a couple 7ah batteries with the red lettering on the side go bad like that including the one that came with my Humminbird. As far as carrying 2 of the 7ah batteries & changing them out on an angry ocean were you have to open up your hull to do so...... I did that once...... jim

Thanks Salty, you obviously know exactly what I am talking about. I do have the red-lettered battery distributed by Humminbird. I know it needs to be replaced, but wasn't happy with the 6-hour life it had when new and was expecting the same performance from a second one. Now I have some hope that a new battery will be sufficient. If not, I might consider replacing the finder with a Lowrance if they are truly more energy efficient, though am not thrilled with that idea as I am sure the mounting holes, etc will not match so I will have to do some repair and what not. Also suspect the Lowrance transducer will not fit in the scupper hole of my OK Trident. And, as you pointed out, switching out batteries while underway in the ocean is not practical. Thus, I am still considering a solar panel charger. I think the panel won't be too hard to mount to the top of the front hatch cover, and I can probably mount the panel power cord to the hatch so the panel can be plugged and unplugged from the cord when necessary. All I would need is a water proof cap for the cord socket...

Matt

Yeah definitely try buying a brand new battery & see if you get enough run time. I was in the same predicament about 2 years ago & found some 9ah batteries that were only a little bit bigger & heavier than the standard 7ah ones that most people are using. There is also some lithium batteries that some people buy on Ebay that are much smaller & lighter but just didn't look durable enough to me. As long as you protect the terminals on the block style 5, 7, 9ah batteries they are practically indestructible. But those lithium batteries really save on the weight. Rather than have to worry about running solar, & all of the extra vulnerable wiring & connections, maybe look into one of those more expensive lithium batteries since you have to get a new battery anyways.

The cheaper & easier solution is to parallel a second battery to effectively double aH's. Solar panels are laminated to glass & will degrade if exposed to salt. Space is such a premium on a yak & less is more if you consider adding a charge controler (absolute nessecity since solar panels don't supply linear voltage) increases splice points that will always require care against corrosion. That's my $0.02.

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Thanks for the paired battery suggestion, b.shadee. Regarding the corrosion problem with solar panels, check out the link to the manufacturers site. These are rollable, and supposedly waterproof (and yes, saltwater proof). Powerfilm is the OEM manufacturer for a panel that Hobie sells for their yacks.

I am going to try a newer and larger battery before deciding whether to go with a panel or not.

Matt

 

  As mentioned, Solar will only work when the solar panel can supply more power than the load connected to it.  Keep in mind, that the whole panel will require full sunlight to produce it's maximum power.  The particular panel being flexible sounds like some part of my not get full sun all the time.  I will say this, though, at 7 watts and your ff only at 4w, that panel should help extend the battery life, and return a charge to the battery when the FF is off.  Course if the battery is hosed, it won't help.
   I think, at an open voltage of 15.4vdc it's a little low, you might find that in direct sunlight you can measure up 21vdc!  however when it's connected to a load (Battery) that open voltage will be lower.
 You might be able to get away without using a "Charge regulator"  It's always safer to use one though.
    I have almost 22A of from 5 different solar panels on my roof (1 LG 720W 7.2A, 2 solarx 64W panels and 2 64W Unisolar panels) and a C30A charge controller, keeping a marine battery (G27) for my 12Vdc water pump charged, this in turn squirts rain water collected in a 500 gallon tank all over my lower yard when I water with it.
  Belive me I've though about solar on my OB many times.
  How much is that panel?  and let us know how it goes.

i say ditch the 7ah and use a 9ah practically the same size with longer life time.

Elite 4 is my suggestion I've went 20 plus hours with out charging my 7ah battery, I do wonder if not all battery manufactures are honest about there specs. 


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Thanks guys. The cheapest price I have seen for the panel is about $130. I am going to start with a new and larger battery and see how that goes. It is really frustrating running out of battery after 3 or 4 hours. I'm now thinking that the larger battery will solve it, but I am still thinking about the solar panel because, well, that's just the way that I am. If I install the panel I will post pics and reviews on how it goes, but I'll start with the battery for now.

Matt