With components in hand or on the way, you should be able to tell us the answer to this question by going for a test cruise.
It doesn’t really tell us much to say that your pontoon boat is lightly loaded, while the motor is rated for a max. of 28amps & 24v. Stating “recharge my batteries while not in use and gone from the 110v for a few days” sends mixed messages and begs other questions. If the boat’s not in use and away for a few days, does it really have 3-5 days to sit and charge by solar without you drawing power from it? Duty cycle is important here. Say you have 100ah of battery capacity---with summertime and at least a 4hr solar day, you could charge fully depleted batteries up with panels that delivered an average of 25amps into 24v, or 600watts. But if you had 3 days to recharge the batteries and didn’t draw them down in the meantime, only 200watts would do it. And if you really only drew the batteries down 50%, a 3-day recharge would only require 100watts average power.
Scenarios are important. Noone can fill in these blanks for you without them.
What are your expectations? What are your constraints?
We really need something like “oh, I want to be able to cruise out to my tiny little island that draws 20amps continuous from my cute little MinnKota and allows me to get to the tiny little island in any scenario in 2 hrs. I plan to sit there in my beach chair and otherwise not motor that pontoon boat for 3 days and the boat will sit there in the sun the whole time. After 3 days, I want to cruise back with a full battery pack. This will be July in Florida and no provision for cloudcover is necessary for this analysis. Area available for solar panels on my awesome catamaran is 6 sq-meters. My budget is limited to $500 for solar panels, mounts and charge controller.”
In this scenario, you’d draw down your batteries about 40 a-h and with 3 days to recharge with probably 5-hrs effective solar energy, you need under 3-amps of charge current average for 5hrs daily. Into 24v, that requires 72watts of power. You could buy 2 surplus 50-watt panels (Siemens, Sanyo, etc.) that no one uses any longer for probably $3/watt…i.e. about $300. Since you went for 100watts when you needed but 72watts, you might be able to ditch the charge controller if the panels each output near max power at 24v---but that’s not too likely as the 50watt panels were typically 17v @ 3amp…in that case, you’d expect about 12v @3amp, or 36v per panel---which really would meet your needs anyway. But you are giving up about 30% of the peak power these panels can produce. The beauty of this scenario is 2 panels on a catamaran might look cooler than 1 panel and you could locate 1 over each pontoon. You could also try to buy a cheap charge controller to either boost the parallel output of these 2 panels to 24v, or, putting them instead in series, you could put a buck converter in place that semi-optimally charges the batteries more efficiently. Either way, that converter might cost a significant portion of your panel cost.
Anyway, you need to do a test run with your pontoon boat, then do analysis. It isn’t hard.
In case this helps-
-Myles Twete, Portland, Or.
From: electricboats@yahoogroups.com [mailto:electricboats@yahoogroups.com] On Behalf Of Doug B
Sent: Monday, February 28, 2011 10:43 AM
To: electricboats@yahoogroups.com
Subject: [Electric Boats] How many watts?
Based on the advice received here I purchased two Group 27 12v batteries for my Minn-Kota 65# 24v transom-mounted motor, and I'm awaiting the arrival of a Guest dual-bank 10 amp Pro Charger.
What I'd like to know now is: how many watts I should look for in a solar panel selection to help in my range while underway, and recharge my batteries while not in use and gone from the 110v for a few days. The motor pulls a max of 28 amps at 24v, I think; and powers a 16' pontoon boat--lightly loaded.
Thanks!
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