Solar powered battery chargers

[Allan Hughes]

Does anyone use one of these for charging trolling motor batteries on camping / fishing trips? I see you can buy 250W units that stack quite conveniently, before I try one I thought I'd ask if anyone knows what the crack would be using one. Camping / fishing trips seem a bit dreamy at the moment I know but hey, we will come out of the other side one (hopefully) sunny day.

[Martin Kuhn]

Hi.
I've used a redarc in my camper and in my prado
Been absolutely happy with them.
Cheers
Martin

[peter birch]

Al having you got a onboard charger for the electric batteries.tou would need at least 250 to start with .

[Simon Gilbert]

All depends on how much power you go through on your motor and how long you have to charge it. Unless you have a huge charging capacity, you probably need 2 batteries. One you are using on the day to run the motor while the other charges.

Looking at the theory, if a 250 watt charger is 100% efficient (thats the efficiency of the rated supply not efficiency of converting sunlight which is arround 20%), it produces about 20 amps in 12 volt (10amps if its 24 volt). Thats 100% under laboratory conditions - not camping on a beach..solar are never that efficient even under direct sunlight, optimum temperatures (optimum from memory is 25 degrees...it's worse if its hotter) and age of the panels . Lets say average of 70% over 4 hours arround midday - so less at other times when the panel isn't pointing directly at the sun (or are you looking at spending more on an automatic tracking panel?). So its putting into your 12v battery about 55amp hours over 4 hours. Lets round that upto 70amp hours over a whole day. Remember 24volt is half of this.

So your motor would only want to use 70 amp hours a day for you to swap batteries each day and to maintain capacoty. Remember you should only draw your battery down 50% for a lead acid deep cycle battery, maximum 70% for AGM.

Remember....i think I'm overestimating how much you will get out of a 250w panel. As I'm writing this it seems to me that you need twice this capacity and still have 2 batteries swapping.

Happy for someone to disagree with these figures... I'm writing this in bed about to go to sleep and havent checked the calcs.

[Simon Gilbert]

Oh and you loose efficiency through a regulator. You MUST use a regulator or you can easily wreck an expensive battery. They don't need to be expensive. As Martin said...redarc is particularly good but you don't need this good if you are only using the panel to charge a battery ( and thats the only thing chrging it)

[Simon Gilbert]

And the other thing....if your battery is 24volts you either need a transformer ( again reducing efficiency) or 2 panels setup in series. Panels put out 18volts. While I've got a fair amount of experience with using them on 12 volt batteries I have none for 24volt.

In writing this i just realised that i stuffed the calculation for amps for 250w...its about 14amps ( because it's a 18volt output). So that will change the calculation above. So perhaps 40 amp hours over 4 hours around midday...estimate 60 amp hours over whole day....probably less.

[Allan Hughes]

Thanks for the input guys, much appreciated. I'm surprised the efficiency of them is as low as you are suggesting, I've no experience using them so great to have your thoughts on this.
Here's a bit more context - on my boat I have four 12v batteries, two of which are dedicated for the 80Lb 24v Min Kota trolling motor. I have a fancy on board 4 bank charging unit that allows me to charge all four batteries when back on dry land by just plugging in one plug to a 240v socket. Nice and simples, no need to remove TM batteries or do anything strenuous, each battery gets charged at the correct charge rate depending on the state of it's charge and most importantly I have lots of little red and green lights that flicker and look really cool. I mostly charge the House and Engine batteries direct off the motor (200HP Yam / 50A) but sometimes I'll draw down the House battery over the course of a day more than the Engine battery because I tend to run the sounders and GPS a lot whilst under power of the TM or when drifting with the main engine (obviously) off.

I have intentionally not connected the TM batteries to the Engine battery and motor charging circuit because like most of us who take risks in white water I want the security of knowing my engine will always have the current it needs to start first and every time. Everything works just fine as long as I can reach a 240v power socket to recharge the TM batteries overnight, but on an extended multi day/night island camp trip I'll run them down. So I was contemplating using a solar power set up to top them up.

Is this practical given how long it would take or is the output too low to provide a reasonable charge boost? Is a better option for me to fit another circuit for the TM batteries to connect to the motor alternator charging circuit so I can grab some Amps on the run back in at days end?

[Dave Bailey]

I have thought about this issue a bit too (have two batteries for my 24 volt Minn Kota that I currently only charge from the mains).

Solar would be good if it was quick enough...I guess if you are getting 10 amps it would at least top it up a bit, even if it is unlikely to fully charge it.

I did consider a charger that worked off the motor briefly, but the options for 24 volt were poor when I fitted mine (about 3 years ago). There were plenty of people doing this with their 12 volt setups at the time, with favourable reports. The situation is probably a lot better with 24 volt on board chargers (than can run from the motor) these days - that would probably be my suggestion.

What sort of cost are the 250 watt solar panels you have been looking at Allan?

Dave.

[Simon Gilbert]

Allan...if your looking at charging during a camping trip (and assuming you are fishing in the boat during the day), you're going to need to swap another set of batteries over onto the shore to charge during the day. You will ever get enough charge into 2 x 12 volt batteries ( I'm assuming the motor runs off 2 x 12 volt batteries in series and not 2 x 24 volt in parallel) over the morning couple of hours of afternoon couple of hours if you are only going to use 1 set of batteries - unless you have a half dozen set of panels.

So for a second set of batteries ( assume good quality at least lead acid), 2 panels, regulators amd wiring ( this can be expensive as you need BIG wires) you are up for a fair wack of dollars. Coupled with the hassle of getting batteries out of the boat onto the shore. Optional stuff like sun trackers, transformers ( if 24 volt batteries) or extra panels may also be included.

Personally I would get a unit to take power from your motor and charge them while you're going along. A couple of years ago I looked at this for a mates 24 volt system. I can't remember the costs ( it was expensive), but lots cheaper than the solar route and a lot less hassle. The 12 Volt shop in Kewdale would be a reasonable place to get an idea - although they are expensive.

But if you want to keep a 12v fridge working while your campng, solar is a great way to go. A few years ago we kept a 60 litre waeco fridge operating for 2 weeks using a 160w panel and a good quality AGM deep cycle battery. Don't forget a quality regulator. The set I have folds in half and fits into an aluminium box - so they are well protected. Brilliant!! Flexible 'panels' have potential as they fold more but are inefficient ( so you need a bigger one for the same power) and you need a way to point them to the sun more directly than simply on the ground plus I think for the same power they are expensive.

[Martin Kuhn]

Hi. In line with Daves and Simon's suggestions, The set up I have is that the outboard charges my start battery. Once that is topped up a relay then steers the power towards a 12 to 24 v step up transformer/charger (Stirling brand) that charges the TM batteries. That way the TM batts get some charge whilst moving between fishing locations. The setup works very well so far (touch wood)

For your scenario charging the TM batteries overnight, I would personally consider getting a small generator that either puts the 240v through a redarc battery management system into your 24 volt TM battery bank, or, one that spits out 12v. ( I vaguely recall that they exist) This is not a cheap solution, but I think more practicable than the solar options. Definitely Not as silent or sexy as a solar system.
I use a similar setup on my camper trailer when the solar panels just can't keep up with the camper electrical loads (especially when it's overcast a lot). Mine is a 12v battery management system, but I know that a 24v version ( for trucks) exists.

Hope those ideas help.
Cheers