Solar panel to top off/maintain batteries

My motorboat has an engine and a service battery of 110A, both lead acid.
I'm considering adding a solar panel on the roof of the pilot house to only top-off and maintain both batteries while I'm away during the workweek (I'm a typical weekend boater)
Besides an MPPT charge controller, I'm not sure what kind of solar panel I need for my purpose >
it seems I need some 25-3oW nominal power, but I notice that some panels are more efficient than others. Maybe I can do with less power, but a more efficient one.
What performance/specifications should I be looking for ??

THANKS,
V.
The Netherlands

 

I'm guessing you mean 110 Amp hour, which is probably a size 29 or 31. Your panel rating is about right for minimal topping off and maintenance with a charge controller, but the controller can be a very cheap one, like $30 cheap. Even if it breaks and shorts though, you have lots of time - months - to notice and replace it. There would be a noticeable, but small water consumption over a couple months. Or you could drop down to anything under about 20 watts and not even bother with the controller because the panel won't be able to cook the batteries. I'd probably go with 15 watts just to be on the safe side. As for the panels, there really isn't anything to bother about for a 15 watt or 30 watt system. If it was 3 kilowatts, then worth fiddling with. You are going to spent more money on the wire and the switch and fuse than on the panel, and they are arguably more important to get right. Panels have gotten cheaper faster than the MPPT controllers, so the size of the array where you start to look seriously at MPPTs is actually bigger now than a decade ago.

I should probably point out that topping up means when you leave the boat after using it, the batteries are at least 80% charged, and preferably more than that. Otherwise, it will take too long to bring them up, and some reduction is life might be incurred. Since the price difference between, say, 30W system and 60W system is trivial, I'd go bigger if you leave the batteries partly discharged at the end of a trip. But overall, this is really small potatoes here, anything is a lot better than nothing, and no-one will be able to tell if you got it perfect or just 80% right.

 

Thank you for your reply.
The idea is indeed to top-off & maintain the batteries (110Ah) after boating, so 80% charge would be the case I expect for the service battery; the motor battery will have much more charge left.
My plan is to install a panel that fits the maximum available flat space on the roof, approx. 2x2 foot, which will be a 25-30W unit I think.
A nice extra feature of an MPPT controller is the battery over- & discharge protection, but cost is a concern.

In the existing electrical installation, the alternator of the outboard motor is connected to an automatc charging relay that feeds both batteries >
BLUE SEA SYSTEMS Starting Isolation Dual Sensing Automatic Charging Relay | West Marine https://www.westmarine.com/buy/blue-sea-systems--starting-isolation-dual-sensing-automatic-charging-relay--8646283?recordNum=4

Can this relay remain, or to be replaced by the new charge controller (if needed) ??
This relay also needs a minimum of 175mAmp from the charger to have both batteries charged in combination: can that be a prob for a 25-30W solar panel ?

THANKS,
V.

 

The relay should be fine. It's going to drain 2 watts when operating. You can hook your solar to either side of it since it is dual sensing. I'd hook the solar up to the most convenient location, battery switch, relay, or either battery. If hooking to to relay, I'd go with the house side of the relay so the relay will operate fewer hours. And it's a good idea to group these little boxes anyway. So maybe mount the solar controller near the relay. But keep the wire runs as short as possible and use fat wire. I'd go with 10ga marine. It's tough and will have practically zero voltage drop at these currents.

 

I have a West Marine 150 watt battery combiner on my Gulfstar 37 sailboat which has given me years of trouble free battery charging. I also have four 65 watt solar panels and a 105 amp alternator. My systems are similar in operation to yours. Just bigger. The most efficient charging occurs when you are charging the larger of your battery banks. This is typically the house battery bank. It does make a difference. You want both the solar panel and the engine alternator connected directly to the house battery bank. Then run a wire from the house bank to the Blue Seas relay and then on to your starting battery. You are going to need an on/off switch and a fuse in the wire coming from the solar panel to the house battery bank. I used a 20 amp breaker in my system. If you use a charge controller you want the switch and fuse between the charge controller and the battery. Why an on/off switch? I discovered that when I was under power and my batteries came up to full charge the solar panels' output would shut down the alternator's voltage regulator and my tachometer quit working. Now when I am under power I turn off the solar panel.
If you have one of those "Off/1/All/2" battery switches you should replace it with 3 separate switches. One switch for the house bank, one switch for the starter battery and one switch to combine the house and starting batteries. That's what I have on my boat.
Good luck with your project.

 

FYI> This a (tiny) scheme I got from Blue Sea. It connects two outputs of the duo solar controller to the positive terminals of both batteries, parallel to the ACR. This seems to enable combined charging of both batteries in case the current from the solar panel drops below the ACR's minimum required level (below this level it only charges the first battery).
The alternator goes to the motor battery, the solar panel to the house battery.



Here's a picture of the 3 battery isolator switches on my mobo , the black one switches the negative terminal (have no idea of it's purpose) >

 

The diagram doesn't show which battery bank the alternator is connected to. The installation instructions for the Blue Seas relay state that the alternator should be connected to the larger battery bank. Which is usually the house bank. What happens is that the voltage regulator for the alternator senses the state of charge for the battery bank that it is connected to. The voltage regulator won't sense the state of charge through the relay for the other bank. So if the bank that the alternator is connected to is fully charged the alternator stops charging and the other bank is never brought up to full charge. The starter battery only goes below full charge when starting the engine and is quickly brought back to full charge. On the other hand the house bank rarely gets to full charge when the boat is in use because something is always drawing power. Thus the alternator is always charging when under way if it is connected to the house bank and the Blue Seas relay keeps the starter battery fully charged.
Hope this makes sense as to why you want both the solar panel and the alternator connected to the larger battery bank.
IIRC the theory for a switch in the negative wire is to prevent stray currents from causing galvanic corrosion.

 

Can I interject some dumb questions? Why would anyone booger up the starting circuit for the house? Wouldn’t it be wiser to provide sufficient solar to charge the house without the engine? What I planned...

Or is this just a method for less solar?

Thanks if I am wrong.

caveat: my starts are like 20’ from house and 25’ from house solar

 

It is all about efficient charging. The large house bank will accept more charge at a higher rate than the starting battery. So you want your solar panels and the alternator to charge the house bank. The battery combiner keeps the starting battery charged. Under way under power your house bank will be discharging to handle the power needs of the boat whereas the starting battery will just be sitting there fully charged. If the alternator output goes to the starting battery the alternator will sense the full charge on the starting battery and stop charging. You will end up with a discharged house bank.
If you need to charge your house bank with the alternator the best way to do this is to have the alternator connected to the house bank. If the charge from the alternator goes to the starting battery you will at best just get a trickle charge through the battery combiner to the house bank.
I hope this is making sense.

 

Thanks Chuck!
in the Blue Sea diagram, the figure depicting an outboard (incl. alternator) is actually connected to the start battery.
On my (new) mobo, both batteries are identical 110Ah, so no larger bank. But I understand the bigger benefit for the start battery by connecting the alternator to the house battery.

Am I correct that both the solar charge controller and the ACR will keep drawing current when the battery isolator switches are turned off?

 

How does one size wire for the above?

I see some blurry numbers, but can’t read, were those wire ratings?

OP-thanks for the post

 

I don't know if your solar charge controller draws any current. As for the Blue Seas ACR the documentations says it draws 175 mamps when combined and 15 mamps when not combined. This is a minuscule amount of amp draw. This would not be affected by the battery isolation switches. One friend of mine who was a marine electrician put switches in the lines from the combiner to the batteries. I did not do this on my boat and have never had a problem with discharged batteries. Your solar panels will always be charging your batteries whenever there is sunlight regardless of whether or not the isolation switches are turned off or on.

As to wire sizes I don't remember what size wire I used. All I remember because I thought that it was odd is that the wires from my combiner to the batteries had to be at least 3' long according to the installation instructions. No explanation as to why this was necessary.

 

A minimal resistance was required apparently.

 

To be more specific I'd recommend a mono crystalline panel over a poly one. That's because the mono has a higher efficiency, especially when being partially blocked from sunlight.

How big? That depends on how you use your boat, how long engine time you have going out and going home and of course how much you use your batteries. But since you only have two 110Ah batteries, one start and one house, it sounds like a boat with more engine than quarters so I assume that you don't have much living of the house battery when you're out?
And about that, do you rely on shore power when you're out? Or perhaps you don't do sleepovers but only day trips?

As for the regulator I'd recommend a PWM over a MPPT. While MPPT has greater efficiency in theory (and some in practice) the complexity comes with a price. In terms of actual Ah being fed to the batteries per dollar you're better of buying a larger panel and PWM over a smaller panel and MPPT.
A bigger concern with the MPPT is that you have to be more careful where you put the regulator. Despite what the manufacturers state (and what regulations they have to follow) it's not uncommon to have interference in VHF and FM-radio. In one boat I got a total block of both VHF and FM-radio and no matter what I did, even swapping the (according to manufacturer) faulty regulator for another one helped. So the high price MPPT now sits on a shelf somewhere and a cheap PWM does the job without a fuss.

 

Thanks Magnus, my boat is a 21' Jeanneau Merry Fisher 605, with a 115hp outboard engine, daytrips only. There's a nice spot on the roof of the pilothouse for a small solar panel like 30W, and want to be able to top off without shorepower. Cost is not my main concern (small unit anyway) but getting the best possible charge out of this panel for the batteries.

Where I live, Victron seems to be a leading brand in this business, this is what they claim regarding their latest MPPT's, is this valid or just marketing hot air ? >>

"BatteryLife: intelligent battery management
When a solar charge controller is not able to recharge the battery to its full capacity within one day, the
result is often that the battery will be continually be cycled between a “partially charged” state and the “end
of discharge” state. This mode of operation (no regular full recharge) will destroy a lead-acid battery within
weeks or months.
The BatteryLife algorithm will monitor the state of charge of the battery and, if needed, day by day slightly
increase the load disconnect level (i. e. disconnect the load earlier) until the harvested solar energy is
sufficient to recharge the battery to nearly the full 100%. From that point onwards the load disconnect level
will be modulated so that a nearly 100% recharge is achieved about once every week."