Solar panel to top off/maintain batteries

Use two small panels of about 15-20W each. Wire them directly to their respective battery. Don't use a controller/regulator. Don't use bigger panels.

Fuse the panels right at the batteries, in the positive lead only. The fuse is there only to protect the wiring in case of a short. If a fault develops, the panels will be shorted with no consequence, but shorting the battery through that wiring will burn your unattended vessel to the waterline.

Don't use switches, relays, diode isolation thingys. Don't use any of that junk.

It won't recharge your batteries if you flatten them, but you don't need it for that. You only want it to put a tiny spoonful of power into the batteries every day just to top off the self-discharge, which it will do safely.

It's easy to understand. Easy to diagnose. Easy to repair. Safe. Inexpensive. You do not want complicated electrical junk on a boat that no one can remember how to fix.

 

Thanks Steve,
I understand the attractive simplicity of your recommendation.
Even a 20W solar panel can -according to the specs, generate a voltage of >18V, but the manual says that the charging voltage for my batteries cannot exceed 14.6V >
how can I solve this ?

THANKS,
V.

 

Yes, I missed that bit out...

The batteries can tolerate a continuous input charge current of some small percentage of their AH rating, without harm. It is something like 0.1*AH or something. I had a quick google and I can't see it immediately. Your manual will tell you what that current that is. As long as your panels never get close to that current, you can completely ignore any regulation.

 

Here's an article about it. This is trickle charging, ie constant current. (The alternative is float charging - constant voltage, requires a regulator.)

Trickle Charger Information http://cs.yrex.com/ke3fl/htm/TRICKLE.HTM

It seems the charge current must be less than one hundredth of the battery AH rating. So your 110AH battery needs only one amp (12-15 watts).

The system has zero fire risk, simple, fault tolerant, abusable, safe, repairable, uncomplicated, and takes up minimal space, AND ridiculously, it's also your cheapest option.

Relays, terminals, regulators, switches, all just ask for trouble. They corrode, get wet, catch on fire, jam on or off, or the system is complicated enough it will find a way to catch on fire and burn. I know of one bloke who watched his homemade GRP plane burn to the tarmac because of "a good idea" - that must have stung. Most yacht electrical systems are way too complicated.

Try find gel-filled or o-ring sealed connectors, or else use butt-splice crimp (use Utilux tool) and protect with glue-filled heatshrink. Don't solder - it point loads the wire and it will break.

Every battery will be different. The description above is only correct for liquid-filled lead-acid. AGM will have a much smaller current. Lithium Ion(and others) CAN NOT be trickled like this - Lithium Ion will randomly light up and BURN if you constant-current at ANY amount, hilariously..

Mount the fuse AT the battery, and test it to make sure it blows when you short-circuit the panel. Yes, wire it all up and SHORT the wiring RIGHT at the panel, and the fuse should pop without delay, as in instantly. Feel free to use as large a fuse as you like (much greater than the panel current is fine) but you must check to see that the fuse blows immediately - if it hesitates more than half a second you are asking to burn. Feel free to blow a few bucks worth of fuses to persuade yourself that it will protect you.

Nothing can happen to you, your yacht, your panel, your wiring, or your battery. You can inadvertently or intentionally do any bad thing to any part of it and it wont do a bad thing back. Sleep well!

HTH

 

Steve,
What's your view on Victron's BatteryLife algorithm thing >>

"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."

 

The example system that they mention is so ridiculously overloaded as to laughable. As to their invented "algorithm" I think it is equally laughable.

Build properly designed PV systems that don't need clever software just to not destroy themselves.

 

About the Victron system - the idea here is that you are using a minimal battery system and working it pretty hard, with an eye to minimizing life cycle cost. If using a generator to recharge, you want to shut the genset off when its load drops below about 25% rated load for a 10kW genset (or 30-35% for smaller ones like 4kW) because fuel consumption per kW absorbed by the batteries goes to hell. So by design, wet lead batteries would be cycled between about 45% and 80% charge. This works for say 2 charges per day for a week, but then you must do an equalization charge or you will damage the batteries. This is the most cost effective routine for wet lead batteries charged via a generator.

Solar presents a different challenge when used as the primary recharging source. It isn't available on demand like a genset is. So you want to design the system so that you are assured of the ability to perform an equalization charge when needed. This temporarily reduces the amount of solar power that recoverable from the batteries since equalization charging throws much of the power away. Because wet lead batteries are more efficient round trip at lower states of charge, they will often settle in a part-charged regime if heavily loaded, or due to weather factors. The monitor function is intended to help you manage your load to prevent damage from too much power transfer at part charge between equalizations.

This stuff starts to be taken seriously when you start talking about 10kWh/day extraction from the battery. But I can cruise indefinitely off-shore or live aboard with two 200W panels. That handles autopilot, nav, and house loads 24/7. My battery is three size 29 batteries, 1 for the diesel and two for the house, which I rarely isolate from each other. I have had to use the alternator to help with charging about three times, when the batteries were getting old. So I'm basically in Steve's camp that the system described is not typical of most cruisers. I have, however, helped deliver a nice little 50' catamaran that was pulling 83 amps at 3 am with me the only soul awake. No idea where it was going, but that was the lowest amp draw I saw on that boat during the 2 week delivery.

So, what do I think about it. I think I'd use it if I had it. Beats alligator clips and eyeballing the batts for two hours and fiddling with a rope tossed on the panels to adjust voltage.

 

Update > I installed a nice 3mm thick flexible 50W Sunbeam solar panel with a Victron Bluesolar 75/15 MPPT controller on (single) a 100Ah semi traction battery.
Assume its overkill, but seems to work fine so far.
I tinkered a USB adapter for a few Euro's that connects the Victron controller with a Victron app on my smartphone, to read out -historical- performance: cool feature, but I don't expect to be using it much.