Using Solar to keep batteries charged over winter

[gibsonih]

I have four 140 ampHour house batteries, one 110 ampHour starting battery and two smaller batteries for a bow thruster. When on board, charging is via two 40 amp battery chargers when using 220v ac. (Shore power or generator) or engine alternator.
For most of the year the boat is on the hard and it is difficult to connect to 220v ac, so is not done as regularly as it should be.
I would like to use solar power to keep the batteries charged during this time.
Presuming the batteries are fully charged to start with, what size solar panel do I need and can it be done with one panel and one regulator? I presume a MPPT regulator is best?

[StuM]

You've got maybe 1000Ah of batteries.

Assuming they are FLA, their self discharge rate will be about 5-10% per month.
Say roughly: 100 Ah per month, 3.5Ah per day, 50 Watt hours per day.

So even a 20 Watt panel and a cheap controller should be enough

[gibsonih]

many thanks

[Sailsgood]

Originally Posted by StuM

You've got maybe 1000Ah of batteries.

Assuming they are FLA, their self discharge rate will be about 5-10% per month.
Say roughly: 100 Ah per month, 3.5Ah per day, 50 Watt hours per day.

So even a 20 Watt panel and a cheap controller should be enough

Yep! My advice; take a few more W (perhaps 30W) to be completely safe .. But no more than 30W, which is already a precaution.

[SecondBase]

Stu and Sailsgood are right on the number. If you keep under 1 watt to 25 AH you do not need a controller. I like West Marine for these units because they sometimes fail and the no hassle return is easy (for me anyway).

[Ardbeg]

I leave my batteries in all winter ( Ontario) for 6 winters now and have my solar panels on all winter. Never had a problem. I have 480 W panels. Overkill I know but it works and batteries are in perfect shape 7 years later. So I highly recommend it.

[Stu Jackson]

Originally Posted by SecondBase

Stu and Sailsgood are right on the number. If you keep under 1 watt to 25 AH you do not need a controller. I like West Marine for these units because they sometimes fail and the no hassle return is easy (for me anyway).

you do not need a controller

This is incorrect. The voltage will rise and damage your batteries. See Maine Sail's website. Don't do it.

[sailcrazy]

Our battery system is a 675amphr "house battery" (actually 3-225amphr batts connected together) and a standard mx-free start battery recharged by a Balmor Duo-Charge from the house battery. We have 4-80watt solar panels and a MPPT controller hard-mounted. We leave the boat up on the hard in Puerto Rico for 6 mos ea year. Although fully charged before we went on the hard, we noted that the battery water was bubbling frequently during that first season on the hard. We disconnected 3 of the 4 panels, and the bubbling stopped; and the batteries were fully charged when we returned for the next season. Now we have done that same procedure for the past 3 seasons....and no bubbling and fully charged upon return. FWIW

[arthur.watson.1]

I have a 140-watt Solbian flexible panel and an MPPT regulator. I didn't want to leave my expensive panel out in the weather all winter, so I found a very inexpensive 30-watt rigid panel on Amazon with the same MC4 terminal plugs. I simply plugged it in and it seemed to work just fine. NB, I have only 150Ah battery capacity, so YMMV.

Best,

Art Watson

[Locquatious]

You didn't mention the type of batteries, nor where you store your boat on the hard.

If you have wet cell batteries, pay attention to the outgas voltage as a function of temperature. For example, in the tropics the battery temperature may be in the 90's and the batteries will begin to bubble above say 13.3V. Over a long storage, the batteries will eventually dry out and you will come back to damaged batteries.

I was not aware of any solar regulators that allow control of the battery voltage during tropical storage, so I made my own PWM regulator that I could set to 13.2V. Our batteries were never dry again and we got a few more years out of them.

Prior to making my own regulator, I shaded some of the cells on my panel with tape till the output voltage into the equilibrated batteries was 13.2V. This worked, but it took a long time to find the equilibration tape/voltage point at max solar.

For AGM batteries, our controller is set for the manufacturer's recommended float voltage at maximum boat storage temperature.

[ArmyDaveNY]

Originally Posted by Stu Jackson

you do not need a controller

This is incorrect. The voltage will rise and damage your batteries. See Maine Sail's website. Don't do it.

West Marine uses, among others, Nature Power solar panels. Although Nature Power says that anything over 12 Watts needs a charge controller, it seems silly not to use a charge controller on any panel even if under 12 Watts. The nature panel charge controller is only $20. Given the cost of new batteries it is money well spent.

I have a 15 Watt Nature Power panel on two of my boats, along with their charge controller for each, and they work reasonably well. They're not sophisticated but they do a respectable job. They are plug and play and West Marine does have a great return policy. The panels are semi-flexible.

I just ordered a 150 Watt Grape Solar kit from Amazon (also available at many hardware stores such as Home Depot) and it too is a plug and play system. The 150 Watt kit is composed of three 50 Watt panels and it includes a charge controller (PWM). They also offer a 50 Watt kit for around $101 US, also with the PWM controller. The panels are rigid and have an aluminum frame.

These are pretty much plug and play systems and may be sufficient for your needs. They are relatively inexpensive and occasionally are on sale at good discounts.

[noelex 77]

As Stu has correctly calculated, you only need a small panel to keep up with self discharge, but given the low cost of larger solar panels, more wattage is worth considering.

This can allow you to run items like dehumidifiers or if leaving the boat in a wet berth, power to run bilge pumps etc.

Given your initial post these requirements may not be necessary, but it may be worth planning for a system that will cover all eventualities given the small additional cost.

The solar regulator need not be MPPT. This type of regulator will harvest more power from a given panel, but considering your requirements it will be cheaper and more reliable to install a slightly larger solar panel with a non MPPT (or PWM) regulator. Although quality PWM regulators are getting hard to find.

It is important the regulator is adjustable. Ideally you want a low absorption voltage and a very short absorption time, combined with an occasional higher voltage to stir up the electrolyte (at least for wet cell batteries).

Unfortunately, such adjustable regulators are not at the cheap end, but need not be super expensive and the bonus is in battery life.

[foufou]

I have a trickle charger I use on a start battery that puts out 21V. About one third amp. It has been in use close to twenty years and has never damaged a battery. they typically last eight years. So I am not certain what other experiences may be regarding charging without a controller.