Battery and Amps

[Grime]

This is not about Do I need a fridge on a boat.  I need one for the type of food I have to eat. The question is about battery and amps.

I have a group 29 marine battery and purchasing a ARB 50 qt fridge/freezer. I would use 110 to get the unit cooled down. Then use 12 volt after leaving the dock. The average amp draw is 1.35 amps.  I also have 60 watts of solar.

Do I need to run 2 group 29 batteries?  Not sure the size of the alternator on the Yanmar YSM 8.

My other elect. requirements
VHF
Chartplotter
nav and cabin lights. LED
Sometimes cig plug to charge cell phone, tablet, handheld vhf, and laptop.

Just plan dumb persons language please. 

[Captain Smollett]

Couple of questions before a reasoned answer can be given.

(1) Does the unit draw 1.35 amps ALL the time or does it cycle on and off.  For example, an Engel only runs about 20 minutes total out of every hour.

(2) Group 29 tells the physical size of the battery, but not the charge capacity.  There is great variation.  Do you know the amp-hour capacity of the battery?

I'm going to guess and say with that battery AND a 60 watt solar panel, you are PROBABLY ok (provided you get sun on the panel, of course).

A 60 W panel will give you something in the ballpark of 4 amps, and I usually figure 5 hours of charging sun per day (you may get more, but keep the estimate conservative).  So that panel will give you something around 20 amp-hours per day.

If the fridge does not run 1+ amp continuously, the panel should be able to keep the battery topped off.

It will depend on how many amps you use running the other gear, though.  No generic answer possible in the strictest sense.  Strictly speaking, you'd need to answer "how many amps for how many hours" for each item on your list.

My answer assumed no charging from the engine. 

[Grime]

(1)  The all-new 50 qt model is able to maintain sub-freezing temperatures in 90? f heat, while only drawing an intermittent 1.35 amps per hour from a 12 volt power source. From the manufactures site.

(1a) I assume it kicks on and off like my fridge at home does.

(2) The battery I have now is 125 amp battery.

I don't understand solar very well but my 2 30watt panels will put out a charge even on a cloudy day. I can pull the battery down using a inverter to run my laptop. Not long after the sun comes up my battery is topped off. Cloudy day the same.

The fixed VHF is not used much. The handheld can use regular double A batteries. That leave the chartplotter. Not sure until I get to the boat over the weekend. 

Would it be best to get 2 6volt batteries instead of 2 12volt?

[Captain Smollett]

(1)  The all-new 50 qt model is able to maintain sub-freezing temperatures in 90? f heat, while only drawing an intermittent 1.35 amps per hour from a 12 volt power source. From the manufactures site.

(1a) I assume it kicks on and off like my fridge at home does.

(2) The battery I have now is 125 amp battery.

Seems to me that you should be fine, then.  Looks like the fridge will draw something less than 20 amps per day (and maybe quite a bit less!) and your battery has enough capacity to power things even if you get a few "no sun" days (again, assuming no engine charging).

I don't understand solar very well but my 2 30watt panels will put out a charge even on a cloudy day. I can pull the battery down using a inverter to run my laptop. Not long after the sun comes up my battery is topped off. Cloudy day the same.

Yes, they will put out SOME charge even on a cloudy day, but it's not their 'full rating.'  They need bright, direct (that is, mid afternoon) sun to produce their full rated output.

I usually lean conservative on my calculations (to build in "margin for safety") so that I use "full sun, 5 hours per day" for calculations.  Anything beyond that (morning sun, cloudy days) are extra.

The fixed VHF is not used much. The handheld can use regular double A batteries. That leave the chartplotter. Not sure until I get to the boat over the weekend. 

Unless that chart plotter is REAL power hog, I'm thinking you will be fine with your current set up.  You can measure the current draw on the plotter to be sure, but with your present battery, you have plenty of capacity for a pretty healthy budget, good charging from your panel and the engine alternator in reserve "just in case" you hit a stretch where it is needed.

Seems to me like a pretty solid set up.

With a 125 amp-hour battery, I would personally budget no more than 60 amp-hours between charges.  That pulls the batt down half way, which is quite a bit, but not 'terrible.'  Let's say the fridge + chartplotter together are 20 - 25 amps per day...just a wild guess, but it puts a number on it for us.

That would give you 2-3 days between charging (ie, running JUST on battery, no solar and no engine) and still not running the battery below 50% charge.

The panel will give your your rough 20 amps per day if you get good sun, so as long as you have mostly bright days, you should be golden. 

The laptop will draw quite a bit (depending on how much you use it, of course), but I don't think the cell phone will change things appreciably.  It's draw is small compared to the fridge and/or the chartplotter.

Now the big question is...lighting.  Do you plan to run at night with navigation lights on?  They can suck a lot of juice, unless you've gone LED.  Cabin lighting I would not worry to much about, as they are usually not on long (unless you are a real night owl and have the cabin lights on all night).

That is, running at night without the engine, of course...sailing.  If motoring, alternator should cover the lights and then some.

Would it be best to get 2 6volt batteries instead of 2 12volt?

Depends on a number of things...and what you want.

The 6 V batteries I have are 215 amp-hours.  In series, they give me 215 a-hr at 12 V.  They are smaller than Group 29, though...maybe Group 24 size.

If you had two 12 V batteries like the one you have now, you'd have 250 amp hours capacity at 12 V.

So...either or, really.  They are similar.  The two 12's are a bit more juice at the expense of a little bit larger cases.

Depending on your lighting requirements, but I see no real pressing need to upgrade to two batteries.  If the batt you have is very old or has been drawn to "parade rest" a few times, I'd consider replacing it, but it seems like you have enough capacity to get good service.

[Captain Smollett]

Quick follow-up to clarify something:

I keep saying "it depends" because there is not single answer.  There are a lot of factors.

Designing a system for true "off grid" power supply is a different ballgame than if you plan to hook up to shore power at a marina every night, once or twice a week, etc. 

It also depends a lot on how often you plan (in the general sense) on motoring.

So, my above comments are for the "off the grid" side of the spectrum.  Any shore power or motoring is going to be added to that.

The other thing to clarify is that generally the question is more about charging capacity than battery sizes and configurations.  The battery just stores the juice so you can use it when you want to use it; but, you have to generate the juice.

If you increase your capacity (by getting a second battery), the only thing that will really change is

(a) Your 2-3 days between charges window will open up to 4-6 days

OR

(b) You keep the 2-3 day window and your budget allows for a 25% draw down rather than 50%...which will extend the life of the battery(ies).

I hope that makes sense.  To answer if the second battery is needed is only for you to answer:  do you think you'll have more bright sunny days than not, or do you mind running the engine for charging a few hours per day every few days if the sun is NOT shining?

If I were coast hopping, motoring the ICW and that sort of thing, I personally would not add the second battery at this time.  I'd try it for a while and see if it is really needed.

[Grime]

Thanks for the great input and advice.

We will be doing the ICW more than offshore so we'll be using the motor a lot.  :(  I'll stick with a single battery for now. I will replace the one I have now so that it is fresh. The one I have now is over 3 years old. I took it off Miss Froggy when I sold her.

Don't plan on doing any night sailing but when we have to all my nav lights are LED. Marina's are to expensive to use often. We'll try to use marinas at military bases when possible or a need to tie up.

[okawbow]

  Not sure the size of the alternator on the Yanmar YSM 8.

My other elect. requirements
VHF
Chartplotter
nav and cabin lights. LED
Sometimes cig plug to charge cell phone, tablet, handheld vhf, and laptop.

Just plan dumb persons language please.

The alternator on my YSE8 is 30 amps.

When I went on a 1000 mile, 30 day cruise; I used 2-100 amp/hr batteries. My 40 watt solar panel, and part time motor use, kept both fully charged. I ran lights, gps, bilge pump and tiller pilot, without worry about running the batteries low.

I like to have 2 batteries on board just in case. However, I can start the YSE8 easily with the hand crank.

[Grime]

My bilge is pretty dry and on a float switch. Rarely comes on. Don't have a helm pilot wish I did.  If I ever do get one I'll have to re figure my amp usage. 

[Grime]

Got my ARB 50qt. fridge/freezer Tuesday last week. Plugged it in to 120 and set the temp down to 29. Within an hour the unit was cooled to 29 degrees. I ran it at that for 24 hours then changed over to 12 volt. I don't have a way to check amps used but my volt meter never got below 13 volts. I wasn't using anything else at the time on 12 volts.  I'm still worried about the battery as its over 2 years old. Thinking of getting a new Group 29 122 amp and using the old one on the trolling motor for awhile.