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Old 08-04-2014, 17:44   #1
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Batteries How Many?

I need new batteries. I have decided on 6 volt golf cart batteries. The question is how many. I have a H36 with a hungry separate frig and freezer. I know more is better but too many is also a problem. I currently have a pair of 6 volts that has been short lived from over discharge. My questions is I can not decide if I need 4 batteries or 6. My goal is to be able to go 24 hours before starting the Honda generator. I have space for 6.

Ps all other loads are small other than autopilot that needs to run no more than 12 hours a day.
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Old 08-04-2014, 17:48   #2
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Re: Batteries How Many?

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I need new batteries. I have decided on 6 volt golf cart batteries. The question is how many. I have a H36 with a hungry separate frig and freezer. I know more is better but too many is also a problem. I currently have a pair of 6 volts that has been short lived from over discharge. My questions is I can not decide if I need 4 batteries or 6. My goal is to be able to go 24 hours before starting the Honda generator. I have space for 6.

Ps all other loads are small other than autopilot that needs to run no more than 12 hours a day.
We can't tell you. You need to calculate how much energy you use on average between recharging events and then size your bank accordingly so that you're not over discharging but can adequately recharge when you're charging. A little more research and some math should give you your answer.

I don't mean to be flip, but this is one of those subject matters that you need to educate yourself on in order to make sound decisions for your specific situation. That education will also allow you to manage your battery bank more effectively going forward, both for adequate energy supply and battery longevity.
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Old 08-04-2014, 18:11   #3
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Re: Batteries How Many?

Yeah, total your amp estimate, and get... what?.. two times your daily usage at least? Just remember, too much excess storage and then you might have trouble refilling it once pulled down! Although this might be the lesser of evils....
But realistically, yes people do it with 4 or 6. If that's your question, you have room for 6 and they are not high in the boat go for 6.
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Old 08-04-2014, 18:22   #4
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Re: Batteries How Many?

What would be amp hrs of the bank with 6? What is your charging capacity?
Solar or wind and from your engine?
I agree that 6 would be better.
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Old 08-04-2014, 18:35   #5
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How do you calculate? Use the rate amp hours or measure actual amps. I find both problematic when all my large loads at so inconsistent. Frig and freezer depend on today's load and auto pilot will be almost nothing on a good breeze day and a awful lot on a windless one.
That brings me back to asking for real world experience.
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Old 08-04-2014, 18:41   #6
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I guess I should add I have no solar (too far north) and no wind, I would like to be on the hook 24 h before starting Honda.
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Old 08-04-2014, 19:01   #7
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Re: Batteries How Many?

Your fridge and freezer should have amperage specs that you can look up. To get amp hours for each you need to estimate how long each runs in a day. 50% might be a good starting point, so 12 hours times the sum the amp rating of these items gives you amp hours/day. You need to recharge well before you consume half the amp hours in your batteries, so you should be able to estimate whether you need 4 or 6. Then, as mentioned above, you need to make sure your charger can charge all those batteries.
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Old 08-04-2014, 19:01   #8
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Re: Batteries How Many?

Refrigeration is the driver. On one boat I had 4 normal deep cycle 12 v batteries in the tropics. At anchor, refrig, lights and maybe an hour of tv/video was the load. It worked and I had to charge at least one hour a day, maybe 1.5 hours every third day with a 100 amp alternator. Again, if I had 6 batteries my batteries wouldnt have gotton so low between charges, but they would have lived in a state of undercharge unless I charged maybe 1.5 hours every day.
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Old 08-04-2014, 19:24   #9
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Re: Batteries How Many?

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How do you calculate?...
You must estimate your average daily use. List everything that draws power, how many amps draw, and how many hours a day it will be on. Add'em up. Estimate best and worst case. Actual will be somewhere in between.

It's easy. Even girls can do it.
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Old 08-04-2014, 19:48   #10
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Re: Batteries How Many?

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How do you calculate? Use the rate amp hours or measure actual amps. I find both problematic when all my large loads at so inconsistent. Frig and freezer depend on today's load and auto pilot will be almost nothing on a good breeze day and a awful lot on a windless one.
That brings me back to asking for real world experience.
I have been through this exercise recently to assess the size of my existing bank and to get an idea of how much solar I needed.

I found this worksheet pretty useful. It was posted by someone else on the forum. I made it into my own spreadsheet because I am geeky like that and it gives you the ability to easily adjust various parameters and see the result. For example, I could work out an estimated amp hours at anchor, on a day sail and on a passage. Just by playing with the number of hours used for each load, you can quickly get an idea of your range.

http://newcontent.westmarine.com/con...r/Elecbugt.pdf

Initially I just made estimates of the amps for various devices to get a ballpark load to see what I was up for. Gradually though, I managed to track down the specified amps (or watts and convert that to amps) for all the various devices aboard. Finally I installed a battery monitor and measured actual amps for the various loads.

You don't need a battery monitor I suppose but I love mine and you really get the full picture on your battery and your loads. Forum member Mainesail has a great article on his site on installing a monitor and I followed his guidance to the letter. With the monitor installed, I was able to run the boat for a typical day and see how many amp hours I actually used. I was within about 10 amp hours of the worksheet calculated one so was pretty happy with that.

Upshot of my calcs was that my battery bank was ok but I needed more solar to avoid extended generator/engine charging.

Hope this helps.
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Old 08-04-2014, 20:14   #11
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Re: Batteries How Many?

Those who have advised you to calculate your daily usage are right: that's the proper way to do it.

A seat-of-the pants estimate is that on a 36ft sailboat six golf-cart batteries would be more than enough, unless you have some very unusual equipment aboard.

I carried four golf-carts on my 42' sloop for years. They were marginally enough. When I added 2 more, giving me a total of 675AH capacity, that left a healthy margin of onboard power.

If you have room, by all means put 6 golf-carts aboard. That's only about a $250 increase from 4 golf-carts.

It would be good to have a healthy charging capability...big alternator and substantial charger. You mentioned the Honda generator. If you have a Honda EU2000i, it can handle battery chargers up to about 75 amps.

Golf-carts are tough, but not indestructible. You need to get them to full charge as often as possible and to charge them routinely at 14.6-14.8 volts DC. Float them at 13.6-13.8 and occasionally kick them up a volt or so to absorption voltage.

Also, cut down your daily power needs as much as you can. Install LED lights if you don't have them already...that makes a big difference.

The largest draws are likely to be the frig/freezer and laptop computer(s).

FWIW,

Bill
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Old 08-04-2014, 20:20   #12
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Re: Batteries How Many?

I'd go for 6. The batteries will last longer if they discharge less. The general rule is go no farther than 50% discharge. Just as deeper discharges will shorten battery life, shallower discharges will extend it. Also, if you have a powerful enough charger, you may be able to recharge them at a faster rate (greater overall charging current) because the charging current will be divided across more batteries and so each will heat up less. Similarly, batteries perform best when discharged slowly. The more batteries you have, the slower each one will discharge.
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Old 08-04-2014, 22:36   #13
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Re: Batteries How Many?

On the other hand, the more batteries you have, the longer it takes to complete the charge. So a too-large bank might never get charged all the way, leading to early failure.

Do the estimate instead of just guessing at how many batteries you need.
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Old 08-04-2014, 23:23   #14
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Re: Batteries How Many?

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1. On the other hand, the more batteries you have, the longer it takes to complete the charge. So a too-large bank might never get charged all the way, leading to early failure.

2. Do the estimate instead of just guessing at how many batteries you need.
1. Uhm, not really. Based on the same daily draw, it simply doesn't. The tradeoffs are simple: larger bank can run longer before reaching 50% SOC, too large a bank can be chronically undercharged if recharging sources are limited.

2. Agree 100%.

Here's one way how:

The All-Important Energy Budget:
Energy Budget

Record of Daily Energy Use of 100 ah per day:

"Breaking In" New Wet Cell Batteries
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Old 09-04-2014, 01:44   #15
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Re: Batteries How Many?

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On the other hand, the more batteries you have, the longer it takes to complete the charge.
A bigger bank is always better - that's the best advice here so far.

Since the OP has no Solar he will only be re-charging to about 85% with his Honda. A larger bank will put in MORE Ah more quickly because he is always in the bulk stage where the charge acceptance of the bank is higher.

So in his case at anchor he will "charge" a bigger bank FASTER.

See my post #137 for why bigger is better:

Size of Battery Charger
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