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Old 13-10-2015, 21:37   #1
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Help Me calculate How Big My House Banks Should Be.

Hey guys. my boat didn't come with any house batteries, I already have my starting battery but now I need to pick up my house battery banks. I made up a chart (see below) with the various systems on the boat and approximately how much power they use and my plan is to pick up 4 trojan T105's which will give me 2 12v 225ah banks, given my calculated ah demand of 266ah/day do you guys think the 4 T-105's would be sufficient?

Screen Shot 2015-10-13 at 2.04.28 PM by Nathan Pablo, on Flickr
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Old 14-10-2015, 01:57   #2
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Re: Help Me calculate How Big My House Banks Should Be.

if you are drawing 250 a day you need something in the 1000ah range. unless you have a way to charge like solar etc.


because A you can only use half the power in a flooded acid battery, and B you want 2-3 days in a battery. a 1000ah battery gives you 500ah useable which is 2 days.


how are you currently getting by with only a start battery?... are you running the engine 24/7?


250 a day is a lot for some basic things you have listed. you might be over estimating everything. except for the water maker. that one seems really wrong. normally you can't run a water maker off a battery because they are so big.


also you don't want 2 house banks, make one bigger one.
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Old 14-10-2015, 03:18   #3
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Re: Help Me calculate How Big My House Banks Should Be.

I think these estimate charts can often be very wrong, but can also highlight potential heavy usage items. You haven't listed computers and phones which can be nearly as high as a fridge. Change the lights to LEDs and check the Watermaker power, we only use ours when motoring. Why is the GPS Current so high?

The best way to determine the Ah needed is to invest in a Battery monitor or a digital amp meter and measure the actual current usage between the extremes of anchoring and sailing at night.

But consider what actually happens on board most cruising boats. Batteries can charge quickly to 80% of their maximum, but take many hours to get to 100%. So if the batteries only get 80% charged most of the time what is the actual useable capacity? It’s a simple calculation - a 100 Ah battery only charges up to 80% = 80Ah. It must be charged when it goes down to 50Ah, so in this situation there are only 30 Ah available for use from a 100 Ah battery. That’s 20 Ah less than the theoretical 50Ah, lower by 40%. So for every 100Ah capacity needed you should get 140Ah.

Charging capacity must also be taken into account to determine house battery size, but be aware that battery charge efficiency is on average 85%. So if you want to raise an FLA bank by 100Ah with an 85% efficiency then approximately 120Ah has to be input. (85% of 120 is 102). With an AGM bank at 98% efficiency then only 102Ah has to be input to raise the bank by 100 Ah.

Solar is very cost effective for charging but most boats can never mount enough panels to get back to 100% because there are just not enough hours in a day.
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Old 14-10-2015, 04:02   #4
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Re: Help Me calculate How Big My House Banks Should Be.

First last column is measuring amp-hrs per day. (but that's a seperate thread).

Once we correct for that...

While it sounds nice to plug in a meter that isn't very practical for a new boat where you need a battery bank now. I would suggest adding margin for error since you don't have actual usage numbers and not all days are equal. Maybe 25% extra, so 266 amp-hrs used each day goes to 333 amp-hrs each day.

I'm going to assume you are looking at a situation where you are away from shorepower and not motoring somewhere daily. Also, I'm going to assume you plan to provide solar or wind power regneeration to fill the batteries back up.

How many days do you want the batteries to last before you must find a dock with shore power or crank up the engine to recharge? If we assume 3 days (you must decide what you are willing to live with), that takes you to roughly 1000 amp-hrs.

Now you have the next choice to make. If you use the full rated amp-hrs of a battery, it will take very few cycles before they are trash. Most people limit it usage to between 30-50%.
- If you are only assuming a day or two before you crank up the engine, I would suggest only using 30% of the rated capacity as you will be dragging the batteries down to the low point pretty regularly. Also, replacing the last 10% of a batteries charge takes a long time so it often doesn't happen so you may often be starting with an 80-90% charged battery.
- If you design a system that can go say 5 days without any charge, it will be very rare to actually go 5 days with no output from the solar/wind system, so dragging it down by 50% is more acceptable because it will happen so rarely.

So lets say with a 3 day supply, you are willing to on rare occasions go down by 50%. That means to draw 1000amp-hrs, you need a 2000amp-hr battery bank.

That's the math once you determine your typical draw.

That's a pretty big battery bank but looking at your table, some things don't make sense (for ease of reading I'll put up a second post)
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Old 14-10-2015, 04:07   #5
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Re: Help Me calculate How Big My House Banks Should Be.

I can only agree with measuring what you are actually using. Your watermaker number looks particularly low. We have a spectra and that uses 8-9 amps to make 30 liters of water. We would not run it 6 hours per day. Other numbers look off also.

AGM batteries can be discharged down to about 30% (some say 25%). So having say 500A of AGM means your usable power will be something like 350 AH.

You also should look at what kind of alternator you have and how much it puts out and for how long.

I realize this post raises more questions than it answers - but having just gone through this entire exercise, I can tell you you need to look at the entire picture. Not just the usage.
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Old 14-10-2015, 04:22   #6
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Re: Help Me calculate How Big My House Banks Should Be.

Quote:
Originally Posted by frozenhawaiian View Post
Hey guys. my boat didn't come with any house batteries, I already have my starting battery but now I need to pick up my house battery banks. I made up a chart (see below) with the various systems on the boat and approximately how much power they use and my plan is to pick up 4 trojan T105's which will give me 2 12v 225ah banks, given my calculated ah demand of 266ah/day do you guys think the 4 T-105's would be sufficient?

Screen Shot 2015-10-13 at 2.04.28 PM by Nathan Pablo, on Flickr
As mentioned, I think your usage is a bit off.

First I think you are mixing "at anchor" with "under way" numbers. (ie: you have the instruments going 24hrs but also the anchor light for 12hrs each day)

If you expect to do multi-day passages, you may want to run a seperate calcualtion for at anchor vs under way as you will use different devices for different periods of time. Then you can pick the one that uses more amp-hrs.

Keep in mind for each 1amp-hr saved here, the battery bank rating can be reduced by 4 amp-hr based on the previous post calculations so even some of the smaller items can make a difference.

Going thru the numbers some items to consider:
- A fresh water pump at say 5gpm running for 30minutes, would go thru 150gal. If these are daily figures, you either need massive tanks, or you will be going to shore to fill up daily.
- Turn the gas valve off when not in use and you can probably save 95% of the amp hours.
- Do you really need the VHF on for 10hrs per day at anchor?
- Depends on the unit but 3.2 amps for the GPS seems a bit high
- VHF, GPS, Autopilot: Will those be purely when under way? If yes, will the engine be running since you show 2hrs for the engine instruments? If not, can you assume there will be wind for the sails and thus power is being replaced as it's being used.
- Why would your nav instruments be running 24hrs per day if you are anchored?
- A large chunk of your amp-hrs is related to lighting. Consider LED replacment lights which can drop that by 70-90%.
- On the other hand what about laptop, tablet and phone charging?

The more I look at the numbers, the more it probably makes sense to break out the calcualtions as "at anchor" vs "under way".
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Old 14-10-2015, 04:28   #7
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Re: Help Me calculate How Big My House Banks Should Be.

I'll chime in with so e real life exp though by no means as knowledgeable as these guys posting before me. We have a 38 foot cat. A large reefer w freezer and a 43qt stand alone freezer. All the toys one would want for live aboard etc. We have 4 trojan 1O5s as our entire house bank. Our power consumption doesn't seem as high we have all led's. And we don't use the water maker often only 3 hours or so 2x a week when out. We have 750 watts of solar and an outback 80 controller. Most days the batteries are fully charged. Some days by 3pm some by 1 some by 5 , some days they are arent . If we have a full day of clouds and overcast we either need to run an engine 1 has a 6O amp alt the other is a 1OK amp with smart regulator, or the genny. I would love to add 2 more Trojans. I am having a hard time to finding a suitable spot. If we had less solar with our usage it would be even harder On our banks to the poi t of an early demise. We could turn the fridges off at night etc and be more conserving etc but we don't.

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Old 14-10-2015, 04:28   #8
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Re: Help Me calculate How Big My House Banks Should Be.

Of course then you need to look at your charging system. It can take a wide variety of forms:
- In the old days, the engine alternator ran daily for a few hours was popular. It's still OK as a backup but as a primary system it puts a lot of hours on the engine and the light loading is considered bad for the engine if you aren't also using it regularly for propulsion in order to get it to full operating temperature.
- A stand alone generator will work but unless you have a major draw like air/con, there are better options.
- A wind generator can put out a lot of amps if you are in a windy area but moving parts require maintenance and the noise of some units can be annoying.
- Solar is the most popular for new systems as the cost has come down and the efficency has gone up. The biggest downside is you need space to mount them without shadows from the rig.

You need to do a similar amp-hr calculation in reverse to estimate how many each system will be putting back in. Most people try for the solar/wind systems to handle the recharging except in rare cases where they might crank up the engine after 2-3 days of calm cloudy weather.

One other thing particularly if you are using alternators or a generator (but also possible with a large solar array): Batteries can only take so much charge at a time and it drops off as you get closer to full. If you update your calculations and decide you only need 400 amp-hrs of battiers, the batteries won't accept the full output from a 200amp alternator. It usually assumed to be around 25% of the amp-hr rating, so a 400amp-hr bank can take in around 100amp. As it gets close to full that drops off.
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Old 14-10-2015, 05:27   #9
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Re: Help Me calculate How Big My House Banks Should Be.

I think you need to go large here, if you have more bank than you need, it is excess weight, but your bank's life expectancy goes up a lot if you don't deep discharge it.

According to the chart in Lifelines battery manual, you'll get 5,000 cycles out of a bank if you only discharge 10%, but 1,000 cycles if you discharge 50%.

Pays I think to have a little more bank than you need.
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Old 14-10-2015, 06:41   #10
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Re: Help Me calculate How Big My House Banks Should Be.

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I think you need to go large here, if you have more bank than you need, it is excess weight, but your bank's life expectancy goes up a lot if you don't deep discharge it.

According to the chart in Lifelines battery manual, you'll get 5,000 cycles out of a bank if you only discharge 10%, but 1,000 cycles if you discharge 50%.

Pays I think to have a little more bank than you need.
There is a point where it makes a difference but your example doesn't work:

Let's say you have a 100amp-hr battery:
- 10% would be 10amp-hr. With 5000 cycles, you would get 50,000amp-hrs out of the battery over it's life.
- 50% would be 50amp-hr. With 1000cycles, you would get 50,000amp-hrs out of the battery over it's life.

Where your point makes sense is if you get much over 50% draw down, the number of cycles goes down faster than the percentage goes up.
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Old 14-10-2015, 09:31   #11
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Re: Help Me calculate How Big My House Banks Should Be.

You may be right in your calculations but if you are you need to be looking at reducing your use unless you are on a cat and can rig a very large solar array. Some of your numbers need attention, for example you have a total of 67A for lighting which is way to high. mine is about 5 at anchor and still under 10 running 24x7. Change bulbs to LED and switch off unused lights it's much cheaper than buying massive batteries. I generally find 75 - 100Ah/day is my high usage days but it is often less. Becoming conscious (and economical) with resources like power and water is part of adapting to life on a boat where these things are not unlimited like they seem on land. If you can't do this you are going to need a generator or you will be constantly having battery problems.
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Old 14-10-2015, 11:42   #12
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Re: Help Me calculate How Big My House Banks Should Be.

thanks very much for all the info guys
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Old 14-10-2015, 12:05   #13
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Re: Help Me calculate How Big My House Banks Should Be.

Go back and recheck your numbers. you're mixing underway items with at anchor items with some numbers that just don't make sense. who has their gas valve ( solenoid) energized 24hrs a day!
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Old 14-10-2015, 12:23   #14
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Re: Help Me calculate How Big My House Banks Should Be.

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There is a point where it makes a difference but your example doesn't work:

Let's say you have a 100amp-hr battery:
- 10% would be 10amp-hr. With 5000 cycles, you would get 50,000amp-hrs out of the battery over it's life.
- 50% would be 50amp-hr. With 1000cycles, you would get 50,000amp-hrs out of the battery over it's life.

Where your point makes sense is if you get much over 50% draw down, the number of cycles goes down faster than the percentage goes up.

My example is straight out of Lifeline's battery manual, and your trying to use total amp hours drawn out, which wasn't the point, number of cycles is.
Now I assume those numbers are essentially un-obtainable outside of a lab, but they do illustrate the point.
Regularly discharge to 50% SOC and you are doing good to get much over two years out of a bank, shallower discharges will mean the bank lasts longer. Plus it's tougher to recharge a smaller bank to full capacity as the acceptance rate is lower.

I'm new and just getting a handle on this myself, but if you discharge a bank to 50% on a cruising boat not on shore power, how are you going to fully re-charge it?
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Old 14-10-2015, 12:28   #15
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Re: Help Me calculate How Big My House Banks Should Be.

Quote:
Originally Posted by frozenhawaiian View Post
Hey guys. my boat didn't come with any house batteries, I already have my starting battery but now I need to pick up my house battery banks. I made up a chart (see below) with the various systems on the boat and approximately how much power they use and my plan is to pick up 4 trojan T105's which will give me 2 12v 225ah banks, given my calculated ah demand of 266ah/day do you guys think the 4 T-105's would be sufficient?
Hi Frozen,

Your worksheet is a good start but I suspect, as others have mentioned, it could use some refinement before using it for decision making.

E-Marine has a good worksheet that might get you to the next level of refinement.

For comparison, we have a bank of 8 Trojan T-105s [900AH at the 20 hr rate] and that holds us for 2 days [~200AH/24 hrs because we have 2 refrigerators and 1 freezer running all summer (guests, fishing, and gluttony) in additional to the typical loads most of us share...]

From experience, I recommend you give careful consideration to maximizing your AH capacity within your bank space constraints. The only way I can increase house bank capacity is to go with larger AH batteries with a similar footprint [i.e., taller... and I can only fit in 4 taller batteries out of 8- unless I lower the hull... ;-]

When you get to engineering design [including wiring] and placement of your new house bank, SmartGauge [among many others] has some good info worth reading.

In addition, if your bank will be difficult to access once installed, and if you stay with flooded lead acid batteries as you originally specified [e.g., Trojan T105] then consider one of the remote watering systems to make your life easier. [And don't forget to vent your battery locker outside the boat!]

In case you might find it helpful, here is a link to my blog post detailing replacing our house bank and installing one of those remote watering systems.

But back to basics: refine your consumption estimates first...

Have fun with your project, and don't fall into the false economy trap of skimping on wire sizes...

Cheers!

Bill
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