Boat Energy Balance

[lawrence_craig]

Hi all,

I bought my boat (Bav47 2007) in September and I’m a liveaboard. Unfortunately I inherited a lot of poor maintenance and poor decisions from the previous owner.

I need to replace my entire service battery bank. Before I do so, I want to reevaluate my energy balance and requirements.

I will then use this to improve my current setup and make decisions on what type and size of batteries I need to buy (rather than like for like).

Does anyone have any sources or info on how to do this?

I have seen some Ah calculators but none that do an energy balance. And other sources I have found are just very ambiguous.

[sailorboy1]

I don’t really understand the question. But once you determine your AH usage you match a supply to meet it.

I’ll give you an easy answer. I would say a typical boat your size will use 175-200 AH/day. You need a 400+ ah house battery bank. If going to use solar primary you need 600-650 W of panels. Reduce the solar if using an addition charging source by your estimate of it.

[newhaul]

Here would be a good place to start
http://www.cruisersforum.com/forums/...ing-34621.html

[evm1024]

Originally Posted by lawrence_craig

Hi all,

I bought my boat (Bav47 2007) in September and I’m a liveaboard. Unfortunately I inherited a lot of poor maintenance and poor decisions from the previous owner.
SNIP.

That is why it is call POS.

(you know - Prior Owners Stuff)

Generally speaking add up all your daily power usage. Then multiply by 2 to get a base usage. (or more or less if you like a greater margin or less).

Then multiply by 2 again (for flooded lead acif at 50% DOD) to get a battery capacity size .

You can change the multipliers as suite your needs/comfort zone as well as for other battery technologies.

Then put on your flame retardant suite for the ensuing discussion.

[lawrence_craig]

Originally Posted by evm1024

That is why it is call POS.



(you know - Prior Owners Stuff)



Generally speaking add up all your daily power usage. Then multiply by 2 to get a base usage. (or more or less if you like a greater margin or less).



Then multiply by 2 again (for flooded lead acif at 50% DOD) to get a battery capacity size .



You can change the multipliers as suite your needs/comfort zone as well as for other battery technologies.



Then put on your flame retardant suite for the ensuing discussion.

Haha yes, I thought this topic might invite a lot of keyboard warriors and flamers.

Good advice tho, I’m looking for a general rule of thumb or bits of key information that will help me make an informed decision as opposed to sticking a finger in the air and guessing.

[GordMay]

ABYC E-11.8 LOAD CALCULATIONS (PDF pages 4 - 8)
➥ https://www.paneltronics.com/atimo_s...11Excerpts.pdf

See also ➥ https://shop.pkys.com/system-calculations.html
And ➥ https://www.go2marine.com/docs/mfr/p...ectrical.shtml

[lawrence_craig]

Originally Posted by GordMay

ABYC E-11.8 LOAD CALCULATIONS (PDF pages 4 - 8)
➥ https://www.paneltronics.com/atimo_s...11Excerpts.pdf

See also ➥ https://shop.pkys.com/system-calculations.html
And ➥ https://www.go2marine.com/docs/mfr/p...ectrical.shtml

Good man, thank you

[Dockhead]

Originally Posted by lawrence_craig

Haha yes, I thought this topic might invite a lot of keyboard warriors and flamers. . . .

I doubt it. But if they appear, just ignore.


You've gotten great advice so far in the thread.


A boat your size with typical equipment will typically have about 400 amp/hours of batteries which will typically run such a boat for about 24 hours without charging. More is better (I have the equivalent of 840 amp/hours on my boat, and it's not overkill). If you have a lot of solar, then there is somewhat less pressure on the battery bank, but lots of solar and lots of battery capacity is a nice combination.


One tip for you -- if you're replacing your batteries, take the opportunity to put in some GOOD ones. Stay away from "leisure batteries" or anything other than true deep cycle ones. The best value by far in batteries for cruisers who spend much time off grid is golf cart batteries like the classic Trojan T-105 or the similar 6v batteries sold at Costco. Such batteries vs. "leisure batteries" will make a huge difference in how well the system works. You might need to build a new battery box since these batts are taller and narrower than typical "leisure" batts, but it's worth it.



Then, make sure you have a proper multistage battery charger, and proper regulation of the alternator. How do you charge your start battery?

[mvweebles]

When I first read this thread, I thought 400ah/day was an enormous amount of energy. It all comes down to whether you are at-anchor or underway. I have recently redesigned my AC/DC distribution system for off-grid cruising. After a ton of research (and some prior experience with solar), here is where I put my money. Not intended to be a recommendation, just what I decided would work for me:

UNDERWAY: ~200AH/Day (~2.5kwh/day)

From the Simrad site, their 4G radar uses 20-watts/hr; and a 12-inch MFD uses a maz of 30W/hr. If you have repeaters, it adds up fast. A/P is somewhere in the neighborhood of 3A-6A (ComNav site), so somewhere in the neighborhood of 50W/hr. Electrical load for underway is around 100W-140W per hour, depending upon how many MFDs you have - over 200AH per day. Of course, going into austerity mode by putting electronics into standby, etc. will cut this, but you get the idea.

IDLE/AT-ANCHOR ~80AH/Day (~1.3kwh/day)

Refrigeration - around 50AH/day (Webasto and also the spreadsheet link). Watermaker - 10AH for 7-gals (Spectra - 4.2W/Ltr)
Lighting and misc - 20AH (guess - all LED). Includes PC/Tablets
Pumps (Bilge, Shower, FW, etc.) - 5AH/day

TOTAL: ~300AH/day underway (~4kwh).
This drops considerably when not underway. Please, I know the math above is rough and doesn't add-up exactly, but the point is you have to figure out what you are designing for - being energy efficient while underway means either (1) being very austere with electronics and A/P; or (2) running the engine or generator for at least an hour each day - solar will not cut it; or (3) some combination of the both.

SOLAR OUTPUT

I have a large hard-top with four 200W panels and get around 3.5kwh/day out of them - this is not possible on most sailboats (confession: I'm actually a trawler guy who self-identifies as a sail-cruiser - 1970 Willard 36 full displacement/ballasted double-ender). Unlike solar on sailboats, I have no shading issues and thus get a ton of solar energy. I'd guess that's about twice what a similarly sized sailboat could generate without solar panels becoming a total nuisance onboard. The good news for sailboats is four smaller 100W panels is probably close to enough to power at-anchor needs

ENGINE/ALTERNATOR OUTPUT

Stock alternators usually have an output of around 70A at 14.2V or so, but output drops off quickly even though the batteries are not charged yet. So a rated output of around 1kwh/hour of run time.......but you will get less than half that, perhaps a lot less at idle speeds. And they are a bit tough on batteries - typical alternators are designed to supply electrical draw from accessories such as blowers, stereos, etc. They are not optimized for pure charging of a depleted battery.

Enter the Balmar staged alternator, which is indeed designed to replenish a depleted battery in a sensible manner. Even if you replace a 70A stock alternator with a 70A Balmar, the actual output will be higher (I have no data on this, but would not be surprised to see a 50% increase in output compared to a stock alternator). Get a 150A Balmar, and you're in great shape. Plus, with staged charging, your batteries will last much, much longer.

Bottom line -

Underway, electronics' draw increases energy usage enormously compared to at-anchor. Easy to consume 300AH/day (4kwh). In practical terms, this can only be partially offset by solar. Either conservation (Standby Mode; don't make water, etc.) or an alternative charge source is needed (run engine 1-hour/day - and take a hot shower).

[a64pilot]

A lot of the advice given above has you discharging your bank to 50% daily.
I say that for long battery life and as a way to deal with cloudy days etc you need on average to discharge no more than 25%.
Now that’s an average and of course changes day to day, and that along with cloudy windy days is a good reason to have excess battery capacity.
Plus if you look the lifespan of batteries increase the shallower they are discharged.
The downside is increased weight, money and room needed for a larger bank.

If you decide to size your bank so that your discharging to near 50% on a daily basis, consider a generator, cause your going to need something to fully recharge for the next days discharge of nearly 50% for those days when Solar isn’t great.

[gamayun]

If it were me, I'd start with a really good battery monitor and turn everything off except whatever device or light switch you want to use. Then see what each device outputs and for how long it cycles (such as the ice box or how long you might transmit on the VHF). If you go for a sail, look at what the auto pilot draws under duress before and after you turn it on. That's a biggie, as is a computer. In my limited experience, this is more real life than knowing the windlass draws 30 amps and you might use it 15 minutes each week. The reality is we forget to turn off certain lights all day that are drawing 1 Ah or we accidentally bump the fridge up to high and it cycles over and over needlessly freezing our lettuce. Once you have reality, then you can monitor the batteries' total charge (not SOC) and make sure you're fully recharging at the appropriate point for the type of batteries you have. A sufficiently large alternator is key if you don't have the solar "oomph" (my technical term for it) to keep up with the daily charging needs to get back to whatever is 100% for your bank. If the regulator does not show it's in float stage, you ain't there yet.

[thinwater]

Originally Posted by evm1024

That is why it is call POS.

(you know - Prior Owners Stuff)

Generally speaking add up all your daily power usage. Then multiply by 2 to get a base usage. (or more or less if you like a greater margin or less).

Then multiply by 2 again (for flooded lead acif at 50% DOD) to get a battery capacity size .

You can change the multipliers as suite your needs/comfort zone as well as for other battery technologies.

Then put on your flame retardant suite for the ensuing discussion.

PO stuff. POS is piece of s___. Of course, this can be the same....

[mvweebles]

For old-school lead-acid, 50% DoD (depth of discharge) is the long-accepted rule. There is benefit to less, but its not significant.

However, Lithium batteries can easily thrive in 80% DoD applications. Some of the higher-end AGM (Odyssey batteries, for example) also do well in higher DoD applications, but rival Lithium in expense but are the size/weight of lead acid.

EXAMPLE: The lengthy post I gave about energy budget came up to around 300AH usage per day. What size battery bank is needed?

1. Lead Acid. Cheapest initial cost is Costco 6v Golf Cart batteries for around $100/each. 200A @ 6v or 100A @ 12V. 50% DoD so the bank needs to be six batteries to give 300AH of usable capacity. Bank will weigh about 550 lbs, and take up a sizeable amount of real estate. They are rated at about 500 duty cycles, and will last around 4-5 years if cared for. About $1.25 per duty cycle, or around $10/month.

2. Lithium. You can get 100A @ 12V batteries for around $650/ea. Because they are 80% DoD, you only need four of them, and they weigh around 30-lbs each and are less than half the size of the 6V golf cart batteries. They are rated at 5000 duty cycles and will last at least 10-years (there are some with lifetime warranty). Initital cost is $3000, but cost per duty cycle is less than half the Lead Acid; with 10-year life, call it $25/month. But because they are much smaller and lighter, flexibility to mount is enormous - back of a seldom-used cabinet, for example, thus freeing up real estate.


This thread originally asked about energy balance. So far, a coarse discussion on energy usage and budget. I'd hazard a guess that for half of the 40-foot cruising boats out there with retired/semi-retired cruisers (versus shoe-string cruisers), a battery bank with 300-400 amp hours capacity is a decent start. From there, replenishing the capacity is the trick - a 150 amp Balmar starts high (bulk) but reduces into float fairly quickly, so will need to run 3-4 hours run time with all the electronics on, 1-hour or so at anchor. Solar can cut that substantially. Conservation, even further.

[Dockhead]

Originally Posted by mvweebles

When I first read this thread, I thought 400ah/day was an enormous amount of energy. ..

Note that if you expect to be able to pull 400 ah out of your batteries without charging, you need an 800 ah bank, and you need to start with a 100% SOC, which is hard to achieve off grid.

[newhaul]

Originally Posted by Dockhead

Note that if you expect to be able to pull 400 ah out of your batteries without charging, you need an 800 ah bank, and you need to start with a 100% SOC, which is hard to achieve off grid.

without going Lfp that is .