How big of a battery bank do I need?

[Reefmagnet]

I've got similar consumption and have spent some time running some practical tests.


You need to figure out how much your batteries draw down each day based on output vs input and how long you'd like to go between requiring the use of external charge souces. For me, I think anywhere between three to seven days is the sweet spot depending on circumstance. So this boils down to a fairly simple formula.


battery rated capacity a/h = daily a/h deficit * days *100 / battery usable capacity %



Therefore if you have a 50 a/h average daily deficit (which is about what I would broadly expect from your setup with little running of the engine) and want to be able to go three days without running the engine for an extended period or other charge sources, using batteries with 50% usable capacity you'd need...


50 * 30 * 100 / 50 = 300


Which is a 300 amp hour capacity bank.


The downside is you'd need to put 150 a/h of charge back into these batteries every three days using the engine or a genset.


The better approach is maybe to increase your solar or add a wind generator. FWIW, I recently tested a 300w solar blanket which I can throw out on deck when at anchor and went from a daily deficit to fully charged in good sunlight conditions (I have 380w of fixed panels). It's only a cheap one with panels that won't last a long time, but the results convinced me enough to plan to fork out for the dearer EFTE version once this one bites the dust.

[Stu Jackson]

Originally Posted by Reefmagnet

>>>>>>>>>>>>>>>>>>>


You need to figure out >>>>>>>>>>>>> how long you'd like to go between requiring the use of external charge souces. >>>>>>>>>>>>>>>>>>>

Those who responded correctly to my earlier post, this ^^^^ is the "other half" of the answer.


Thx, reefmagnet.

[tradrockrat]

The simplest way it's ever been described to me is as follows:

"Rule of thirds and always round up."

A balanced bank / solar system prolongs life of batteries and supplies power when the environment isn't cooperating.

You use 150 ah a day? That's 1/3 of your needed bank
Your bank is (150 x 3=450) Round up to 500 even. Your bank should be 450-500 a/h

Solar provides about 1/3 of it's rated Watts in a/h. So you need (150 x 3 = 450) Round up to 500. 450 - 500 watts of solar.

Your solar should hopefully put in what you take out - but it won't. Assume you lose 1/3 of the a/h you budget for the day (150 / 3 = 50). 50 a/h deficit a day. This is acknowledging you're gonna be cloudy sometimes. Bright stretches might result in virtually no loss at all.

When your bank gets to 2/3 full (500 x.66 = 330). Round up to 350. At 350 you need to run your alternator / genset in the morning to top it off.

That's 500-350 = 150a/h you can lose

150 / 50(daily loss) = 3. Three days between top offs in general.

Sunlight changes that by - you guessed it - about a third. So you might get 1 day in horrible weather or 6 days in sunny weather - but this way of thinking never puts you going below 66% battery.

NOW - do you have the space for that bank and the space for those solar panels? Whole 'nother question (not to mention wind/water gen).

Summary - 450-500 ah battery bank and 450-500 Watts of solar for 150 ah / day usage is balanced and robust. Now you can adjust for space, weight and compromises you feel comfortable with.

Of course, this is all JMHO, but it was taught to me by a friend who installs solar systems in boats and RV's for a living. Feel free to ignore or disagree.

[Don C L]

So I just got 2 6v Trojan T-105s (225ah) and I have very little electrical demand so I have plenty now! The math here it looks to me like 4 of those would do it for you, if I got the math right...

[a64pilot]

A Honda 2000 can run a 100 amp 12V Charger and stay within its 13.3 amp max continuous power, a Honda can actually make more power, but it gets loud if it does.
Amps and amp hour are actually different things, like miles, and miles per hour.

[StuM]

For crying out loud, how many times does it have to be said.


It's AMP HOURS per day, not AMPS!


And they are abbreviated to Ah, not a/h!

Amp is a RATE of consumption, not a quantity of energy.

[barnakiel]

It is possible some of your figures are too high.


Mind max ratings vs. average or continuous ratings.


This applies to chargers, auto pilots, fully loaded fridges, etc.


It is better to err on the high mark but make sure the figures you apply are not grossly overrated.


About the only time an AP uses near max rating power is in very very heavy sailing where it struggles a lot.


And most equipment chargers tend to be at max only the first couple of minutes.


etc.


b.

[arsenelupiga]

we have 360W solar matched with 400 AH lithium. Meaning we can spend 320 AH. The other day when sailing north for 3 days there was not enough sun due to sail shade. We also had several cloudy/rainy days. However did not have to turn on any charging as solar brought back to full on day 10. We dipped to minus 280 AH. Actually now in 2 years of lithium I used generator 1 x only to top up batteries. Else using exclusively 360W solar.

We do however make water using generator. I feel we have space for another fridge, but at present there is no need for it.

As lithium getting cheap, would err on extra lithium capacity than on extra solar so you can use energy from sunny days to get over bad periods. But if you have good space for soar, go for it.

[Dockhead]

Originally Posted by tradrockrat

. . . . Summary - 450-500 ah battery bank and 450-500 Watts of solar for 150 ah / day usage is balanced and robust. Now you can adjust for space, weight and compromises you feel comfortable with.

Of course, this is all JMHO, but it was taught to me by a friend who installs solar systems in boats and RV's for a living. Feel free to ignore or disagree.

Looks very sensible to me


1/3 of the bank's nominal capacity is in fact what you might hope to practically get out of it in practical everyday use. Either drawing down to 66% from 100% or to 50% from 85%, which is about where most people stop charging with internal combustion charging sources, since the acceptance rate of lead batteries falls way over after that.

[tradrockrat]

Originally Posted by Dockhead

Looks very sensible to me


1/3 of the bank's nominal capacity is in fact what you might hope to practically get out of it in practical everyday use. Either drawing down to 66% from 100% or to 50% from 85%, which is about where most people stop charging with internal combustion charging sources, since the acceptance rate of lead batteries falls way over after that.

Thanks - my buddy told me he keeps it simple because guys like me start to have eyes glaze over and ears stop working when he gets deeper into stuff like acceptance rate and slowed charging etc...lol

I also apparently have no idea how to accurately abbreviate amp hours, but I'll try harder next time...

[powsmias]

nice thread.. thnx all

[stormalong]

If you plan to do a lot of passage making you should consider additional alternate energy sources. Solar is often shaded by sails and your course and angle of heel do not optimise output. Your 350 watt panels are only 350 watts under ideal conditions. My 320 watts of solar didn't cut the mustard.

There are two additional sources of electricity that worked well for me when I was doing ocean passages - wind and water. Wind and water work 24/7, not just when the sun shines. I used an AirX with Air+ blades, there are better products available today. I also used a towed generator.

In trade wind sailing the the wind generator produced pretty steady output. The towed generator produced about 8 amps at eight knots. Before I had the wind and water power I had to run the engine about two hours every other day to keep the batteries up. With that setup I sailed tens of thousands of miles without running the engine at all.

My loads include: electric autopilot, twin refrigeration systems (one refrigerator, one freezer), occasional radar, plus the usual lights, radios, etc.

It was so nice to be able to use power without worry.

[Pizzazz]

Originally Posted by TrailerCruiser

How big of battery bank do I need?

I'm trying to determine how big a bank of batteries I would need to run 24 hours per day, say to cross large bodies of water. Note I also have 350 Watts of solar on a MPPT controller (might even add more).

Here are my power consumptions estimates. What am I missing or how far are these off? Based on these I will consume around 150 amps per day. Does this seem about right?

Overall budget sounds about right, 150 AH is typical but some of your estimates are off. A typical led light fixture is 0.3A, it is unlikely you have 30-35 of those to reach 8A. You should also change your anchor light, etc. to LED. Basically with LEDs, you can easily drop below 2A for all your lighting needs.

Short answer, you need four golf cart batteries.

Long answer... you size your batteries to the max current draw at any point. Assume for now you have golf cart batteries which are the best value (brand does not matter). Your house battery size would be 220, 440 or 660 AH (2, 4 or 6 batteries). Batteries do not like to be drawn faster than about capacity/20 which means your max current draw will be 12A, 24A or 36A respectively. You can always exceed this briefly, say running the inverter, etc. but then the available capacity shrinks very fast. If you are running a fridge, that cycles often, with a high starting current, it is advisable that you get at least 440 AHrs (per my compressor's installation manual). Next, you calculate your available capacity. If you mostly stay below the max current stated above, a golf cart battery will give you good performance between 30% and 80% state of charge (this means it will rapidly accept charging current and the voltage will not sag too much when you load the battery). It is unlikely that you will charge your batteries above 80% on most days (most cruising boats manuals advise you to accept this fact vs. trying to fully charge the batteries every day) which is slow and painful as lead acid battery acceptance falls quickly above 85%. If you can fully charge the batteries once a week and equalize once a month that would be great. Below 30% SOC, the voltage sags very quickly, you can still run the boat OK but you may need to be more careful about your max draw.

Next, you need to make the decision, do you stay with golf cart batteries and accept that you will need to replace them every 2-3 years with the above heavy usage (in this case it is easier to replace four batteries than six or eight) or do you double your capacity and weight in order to reduce how often you drop below the 50% state of charge or do you just go for lithium where none of these restrictions apply. Based on many discussions in this group, if you run your batteries hard (in the 30-80% SOC, as described above) they will last 2-3 years, while running them in the 50-80% SOC will double their lifetime. So it is cost neutral but you need to balance the additional weight vs the hassle of replacing batteries. Lithium will double to treble the cost once more, so it is a longer-term investment.

SV Pizzazz

[Pelican 2]

Small batery banks of lead acid will not last with average use. Manufacturers often put around 200 amp hours on board from my experience which is great for a day sail. To be off grid I would say the minimum is twice that and 600 watts of something to charge with. We have almost 700 amp hours of lead acid excluding the engine battery, 600 watts solar and 600 watts wind turbine. Wind turbine really comes in to play when you are sailing so if sailing through the night its good to have. At anchor the solar is most of the charge. We occasionally use the engine to top the batteries. at anchor about every 4th day or so. We do not carry a separate gen set. When sailing we do not need extra top up normally. I find if we move every day the windlass adds a bit of draw as its close to 2 KW but I cant say whether we use the engine more as we only sail off the anchor in light breezes and where there is lots of room so the engine tends to be used anyway.

[sailcrazy]

With 2 of us aboard, our energy use is near yours. We built our system around 6 T-105s connected into 3-12v banks...usually easy to find/replace when needed, reasonably priced, good power(rated at 225Ahs), easy to use, older but known technology. We also use a separate, isolated conventional start battery for both the genset and the engine. We supplement our alternator with 4-80watt solar panels(they need to be replaced with new, bigger ones) and a wind gen. Here in the Caribb the wind gen runs 24 hr/day, while the solar runs about 12 hrs/day but only provides significant power about 4 hours/day! (ours are fixed, vs tilting). Solar is good, no moving parts, and quiet....but it really depends on where you intend to sail-add a D-400 wind gen to your thinking! For power management, we usually run the genset when we anticipate a heavy load(windlass, watermaker, etc). Everything's a trade-off.