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Cruising Boat Electrical System Design, Part 3—Specifying Optimal Battery Bank Size

In Part 1 and Part 2, we took a deep and considered dive into analyzing the electrical loads on our boats and thinking about ways to reduce said loads through smart systems thinking.

(If you have not read those articles, please do so now, otherwise this one will make no sense to you.)

In this chapter we get the payoff for all that hard work: How to calculate optimal battery bank size or, alternatively, how to live with the bank we have. And I have built a spreadsheet to make the whole process easier.

Note: To help things simple, I have assumed 12 volts, but if you have a 24 Volt system just divide by two.

Let’s dig in:


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More Articles From Online Book: Electrical Systems For Cruising Boats:

  1. Why Most New-To-Us Boat Electrical Systems Must Be Rebuilt
  2. One Simple Law That Makes Electrical Systems Easy to Understand
  3. How Batteries Charge (Multiple Charging Sources Too)
  4. 5 Safety Tips For Working on Boat DC Electrical Systems
  5. 7 Checks To Stop Our DC Electrical System From Burning Our Boat
  6. Cruising Boat Electrical System Design, Part 1—Loads and Conservation
  7. Cruising Boat Electrical System Design, Part 2—Thinking About Systems
  8. Cruising Boat Electrical System Design, Part 3—Specifying Optimal Battery Bank Size
  9. Balancing Battery Bank and Solar Array Size
  10. The Danger of Voltage Drops From High Current (Amp) Loads
  11. Should Your Boat’s DC Electrical System Be 12 or 24 Volt?—Part 1
  12. Should Your Boat’s DC Electrical System Be 12 or 24 Volt?—Part 2
  13. Battery Bank Separation and Cross-Charging Best Practices
  14. Choosing & Installing Battery Switches
  15. Cross-Bank Battery Charging—Splitters and Relays
  16. Cross-Bank Battery Charging—DC/DC Chargers
  17. 10 Tips To Install An Alternator
  18. Stupid Alternator Regulators Get Smarter…Finally
  19. WakeSpeed WS500—Best Alternator Regulator for Lead Acid¹ and Lithium Batteries
  20. Smart Chargers Are Not That Smart
  21. Replacing Diesel-Generated Electricity With Renewables, Part 1—Loads and Options
  22. Replacing Diesel-Generated Electricity With Renewables, Part 2—Case Studies
  23. Efficient Generator-Based Electrical Systems For Yachts
  24. Battery Bank Size and Generator Run Time, A Case Study
  25. A Simple Way to Decide Between Lithium or Lead-Acid Batteries for a Cruising Boat
  26. Eight Steps to Get Ready For Lithium Batteries
  27. Why Lithium Battery Load Dumps Matter
  28. 8 Tips To Prevent Lithium Battery Black Outs
  29. Building a Seamanlike Lithium Battery System
  30. Lithium Batteries Buyer’s Guide—Part 1, BMS Requirements
  31. Lithium Batteries Buyer’s Guide—Part 2, Balancing and Monitoring
  32. Lithium Batteries Buyer’s Guide—Part 3, Current (Amps) Requirements and Optimal Voltage
  33. 11 Steps To Better Lead Acid Battery Life
  34. How Hard Can We Charge Our Lead-Acid Batteries?
  35. How Lead Acid Batteries Get Wrecked and What To Do About It
  36. Equalizing Batteries, The Reality
  37. Renewable Power
  38. Wind Generators
  39. Solar Power
  40. Watt & Sea Hydrogenerator Buyer’s Guide—Cost Performance
  41. Battery Monitors, Part 1—Which Type Is Right For You?
  42. Battery Monitors, Part 2—Recommended Unit
  43. Battery Monitors, Part 3—Calibration and Use
  44. Battery Containment—Part 1
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Ted Scharf

Lot of interesting information there. I like it. But I think you made a mistake here:
Wind: zero when at anchor and say 10 Ah/day sailing in lightish air downwind.
Solar: Assuming a reasonable sized array without much shading on a sailboat, but worst case conditions, 20 Ah/day.
Wind generators: given the poor reliability track record of these units, I would not take them into account at all—if it dies at sea, we will need the same battery bank size that we would without one. If it works, cool.

I would consider wind and wind generators as the same. Was the first one water generators?

Paul Koenig

I used a Watt & Sea hydrogenerator to provide my power on a race / trip to and from Hawaii and it worked great. Underway it more than handled our electric needs for that 13 day trip. However one issue with 100% renewables is that you need another way to heat domestic water since the engine heat isn’t available. We ended up running the engine for a short time each day just to make hot water.

Roland

Running the engine a short time once/day on an ocean passage is good practice to detect
problems.

As Captain Ron said. “If anything is going to happen, it’s going to happen out there”

Marc Dacey

This is a handy spreadsheet, and, unsurprisingly, I do not question your figures. We have one of those big banks, plus four panels in an arch over the stern and a yet-to-be-installed wind generator. But the “fast boat” thing was never going to happen with a full keeled steel motorsailer as we have, so having a circa 1100Ah bank directly beneath the center of effort (basically three metres under the gooseneck) is part of the internal ballasting and plays a role batteries usually do not. We’ve chosen small generators (a pair of Honda 2200s) as “tertiary” charge sources and ways to run standard power tools without resorting to the “lossy” inverter process. And I’m going to very likely get a Mark Grasser externally regulated alternator to make amps while motoring, as that’s unavoidable and I may as well make water then, too. And hot water, then, too! That said, our projected energy budget is lower than most, because we don’t have the appliances some cruisers sport.

These measures work to the strengths, such as they are, of the boat we have, which will let us cross oceans at five, instead of seven, knots. So we’ll need a drogue, too, because we can’t outrun weather so well. The logic for an oceangoing catamaran or a “fast” mono with shallow bilges would be necessarily quite different when it comes to positioning and keeping charged big boxes of acid-covered lead. Your spreadsheet is very useful in helping to focus those concerns, even if it leads some to rethink refrigeration and the value of a wind vane!

Marc Dacey

Anything that helps sailors to think critically about what’s important to have on their boats is worthwhile. Perhaps an extra comment would be that the loads bigger alternators draw from a running diesel may drive a lot of other decisions. When we went from a 52 to a 60 HP diesel, I always planned to use that margin to spin a bigger alternator, which was feasible thanks to the acceptance of a larger bank and heavier cabling. A very rough estimate is that 200 amps @ say, 14.4 VDC “costs” five HP, although I forget the actual method of calculation and am going off a Beta Marine tech’s recall. While nothing one bolts to the engine is trivial, that’s acceptable to someone in a motorsailer, and maybe less so to someone with a smaller diesel and a lighter hull.

Eric Klem

Hi Mark,

A good rule of thumb on an alternator is that they are 50% efficient. Taking that, 14.4V*1A=14.4W=.02hp. Including the efficiency gets you .02hp/0.5= .04hp/amp of alternator output. So a 90 amp alternator output would take about 4hp to run when flat out.

When it comes sizing the system, it gets a bit trickier. The recommended sizing seems to be to take your shaft power at full speed and add your accessory power and get the engine requirement. But how often do you motor at full power with your alternator at full output? If we sized for normal usage, most cruising boats have large enough engines and they are matched to the boat such that there is >5hp available for an alternator at cruising rpm. The problem with this is what do you do when you do need full rpm and your batteries are low enough that you will bulk charge as you won’t be able to reach full rpm due to being power limited. The clever solution to this is to have the system back off on alternator power when you get to really high rpm or even better high load and I have heard of a few people who have put in a manual switch for doing this but I don’t know of any system that does it right out of the box. Even this is a simplification as for boats with good sized props that really bite, your load varies not just with engine speed but with boat speed. The best would be a regulator that depowers the alternator at high commanded engine load and a variable ratio transmission but I don’t know of any cruising boats doing that. Of course, we need to keep it all in perspective and if you look at a fuel island plot for a diesel engine, the efficiency doesn’t vary that much in the normal operating range so while there is some to gain, it is much less than most people would expect.

Eric

Dick Stevenson

Hi Marc & Eric,
I installed a field wire disconnect back-when for two reasons: we had a big alternator and only an adequately sized engine and I worried about the amount of hp that might not be available to me in a fire drill of some sort if the batteries were low. Luckily, this was never necessary.
The other reason (used a lot) was that the PO had put on a microwave, which actually has proved very useful and has earned its place. However, when motoring and wishing to use the microwave, I worried that the inverter draw off the batteries would make for a shock load on the alternator and, more important, a shock load on the belt that might cause a problem. So, we would take the alternator off line (a field wire disconnect switch) while using the microwave under way by power, use the invertor to run the microwave off the batteries and then bring the alternator back on line which was done with a soft ramped-up charge curve.
Worked a charm.
My best, Dick Stevenson , s/v Alchemy

Eric Klem

Hi John,

Just to be clear, that rule of thumb includes the losses of a v-belt and is on the conservative side. At low power levels, the efficiency can be a lot lower as the parasitics never go to 0 but things are sized around full output so the rule of thumb is a decent guide there. I hope that Calder’s system is far more efficient than 50% given its power levels. A synchronous belt should be capable of >95% efficiency and a decent generator >85% meaning I would think it should be >80% efficient if they put the effort in.

Eric

Eric Klem

Hi Dick,

Interesting, thanks for the report. I have contemplated doing that but never have. Even for people who have not “overpropped” and simply tune the pitch so that they can get full rpm in flat water with the alternator putting out, this might be a way to deal with the issues of prop loading related to weather. That said, not many boats that I have been on can really put down all their power anyways.

Eric

Marc Dacey

Thank you, Eric, for those numbers and the formula. And Dick, the “disconnect switch” is a simple, very good idea I’ve heard of before; such a thing could even be at the helm for when you absolutely have to have every HP available for thrust. I would not motor at full power customarily otherwise; I would motor at the 75-80% point for best fuel efficiency. In fact, I’m flattening my Variprop’s pitch this winter because I want to get more RPM out of my wide-open throttle…at least in forward. I’m quite pleased with the rather torquey reverse. I also wish to have enough flexibility to equalize under power, which I would only attempt during pretty calm-state, daylight motoring for obvious reasons, as I would effectively go to handheld instrumentation for a few hours. So, no equalization runs in a TSS.

As is axiomatic, everything a trade-off and I have designed my setup to operate (ideally) within a fairly narrow (70%-100%) band of battery discharge well above the 50% SOC on which a lot of charging regimens are premised. Whether panels, wind gen and alternator can restore what we draw down is another question, but we are going to try hard to be sensible and ever so slightly Amish in our use of electricity aboard. Now, to order that hand-powered coffee grinder…

Eric Klem

Hi John,

For users who have kept their loads down, I think that the time where renewables get really interesting is when you have a battery bank and solar system that are well enough balanced in size to get you where it is not a day between recharges but multiple days. The key to the battery capacity is that if you have a low output day or 2, it needs to be able to get you through with minimal renewable input. If you can slowly step down your charge over a few days but stay within an acceptable range, then you will likely get to a stretch of better weather or need to motor anyways which will bring you back up towards the upper end of the charge range. If you look at the extremes with renewable only generation, a very large battery bank and renewable that can just hit your average consumption works as does a battery just large enough to get you through a night and renewables that are enough to get you back up to charge each day in any weather works also. Of course, you ideally want to be in between these 2 extremes and to really figure that out, you need a basic time-stepping analytical model and some basic climate data which is beyond the scope of this post. I quickly put together one of those models when figuring out the backup battery bank for my parent’s solar system many years ago as they can have long outages where they are located. At one point I looked at it for our boat and decided that our batteries were actually pretty big and our solar pretty small so if we ever need more power, more solar will be the answer for us but for the time being, our modest solar works quite well thanks to the large battery bank.

For people unable to get to the consumption, storage and generation levels for a mainly renewable boat, timing is key and I still think that renewables have a place. With solar, running the generator first thing before the solar starts putting out is really interesting as it lets you use more of your battery capacity and keeps generator run times short an efficient. And for weekenders, solar is really compelling on boats that live on moorings as you can leave the boat not at full charge without wrecking the batteries and come back the next weekend to a full charge all with a pretty modest panel size.

Eric

Adam

Thank you for this comment, Eric. It introduces a very valuable angle into John’s reiteration of “be realistic”. Readers can include in their own assessments the consideration of whether they can realistically accept sub-complete charging and make the rest up a bit later in scenarios similar to those you’ve described.

Mike McCollough

As always there is quite a bit of good, enlightening and informative material. After looking at several years of fixed solar panels on top of our house I have used the formulae for sizing 75% output for 5 hours a day. This takes into account thermal losses due to the panels getting warm. This square wave fits nicely around the curve showing our daily usage. For a 12V 100W solar panel system this means there will be 75W for 5 hours. Converting 75W for 5 hours into AH, 75/12 = 6.25A, 6.25A for 5h = 31.25Ah. Each solar charging cycle you must supply the needs plus replenish the batteries usage from the night before. Assuming a 12 hr day and 12 hr night and a 10% inefficiency, loss, means I only have 1/2 of 28Ah available for usage and 1/2 of 28Ah for charging. This shows how little solar panels actually resupply during a good day. Things deteriorate quickly when the days are very dark or cloudy.

Roland

Mike,
Just want to confirm your numbers.
Today was a good day for solar here on Martinique. My 640 W solar produced 190 Ah.
Yesterday we had a day with many showers. Still I got out 150 Ah. The panels are fitted horisontal and there will be some shadows occasionally. I’m actually very pleased with the result as that means I only have to run the engine one hour every second day to charge, make water and generate hot water.

Mike McCollough

Roland,
Your numbers fall within expectations. Assuming you have a 12V system, 75% of 640W = 480W, 480W / 12V = 40A, 190 Ah / 40A = 4.75 hrs. My formulae is for sizing how big a solar array does one need. The other item in the equation is what is your load is in the day and at night. With 190Ah you should be able to replenish your night time usage if you have an average hourly load around 5 Amps. Anything more will require some other method to top off the batteries periodically. ( Assuming 12 hours of daylight and 12 hours of night).

Roland

John,
By today’s standard 640W is not that much. Many boats I see have 900-1000W. I would say 90 % of the boats in the Caribbean are equipped with an arch in order to increase panel area.

We have 2 x 100W on the lifelines,
240W on the Hardtop and finally 200W on the Bimini. Shading is not a problem when on anchor. Under sail shading can be an issue as I can not bring out the boom as I want. But normally I have ay least 400W not shaded when under sail.

Dick Stevenson

Hi Roland,
I share John’s concern about items attached to lifelines: solar panels are a big concern, but I extend my concern to jerry cans, kayaks and the like. John has expressed one element of concern well, but a seldom considered concern is the stress on the stanchion bases/bolts/backing plates which were not designed with tolerating repeated sharp loads of this kind. Think, if you will, of the loads going to wind in a breeze heeled over 20 degrees that the weight of solar panels puts on the stanchion bases with each “slamming” into a wave, especially if there is some water flying around (or exchange solar panels for full jerry cans etc.). If you hear of stanchion bases leaking (and I have come across this 2 times, at least), there is always the chance that the lifeline stanchions were actually stressed saving a life, but it is far more likely that were items attached that caused multiple thousands of small shock loads on the stanchion bases till they loosened enough to start to leak. And, if lucky, the leak did not migrate into a balsa core.
My best, Dick Stevenson, s/v Alchemy