The Offshore Voyaging Reference Site

Battery Bank Size and Generator Run Time, A Case Study

I recently had lunch with a friend named Hans who owns a very nice 54-foot offshore sailboat. Hans does some pretty serious miles, with a bunch of trans-Atlantics and countless voyages from Nova Scotia to the Caribbean in the fall, and back home in the spring, to his name.

Hans shared that he was considering replacing the house bank on his boat with lithium batteries to dramatically increase the effective bank size from its already large capacity, about 800 amp hours at 24 volts (1600 amp hours at 12 volts).

When I asked why he would take this expensive step, he shared that his goal was to reduce generator run time from its current several hours a day when sailing offshore. (Hans has two AC generators, one of 3.8 kw and the other a monster 14 kw.)


My first thought was that increasing the bank size would not decrease generator run time since the amp hours used have to be put back eventually, regardless of bank size.

OK, I Get It

But then I started to see his logic for his typical four to six-day passages: Nova Scotia to Bermuda and Bermuda to the Caribbean, and return.

In this case a massive house bank (lithium or not) would increase the chances that he would not have to disrupt his sailing with long and frequent generator runs since:

  • He usually goes alongside at the beginning and end of a passage and so can be fully charged before leaving and then recharge from shore power on arrival.
  • These passages typically, or at least often, include some motoring with the attendant battery charging.

And, by going with lithium, the possibility of several days of noise-free sailing goes up substantially, since the house bank could be safely discharged to about 25% of capacity, rather than the 50% that is prudent with lead acid.

We didn’t get into the details of Hans’ usage at sea, but knowing that his boat is fully tricked out with all the gadgets—plus autopilot, watermaker and big freezer—and based on our own usage at sea, I would estimate about 200 amp hours at 24 volts each day (400 amp hours at 12 volts) and certainly no less than 150 amp hours at 24 volts.

So, for the sake of discussion, let’s assume the larger usage and that Hans installs a 1300 amp, 24 volt lithium bank. That would supply about five full days of sailing, based on the above, before he needed to run the generator or plug in—problem solved.

(A lead acid bank of any type—liquid filled, Gell, or AGM—would be impractical, since to have the same effective capacity it would have to be over 2500 amp hours at 24 volts or 5000 amp hours at 12 volts!)

We Gotta Think

Hans’ story is interesting because it illustrates how each of us needs to really think about our usage profile before selecting battery type and capacity, not just:

  • Make a decision based on some rule of thumb,
  • or what someone else has done,
  • or just assume that a bigger bank is always better.

For example, this change would make little sense for a sailing cruiser trying to reduce generator (or main engine for charging) run time, who:

  • Spends a lot of time at anchor and rarely goes alongside.
  • Regularly makes longer passages of a week or more in the trade wind belts where motoring is much rarer.

In both these cases a massive bank (lithium or lead acid) wouldn’t help since the generator (or engine for charging) run time would remain the same, all that would change is the interval between run times.

On the other hand, massive banks can make good sense for motorboats (or sailboats that motor a lot) who wish to remain quiet when at anchor and then will have lots of charging time while moving to the next anchorage.

Big Money

I digress, back to Hans. Of course, all of this assumes that he is willing to spend the some US$10,000 to US$15,000 that such a lithium bank and associated modifications of the boat’s electrical system would cost, not to speak of the added complications and fragility of lithium. (See Further Reading.) None of which he was keen on.

A Simple and Cheap Solution

Given these drawbacks, we chatted a bit more about other ways to solve his problem, during which I asked:

How about if you could reduce generator run time to half or even a third of what it is now for less than 10% of the cost of lithium and with minimal complication and installation hassles?

Hans perked up and allowed how that would be a perfectly acceptable solution.

A Quiz

So here’s a quiz for you: What is my simple and relatively cheap solution?

Read on to get the answer:

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. 11 Steps To Better Lead Acid Battery Life
  33. How Hard Can We Charge Our Lead-Acid Batteries?
  34. How Lead Acid Batteries Get Wrecked and What To Do About It
  35. Equalizing Batteries, The Reality
  36. Renewable Power
  37. Wind Generators
  38. Solar Power
  39. Watt & Sea Hydrogenerator Buyer’s Guide—Cost Performance
  40. Battery Monitors, Part 1—Which Type Is Right For You?
  41. Battery Monitors, Part 2—Recommended Unit
  42. Battery Monitors, Part 3—Calibration and Use
  43. Battery Containment—Part 1