The Offshore Voyaging Reference Site

Choosing The Right Alternator, Part 2—Other Criteria

In Part 1 we took a deep dive into alternator efficiency and concluded that we have to be careful to separate attainable alternator efficiency from marketing hype.

So are we all done? Nope, there are two other criteria to consider when selecting an alternator, and then two other system issues to take into account before pulling out the credit card.

And if we are considering an alternator with very high output instead of a stand-alone generator, we need to, once again, evaluate rationally and separate marketing claims from reality.

Alternator Selection Criteria

Let’s start with the two more selection criteria:


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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. Choosing The Right Alternator, Part 1—Efficiency
  18. Choosing The Right Alternator, Part 2—Other Criteria
  19. Right Sizing an Alternator
  20. Making Large Alternators Safer With Whitespace Programming
  21. 18 Tips To Install A Cruiser’s Alternator
  22. Stupid Alternator Regulators Get Smarter…Finally
  23. Wakespeed WS500—Best Alternator Regulator for Lead Acid and Lithium Batteries
  24. AC Chargers For Lead Acid Batteries
  25. Efficient Generator-Based Electrical Systems For Yachts
  26. Replacing Diesel-Generated Electricity With Renewables, Part 1—Loads and Options
  27. Replacing Diesel-Generated Electricity With Renewables, Part 2—Case Studies
  28. A Simple Way to Decide Between Lithium or Lead-Acid Batteries for a Cruising Boat
  29. Eight Steps to Get Ready For Lithium Batteries
  30. Why Lithium Battery Load Dumps Matter
  31. 8 Tips To Prevent Lithium Battery Black Outs
  32. Building a Seamanlike Lithium Battery System
  33. Lithium Batteries Buyer’s Guide—BMS Requirements
  34. Lithium Batteries Buyer’s Guide—Balancing and Monitoring
  35. Lithium Batteries Buyer’s Guide—Current (Amps) Requirements and Optimal Voltage
  36. Lithium Battery Buyer’s Guide—Fusing
  37. Lithium Buyer’s Guide—Budget: High End System
  38. Lithium Buyer’s Guide—Budget: Economy Options
  39. 10 Reasons Why Hybrid Lithium Lead-Acid Systems are a Bad Idea
  40. 11 Steps To Better Lead Acid Battery Life
  41. How Hard Can We Charge Our Lead-Acid Batteries?
  42. How Lead Acid Batteries Get Wrecked and What To Do About It
  43. Equalizing Batteries, The Reality
  44. Renewable Power
  45. Wind Generators
  46. Solar Power
  47. Watt & Sea Hydrogenerator Buyer’s Guide—Cost Performance
  48. Battery Monitors, Part 1—Which Type Is Right For You?
  49. Battery Monitors, Part 2—Recommended Unit
  50. Battery Monitors, Part 3—Calibration and Use
  51. Battery Containment—Part 1
  52. Electrical Tips
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Mark Hamstra

I count more than just two issues with 48v on a cruising boat: https://panbo.com/abyc-examines-the-48-volt-boat/

Mark Hamstra

Even if one does everything DIY and thereby avoids any issues arising from occupational safety regulations, the lack of common components rated to handle the nearly 60v present in a nominal 48v system presents significant challenges — and not just to boaters, but also to those installing nominal 48v terrestrial solar systems. For example, I am not aware of any 1-2-BOTH battery switches that are rated for more than a maximum (not nominal) voltage of 48v, and I only know of a single ON-OFF battery switch that is rated to 60v and has published certification to that effect. Even that one Victron switch isn’t ABYC C-7 compliant, so I literally know of no way to equip a boat with a 48v system that is fully compliant with the current ABYC standards.

Yes, safety regulations and industry standards can be ignored, and under-spec’d components used with the hope that they will prove adequate. What I am saying, though, is that 48v systems on boats still present many issues that 12v, 24v and even 36v systems do not.

Mark Hamstra

A correction: With the recent update to E-13, a system like the Safety Relay functionality in some Mastervolt batteries should be ABYC-compliant without the need for a separate battery disconnect switch. Mastervolt’s Safety Relay system can be used with two of their 24v lithium batteries in series, so there is at least one way to build a fully compliant 48v system.

Mark Hamstra

Using a Victron Lynx BMS still requires a battery disconnect switch as close to the battery as practical in order to be ABYC compliant. Let’s not forget that I was not wrong before the new E-13 came out, and I am still not wrong on this issue.

I would be much happier if you would post a link to a 60v-rated C-7 compliant 1-2-BOTH switch instead of writing that building the kind of 48v battery system that you advocate is not difficult or misrepresenting what I wrote. I still do not know of fully compliant battery switches. That is still not a claim that they absolutely do not exist.

Mark Hamstra

And on the UL-compliance of Victron inverters, it was I who carefully followed-up and corrected my initial overstatement.

Mark Hamstra

And if you want to get really picky about it, maybe that one available Victron switch is C-7 compliant in the sense that the same, unaltered switch may eventually pass C-7 testing, but it is not currently C-7 compliant because there is no published certification of its having passed the C-7 tests. Finding C-7 compliant switches can be tricky for even 12v and 24v. As far as I know, it is currently impossible for 48v.

Mark Hamstra

[emphasis added]

“So my strong recommendation is that all 48-volt banks should be made up of at least two series strings, in parallel, of smaller-capacity batteries or cells, rather than a single string of high-capacity cells.

Not difficult to do, but worth thinking about before we buy a bunch of expensive batteries.”

It actually is difficult to do — or at least it is very difficult to source adequately spec’d 1-2-BOTH battery switches. Again, I’d be overjoyed to have some pointed out to me.

I didn’t start out by criticizing what you wrote. I simply and as succinctly as I could pointed out that there are more than the two issues that you mentioned in building a 48v system. In fact, the lack of availability of components like battery switches that the article I linked discusses bears directly on one of the two issues that you did mention.

You chose to expand the matter in a rather defensive manner instead of doing something more neutral like simply acknowledging that there are more issues and providing references to where you had previously discussed the issues raised in the panbo article.

In responding to you, I again avoided criticizing what you wrote and tried to stick to the facts that I thought were not being adequately addressed — viz. that building a 48v battery system as you described actually is a difficult thing to do.

In responding, you chose to go into what felt to be at least approaching ad hominem arguments.

I do not post carelessly or irresponsibly. If I make a mistake, I try to correct it as soon as I recognize my error. Yet no matter how I post, even if I very carefully couch what I am saying in terms of the limits of my awareness or knowledge, it certainly feels as though, if you are unaware or unfamiliar with the matter that I am posting about, that you all too often get defensive, assert doubt, disbelief or denial before even checking into the facts of what I am saying, and indulge in unfair attacks on my person that frequently repeat your fixation on my supposed “tone of certainty in [my] pronouncements” even when I have carefully avoided any such assertion of certainty. You did and have yet again misrepresented what I wrote.

If you make a mistake, if you misstate or overstate something, I wish you’d have the decency to admit it instead of diverting into defensiveness, irrelevancies or personal attacks. It gets tiresome. If you think the issue that I am addressing is not germane to your main points, then just point out where you’ve addressed my issue elsewhere; else clarify or correct the side issue as succinctly as possible without indulging in these unnecessary blow ups.

Alwin Bucher

“ So my strong recommendation is that all 48-volt banks should be made up of at least two series strings of smaller-capacity batteries or cells, rather than a single string of high-capacity cells.” Should perhaps read “parallel”? Great article in any case! Love it when the takeaway for me is “don’t bother, go cruising”…

Arne Mogstad

Hi, maybe slightly off topic, but would adding a higher voltage alternator and lithium bank (say 24 volt to stay within requirements), and then a DC/DC converter down to a 12 volt lead acid bank that feed the critical loads, be a reasonable way to fulfill the requirement to have a lead acid backup?

My boat is currently 12 volt, and I would say that 90% of the loads I have today is critical with the exception of the fridge. I would run an inverter and some other things from the 24 volt system. I’m probably missing something, but it does seem like a decent way of getting the benefits of both voltages, no need to change a lot of existing equipment, and getting a lead acid backup.

Also very timely articles as I am in the need for a new alternator (a couple days ago, a seawater hose came off and drowned my current alternator, which now gives me 5-10 amps… whoop whoop)

Matt Marsh

If 90% of your loads are “critical” then either your assessment of what “critical” means needs a re-think, or your boat’s systems are simple enough that you won’t really see a great cost to benefit ratio from 24V, lithium, etc. upgrades.

For all their flaws, simple lead-acid 12V systems have a really good cost to benefit ratio as long as they accomplish the tasks you need them to do. If the alternator in a 12V system blows, and the system generally met my needs otherwise, I’d normally just install the best 12V alternator I can get that’s well matched to whatever charging current my lead-acid batteries are able to accept.

I don’t recommend dual-voltage systems. They add complexity and cost, and they ruin your ability to MacGyver-up some kludge if things break 500 miles out from Tora-Bora. If I’m specifying a system as 24V then it is 24V throughout all main panels, all main batteries, & all main wiring; only a handful of low-power loads (e.g. electronics that are only available in 12V versions) get run via individual DC-DC converters or a DC-DC converter feeding a small 12V subsystem that is physically and logically local to that set of devices.

Arne Mogstad

Hi, thanks to both! Yeah I don’t know exactly if 90% is correct, but the few things I have under passage that’s unnecessary (like the music player, some of the lights, phone charger…) they’re all easy to disconnect. Apart from the fridge (which I could easily turn off), I do think that 90% of the current leaving my batteries goes to essential things (nav systems, communication, autopilot…). I honestly can’t really think of what else could be running that would not be considered essential? But yes, I do think I have a fairly simple boost in that regard. At least not a lot of unnecessary power draw.

So yeah, I think the conclusion is that it’s not much benefit for me to switch to a dual voltage system. I do see some benefits with a lithium setup, but honestly, an appropriate alternator and regulator would solve most of the issues. I was thinking running a larger inverter-charger on a 24 volt system, but it’s not really something I NEED. More a thought as I’m already buying a new alternator and regulator.

Thanks a lot for both of your really helpful and direct replies!

Kindly, Arne

Stein Varjord

Hi John,
I agree with the conclusions here, of course. One slight problem, however, about the recommendation to split a lithium battery into two separate batteries run in parallel. That’s fully possible, of course, but it makes this double bank totally dependent on electronics that have proven their lack of reliability: Balancing circuits.

LFP cells treated right are inherently extremely stable. If the cells are properly matched, top balanced, and correctly connected (often not the case), they will usually stay perfectly balanced for years of heavy use. We need to monitor them and have a BMS to make sure that is indeed the case, but if all is right, they’re surprisingly smooth.

Pretty much all current LFP batteries have balancing circuits to make them work with poorly matched cells, that drift out of balance. This is not ideal, but works. When a battery of this type dies, it’s usually due to slow overcharge, but second is that a piece of the balancing circuits failed, killing a cell.

With two independent banks run in parallel, we need a way to keep the two banks balanced. That’s the same principle of circuitry, but running 14 times higher Voltage and a hell of a lot more power. It can be done, no problem, but is the end result more reliable? I don’t know. Myself, I’d prefer a single battery bank that was as simple as possible and that I understood and watched well. Much fewer points of failure.

I think the topic of this article will be far more relevant in the fairly near future. People want more and more stuff on their boats, and think their boat is a house. Electric propulsion is in its infancy, but it’s coming. This means higher voltages will be introduced. For relatively small sailboats we’ll be fine with 48V, but faster motorboats and bigger sailboats will need more. Probably the EV standards will move onto boats. That means many hundred Volts, locally.

Also the Sodium batteries mentioned another place, and some other chemistries, miss the decently flat Voltage curve of lead acid (11,8-14,4V) and the extremely flat curve of LFP (12,8-14V). Sodium for an imaginary nominal 12V package would seemingly go something like 8V to 16V. The benefit is that it’s easy to know the state of charge, SoC, but it will be impossible to use power directly from the battery bank. We’ll depend on converters or similar.

My guess is that within 5 years, we’ll see much higher voltage banks entering boats. They will operate more separate from the boat systems. More like a power plant onboard. Everything going into it or out of it will be transformed into what is needed for that purpose. The cells will be much cheaper than now, and last many times longer, like forever, but the systems as a whole will be more expensive, more complex and probably illegal for DIY. It will give new possibilities and much more efficiency. However, is it generally good for long distance cruisers? I strongly doubt it. I think it will be gradually harder to keep a boat simple, reliable and serviceable. Electric motors are so incredibly simple, reliable and cheap (really) that the industries are desperate to find other channels into our wallets.

Stein Varjord

Hi John,
Yes, Victron are among the few who can do this in a probably quite good way. Partially because they use external BMSes and proper circuitry, like contactors rather than diodes. I have no doubt that it’s good.

For DIY LFP builds (not for the non nerds), it’s strongly recommended to put cells in parallel first, to get the required capacity, and then put those packs in series to get the required Voltage. The reason being partial self balancing, no electronics needed between cells in parallel.

However, my issue isn’t the details, which parts are used and how they are used, but rather the main principle. Two independent strings that are run in parallel needs something to keep them in balance, the exact same problem as between the cells in each separate string, but at a tens of times higher load.

The system needs an extra level of electronics. One more possibly weak link in the chain. Also, one of the basic KISS principles: “two objects fail twice as often as one object.” 🙂 We get two complicated battery banks in stead of one.

I don’t think it’s a bad solution to have two separate strings in parallel. I just think it’s worth considering how it actually improves total reliability. I’m not sure it does.