The Danger of Voltage Drops From High Current (Amp) Loads

Reading Time: 6 minutes

In the last three chapters we figured out what battery bank size we need for normal usage. But what about really big loads like induction cook tops.

How big do our batteries need to be to supply the peak loads we will be taking on without the voltage dropping below acceptable levels, even for an instant?

Loads We Need to Think About

To calculate that we need to be realistic about:

  • The worst case total load if multiple high-load devices are used at the same time.
  • The acceptable lowest voltage.

Not Just Electric Cooking

And further, this is something we need to take into account before installing any of the high-current-draw electrical gadgets becoming prevalent on boats today:

  • Electric cooking
  • Electric winches
  • In-mast and in-boom roller furlers
  • And on it goes.

That said, we probably don't need to take into account loads like:

  • Bow and stern thrusters, since the main engine will be running and supplying amps, at least as long as there is a big reliable alternator installed.
  • AC loads like washing machines and dryers, as long as we are willing to restrict their use to when the generator is running as part of our daily charge cycle.

(Note that I'm not advocating for installing either of the above, just being even-handed.)

Lowest Acceptable Voltage

11.75 volts is the absolute minimum we ever want to see on an offshore cruising boat's battery terminals, even for a moment.

Why that number? If batteries get below 11.75 volts, it's likely that the voltage at the inputs to important gear like radar, autopilots, NMEA 2000 networks, etc., will dip below 10.5 to 11 volts, depending on losses in the feed cabling (more on that in the next article), which is the level where said gear will get flaky, or even shut down completely.

It Matters

To see how important this is, just imagine we are broad reaching along in big tradewind seas while the cook is fixing dinner with two induction rings on—pasta and sauce—and the crew on deck trims the jib with an electric winch.

...The NMEA 2000 network chokes, the autopilot loses heading input from the compass...wipe out...dinner splattered all over the galley...cook is severely pissed off and goes on strike...survive on sandwiches for the rest of the voyage.

And that's a comparatively benign result. If we were running off, there could easily be a crash jibe, and if the preventer parts off—just happened to some friends of mine—horrible injuries or even a fatality are a real possibility.

Max Load

So let's use that scenario to calculate some average and maximum current loads (amperage):

So which number should we use: maximum or average? I guess that depends on how scared we are of crash jibes. Me? Going with maximum every time.

Battery Behaviour

So the next thing we need to think about is how big the battery bank needs to be to stand up to these kinds of loads without the voltage dropping lower than 11.75.

  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. Do You Need A Generator?
  22. Efficient Generator-Based Electrical Systems For Yachts
  23. Battery Bank Size and Generator Run Time, A Case Study
  24. Battery Options, Part 1—Lithium
  25. Battery Options, Part 2—Lead Acid
  26. A Simple Way to Decide Between Lithium or Lead-Acid Batteries for a Cruising Boat
  27. Eight Steps to Get Ready For Lithium Batteries
  28. Why Lithium Battery Load Dumps Matter
  29. 8 Tips To Prevent Lithium Battery Load Dumps
  30. Building a Seamanlike Lithium Battery System
  31. Lithium Ion Batteries Explained
  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. Hydro Power
  40. Watt & Sea Hydro Generator Review
  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
  45. Q&A—Are Battery Desulphators a Good Idea?
Subscribe
Notify of
59 Comments
Oldest
Newest
Inline Feedbacks
View all comments
Drew Frye

One more thought, which because of your northern location I am SURE you are aware, but I did not see mentioned and some readers may benefit.

Battery voltage drops with low temperatures in a very predictable way. I always keep a temp/SOC table taped to the panel. In the winter it takes far fewer amps, combined with partial SOC, to cause electronics to drop out, as John describes. The batteries are not dead. Amp-hour capacity is unchanged. But the useful portion of the charge becomes much smaller in the winter. Loosing the autopilot is a drag and solar charging is limited in winter (short days, lots of clouds, low sun angle).

In part I am aware of this because the battery banks in many multihulls are in unheated areas of the bridge deck and can go sub-freezing, just like a car.

Gari Williams

One other advantage of lithium in this article’s context is that a 4 cell LiFePO4 battery has a in use voltage of 13.3 volts. Thus not only is the voltage sag less, the starting point is more than 0.5 of a volt higher than LA. I am a lithium convert and have an 800 Ah bank installed on my boat, but I accept your reasons for rejecting the technology, John.

Denis Foster

Hello John,

Good article, waiting for the rest to come.

Is it true that carbon foam lead acid lke Firefly can have a deep voltage sag when already discharged over 50-70% and get the electronics system to not work suddenly.
Is this a good argument to separate banks that supply critical equipment and the service bank of non essential items like cooking etc…
On of the advantages highlighted by Firefly of carbon foam is that you can go further in dicharge than other lead acid technology AGM or Gel.

Thanks

Denis

Drew Frye

This may be slightly off topic, but this is an adventure cruising site.

A disadvantage of lithium batteries is that they can be seriously, perhaps fatally, damaged by charging while below freezing. Tesla solves this by heating the batteries and bilges seldom get that cold. However, it is one of the reasons I have held back on lithium, since my last two boats had battery boxes that regularly went below freezing. Solvable, but complicating.

I’ll leave this for discussion.

Dick Stevenson

Hi John, Administrative question. I don’t see any more the option of asking for a stream’s posts to be sent to me without leaving a comment: a feature I used regularly. Is that no longer possible? Thanks, Dick

Stein Varjord

Hi Dick,
The last words of the article, (in the part most skip), are these:
“Sorry, the subscribe to comments feature is currently not working. We found a nasty bug and so had to shut it down. The developer is working on it and hopes to have a fix this week.”

Dick Stevenson

Hi Stein & John,
I do skim at times and missed that notice. Thanks, Dick

Stein Varjord

Hi John,
I strongly support one of your core messages here:
Equipping our boats as if they are grid connected houses is a bad idea, for lots of reasons, and as explained in the article, even fundamental reliability and safety is at risk. I really think we should try to keep systems as simple and reliable as possible, which means we should accept some degree of “camping” attitude in our demands for creature comforts.

On our boat we have space (and battery capacity) for a dish washer, but that will only enter the boat over my dead body. I’m perfectly happy with doing the dishes by hand. It’s not really much more work than loading and unloading a machine. We live full time on our boat.

Same with airconditioning. We can’t do that on our batteries, and also don’t need it in northern Europe, but with good shading of the decks and good air flow through the boat, even really hot climates become totally fine. I also assume that the reason for going to really hot places is that we actually like heat? I do.

We have no use for electric winches either. If a sail is too heavy to handle manually, it’s too big or the systems are not good enough. On our 40 foot cat, there are zero tasks anywhere near too heavy for my normally strong girlfriend.

Our only notable electric assistants are autopilot, anchor windlass and soon a small washing machine. The latter totally goes against my stated principles, but it’s such an addition to practicality and comfort for living aboard that it’s worth it for us. Wall mounted Daewoo Mini. It uses well under 20 Ah in total on the programme we’ll use when off grid. Then we’ll feed it engine heated water.

We have never needed a generator. Solar alone is enough, 1100 Watt nominal, with engines as backup. Our boat is very comfortable, and still has very low electric needs. We can run our boat just fine on an old fashioned 240 Ah 12V semi traction flooded lead acid bank. Still I’m a total fan of lithium.

However, a warning: Be very wary of 12V lithium batteries. “Drop-in” batteries. They can theoretically be almost as good as the alternative, but VERY rarely (never) are. Use separate prismatic cells and external BMS. The latter typically lasts twice as long, is way more powerful, way more reliable and costs anywhere from the same to 70% less. They do need a bit more knowledge to build the system, but not hard to get info. I won’t go further into this, as it’s the wrong thread. Just mentioned it here because it’s an issue I think more people should be aware of.

Mark Wilson

Hi Stein

Not quite sure what your final paragraph means but fully concur with the preceding para’s.

Dick Stevenson

Hi Stein,
I have lots of friends who are completely committed to creature comforts, but some occasionally ask what it is like to live on a small sailboat boat. After waxing ecstatic about all that I appreciate, I then comment that I (with Ginger) have washed every dish, pot and pan by hand for much of the last almost 20 years now using a minimum of water use. Their reaction to that comment usually defines whether I go further in the description of life aboard.
And agree: doing wash by hand is no big deal.
My best, Dick Stevenson, s/v Alchemy

Michael Lambert

Between diesel, propane, and gas, the fuel I’ve been keen to ditch is gas, since grilling zucchini and sausage is almost one of the reasons I like to cruise. The last two seasons I’ve run a torqueedo travel, we have been limited to a week or less so it works well in that respect, charging while it’s on the mooring with no other demands. But I’m not sure it would be sustainable for long term cruising without drawing too much off the bank. The reliability aspect leaves a bit to be desired what with breaking sheer pins and other issues, so now I’m wondering about a propane outboard.

For me keeping demand low enough that one never burns fossil fuel just for electricity is more critical than ditching propane.

Michael Lambert

In that case I’ll add my other gripes, which are: the “latch” or pawl that holds the outboard tilted up is weak, and gets stuck in an unhelpful position, also the cable connection has one of those plastic nuts that WANTS to cross thread, plus if it happens it won’t back out without channel locks
or the like. But the worst is the shear pin, which breaks if you run it within sight of any floating seaweed. And removing the propeller nut after seems to require running the motor while holding a socket wrench next to the spinning prop. I’ve taken to coating the pin in tef gel but I don’t know if that makes a difference yet. I wonder if epropulsion is better, or if I should do propane…..

Matt Marsh

The issue there, I think, is that you are dealing with a “New and Innovative” company that, while it has a very clever product, has not yet learned many of the lessons that existing manufacturers learned decades ago. The shear pin, for example – a conventional 8 hp outboard will usually have a rubber spline hub, which can slip on impact and then re-engage, so most slips are self-correcting and, in a big hit, you can still get home at low speed even if a couple of blades are mangled. The plastic nut – again, solved decades ago with mil-spec connectors and their industrial derivatives, but someone didn’t realize why we do it that way and so took a cheaper route.

That’s always going to be an inherent problem with the first couple of generations of any new technology from any new manufacturer. It takes time and iteration to learn how to work out those bugs.

Michael Lambert

I don’t know. Every time I walk by one at a dinghy dock I look for changes but haven’t actually researched it.

Eric Klem

We are primarily rowers and only bring our outboard on trips where we have enough people on board that we don’t all fit in the dinghy at once or when we are going somewhere that we feel we need to be able to stow the dinghy below and therefore are using our inflatable which doesn’t row well.  However, with kids and the many more shore trips associated with that, I have been toying with the idea of using an outboard more.

I ran a very basic simulation of our dinghy usage and charging profile and concluded that an electric would likely meet our needs.  I concluded that the right way to set it up for us would be to have it start to charge once the engine was running but not when only the solar or nothing was.  Even though we actually start the engine less than half the days that we are underway, this would still be able to just keep up in the simulations of all the trips that I have data for and that was assuming a single epropulsion battery and no rowing.  Realistically, we should start by bringing the ancient outboard we already have on a trip we normally would leave it home for and see if we find it worthwhile to have along and then I can go through the trouble and expense to set up the boat for an electric.

Eric

Dick Stevenson

Hi John and all,
Perhaps of interest to some:
I have a good-rowing, hard-shell dinghy which brings me great joy when poking around anchorages. With it I also have a 4+ yo Torqeedo outboard which is primarily used for long excursions and windy conditions with chop.
My take is that a slippery hard dinghy is going to get far more mileage out of an outboard than any inflatable.
My memory is that at medium to low speed (the equivalent or faster than rowing), I could easily do a round trip of 5km, but maybe not 5km out and 5km back. I did not worry about running out of power as I always had my oars and, with the oars, I knew I could always get back. This is less true (or not at all true—see below) for an inflatable with an outboard that gets finicky or runs out of fuel.
It is expensive, but it is possible to have an extra battery.
The faster you go, the less mileage: at low speed, (all one needs for poking around anchorages and going ashore) the battery lasts a very long time. At high speed, especially with a load, one can deplete a battery surprising quickly.
My best, Dick Stevenson, s/v Alchemy
Ps., Most think that a hard-shell rowing dinghy is just a quirky choice for a cruiser. I would offer one safety reason: I could not get upwind in ~~ 20 kn of breeze and chop in an inflatable when rowing. And, it was a decent rowing dinghy, a rib. One problem was the big tubes which I loved as they kept the dinghy really dry. The down side is they caught the wind and chop and stopped the dinghy cold or spun it around. Unless I had the outboard (gas at that point) and it was working (unfortunately not always the case for me), I might just not be able to get back to that boat. At least once this was a scary scenario and I worried that I could not return to the boat in a rising wind situation.

Michael Lambert

I for one would love to see that analysis. In theory electric outboard are great, despite my gripes. I love not having to start it, it’s absurdly easy to shlep, you can chat with people underway, you can dip just the prop in the water in the shallows, and no gas of course. But I forgot one gripe: there is a ~1 second delay on the throttle. Later this summer my 7yo was driving, and when nothing happened when he turned the throttle, he turned it more. So it shot straight to fairly wide open, and he almost went over the transom onto the prop. It was scary. And yes, don’t buy new tech, but my brain, despite years of experience, is stubbornly trusting. “They are smart, I’m sure they figured it out”. So of course I just bought an electric foil board. It was far too tempting.

Marc Dacey

Since I put in battery monitoring and some solar panels, I’m able to review the “power hogs” during the rare times we use the inverter. The drawdowns for the vacuum cleaner can be rather dramatic, so we tend to vacuum at noon, while underway during motorsailing. If I need to use AC power tools, I either restrict this use to 10 minutes of running on the inverter, or I start a Honda genset to avoid drawing down the batteries. I think the whole argument about induction has very little to do with its innate qualities, and highlights the need to know one’s power setup and battery capacity. We have a standing order not to go below 12.3 VDC and I prefer to just use the first 25% of the battery bank’s capacity. This has meant I peer at my “numbers” regularly and now have an understanding of which loads are trivial (most of our DC side, save the autopilot and, to a lesser extent, radar) and which are “expensive”, such as almost all inversion, air conditioning/heat pump usage and power tools or non-LED AC lights. We’ll see the cost of the watermaking process in the spring, but because we keep a decent log, it won’t be a surprise. As an aside, using our 700 watt microwave for three or four minutes doesn’t draw very much and we “recover” with even our stock alternator within minutes. But if you don’t measure comparatively, you won’t know.

Marc Dacey

That’s very much as planned, John. I carry six L-16s right under the mast that rarely go below 75-80% SoC. If I need to weld something, I can, but we are set up to use less than the average boat. As I put in a larger alternator and get the wind generator online in anticipation of fewer shore power days and more anchoring out/passagemaking, I want to maintain that percentage so I can get many years out of our bank. I need to get some sort of battery monitoring on the two Group 27s I keep forward for the windlass. Getting down east this year saw a fair bit of windlass use, but I didn’t notice any startling drawdowns of that bank, either.

Induction rings strictly from a usage standpoint seem like a solution to an already solved problem. We have a small electric grill we’ll use at dock occasionally, but it wouldn’t occur to us to switch on the inverter to run it at sea. The electric kettle, maybe.

Eric Klem

Hi John,

Very good point on voltage drop and the need to check this as well as just the Ah that you will use when sizing your bank if large loads are expected.  One other source that people should watch for is if they use their house bank to start their engine, it is a momentary drop but can be quite substantial causing issues like having to reboot everything right before entering a harbor.  We do this and have never had any electrics drop out but it is a relatively small engine with a house bank of 4 GC2s.  I did measure the voltage a while ago during this but can’t remember the number.  At the time, I went through and checked all the electronics for this but I do need to remember to check whenever I next replace something.

For people who really insist on multiple large loads that could potentially occur at the same time, I suppose you could try to mitigate this by putting the large, non-critical loads like cooking on circuit(s) that are either fitted with low voltage cutoffs or relays to keep them from all operating at the same time.  You would need to take a real look at usage and make sure that you were not going to constantly be locked out by these.

I remember the first time we went full throttle on the first electric car that I built and watching the voltage go from ~152V (144V nominal) towards 100V and thinking something was really wrong.  Of course, that wasn’t the case at all, it is just that we were pulling close to 1000A on 24 Trojan T-145s wired in series.  Having since built a vehicle involving capacitors, those are even worse but at least our simulations ahead of time had told us that and we could design to mitigate it.  Capacitors have basically no voltage sag under load but voltage and state of charge are linearly related so our brushed DC motors became really speed/torque limited once you were down off full charge on the accumulator bank.

Eric

Matt Marsh

I’d like to take a step back and consider this issue not from an electrical system sizing standpoint, but from a design principles standpoint.

The example scenario is: High current demand –> Main DC bus undervolt –> sensor dropout –> no heading data on main data bus –> autopilot loses control –> crash jibe –> injury.

Breaking the first two steps in the chain (High current demand –> Main DC bus undervolt) seems obvious. But looking farther down the chain….
– Why is the autopilot allowed to lose control if a particular data packet fails to arrive?
This speaks to a level of redundancy and fail-safe / fail-dangerous character that is more appropriate to consumer toys than to critical systems. In aerospace & automotive, equipment is expected to continue to perform safely under extreme under-volt and missing-data conditions, and to fail in a non-deadly way.

And, another step later,
– Why is a loss of autopilot control allowed to lead to a crash jibe?
This speaks to an inherent property of the rig design, which very quickly leads one to consider whether the common sloop / cutter design with standing stays & shrouds & spreaders is chosen because everyone else does it, or because it’s really the best solution. I have seen precious few cruising boat designs where the rig is set up to remain inherently safe in case of such a failure.

This kind of “keep asking WHY is it done that way?” at each step of the failure chain is, I think something we often forget to do, particularly if the first couple of steps of the chain seem easy to fix.

P D Squire

Sounds like another argument in favour of E-wincher over powered winches.

Unless perhaps they have big current draws while charging.

Petri Flander

Hi, good point on voltage dips/brownouts.
This will be a bit beside the point, but…
On yard, we ‘hacked’ one of these cases (3kW inverter, tired bank) with DC/DC converter between main DC bus and nav electronics. AP pump was left to main bus since it didn’t seem to mind the ‘brownouts’. Since the boat was going to be overhauled on next winter anyway for charter classification, this hack was just more expedient. Also, for that season, we misused radio battery (charter req.) by parallelizing it to nav DC/DC converter output via schottky diodes. That way, if converter goes on strike, radio battery kicks in seamlessly via diode and feeds the nav chain.
We could have added third DC feed from main bus for redundancy and let the rado battery be, but we got only two leftover diodes and radio battery wasn’t required on that season so it had nothing else to do. Later, we did the triple feed and there was no complaints about resets. Cheers.

Alan Miegel

Just a thought and I am considering this as I have to redo my electrical system on my new to me boat. Why not a dedicated battery or bank for autopilot and electronics? Used with a battery combiner or charge relay it could effectively be separate from your house bank and not subject to high amp loads/voltage drops.

Matt Marsh

What it comes down to, really, is that a typical 12 V system has a practical cap at about 1.5 kW total load; i.e. it’ll work for engine starting and a few light loads (nav lamps, radio, plotter, cabin lamps, etc.) but if you start adding a power windlass, power winches, inverters, etc. you end up grafting hack-ish partial fixes onto a fundamentally inappropriate architecture.

Splitting up a bank is inefficient in terms of weight and cost. The autopilot is usually the biggest consumer while underway; it takes nothing while at anchor. A dedicated bank for AP & instruments plus a dedicated bank for house loads will be more batteries, more weight, and more money than a single large bank that can handle both in realistic use.

Evan Effa

Thanks John for this topical discussion.

We have a… (cough…ahem) power boat (Nordic Tug 37) so our big battery bank draws are when we are at anchor. We are often out in the winter months and enjoy having lovely anchorages that can be very crowded in the summer months, all to ourselves in November or February.

I was under the impression that I should not let the house bank voltage drop below 12.0 V. I tend to be thinking that I should fire up the Generator when we approach 12.1 V on cold winter mornings but when I look at the various 12V instruments I would have active at anchor, (NMEA 2000 network with GPS, heading, weather, Depth sounder etc.) I’m not sure that I can point to any that would not tolerate that or even lower voltages. (I note that our I was told that the Wallas DT40 diesel heater will not tolerate low voltages but I see that it will accept voltages as low as 11.0V if it is already running but shouldn’t be started up at voltages less than 12.6V.)

We have a ~660 Ah Carbon Foam house bank. Once our SOC is down to ~ 55% we are flirting with 12.1 V & I’m feeling nervous.

(In sunnier, more summery conditions, the solar panels kick in by early morning and bring up the working voltage somewhat allowing a more elective charging time.)

Should I be willing to accept, say 11.9 V and get another 100 Ah out of the bank?

-evan

PS:
We avoid any heavy AC current draws on the inverter, like the microwave or vacuum cleaner when on the house bank for the very reason you cite above: Significant voltage drops.

Evan Effa

Thanks for the reply.

I see very little drop in Voltage with any of our DC loads at anchor. (Water pump, Vacuflush pumps would be the only higher draw intermittent loads. ( Big draw items like thrusters, windlass etc have their own batteries and are only in use with the engine running.)

The outstanding question I still have is: What is the lowest acceptable / safe Voltage (even intermittently) for the HouseBank to get to before re-charging?

-evan

Evan Effa

OK.

Sure.

It’s helpful to consider that the arbitrary lower limit value of 12.1V (even in the momentary dips) might be too conservative.

BTW: I am not just looking at Voltage. I have a Victron shunt with the BlueTooth App and monitor my usage and SOC quite closely…(probably too closely.)

I will plan to go through all my 12V devices including the inverter to confirm that they are rated for say, a potential 11.9V lower limit.

Kevin Connell

Perhaps a better setup is a separate bank for the autopilot, mfd, and perhaps radio, the mission critical stuff.

Ernest E Vogelsinger

Perhaps not necessary to have a separate bank? I am planning of having two positive bus bars, one for “regular” appliances who are not susceptible for sudden voltage drops, such as cabin, cockpit and nav lights, or high-drawing items such as autopilot rams, inverters or USB chargers. The second bus bar would be fed by a DC/DC converter, e.g. Victron Orion Tr-12-12-9, and deliver constant voltage for sensitive equipment (which usually doesn’t draw too much). As to my calculations the above mentioned Orion could easily power all nav and other electronics equipment, including a radome (you could have two of these in parallel if necessary).
In case you have other load-prone equipment, e.g. a bow thruster, there would be the need for a separate battery which should be isolated from the rest. Again a smart DC/DC charger could keep these charged as they may only need to be topped-up anyway.

Peter Rowland

John, your article implies that on a smaller vessel induction cooking may not be possible, but I wonder if that is true? Or at least can induction be used to supplement Butane (more popular in the UK than Propane). The reason I am wondering it is I am hoping to do it hence the interest in your article.

One advantage of a smaller yacht, ours is 5Tonne 31ft, is that the other loads are none existent. The winch is my right arm, the autopilot a low current wheel drive, manual windlass, bow thruster? ha I wish.

That has to be offset against a smaller house battery bank and this is were I am struggling a bit despite playing around with your spreadsheet so would welcome a couple of minutes of your time. If we assume a single induction hob and we keep it down to 1kw rather than full power, the current at 12v about 80A. I believe the inverter is 85% efficient which your spreadsheet suggests a load of 96A.

So the question is could a pair of charged T105s with 225Ah capacity support a single 1kw hob for say 30 minutes. That’s a curry, Spag Bol or 2 x omelettes, without dipping below11.75v?

Is this a worthwhile goal? well 3 years ago in August at the height of the holiday season, SW England experienced a huge shortage of Camping Gaz, one of the two major brands of gas available. No chandlers, petrol stations or caravan/camping sites had any across the whole of the SW. We unknowingly sailed right into this with an empty cylinder. We were lucky enough to find one small cylinder which cost a fortune and I promised never again, in future we would have a back up plan, just in case.

Peter Rowland

John, thanks for the reply and yes this is very much a back up system to be used as mix and match. Lots of sun, sure use the induction hob to cook a meal with. Day 3 of a gale with lashing rain and wind, not exactly an unknown occurrence in the UK, then on with the gas. The other way to look at it is fresh fish is going on the hob because its delicate and quick to cook. We seek out fresh fish markets. However, that shoulder of Welsh lamb, well that’s headed for the gas oven down low for the afternoon. As we did with the turkey for Christmas Dinner whist sailing across an empty Solent see piccy .
[IMG]https://www.cruisersforum.com/forums/members/27361-albums1537-picture33314.jpg[/IMG]

You will be horrified 🙂 but we manage with a house bank of 160A, that’s 2 x TMX24 and have done for two years. This is small boat cruising 2 to 3 weeks at a time coastal hoping. Done the Atlantic prefer the quiet spots others can’t reach, we draw 3’8″ in old money.

[IMG]https://www.cruisersforum.com/forums/members/27361-albums1537-picture33315.jpg[/IMG]

Changing to 2 x T105s would be an upgrade. LFP another possible option. 200A of LFP would do, no point in going bigger for us because lithium is just a store of energy at the end of the day and that energy has to be replaced.

What still confuses me is this short term voltage drop effect on a flooded lead acid battery. Does it really matter if I draw 80A for 30 minutes? That’s 40AH. Assuming you start with well charged batteries and monitor/manage the DoD.

Robert Chapman

Please stop using the term “amp/hour” in your spreadsheets and at least one place in the text. It’s meaningless. Perhaps you intend to use “amp hours”, but it’s not always clear what you do mean.

Andrew Todd

I think I we need to be careful with the choice of induction tops. I think some if not all pulse the coil at full power to achieve lower percieved power settings.

Brian Russell

We have been using a NuWave induction hotplate (US$69.00 on Amazon)for the past 4 years as a supplement to our propane range. As well we boil 2 kettles of water in an electric kettle every morning. Propane has been in short supply in the UK where we spent summer /fall 2021 and summer/2022, so the induction has gotten a good workout. We have 56o AH of Firefly batteries, 640w of solar (fairly useless in Scotland and Norway!), a wind gen, can charge the house bank at 120A with our 150A Balmar alternator and also have a 9kw diesel genset. Yeah, I know, but i have a welder onboard… Anyway, our NuWave has a selector for 600, 900 or 1300w input. I just ran a little test after readi g the above comment about “pulsing” at full power to achieve these input figures. My results at the beginning of heating 500ml of cold water are:
1300w= -64.3A
900w= -54.7A
600w= -28.00A
Setting was Med/High 375˚F
All of these are at about 13.4V, we are plugged in with a small battery charger feeding the batteries which are at 100%. The amp draw would certainly be higher at lower voltage but the point is there does not seem to be any pulsing at full power going on. Additionally, I can discern little difference in boiling time between 900 and 1300W settings. The unit reduces amp draw drastically once temp is reached and things are simmering.
One does not have to “max” the cooker in oredr to have practical results. I will say that trying to cook a multicourse meal on 1 burner is a juggling act. Potatoes, rice and pasta just need to be brought to the boil then the pot can be stuck inside the tea cozy with a tea towel and the cooking will continue. Also, use much less water then called for on the package with pasta! Practicing energy conservation is half the game!
Another Top Tip we just received from a Danish cruiser here in Porto Santo is to place one of those silicone baking mats (SilPlat)between the pot and the smooth slick induction top to act as a non-skid. Works well in mild sea conditions, not as good as fiddles on the gas range, but better than nothing. By the way, all 5 of the boats (English, Swedish, Dutch) we are hanging out with here are using induction as much as they can.
Next will be a combi-microwave convection electric oven…