Rethinking power management

[goboatingnow]

What a nonsense argument.

For a boat we are typically using a “ nominal “ voltage ie 12 or 24V. Hence Ah ( which is an SI Unit by the way , ie coloumbs ) hence Ah is a useful number to compare battery capacity and its usage.

EVs use kWh because the internal battery voltages are not standardised across cars. But boats are typically standardised.

Hence Ah is fine for comparisons. Both Wh and Ah contain inherent inaccuracies but it’s ridiculous to start the Wh angels on the head of a pin argument.

My system can log ( at 30 secs intervals , voltage and current , consumed by devices etc) but in reality Wh versus Ah comparison is bunkum primarily because theoretical battery capacity in Wh is as useful as Ah.

[s/v Jedi]

Originally Posted by goboatingnow

What a nonsense argument.

For a boat we are typically using a “ nominal “ voltage ie 12 or 24V. Hence Ah ( which is an SI Unit by the way , ie coloumbs ) hence Ah is a useful number to compare battery capacity and its usage.

EVs use kWh because the internal battery voltages are not standardised across cars. But boats are typically standardised.

Hence Ah is fine for comparisons. Both Wh and Ah contain inherent inaccuracies but it’s ridiculous to start the Wh angels on the head of a pin argument.

Nonsense? Then it’s gonna be easy for you to tell us how long a fridge will run from a battery when the fridge consumes 700Wh per 24 hours and the battery is 225Ah

[goboatingnow]

Originally Posted by s/v Jedi

Nonsense? Then it’s gonna be easy for you to tell us how long a fridge will run from a battery when the fridge consumes 700Wh per 24 hours and the battery is 225Ah

I use a nominal voltage and perform the comparison because in particular battery Ah ( or Wh) is a highly variable number.

Hence in my case I convert at 12.5V as this is the average nominal value of my 12v system

Any errors in my fridge consumption by converting to Ah pales in comparison with the inaccuracies in Wh Ah battery computations

Precision and accuracy get mixed up by a lot of people including this thread.

[Dockhead]

Originally Posted by Na Mara

I have been on a few threads here arguing variously for questioning received wisdom on managing power onboard. Some of my ideas were wacky and some ultimately flawed, but I still think the received wisdom needs challenging.

The typical cruiser power management setup is geared toward a 24hr cycle. I.e. systems should be designed so that, on average, consumption and charging should be equal in any given 24hr period. Of course sometimes conditions won’t be average and adjustments on the fly (running the engine, pumping to water heaters or load resistors) will be necessary, but the base case is a 24hr cycle.

This made a lot of sense when lead acid batteries were the norm as only 30% of the total capacity of a bank was available for use away from shore power. It then made sense to budget on a 25% DOD to keep the batteries sweet. On a boat with charging primarily from solar or one that motor/genset charges once a day, that 25% needs to cover about 20-18hrs a day. Assuming an average 40ft boat with gas cooking uses on average 1,2kWh a day then that 25% of capacity needs to be about 1kWh or about 85Ah at 12V, which entails about 350Ah of total capacity.

Indeed 350Ah of lead acid is about what most blue water 40ft boats could sensibly carry in terms of weight, volume, cost and engine charging capacity. Hence it made sense to go for a 24hr cycle as this is what the battery storage could accommodate.

But battery tech has moved on a ways and so the 24hr cycle needs rethinking. On my boat I can fit 800Ah of Lithium in the same space I fit 350Ah of FLA previously and at a third of the weight. Fully 80% of that capacity is safely usable giving me 640Ah of capacity at 12 V. That is roughly 7.7kWh, which is enough electrical energy to run an average 40ft yacht for nearly 7 days. And this is without taking into account the charging and discharging efficiencies of Lithium.

So instead of designing a system around a 24hr cycle we should be designing them around a multi day or even weekly cycle. This brings great benefits. For a start, in any given week you are likely to be maneuvering the boat a bit under engine. With an 100A externally regulated alternator that’s a kWh of energy put back every hour the engine runs. Say in any given week it runs 4 hrs for maneuvering on average. Then, including parasitic engine charging, the battery capacity stretches for nearly 2 weeks.

Consider also that the average cruiser ducks into a marina every so often for food, water and fuel and can plug in when they do so. Let’s say they do this every 4 weeks on average. Then assuming they start out with 8kWh of usable energy in the bank, and they put back 4kWh a week with parasitic engine charging, and that they use 8kWh a week, then over the 4 weeks the deficit between what they charge off engine together with what they start with (8+4*4) and what they use (4*8) is only 8kWh!.

That small shortfall can be met any number of ways. A single 55W solar panel would do it. Or you could run the engine 2 extra hours a week. A wind generator would more than do it (average daily output of a superwind 350 where average wind speed is 10kn is roughly 1kWh a day. Only a quarter of that output is required here over a four week period). Or a hydro generator used when sailing. In short, the extra cost of the battery pays back on the reduced need for charging systems.

My general point is this: battery tech is now at the stage where usable capacity can bridge up to a week at a time. Thus it makes sense to switch from thinking I terms of inputs and outputs equalling out over every 24 hr period more or less, to thinking about inputs and outputs equalling out over weeks. This completely changes how we need to think about charging.

For instance, the efficiency of any wind turbine will increase substantially when it’s used to charge a larger bank. Wind is boom or bust. There will be many days when a wind gen disappoints and then there will be day when it rapidly exceeds the capacity of a standard bank and the extra energy gets dumped. With a much larger bank, more of the energy in the boom times can be stored to bridge more of the bust times. thus more energy from the turbine is put to good use and so greater efficiency is obtained.

I think there is a flawed assumption here -- that a 24h cycle is somehow a benchmark, which power systems are designed around.


I don't know anyone who designed their systems that way. Everybody I know simply packed in as much battery capacity as reasonably practical, and then designed the rest of the system around that. My boat as 440AH @ 24v of golf cart batteries, and that is good for about 12 hours of average consumption excluding high power devices which are run only when a diesel engine is running. 24 hours would be a dream.


If you can get 48 hours, even better.


But 1.2kWH per day? I don't know anyone who uses so little. That's like one tenth of consumption on my boat. That's about 1/2 to 1/3 of what we used on my previous very spartan (electrically speaking) boat. I don't think you can run refrigeration on that, much less nav electronics, autopilot, etc.



And anyone who does use only 1.2kWH per day and suddenly has 10kWH of stored power will, I can just about guarantee you, start using more power than that.


And that's fine -- these are dynamic systems. If you have little power, you take stronger measures to use less. If you have more power, then you allow yourself more.



Another factor in this dynamic relationship is power sources. If you have a heavy duty generator, then you are much less fussed about how much battery capacity you have, since it's not big deal to crank it up when you're short. But if you have no generator, no large alternator, and say a small solar installation, then you will be very fussed about not getting short. I don't think there are any simplistic rules or formulas which can be followed here.

[s/v Jedi]

Originally Posted by goboatingnow

I use a nominal voltage and perform the comparison because in particular battery Ah ( or Wh) is a highly variable number.

Hence in my case I convert at 12.5V as this is the average nominal value of my 12v system

Any errors in my fridge consumption by converting to Ah pales in comparison with the inaccuracies in Wh Ah battery computations

I see words but nothing about how long the fridge will run on that battery.

12.5V average?

I’ll show you my math, so you have a bit more time to do yours:

My battery is 10.5kWh but I don’t like to discharge it more than 80% so that’s 8.4kWh / .7kWh = 12 days.

Let me know when you have your number

[goboatingnow]

I have a fridge , approx 60Ah per 24 hours. Ie at a nominal 12.5 , 0.750kWh , this is with the boat not underway. Other loads are very nominal ( all lights are led etc ) invertor usage is momentary as to not add up to much.



Total consumption while under way rises to approx 1.2 kWh per 24 hours largely a function of AP usage ( I’ve not done 24 hours sailing no motor with this boat yet )

[goboatingnow]

Originally Posted by s/v Jedi

I see words but nothing about how long the fridge will run on that battery.



12.5V average?



I’ll show you my math, so you have a bit more time to do yours:



My battery is 10.5kWh but I don’t like to discharge it more than 80% so that’s 8.4kWh / .7kWh = 12 days.



Let me know when you have your number

You clearly are arguing for arguments sake.

If a load has a nominal consumption ( energy ) of say 100Wh and a source has a nominal charge capacity of 100Ah I can convert from one to the other by using a nominal system voltage.

My batteries have 1200Wh or my load has 8.3Ah , ie 12 hours [emoji106]

Both these numbers are very nominal because the load figure maybe averaged on based on either standardised test formulas , particular ambient etc.

Again using Ah or Wh is irelevant in a system with a nominal voltage. You’re just converting from one representation to another. The inaccuracies in load and source energy ( or charge ) specification mean trying to get more accurate by using greater precision is a waste of time.

[Na Mara]

Originally Posted by Dockhead

But 1.2kWH per day? I don't know anyone who uses so little. .

.

Well now you know two. As I also use about this much sailing and around half this at anchor.

If your cooking with gas, heating with diesel, relying on passive ventilation for cooling, and only have a single sub 100l fridge, then with modern equipment it’s easy to get by on less than 1.2kWh a day on average.

I go sailing to get away from it all, not to take it all with me.

[Dockhead]

Originally Posted by Na Mara

Well now you know two. As I also use about this much sailing and around half this at anchor.

If your cooking with gas, heating with diesel, relying on passive ventilation for cooling, and only have a single sub 100l fridge, then with modern equipment it’s easy to get by on less than 1.2kWh a day on average.

I go sailing to get away from it all, not to take it all with me.

Well, to each his own.

I have a large front-opening fridge, and a large freezer. The freezer uses easily double the power of the fridge.

I have a lot of navigation equipment which runs 24/7 whether underway or at anchor, including radar.

Lighting is all LED, but we run various other gear, and with a large crew on board, a lot of power goes to charging mobile devices.

We do mostly cook with gas when off grid, but use electric kettle for the oceans of tea etc. drunk on board. Cooking is mostly on induction when an engine is running, and occasionally we use induction off the batteries.

It adds up.

With a heavy duty generator on board, and a heavy duty second alternator on the main engine, it's not a problem to supply those amounts of power. I run the generator twice a day when under way under sail, and once to twice a day at anchor. The generator runs at 1500rpm and is almost inaudible in its sound enclosure which is in turn inside a sound proof engine room.



YMMV with different setups.

[sailingharry]

Originally Posted by s/v Jedi

I have never heard of people converting their solar arrays to Ah numbers, nor for gensets, wind generators, appliances etc. They are all rated in W, not A.

For future reference, you now have! Because I have nothing on my boat that can measure W, I mentally convert my wind and solar to A. It's not like I've written it down, but I know that my 360W solar array "should" able to do about 25A. And so when I see the dedicated A meter on the solar reading 8 or 10, I know those crap Solbians need to be replaced. Likewise when the D400 Wind generator is pushing out 2, 3, 4 A, I remind myself it will only make the rated 30A in a hurricane (it did, actually...). Or when my 500W Watt&Sea is cranking out 20A, I'm around half it's 40A.


I don't do the conversion with a calculator, and I don't even do it because I think it is the "best" answer. I do it as a necessary evil to work with the world we live in. If I had a W meter, I could convert the other way -- multiply my manufacture's nameplate data on the alternator, fridge, and battery by 12 or 13.

[noelex 77]

Using Whrs can be done but the biggest problem is calculating how much energy the battery is gaining from an input. Therefore knowing how much energy you have stored is going to much more difficult than using AHrs.

Batteries store amps not watts. This is why they are rated in Ahrs.

Using Whrs in the calculation creates some problems. As the battery voltage rises during charging, using Whrs indicates the battery is storing more energy, whereas the opposite is true. As the battery voltage rises, battery efficiency reduces. Using Ahrs, the rising battery voltage is ignored and only the Amps are counted. This more closely follows how batteries behave.

This is why all batteries are rated in Ahrs and most battery monitors report remaining capacity in Ahrs. You can do the maths and fudge the numbers. This would involve employing a battery efficiency quotient that decreases substantially as the charging voltage increases, but why bother.

That is not to say watts and Whrs should not be used on a boat. When voltages are converted, watts are particularly useful.

[mako]

Watts, watt-hrs, volts, amps, amp-hrs….

Guys, what was it again that the OP was actually asking about originally????

[Joh.Ghurt]

Originally Posted by mako

Watts, watt-hrs, volts, amps, amp-hrs….

Guys, what was it again that the OP was actually asking about originally????

Don't you know that this is of little importance? All power problems on a boat would be solved if only you would use the correct units.

[s/v Jedi]

Originally Posted by noelex 77

Batteries store amps not watts. This is why they are rated in Ahrs.

That’s one to remember

[s/v Jedi]

Originally Posted by goboatingnow

You clearly are arguing for arguments sake.

My batteries have 1200Wh or my load has 8.3Ah , ie 12 hours [emoji106]

1200Wh battery, 8.3Ah load, 12 hours