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Old 02-09-2021, 09:51   #1
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kWh to Ah

Am I correct in calculating that a 264 kWh battery is the equivalent of 22,000 amp hour battery at 12 volts?
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Old 02-09-2021, 09:54   #2
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Re: kWh to Ah

Yes.

264*1000/12 = 22000Ah@12V (a bit less if you use 12.4 or 13.8 or whatever).
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Old 02-09-2021, 10:15   #3
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Re: kWh to Ah

So, one kWh about eauals 83 Ah, again at 12 volts.
(as an aside, I wonder why the tradition is to measure batteries, at least lead acid, in Ah instead of kWh. Simplified calculations?)
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Old 02-09-2021, 10:31   #4
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kWh to Ah

Wiring is sized based on amps and length of circuit.
Keeping every thing in amps meant that there were fewer opportunities to make math mistakes.

This is true even for power tools running in AC. They are always marked in amps so you can tell what size extension cord you need to use.
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Old 02-09-2021, 10:34   #5
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Re: kWh to Ah

Yes, 83 is the number.

Ah are useful because going in ≠ going out in Wh. Ah counts electrons, and electrons in = electrons out. But electrons going in have greater energy (13.8V as an example) than those coming out (at say 12.4V). So 1Ah at 13.8V = 13.8Wh, while 1Ah@12.4V = 12.4Wh. The difference is the inefficiency in the battery system (and generally shows up as your battery getting warm during charging). By measuring Ah instead of Wh you can avoid many of those complications. Peukert throws his ugly math in there to complicate things (simplified - internal battery resistance can steal power during discharge as well as during charging).

Just on voltage differences alone the charge efficiency of lead-acid batteries is about 85%, so if you measured in Wh you'd be off by ~15% comparing the amount you put in with the amount you take out. The numbers become better as you move to newer battery chemistries, but they're still not 1.0.
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Old 02-09-2021, 12:45   #6
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Re: kWh to Ah

Quote:
Originally Posted by Adelie View Post
Wiring is sized based on amps and length of circuit.
Keeping every thing in amps meant that there were fewer opportunities to make math mistakes.
Ah. Thank you.
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Old 02-09-2021, 12:51   #7
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Re: kWh to Ah

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Originally Posted by Dsanduril View Post
Yes, 83 is the number.
Thank you.
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Old 02-09-2021, 17:26   #8
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Re: kWh to Ah

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Am I correct in calculating that a 264 kWh battery is the equivalent of 22,000 amp hour battery at 12 volts?
So, are you considering putting an E-Semi truck drive battery in a boat?
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Old 02-09-2021, 18:50   #9
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Re: kWh to Ah

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So, are you considering putting an E-Semi truck drive battery in a boat?
Ha! We read the same news articles!

No, but I am trying to wrap my head around the relationships and equivalencies.

My boat is long and thin, has a 10.1 knot hull speed, carries 250 gallons of diesel, 350 gallons of water (no water maker). I've installed 1.08 kW of solar panels on the arch, and have room for another 1 kW, maybe 2 kW more.

I'm looking at buying Victron 250/100 because my current Victron controller limits output to my old lead acid battery bank of about 1,250 Ah to 700 watts. It's a silly purchase, what I have handles all of my electrical needs and I've not used the gen set in years, but the larger MPPT controller might come in handy during periods of intermittant sun or if a back-up controller was needed.

A friend and I just built a dinghy and I was considering electric motors. But the best 10 horse were stupid expensive, and the batteries … same. So I stuck with my old Yamaha 15 horse two cycle. Though I don't like gasoline, it always starts and I do like its power to weight ratio. But I learned that a 10 hp motor is about 7.5 kW.

My 30 year-old Yanmar 81 horse 4jh-dte has started to make a rhythmic squeak, probably nothing big but needs attention more expert than me. As I was looking into repair, friend I built dinghy with, and who had just replaced (with very little help) the Volvo in his C&C 40 with a 4 cylinder Yanmar said, "before you put a lot of money into that old engine, the new ones are smoother, cleaner and a lot better."

Which caused me to cycle back to wondering how much electric motor I would need to replace the Yanmar, and how much battery I would need to replace the diesel, with no loss in performance.

80 horsepower nominal = about 60 kW, though I throttle back to 80 percent. One gallon of diesel provides about 55 hp-h, I burn about 1.8 gals per hour at a water speed of around 8 knots.

I have 1,750 pounds of diesel fuel weight I could use for batteries (ignoring what's needed for my Dickenson heater), plus probably another 1,750 pounds of water weight capacity I could use if I installed a water maker, plus replacing 500 pounds of lead acids with Li-??, plus replacing the weight of the diesel engine, plus the weight of the gen set … It seemed doable, if one only looks at the numbers.

But it's an old boat and I'm an old man. I doubt there's enough left in either of us for a project of this magnitude, and not nearly enough in the bank for the probable cost.

The Volvo battery pack set my brain back into gear, so I thought I'd clarify the equivalence of kWh to Ah. It's more than you wanted to know, but thank you for asking. You've helped clarify my thoughts.
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Old 02-09-2021, 19:49   #10
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Re: kWh to Ah

What kind of boat do you have? 57' waterline?

If you want to go EP (electric propulsion) you need to accept a couple things about it.
It's good for getting in and out of marinas and harbors, for setting anchors, for getting out of the way of other vessels that might hit you, to help you thru tacks in very heavy weather, for going hull speed for very short distance (2-3nm) or for going moderate distances (20-60nm) at slow speeds (2.5-3.5kt.)

If that is something you don't want to live with then you could go EP-lite, put in an EP system for the above plus mount a moderate sized outboard (high-thrust model) that could push your boat at 5-6kt in calms.

Next step up is to re-engine with a parallel hybrid, Beta-Marine.

Regarding your solar controller. My feeling is that a small controller for each panel is better than a big controller for all the panels or bigger controller for groups of panels. Whenever a panel is partially shaded it draws the voltage/amperage down for all the panels that share a controller. I believe that in general many small controllers will provide more total Whr from the same panels. Large controllers are more suited to shoreside installations where the panels are sited to never be shaded.

I would suggest updating your profile with your general location and your boat make & model in the "Boat" category. This info shows up under your UserName in every post in the web view. Many questions (like this thread) are boat and/or location dependent and having these tidbits under your UserName saves answering those questions repeatedly. If you need help setting up your profile then click on this link: https://www.cruisersforum.com/forums...ml#post3308797

I would happily help more if the link above is not enough.
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Old 03-09-2021, 03:30   #11
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Re: kWh to Ah

Don’t forget when you bring AC into the frame that you have to deal with the fact that kw is kva x power factor, typically about 0.8 for things like induction motors such as you may have in the air con. So you don’t have anywhere near the power you thought you had. Even worse if you invert it first and lose another 10%. Worse still losses from charging and storing on top.
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Old 03-09-2021, 05:05   #12
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Re: kWh to Ah

Amp-Hr were used because historically everything related to the battery system was 12v (nominal), so you could be sloppy and it was good enough.

While amps do play into sizing circuits for day to day use, it's not the best.

KWH is more accurate and gaining ground as higher voltage DC systems and inverters become more common. Once you start having to deal with multiple voltages, it makes more sense to do the calculations correctly.
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