Getting a handle on kW usage

[-TX-]

So, anyway, what's the answer here? With your wealth of MSEE, PE knowledge, what size battery would support a 3.7 amp AC draw running 24/7 through my inverter? Just curious. If battery depletion rates aren't available on my GC15, apply a similar Trojan. Thanks.

[Sunking]

So, anyway, what's the answer here? With your wealth of MSEE, PE knowledge, what size battery would support a 3.7 amp AC draw running 24/7 through my inverter? Just curious. If battery depletion rates aren't available on my GC15, apply a similar Trojan. Thanks.

Does not take a Train Driver to answer that. A 5th grader can do it. Your question has already been answered. I am trying to teach you how to fish rather than do the work myself.

Amp Hours = Amps x Hours
Hours = Amp Hours / Amps
Amps = Amp Hours / Hours.

OK 3.7 amps @ 120 VAC with and Inverter efficiency of 88% will draw 21 amps from a 24 volt battery. If that battery AMP Hour Capacity were to be 690 AH then use the formula above in RED for hours.

690 Amp Hours / 21 Amps = 32.8 Hours aka C/32.8.

There is a issue with your question. You asked until Depletion. You would never ever want to discharge a battery completely, you never want to discharge more than 50%. So instead of 32.8 hours becomes 16.4 hours. Even that is problematic because with just one cloudy day means you have to shut down and wait for a full sunny day to return to recharge the battery. So proper practice dictates your battery capacity should have 5 full days. In practice gives you about 3 cloudy days of use until you have to recharge. Thus is why you would want a generator or engine alternator to save your butt.

Based on that if you pull 500 watts 24 hours a day is 12,000 watts per day. That means the battery requirement is 60 Kwh or 60,000 watt hours. At 24 volts is a 60,000 wh / 24 volts = 2500 AH battery. That monster weighs 3600 pounds and will cost you your roughly $150/Kwh or 60 x $150 = $9000 every 3 to 5 years.

Now before you start ranting again understand your question is invalid, or at least it should be. If the AC is sized properly it does not run 24 hours a day. It cycles on/off. In fact a DC powered AC uses a VFD drive which means it does not use max power when it is running. It can vary the power to generate just enough cooling to do the job. That is why it is important for you to use a Watt hour meter for a week to see what your real Kwh usage really is.

[-TX-]

Great post, thanx. What I was addressing, specifically because I'm pretty comfortable with your math, is rate of discharge. You said that depends on draw and was curious what kind of draw 21 amps is.

[Mike90250]

> curious what kind of draw 21 amps is

And that is the crux of the matter. You need to define it more. I'll assume it's an air conditioner & 40% duty cycle. That changes 24 hours draw to 9.6ah . With a 24V battery bank, that is 240w x 24hr (earlier I just converted the amps to 40%, easier than converting time) = 5,760 watt hours in a 24 hour period, That's the simple load. Your battery must be several times larger to both reduce the Peukert's_law effect & to not be completely depleted.

[-TX-]

I only quickly tested the AC (you're right, air conditioner) and was surprised with its draw because I have it set on 50 degrees, high. So I could be mistaken but I think 3.7 amps is its full draw (except for a second of 12 amps when starting up). Although it's listed at 6.2 amps.

Battery size is 24v/690ah/16,560wh

I'm getting pretty optimistic about all this because I'll actually only be using the AC for 10~ hours. I'm down to around 10 kW per day total including AC

[SunEagle]

I only quickly tested the AC (you're right, air conditioner) and was surprised with its draw because I have it set on 50 degrees, high. So I could be mistaken but I think 3.7 amps is its full draw (except for a second of 12 amps when starting up). Although it's listed at 6.2 amps.

Battery size is 24v/690ah/16,560wh

I'm getting pretty optimistic about all this because I'll actually only be using the AC for 10~ hours. I'm down to around 10 kW per day total including AC

Ah you found the leak. You can turn off the bilge pump and stop bailing.

Glade to hear you are getting close to managing kWh usage.

[Sunking]

Great post, thanx. What I was addressing, specifically because I'm pretty comfortable with your math, is rate of discharge. You said that depends on draw and was curious what kind of draw 21 amps is.

It is called C/Rate.

Most Consumer Battery AH is rated at the 20 hour rate or C/20 where C = the battery AH capacity, and the number 20 is Hours. Example if your battery is rated at 100 AH means it can supply 5 amps x 20 AH. We keep going over the same ground.

Amp Hours = Amps x Hours
Hours = Amp Hours / Amps
Amps = Amp Hours / Hours.

So if your battery is 690 AH and you apply a 21 amps load the C/Rate = 690 AH / 21 A = 32.8 Hours so the C/Rate = C/38.2 meaning the battery can supply 21 amps for 32.8 hours IN THEORY. See the numbers have not changed. No BS.

In practice Mr Peukert will change things. Peukert Law states the faster you discharge a battery, the less capacity it has. This goes back to the numbers game manufactures play with Amp Hours. I said Consumer batteries are rated at C/20 or 20 hours. Industrial and commercial batteries will be rated at C/8, C/6. and C/4. A quality manufacture like Rolls and Trojan will list the Peukert discharge curves. Here is a battery made by Rolls a top quality battery Spec Sheet. Look at it. It is a Rolls 6CS21P a 6-volt 683 AH battery at the C/20 rate About the same capacity you have. Look at the column of Ah specs you wil see the 20 hours discharge rate is 683 AH meaning it can deliver 34.15 amps for 20 hours. Now look down at say the 4 Hours discharge rate. What do you see?

That same battery is now a 423 AH battery meaning it can deliver only 105.87 amps for 4 hours. Now look up at the 100 hour discharge rate and you tell me what that battery can do if discharged at the 100 hour rate. That is Peukert Law.

So the exact same battery can be called anything from a 232 AH to a 963 AH battery. It is advertised as 683 AH at the C/20 discharge rate.

[Sunking]

I only quickly tested the AC (you're right, air conditioner) and was surprised with its draw because I have it set on 50 degrees, high. So I could be mistaken but I think 3.7 amps is its full draw (except for a second of 12 amps when starting up). Although it's listed at 6.2 amps.

Battery size is 24v/690ah/16,560wh

I'm getting pretty optimistic about all this because I'll actually only be using the AC for 10~ hours. I'm down to around 10 kW per day total including AC

OK your battery cannot really do that.

With an RV assuming you have a generator, well any off-grid system requires a generator, you can get by with 3-day reserve capacity with the understanding you will shorten the cycle life. So if you use 10 Kwh/day means you need a 30 Kwh battery. Do no task me how many Amp Hours that is, you tell me.

AH = Watt Hours / Battery Voltage.

[-TX-]

Ah you found the leak. You can turn off the bilge pump and stop bailing.

Glade to hear you are getting close to managing kWh usage.

Posted upthread; I'm down to 60kWh per week! and I really haven't started trying yet, only finding the hogs and addressing them.

[-TX-]

Christ, I should have studied harder!

Now I understand what C rate is. I knew it was a measure with the way you applied it, but now I know specifically what's being measured.Thanks. My Decas have a C rate of 395 minutes @ 25 amps until discharge, I should use that because 25a is close to my usage.. So I'm guessing at that rate I have around 200 minutes on a single battery until 50% discharge. Times 10 = 2000 minutes.

That's over a day so I doubt it's correct. But if I add solar charging to that mix it sounds as if I'm not too far off on my goal.

Between you and I, I really don't give a rat's behind about my battery bank. You've already responded that I should have bought Trojans anyway, and battery tech hopefully will improve in the next few years leaving this bank archaic. If I blow this $1500 bank in the next couple of years, so be it. My priority is being able to to sit offsite for 2-3 days setting up a few of the dohiggies in my profile pic about a dozen times a year. That's pretty much it. Not to say I won't protect my batteries when I can, but if I can't, so be it. Burning them to the ground is still better than driving hours to the nearest $100 Pakistani ****el and then hours back again.

[Sunking]

Posted upthread; I'm down to 60kWh per week! and I really haven't started trying yet, only finding the hogs and addressing them.

The hog is the Air Conditioning.

[-TX-]

it's not factored in yet. It will be the hog but depending on my use of it, who knows.

[Sunking]

Christ, I should have studied harder!

Now I understand what C rate is. I knew it was a measure with the way you applied it, but now I know specifically what's being measured.Thanks. My Decas have a C rate of 395 minutes @ 25 amps until discharge,

That is NOT C/RATE, that is a spec called Reserve Capacity defined as How many minutes a battery can supply 25 amps. It is related to AH but has nothing to do with C/Rate. It does factor in Peukert if the battery is less than 500 AH. Stop and think. 25 amp sis a fixed number of 25 amps. What is the C/Rate of 25 amps. You cannot answer because you have not defined the AH capacity which is a variable. Like asking how fast is 5?

Between you and I, I really don't give a rat's behind about my battery bank. You've already responded that I should have bought Trojans anyway, and battery tech hopefully will improve in the next few years leaving this bank archaic.

Don't hold your breath Pb battery technology is 150 years old and fully matured. It is as good as its going to get. Lithium holds promise but as of now and the foreseeable future is not feasible. Right now it is 400% more expensive and prone to catastrophic failures including fire and death if you have paid attention to recent news events.

[-TX-]

A little faith please

[solar pete]

Your a funny guy TX