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#1
02-17-2008, 12:58 AM
 mkphotohawaii Guest Posts: n/a
Amp Hours to Kwh

I live off the grid and have 16 175 watt panels and 8 Interstate L-16 batteries and continue to be concerned that I'm not getting my Kwh's into my batteries or they are not holding on to them for long once they are there.

Does anyone know how to convert KwH to Amp hours?

Or maybe better, if I have 8 360 amp hour batteries, how many Kilowatts is that good for?

Thanks.

Aloha, Mike.
#2
02-17-2008, 03:12 AM
 JohnS Double Ultimate Supreme Member Join Date: Dec 2007 Location: SE Michigan, USA Posts: 9,233 Rep Power: 18
Re: Amp Hours to Kwh

You need to multiply the amp-hours by battery voltage. That will give you watt-hours. Divide by 1000 for kWh.

To go the other way, multiply kWh by 1000 to get Wh, divide by voltage to get Ah.

You may want to install an ammeter to be sure what current you are getting out of the solar cells. At max rating, you have 2800 w (2.8 kW) for a few hours per day.

The hours per day will depend on whether you have a tracking array and can keep it aimmed at the sun, or have to accept "off-angle" reduction in power. Also, you have clouds there, right? The panel rating is "high noon, direct sun." Most of the time, you are getting less. You probably get less power over more hours, but you are lucky to get the equivalent of full power for 4-6 hours.

Batteries are not 100% efficient. Because of overvoltage required for charging and current loss, you are lucky to get back 70-80% of the power you put in.
#3
02-17-2008, 10:42 AM
 mkphotohawaii Guest Posts: n/a
Re: Amp Hours to Kwh

John,

Thanks for your time... I understand this to some level, but I get confused when I have a 10 kwh day and then a 12.5 day, and at the end of the day, I have about the same amount of stored volts.

Understood that it depends on usage, but my usage was almost the same, within 10% for sure, but yet, the same volts... sometimes I feel that charging to 28 volts, absorbing, floating, etc. should be a once in a while thing, not a daily thing and just have more batteries chaged to 24 or 25 volts.

Do you have any thoughts on this aspect?

Thanks again.

Aloha, Mike.
#4
02-17-2008, 12:04 PM
 JohnS Double Ultimate Supreme Member Join Date: Dec 2007 Location: SE Michigan, USA Posts: 9,233 Rep Power: 18
Re: Amp Hours to Kwh

I have a few thoughts. First, I failed to answer this:
Quote:
 Or maybe better, if I have 8 360 amp hour batteries, how many Kilowatts is that good for?
6 V x 8 batteries x 360 Ah = 17280 Wh = 17.28 kWh

If you typically get 10-12 kWh of solar per day, you may not have enough storage. Although it will depend on how much of your power usage is during the day, while making power, vs at night, drawing from storage.

I note the batteries are 6 V batteries, so I assume 4 are wired in series for 24 V nominal, and two such banks are in parallel. Around nominal, battery voltage is very sensitive to whether it is charging or discharging, so it is hard to comment on the voltages you mention.

24-25 volt may be acceptable after charger is done for the day (sundown figure). It won't charge those batteries during the day. My experience is 12 V batteries and automotive, so double everything I say.

For a 12 V stack, heavy duty charging is done at about 14.4 V (@25 °C, we temperature compensate). Batteries on continuous charge (for emergency operations) are floated at 13.5 V. Fully charged, but off the charger, they run about 13.2 V. After a BRIEF heavy load to clear polarization, and then unloaded, about 12.7 V; below that, they are somewhat discharged, with 11.9 V typical of a discharged battery (all no load figures). A loaded battery will measure less; a moderate load of 10% of amp-hour rating might knock 0.5 V off those numbers.

Unless you are careful about how you measure, the voltage tells you more about whether you are charging or discharging. You'd like to hit (doubling those numbers) 28.8 V around noon (and 2 hours either side) and still be around 27 V at the end of your charging day. As soon as the sun is off the cells, you'll be below 26 V, and very likely 24-25 V, less by morning. You don't want it under 22 V, loaded, just before sunrise.

It is not realistic to shut off all your loads to let the battery rest for hours for a voltage check. I recommend a battery hydrometer to check state of charge occasionally. Be sure to compensate for temperature of electrolyte. This article might help: http://www.batterystuff.com/tutorial_battery.html
For a system continuously charging/discharging (whether car or solar), the terminal voltage stuff is pretty theoretical.
You have to "make hay while the sun shines" and I have to charge while the engine is running.
#5
02-18-2008, 02:05 AM
 mkphotohawaii Guest Posts: n/a
Re: Amp Hours to Kwh

John,

Thanks again for your time... seems you know what you are talking about.

Tomorrow I will test my batteries with no load on them at all, just to see and get a hydrometer as well.

I love being of the grid, and I'm just trying to get it as good as I can.

What do you do for a living?

Aloha, Mike.
#6
02-18-2008, 04:18 AM
 JohnS Double Ultimate Supreme Member Join Date: Dec 2007 Location: SE Michigan, USA Posts: 9,233 Rep Power: 18
Re: Amp Hours to Kwh

I'm retired. I was an engineering manager in the electronics division of one of the Big Three auto companies.
#7
03-05-2008, 12:31 AM
 mkphotohawaii Guest Posts: n/a
Re: Amp Hours to Kwh

John, sorry for my delay to reply... it's been a busy few weeks.

I recently added four more batteries, so now 16 175 watt pannels and 12 Interstate L16's and the batteries seemt to have really helped.

I have a sense that batteries have a "fat zone" or a "charge zone" perhaps that they are most effective in storing energy. You said to times my amp hours by the battery voltage, but that number seems really high to me. And I reduced it by 70% since batteries not efficient, but still seem high.

Interstate says 415 amp hours times 6 volts, that's 2490 divided by 1000, or 2.4 kwh. If I take that at about 50%, then it seems close, giving me 14.4 "usable" kwh from my system. It seems really important to know what your storage is since if you always have too little storage, then you are just thowing away good power on running up to 28 volts each day, absorbing... and then have 24.6 after the Sun goes down.

I'm almost now thinking about another four battereis, so I have close to 18 kwh of storage that I can use as we do get rain here in Hawaii.

If you have time, can you share your thoughts on this with me?

Thanks.

Aloha, Mike.
#8
03-05-2008, 04:26 AM
 JohnS Double Ultimate Supreme Member Join Date: Dec 2007 Location: SE Michigan, USA Posts: 9,233 Rep Power: 18
Re: Amp Hours to Kwh

28 v by day, 24.6 v by night is the reality of battery charge/discharge. In that range, it just is not a reliable indicator of state of charge, only whether it is charging or discharging.

Did you get the hydrometer and check the electrolyte? That is probably a better measure of state of charge. However, recording terminal voltage at sundown and just before sunrise may tell you a lot too. (You needn't follow technical definitions of sunrise and sunset, I mean relative to the solar cells, sun just off the cells and battery supplying load for a short time, maybe 30 minutes, and again before sun returns in the morning). I would plot or graph that for a period of time.

You probably need to analyze your night time load relative to storage. At least on a sunny day, your daytime load is running on the charging voltage. The other issue is how much extra storage you need for rainy days. I have seen recommendations of 3-7 days of storage, but that may be for places with worse weather. (I think in Detroit, I'd need 3 months. Sun is rare in the winter.) The issue of storage and "worse case weather" is one that makes "off grid" really tough.
#9
03-06-2008, 01:59 PM
 mkphotohawaii Guest Posts: n/a
Re: Amp Hours to Kwh

John,

Thanks... and how about the calculation for the Interstate L-16 6 volt battery.

Interstate says 415 amp hours and at 6 volts that's almost 2.4 kwh, but that so hard to image based on my experience with these batteries anyways.

Is that the number you get?

Aloha, Mike.
#10
03-06-2008, 02:32 PM
 JohnS Double Ultimate Supreme Member Join Date: Dec 2007 Location: SE Michigan, USA Posts: 9,233 Rep Power: 18
Re: Amp Hours to Kwh

Yes, that's the number I get.

Two caveats:
*You have to get it fully charged. I don't know if you are suceeding in that or not. You need to look at electrolyte SG, or look at terminal voltage after sundown. (Electrolyte is better)

*While that capacity is available if you total discharge the battery, doing so often will dramatically shorten battery life. Even deep discharge batteries shouldn't be regularly discharged below 50% if you want decent life. So, suddenly, you are at half the capacity effectively.
(An occasional (few times a year) deeper discharge is not a catastrophe, but daily would be).

Do you have a sense of your daily load, and particularly the average night load, when batteries are discharging? That really will define your minimum storage need. The maximum comes from the protection you want for extended inclement weather.

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