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I can follow the math portion of things, I guess I just don't understand the significance of the 3.84AH or 6.4AH. I think that with 50% of my battery used, this means I have 3.84AH remaining, and with 30% drained I have 6.4AH remaining. How do I go from there to battery selection? I tried looking for a sticky or something but didn't find one or missed it.
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There is a sticky in the off-grid selection for designing the battery portion of an off-grid solar system. It will help you a lot if you could give it a read over!
Here is the link to the Solar Off-grid Battery Design in the off-grid section.
Here is a link to the different types of batteries available The Deal With Batteries Tutorial.
Nope....those figures, 3.84AH and 6.4AH, are the battery capacities that you require for the 50% and 30% DOD.
See the first number we work with, 23.04WH, that is the power we need. We then divide that number by the battery voltage, 12V, and it gives us the AH that we require.
As an example, we have 23.04WH / 12V = 1.92AH. So we use 1.92AH per day. Unfortunately you cannot completely discharge batteries, otherwise they will be permanently damaged, as well as reducing their lifespan. For most batteries, the maximum DOD that you should do is 50%, with 30% being the recommended amount.
So to get a 50% depth of discharge, for a power requirement of 1.92AH, means we will need a battery of minimum capacity of; (1.92AH/50)100= 3.84AH @ 12V
Working backawards, if you had a battery of 3.84AH, a 50% discharge will be; 3.84/50%=1.92AH, which is exactly what we require!!
Does that help at all?
Have a look at the threads that I have posted a link to above. Once you have read them a few times, and are still a bit confused, let me know and we can take it from there!Comment
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Thanks for those stickies! It boggles my mind how this can come so easily to some, and then for people like me.... well, you know. So I'm grateful for the help.
This confused me a bit. Where is the 20 hr discharge rate coming from? And why are we dividing by 8? This feels like algebra all over again, I can follow the math portion of things, but I'm lost when it comes to where certain numbers come from.As a Rule of Thumb you want to limit the maximum charge/ discharge rates of Flooded Lead Acid (FLA) to C/8-Hour rate. Where C = the Amp Hour (AH) specification of the battery at the 20-Hour discharge rate. For example a 100 AH battery C/8 = 100 AH / 8 H = 12.5 amps. So if that battery is 12 volts @ 100 AH it can handle a maximum inverter of roughly 12 volts x 12.5 amps = 150 watts.!
So I if I have a 12V 7.2 amp hour battery without any charge going to it, and I use 1.92AH/day in my application, I could get 7.2AH/1.92AH = 3.75 days of continuous use (not accounting for any system losses anywhere)? So if I had a 24V battery I'd basically double those times? I always believed that battery voltages stack, so if I had two 12v 7.2AH batteries together, in series, I'd have a 24V 7.2AH battery equivalent. Or do I still have a 12V battery but now with 14.4AH?Nope....those figures, 3.84AH and 6.4AH, are the battery capacities that you require for the 50% and 30% DOD.
See the first number we work with, 23.04WH, that is the power we need. We then divide that number by the battery voltage, 12V, and it gives us the AH that we require.
As an example, we have 23.04WH / 12V = 1.92AH. So we use 1.92AH per day. Unfortunately you cannot completely discharge batteries, otherwise they will be permanently damaged, as well as reducing their lifespan. For most batteries, the maximum DOD that you should do is 50%, with 30% being the recommended amount.
So to get a 50% depth of discharge, for a power requirement of 1.92AH, means we will need a battery of minimum capacity of; (1.92AH/50)100= 3.84AH @ 12V
Working backawards, if you had a battery of 3.84AH, a 50% discharge will be; 3.84/50%=1.92AH, which is exactly what we require!!Comment
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No problem. It all comes down to practice!
The 20Hr rate comes from the battery manufacturer. Battery manufacturers rate their battery capacity over a 20 hour period. So a 100AH battery can provide 100AH over 20 hours, that is to say, 100AH/20hr=5A per hour for 20 hours. If you use more than that per hour, the battery actually releases less energy as you are drawing more than what it was designed for (peukert's law comes into play then). So on a 100AH battery, to get the full 100AH out of it, you can only draw 100AH/20hours=5A per hour. If you draw more than 5A, then the battery will not provide the full 100AH. This goes for most batteries, as the 20Hr rate is a standard rate. You do get the odd battery that is rated at the 10Hr rate, but that is rare.
If you want to know more about that, the best would be if you did a google for "peukert's law". You will find lots of info!!
Yes, that is correct. But remember, doing that means you will be totally discharging the battery, which will damage and/or shorten the battery's life substantially! The absolute maximum discharge for cycle use batteries is 50%. 20-30% is usually recommended for maximum life.
Assuming you are refering to a 24V battery at the same capacity as the 12V in comparison, the answer is...not necessarily. It depends on how the system is designed. It may actually yield the same amount of runtime just with more wasted power in the conversion. It all comes down to design.
There are two different ways to connect the batteries, in series or parallel.
In series : The voltage adds, and the AH stays the same.
In parallel : The voltage stays the same, and the AH adds.
Example: You have two 12V 7.2AH batteries...
In series setup : you have 24V @ 7.2AH (voltage adds, and the AH stays the same)
In parallel setup: you have 12V @ 14.4AH (voltage stays the same, and the AH adds)
In both cases the actual power available is the same. Power is calculated by Volts times Capacity. So.....
24V x 7.2AH = 172.8WH
12V x 14.4AH = 172.8WH
Same power for both configurations. It all comes down to how you design it, and what application it is intended for.Comment
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Thanks again for all the help. Between the stickies, this thread, and Youtube I'm starting to learn just enough to be dangerous... well, maybe not quite, but getting there.
I think my next step will just be to buy a few components and do some home tests, then field tests, then -- real world application. Is it better, more economical, more efficient, etc to buy a premade panel such as this:
Or buy a bunch of littler ones on eBay and tab them together creating a larger array. Personally I think this option is more work because I'd need to build a frame of sorts and have a protective layer over the cells. However, if you haven't found out by now, this is hardly my area of expertise.Comment
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Now the fun starts!
Well, personally, I wouldn't go for the whole solar cell tabbing thing as it is a lot of work! Plus you have to ensure that it is sealed correctly, that you don't break any tabs etc etc. You may also find that the quality of the cells may be low as well, causing future headaches. So ya, IMHO I would rather go with a pre-made panel (the other small ones you were refering to are cells, that when combined togethor make a panel). It may also be easier and better for you to go with a pre-made panel, as you are still starting out (plus it is less headache!!!).I think my next step will just be to buy a few components and do some home tests, then field tests, then -- real world application. Is it better, more economical, more efficient, etc to buy a premade panel such as this:
Or buy a bunch of littler ones on eBay and tab them together creating a larger array. Personally I think this option is more work because I'd need to build a frame of sorts and have a protective layer over the cells. However, if you haven't found out by now, this is hardly my area of expertise.Comment
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Be careful of what you believe on Youtube, as there is a lot of misinformed people speaking about things they really shouldn't!! A lot of it is hype, scams, and ignorance.....with very few videos that have actual facts and information that is safe to use. It is better to do the research yourself, and make sure that it will work, is designed to work, and won't cause you to go bankrupt and/or death!
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Again, thanks for the assistance all. Everyone here has been very kind and knowledgeable. As I move forward if I run into problems I know where to ask for help!Comment
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Right - It will be interesting to hear how it all turns out.[SIGPIC][/SIGPIC]Comment
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