jek10 said:
You all have great information that I learn today. I'm sorry my questions was so noob, I'm just new in this stuff and I have Idea but I don't know where to start.
My idea here is to create a modular battery pack/power wall connected to solar panels that can power small devices like lights in house and usb devices maybe 250 or 500 watt per module coz here in my country we don't consumed much power per day. If I want more capacity I will just connect another module.
I don't know how to arrange those 18650 Battery if I want that capacity and what voltage configuration is ideal and efficient for home, i know there's also an inverter needed, charge controller and BMS. But I need to focus first on Batteries.
500 watts? Holy cow! I rarely need 500 watts of power!
Let's use 500W at 12V with Lithium Iron Phosphate (LFP or LiFePO4 for short)cells.
NOTE: Your constants and variablesprobably will be different, unless you're using the same cells that I'm using.
LFP cells have a nominal voltage of 3.2 Volts Per Cell (VPC), so for a 12V system you need 4 cells in series for 12.8V. For LFP, you should go no lower than 3VPC, or you go into the steep part of the discharge curve, where falling over the cliff can damage your cells. That means that you have a 0.8V drop before you're in danger when your SOC is at 3.2VPC. If you want 500W of power, Ohm's Law gives 500/12.8=39.1 amps, so that's how much current you need. Now if you have 5Ah cells and the recommended C-rate is C/5, you allow yourself 1 amp per cell, so the simple answer is that you need 40 cells in parallel, giving you 4s40p.
However, you aren't limited to C/5, and if you want a modular system you might not want to design it for C/5. You don't use a lot of power, so you're probably not using 500W all day. And actually your LFP cells can deliver 1C without a problem, as long as it's not a full time job. If 1C is the maximum power that you'll need once in a while, now you only need 40/5=8 cells in parallel (4s8p), a 12V 40Ah battery.
But we have to consider your LVD(low voltage disconnect) of 12V, which is dependent on the complexresistance in your battery. If you want to allow yourself a 0.8V drop at 40A, you can only have 20m? of total resistance in your battery. Let's see if 8p works, and for this you need to understand resistance in parallel circuits. Rtotal=sR/p, where R is the resistance in each branch, assuming they're all identical. Rearranging the equation tells you, p = sR/Rtotal. Now, the manufacturer claimsthat each cell has an internal resistance of about 12m?, and a Powerlab hobby charger comes up with something close, so let's go with that. Also, you'll have contact resistance and hopefully fuses, so let's bump that up to 40m? per branch. p=4 x 0.04 / 0.020=8 cells in parallel, for a rough estimate, which agrees with the simple calculation. So far so good. However, the manufacturer and your Powerlab might not agree with reality. This is only an estimate. You really need to take somemeasurements and to experiment. It's not an easy question to answer.
Do you understand my explanation?
By the way, that was a quick and dirty calculation, and it only looks at Power requirements. Usually when designing a battery bank you start with daily energy usage reuirements. The example of a 12V 40Ah battery only gives you about 500Wh, not accounting for the 80% DoD rule of thumb, which gives you 400Wh. For 1kWh, you'll double it to 4s16p, but that only gives you 1 day of autonomy if your charge controller breaks. The rule of thumb is 3 days of autonomy, for various reasons, so you'd triple that, giving you 4s48p. That assumes that you really need 1kwh daily AND that you're using 5Ah LFP cells which you're probably not.BUT that doesn't include charging efficiency, inverter efficiency, etc., etc. There are many variables to consider in designing a system.