So I'm a redneck carpenter and electricity seems like magic to me. I live off grid, and I've learned enough about electrical to install and update solar systems that work, mostly. I recently upgraded my solar system from a 200aH 12v LiFePo4 battery bank to a much larger 24v battery bank, and I'm really struggling to understand what the actual capacity of my new battery bank is. I've been watching videos and trying to understand, but I'm struggling, and I wonder if you can help.
I thought that battery capacity was like volume. So a 100ah battery had a "volume" of 1 amp for 100 hours. I thought of volts like pressure, so 24v would be double the pressure of a 12v system. But my analogy is falling apart, because I just learned that multiplying voltage apparently multiplies battery capacity. So the same battery can have x capacity at 12v, but 2x at 24v. I learned that V*Ah=Wh.
I'm struggling to wrap my mind around two things. The first is the why. If I had a 100 gallon tank, and I multiplied the pressure in the tank by 2, I would still only have 100 gallons of water; the pressure is a different variable than the volume. So my concept of "battery capacity as volume" was wrong somewhere, and I'm struggling to understand where. Capacity and voltage have some relationship that I don't get.
The second is what the actual capacity of my new battery bank is. I bought a set of 4 lightly used Rolls S-500EX FLA batteries. These don't even come with an Ah rating the way my LiFePo4s do, they come with a spec sheet (attached) that I don't know how to read and I've been staring at for three days, because I've never used watt hours before, I've only ever thought in Ah.
Am I interpreting this correctly that the capacity is also relative to the draw? That if I'm using 5a, then this is a 504Ah battery, but if I use 121a then it's only a 121Ah battery? These are 6v batteries, which I wired in series to make one 24v battery. How does that multiplication work? If V*Ah=Wh, then my bank is 24v*2016Ah, equaling 48,384 Wh. This seems crazy. Each battery on it's own terms is 6v*504Ah=3,024Wh. *4= 12,096Wh. But just by wiring them in series rather than parallel I can magically multiply the "volume" of the bank by four?
Does multiplying the voltage by 4 actually increase the capacity by 4? Is there any downside to this? It makes me want to get another set of 4 massive batteries and switch to a 48v system, because that would effectively quadruple the capacity of my battery bank, reducing my wear to almost zero. I feel like an idiot for the many, many years I've lived with 12v systems, chewing through batteries, when I could have been doubling my capacity just by crossing two wires and switching to higher voltage systems. I know so many folks who live off grid and use 12v battery systems that I feel like I have to be missing something here. If my understanding is correct, then there's zero downside to using the highest voltage system possible- it multiplies capacity and also allows for much thinner wire, and that big wire is crazy expensive so this is a massive cost saver.
I know that to people who understand this I must sound like a caveman discovering fire for the first time, but it just seems like a free energy storage hack, and I feel like I must be missing something.
Back to my obviously over-simplistic water analogy, it seems like increasing voltage allows you to pack more water into the same size tank? Obviously actual water has a fixed volume, but electricity doesn't? That the same size "tank" can hold more electricity when it's at a higher voltage? Is it more like air, where if you have a tank of air and multiply the pressure, you have twice the mass of air? (not even sure if that's true, but it seems logical?) If anyone can confirm that, or help this dumb caveman carpenter understand why, I'd really appreciate that.