18650 power wall build

Hi everyone, this is my situation. Im new to the power wall build and have an issue. I have multiple sources that I reached out to to build me a 300ah power wall. I need a 12v bank that will be changed by solar and connected to a 2000 pure sine inverter. I keep getting this question from the builders and have no idea how to answer the question. I cant get a quote till I answer. Can someone please help me out.
My inverter max continues is 1800w and surge is 3000w. Input V is 10.5-15.5.

1. what max discharge and charge current do you need?



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Your max discharge will be determined by the inverter. So, at 3000W and 15.5V, that's 193.5Amps.

However, I doubt you'd be running at a full 15.5V. Going with 18650 cells, your nominal voltage will be 3.7 * 4s = 14.8V. So, at 3000W that'd be 203A.
Running with that kind of configuration, you would need a lot in parallel to be able to handle the surges from time to time. So, at 1A max per cell, that'd be 200 cells in parallel per pack.

Now, that doesn't mean you have to make a pack that large. You could check your power usage and see that you only ever draw a max of 2400W in a brief moment as the fridge, or washer, or what not kicks on. So, if 2400W is the max, you'd only need to build for 162A draw.

If you haven't bought the equipment yet, I would highly recommend going with 24V. Or, if it's possible to buy a 24V inverter, go that route. That cuts your amp draw in half. So instead of 203A/162A, it'd be 101A/81A respectively of what I quoted earlier.

Charge current is not based on inverter. It's based on what the cells can handle first and foremost. Then it's based on your charge controller. You can't charge higher than what your cells can handle. Different cells manufacturers and chemistries have different requirements. You must check the datasheets to find out what the max current input is for those cells.
With that said, if you build a pack that can handle 100A discharge, and you have 100 cells in that pack, each cell with discharge/charge at about 1A each if you charge with 100A.

What ever your input charge/discharge current is, it will be divided by the number of cells you have in the parallel cells, not series cells.

I need a 12v system it for my RV. My inverter is 2000w. I will not be running 2000W at a give time I would say max 1000W with my heater or ac on. Also the surge from the microwave or refrigerator like you said. Think you could calculate that for me. [emoji16]


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I can't calculate it as I have no idea what the specs are on your devices. That's something you gotta figure out as that can even differ from RV to RV.

To get the most accurate readings, I would take a shunt and put on your current battery input and measure the amps being used. Or, alternatively, you could be a 100+Amp ammeter in line to do the same thing. Then just monitor it over a period of time when different devices load up and such. The advantage with the shunt is that all the data is logged. Using the ammeter, you have to physically watch it when things come on and take note of the measurements. Plus, the shunt is far more accurate especially with surges as the ammeter may not see some surges as they happen to quick.