zzoing said:
A friend is a sound stage engineer and he wants a battery to take gear on the road to supply about 1000w for 2 hours at 240v AC.
1000w for 2 hours is 2kWh of power. Depending on the efficiency of conversion by your inverter (to create the 240VAC), you might need up to 4kWh of battery to create that. For this discussion I'll assume it's around 66% efficient, so you'd only need 3kWh of battery.
Depending on the actual cells used, and the rate at which they can deliver current vs the rate at which the inverter needs it, you might need more battery than you're actually going to use. As long as the C-rate of the battery is high enough to meet the inverter's demands, then that won't be an issue. But if it has a low C-rate it'll need to be bigger.
Also note that as the pack ages, it will deliver less total capacity, and will sag more while doing it, so the bigger the pack is (beyond what is actually required), the less of a problem either of those will be.
I probably have to recommend him an inverter of 12v because 24/48v inverters are a rarer and more expensive.
But they also draw less current (1/2 or 1/4) so they are easier on the batteries.
The main thing for the inverter though is that it needs to be a sinewave type (not modified sine), or you may end up with buzzing (or worse) in the amps and such (depends on how good their power supplies are).
So i have to consider welding 50-100 18650's into a 12v pack... that's a 30s 3p battery.
If you use 30s and only charge it to 12v, I don't know of any cells that will worl for that, because they'll only be charged to 0.4v each. 30s charged to it's proper voltage (assuming LiCo, LiMN, etc) would be 4.2v/cell x 30 =126VDC at full charge. Probably not what you're after.
I think you mean a 30
p (parallel) battery, at 3
s (series), if you want 12v nominal (4v per cell), fully charged to 12.6v (4.2v/cell).
Assuming you use 2Ah/cell 18650s, and need 3kWh of battery, that's 119 cells total in each parallel group, round up to 120 for neatness. Then you need 3 of those groups in series, so you need 360 total cells.
120 cells * 2Ah = 240Ah
240 Ah * 4.2v = 1008Wh
1008 Wh * 3 series modules = 3024 kWh
If you use cells with greater capacity, (or you need a smaller pack) you'll need less of them in parallel. (assuming they can still deliver the current the inverter needs).
Also, you might want to use the nominal voltage (3.7V-ish) for the cells to calculate Wh, rather than the full voltage like I did above, which means you'd actually need 408 cells to get the same 3Kwh (136p 3s).
Another possible issue:
"12v" for most inverters is really 13..6-14.4v, as they are expecting SLA / automotive voltage levels.
They may even have a cutoff (LVC) that is between 10v and 12v, which means the cells in the battery would still have charge left (10v would be 3.33v/cell, and 12v would be 4v/cell). If that's the case, then you will need an even larger battery pack.
If you use a 4s LiFePO4 pack instead of a 3s pack of other Li chemistries, then it will much more closely match the input voltage curve expected by "12v" inverters.
You'll have to check the specs of the inverter you intend to use before you bulid the pack for it. That's your starting point, which you have to know before you can figure out what voltage the pack needs, and what size the pack has to be.
because all the batteries keep each other at the same voltage in a series,
Batteries don't keep each other at the same voltage in series, only in parallel.
