I have seen conflicting opinions wrt connecting LFP batteries in series(12x2=24) and in parallel to double the Ah. Obviously I’m ignorant wrt this topic. Please advise
There is no simple answer to this.
Does it work? It all depends on the BMS put in the battery. If the FETs used in the BMS can handle the full 24V (nominal) voltage, then it can work. Check the specs of your battery and see if it can be put in series. Many
but not all 12V LiFePO4 batteries can be put at least 2 in series.
Can there be problems? Yes. Before you put them in series you will top them both off so they will be balanced relative to each other. Over time they will drift out of balance. To counter that requires one of two things: 1) use a battery level balancer or 2) Every once in a while you take them apart and top them both off again. How often. It depends on the battery and use pattern so I hesitate to give a number. It could be every few months or it could be one or two years.
Second Q is: How to calculate the battery bank energy required for a given inverter… mine is an Outback 3500W Inverter. I do not have any high surge demands over the typical skill saw and other woodworking had tools.
There are two important calculations: 1) Current and 2) Energy.
Current: The battery must be able to supply enough current to satisfy the inverter. Without considering surge, the current could be as high as 3500W/24V = 145.8A to power a 3500W inverter. This would be the minimum continuous current the batteries should be able to provide. I don't like to run things at 100% so I would want the batteries to be able to pump out at least 200A continuous
Surge currents could make that considerably higher. With woodworking tools, I would assume some surges that could go higher.... particularly if a lot of other things are running at the same time. Some batteries don't allow much surge, while others do. It depends on how aggressively the BMS decides the surge is a bad thing and shuts down.
Energy: (Amp Hours or Watt Hours) The information provided is not sufficient to answer this question in any meaningful way. This is a factor of energy used by all of your loads, not the size of the inverter.
If you have a 100Ah battery and your average load draws 10A, the battery is good for 10 hours of run time without being recharged.
If you run the 3500W inverter full-out, a 100Ah battery will drain in about 100Ah/145A = .685 hours or about 40 Min.
