Whereas how many Lfp batts do I need. More math so 3,2v 300ah X 4 in series = 12.8v @ 1200ah/4=300ah?? Rossw Help! I am so used to 12v batts with 6 cells add up parallel halve in series.
LFP is nominally 3.2V/cell, so 4 in series is a "12-v equivalent". 8 in series for 24V, and 16 in series for 48V.
Being SLIGHTLY higher in voltage reduces the current slightly - eg, in my old AGM days, 24 series cells gave me almost exactly 48V, so a 1KW load took 23.2 amps from my batteries (including losses).
With my LFP, sitting at 52V, that drops to 21.3 amps. It may not SOUND like much, but it's a 10% drop in current - which is immediately a 10% increase in amp-hours capacity if you stop and think about it!
Running series/parallel with LFP is exactly the same as with lead-acid, except the higher cell voltage means you only need 2/3 the number of cells to get the desired voltage.
Rossw if I use 8 3.2v 300ah lfp in series to get my 24v (25.6 nominal) to the inverter(setting my PJ to 26v?) what's the ah for just that set?
8 x 300AH cells in series is still only 300AH
I would still like 2 batt banks for maintenance and reserve purposes.
Yes, that's still a nice idea. For my money though, I'd rather have two banks run permanently in parallel, but wired such that I could (if I needed to) isolate one bank. Two banks in parallel makes it easier to charge them, and reduces the depth of discharge on each, giving you better battery life.
If I could get 10 years out the batteries thru constant upkeep and attention thru monitoring that would work
10 years from AGM is probably a big ask IMO, unless you really mollycoddle them. It's not impossible, but it's a big ask.
The LFP are pretty much guaranteed to still hold 80% capacity (or better) after 7 years of discharging to 85% DoD (ie, only 15% left in them) EVERY SINGLE DAY. Your typical AGM probably won't last 6 months under those circumstances.
The trick to long life with lead-acid is to not discharge them too deeply, too often. The downside to that is with a bank big enough to only take them down 20-30% DoD (maximum), is a very big, expensive and heavy bank, that will also take a lot of power to charge properly - which means lots of PV (or generator), big wiring, beefy charge controllers etc.