As those who are off grid know, the first sign your batteries are going into terminal decline will be the occasional beep of the low voltage alarm on your inverter....progressively getting worse until you have to replace your bank. This usually shows up when the pressure pump kicks on or fridge compressor tries to start and because the batteries are older and worn they just cant source that millisecond need for a large enough punch of amps to turn over the motor anymore. Most people try to limp along for a bit to postpone the cost. I think I know someone who limped along for over a year like that, maybe 1.5yrs, finally couldn't take it anymore and bought a new bank. The thing is that is often the prime reason for replacing a bank...just motor starting problems.....and thats not really a problem.
You can extend the life of a old bank by quite a bit if you can add capacitance to the input of the inverter. Now inverters dont come with the kind of capacitance I am talking about...I mean capacitors bigger in diameter than your wrist, or fist. If they had that much capacitance they would be twice the size and 500+ in cost and frankly no engineer in todays competitive enviroment would build such a thing where the only advantage would show up 6 plus years (likely more) in the future, and only when starting large loads.
You on the other hand can add this capacitance for little to no cost to your inverter when the time comes to extend the life of your battery bank. I have been hauling away surplus scrap metal to the scrap yard.....The scrap yard is where everything goes to die..metal and electronics, industrial equipment....everything can be found there. Expensive equipment filled with good parts. The pick below is about a hours work, easily 1000+ dollars in good used parts in that bin, but of interest is the capacitor bank....thats over 120 000 uf of capacitance, complete with buss bars that i will just put away until the day comes that my batteries need a life extension.

I googled the part number on those caps and they retail at over 100 dollars canadian each!
Cost was my time and a box of timbits, I showed up just before lunch (best time). Pro tip here...show up in steel toed boots,and vest, look like you belong in a junk yard...that helps a lot...Make it clear you will just stay for a hour, that way when they come back from lunch they don't have to worry about you being in the yard while they drive around with heavy equipment, Not every place will let you help yourself due to liability concerns so when you find one that will treat them well!
anyway, you dont have to open up your inverter, you just hook up the capacitor bank right at the input terminals of the inverter. Its just that simple...
Think of Capacitors like this as Viagra but for lead acid batteries....They will keep your DC buss stiff (er).
(I will replace the aluminium buss bars with copper, just cause I can)

Now keep in mind I am a unlettered rube....my opinion is 100 000 uf is the minimum needed to make a real difference at the input . If one of those slide rule types says more or less, go with what they say...I always defer to those who have earned the ring.
I have only earned the right to supper size my fries, and claim no other knowledge than experience has given me.