Battery Power for Lawn equipment -- is it time?

third year with the ryobi lawn tractor. batteries holding up well, but in the wettest spring we've had in awhile, i had to mow half one day and half the next to keep them from running down too low.
researching lithium replacements, someone makes a kit for $1,000.

 

My daughter has a black and decker cm1836. I got it for her in mid 2009. Still works well on original agm lead acid batteries. Has excellent power and is easy to use. I tell her those batteries won’t last forever and since they probably aren’t available from b d, lets think about getting a plan for a new much more expensive electric mower.
A couple weeks ago someone put out the same mower on the sidewalk for free. Everything came with it. I looked up batteries after taking the cover off and seeing what they are, and found lo and behold they are common, used in many things like electric scooters. Got a new set of three which gives the 36 volts for under $70 with tax. Got from home depot online but amazon has them. Home depot has better return policies.
So I find that interesting as buying new lithium ion for mowers and tools can easily be more than $70.