I looked at LTO cells and did some maths. They are far less energy dense than lifepo4 and so I discounted them.
And like lenny. I estimate that I will be dead long before any of my lithium packs lose enough capacity to matter as long as they are charged properly.
Look at it like this.
I am about to order 8x 230Ah cells. You get around 40Ah from a group24 lead battery before the chair stops. The rest of the capacity is unavalable to use at the rate we use the battery. And the voltage drop at peak loads make this even worse as the voltage and resistance both get worse as the pack is discharged beyond this 50% point. So call a set of group24 batteries 40Ah accessible capacity as this is about all you will ever get for our use case. Even
this rapidly declines over a few months!
Thats 230Ah LiFePO4 div by that typical 40Ah from a set of lead bricks = some
5.75 times the usable range of lead. Or if you prefer the same as almost
6 sets of lead (12x MK Gels)...
Now even if you charge that LiFePO4 pack once a week, you will still have more range every day than you probably ever did on your longest trip ever with lead previously.
But you dont do marathon trips every day. So in reality you will get maybe 6000+ cycles at averagee 50% charge. Thats 6000 weeks charged weekly. Thats 115 years...
They definitely last longer than 25 years as I have a couple of headway packs that are 16 years old that still show practically full capacity!
Then consider this.
With 230Ah, even if you lost a massive
FIFTY PERCENT of initial 230Ah capacity, it will still be 300% better than a set of brand new MK gels...
Charge them properly, never discharge them too deeply (with 230Ah you cannot actually do that realistically) and they will outlive any chair or chairs you buy many times over. And you will eventually swap them out, not because they are knackered, but because by then something even better may be available.
Now lithium ion cells as used in teslas, and laptops etc have much less cycle life. 200 to 800 depending on type. So to make thoose last an acceptable time in a tesla, only 80% of available real capacity is used. (+10 to +90% typically). That has a huge effect on service life on those types of cell. But that means that a pack needs to be around 20% oversized and full capacity is not used. LiFePO4 behaves the same but starts out with a 2000 to 3000 cycle life anyway.
and backups matter!