>charge. Your Battery Tender appears to charge at 24V 2.5A, which gives 20Ah
>in 8 hours and 30Ah in 12.
I have the 3 amp, 12V, unit. This brand is reasonably "smart",
and basically pumps out (a nominal) 3 amps until it detects
the battery is almost charged, then [some smaller amount]
until 99%, then trickle charge to keep topped off.
A related issue is that... when the battery is dead, dead,
and the Battery Tender doesn't "see" enough voltage, it'll
refuse to send any charging current.
One hobby I had in my former area was going to [big facility]
auctions, where they were selling the UPS's for $5 or so.
About 3/4 of them were ok and only needing a battery swap.
And on about half of those... simply removing the old
battery and hooking it up to a "dumb" charger, so as to
bring it up to (a nominal) 10%, then got the UPS itself
to bring it back up to, say, 50% of factory new capacity.
Which is more than sufficient for plenty of my uses.
The math isn't exact because lead batteries
>charge inefficiently and draw less current as they near full charge. 2.5A is
>a reasonably safe current if left connected for a long time, like a battery
>maintainer, but not a fast charge.
>I can't recommend any particular charger because I build my own, with volt
>and amp meters so I can fast, slow or taper charge and manually correct for
>battery type, temperature and condition. I made my 24V 20A charger from an