My battery tender is an older model and for sure does nothing but charge, and even that is rather lackluster to say the least. I've put a voltmeter across it while charging, and it never gets close to 14.5v to properly charge a new battery, and in storage mode stays up about 13.4v which is a little too high to keep plugged in for weeks on end.
I also have a Pulsetec charger that is supposed to have desulfate, and I can read on a meter the volts pulse up to a higher level. I guess this is supposed to desulfate. I haven't had a weak battery that either charger improved performance, and have decided keeping hooked up all the time neither charger extends battery life, actually I believe battery life was less.
I still like my old 2A Century charger I bought 30yrs ago at Wards works better, but the best charger is a 0-5A adjustable lab power supply. You can buy various Chinese units for $50 or so. Dial in your own amps and volts.