ramming a hefty charge like that into a Lead-Acud battery, is a sure fire way to shorten it's life. Normally, I'd expect the charge rate to drop significantly after ~5 minutes.......the battery voltage rises quite fast, opposing the alternator output and thus limiting the charge-rate.....you could always stick a voltmeter across your battery to verify it's voltage "12v"...~14.2v. (double for a"24v" system ) If you've "flattened the battery seriously, to the point where it's struggling to crank, i'd recharge from an external battery-charger with a long low trickle- charge. A ballpark rate is 10% of the battery's nominal capacity....so, a 38 Amp-Hour battery should have 3 1/2 to 4 amps for about 12 hours)...it should start a bit lower, build up to it's peak rate after a couple of hours and then tail-off as the battery becomes fully charged.
heavy charge-discharge cycles take their toll on battery life!