Have a battery charger with 2 amp. or 12 amp. setting and switch for maintenance free or alternate type. What do I charge my 21 hp craftman mower at? How long should it take for full charge?

Question edited for a lot more clarity. All questions need FULL
'make model (what is is)'.

There is no manual for that particualr model

https://www.google.com/search?q=napa+85-437+battery+charger+manual

Looking at the picture the meter is pretty standard on good chargers and shows the amps going into the battery. As the charger is smart, it reduces the charging amps, so on a 6 amp setting it will show 6 amps at first and slowly drop.

My own charger has a switch for 6 amp charge or 2 amp trickle charge. The meter is dual so one end is high 6 amp charge, and the other end is high 2 amp charge. When fully charged, the needle sits near Zero (0) in the centre like this

amps 6. 5. 4. 3. 2. 1. (0) .5, 1, 1.5, 2 amps

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NI-MH batteries a re very different than car batteries and even tho voltage may be similar, you risk destroying your battery, camera or worse, an explosion. Your batter may only need a limited amount of amperage to charge (likely not much more than 1500ma or 1.5 amp) a car battery charger may put out twice or even ten times that much - hence the explosion risk! Also these kind of chargers detect when your battery is full and will quit charging or switch to a maintenance mode. I'm afraid you're pretty much stuck with using the charger meant for it.

not exact, to charge apropiate needs 10% more than nominal voltage. if not, will not charge the batteries.
And the charger its very important because you can over charge the battery.An example,if you have a 12 Amps Batteries each one if you put it in series will be 24 Volts.at parallel conection will be 12 volts but the double of Amps.
At serial connection will be 12 Volts the amps will be the same but the volts will be 24.

The charger at 12 Volts must be 13.8-14.5 DC Volts and divide the Amps in Hours.
The charger at 24 Volts must be 26-28 Volts. to charge well.

The charger for 12 amps batt example must charge 2 amps per hour at 6 hours = 12 Amps.(2 Amps Charger by 6 Hours Charging= 12 Amps.(Fully Charged.
3 Amperes Charger needs 4 Hours to charge 12 amps Batt.
5 Amperes Charger needs 2 Hours charging an a quarter
All of this is at DC Current.

Luck.

If the battery is below around 11.8 volts it never will charge. The charging systems on these mowers are made to "maintain" the charge. Check the battery, if below 11.8 volts charge the battery with a battery charger. Then see if the mower will maintain the level. If not, you may have a bad battery. Your local automotive store can check it for you.

First thing i would do,check the water level in the battery,don't have to on a " Maintenance Free " battery. Remove both cables,from the battery,clean the cables with a wire brush,especial made for this.There can be a small film of acid oxidation deposit forming,that get into the cable clamps and the battery posts.This causes the cables not to make good contact with the posts. There is something else you can do and that,is use a portable battery charger. Try to charge up the battery with that,this would eliminate the tractor's charging system causing the problem. If this battery won't take a charge or hold a charge,the battery is bad.Hope this helps.Sincerely,Brad Locke DTD 12/04/15

I have used a DieHard portable battery charger (model 200.71222) for several years to recharge my 24-volt, 18-year old Craftsman mulcher, model 315.370270. This is after the on-board charger died. I just disconnect the link between the two 12-volt batteries and remove all other battery connections. Then connect the charger leads to the positive and negative terminals on one of the batteries before plugging in the charger. I put the settings on the charger at conventional low maintenance and 2 amps. Charge one battery and then the other (assuming you have two 12-volt sealed lead acid batteries like my mower). I use binder clips from an office supply store to hold the battery wires on the terminals when I'm mowing...makes it simpler to remove and reattach for charging instead of the nut-and-bolt connectors that came with the mower.

I would say so. Recharge the battery and load test it. It probably will say to re place it.
You can have a auto repair center load test it

The electric clutch needs full power in the battery to stay engaged. I would recommend you check your charging system before replacing the battery. Put a digital volt meter across the + and - terminals of the battery and while running at about 1/2 throttle you should be over 13 volts DC. Probably around 13.5 or so. If not you have a charging problem. Lawn mower batteries I have seen seem to last only 2-3 years. I would also recommend a trickle charger for extended periods of non use. Don't forget the Stabil also.

Hope this helps.

it could be a fuw things doing this,,,
check that the engine is charging the battery up when its running,,use a volt meeter across the battery it should show 13.5 volts if its charging up ok,,,,

then check that the voltage regulator is not draining the battery when the engine is not running,,,

fully recharge the battery first then check that it holds this charge,, take off the battery lead and leave the mower for a week or so if the battery has gone flat then you need a new battery!! winter time kills off batterys if there not kept on trickel charge,,,
there's a tip,,,,buy a solar charger,,,and hook it to the battery when your not using the mower,,it will keep the battery toped up for free???

For charging a car battery that is run down, use the 6 amp setting. The 2 amp setting is for smaller batteries (like a motorcycle or lawn mower battery) or if you are using it to maintain a charge on a battery that is used infrequently (like a boat battery).