What does the battery of a tractor have to have if the tractor is a diesel, when trying to crank it?

I believe you mean 12.5 volts, or at least hope you mean volts. Although that would indicate a full charge, it is amps that give the battery the umph to kick the starter over. A typical battery is rated between 550 and 850 cold cranking amps. The higher the amperage the more strength the battery is able to put into the starter, and the better at overcoming issues like worn brushes in the starter and corrosion in and at the wire connections. I believe if you were to load test your battery it would fail. Make sure all contact points are clean and free of corrosion. Make sure the battery cables are in good condition without corrosion inside of the insulation. And make sure the starter is not pulling too many amps and your tractor should start fine once again.

Probably starter or a discharged battery. Try charging up the battery first. Anyone with a voltmeter could check the battery condition. If it is at 12.2 volts or higher, it should turn the engine over at normal starting speed. If under 12 volts, it is discharged and maybe too low to crank the engine over.
If your battery is at full charge, you probably need to pull the starter off and have it tested for free at any parts store.

What is year--make--model?
Have you done any testing for the charging system? If you use a digital multimeter across battery posts, everything off, the generic spec for a full charge is around 12.6 volts.

If it cranks good, but, won't start, check for fuel--spark--compression, where applicable, a diesel doesn't use spark plugs.

Compare cranking speeds cold to warm most diesels require about 375 RPM's cranking to start. NOW possibly a bad starter (dragging) voltage drop (high resistance) in cables. loose or dirty battery cable ends. To test voltage drop requires 2 people one cranking engine other watching volt meter. Place one meter lead on battery cable end and other lead on the other end of cable. Do this to all 4 battery cables. What voltage that can not go through cable will go through meter. So the closest to zero is the best. good readings are .02 volts for connections and .05 volts per foot of cable.

no crank!, if it cant crank, it can never start.
can be many thing.
1: dead battery or dirty terminals.
2: discharged battery (charge it or try test known good battery)
3: bad starter system.
Every way, possible , is here
http://www.fixkick.com/nocrank.html

ARE YOU HAVING PROBLEMS STARTING? WHAT IS YOUR CONCERN? 13.8 VOLTS IS PRETTY GOOD FOR DIESEL ALTERNATORS I'VE SEEN IN MY EXPERIENCE WITH THE DODGE DIESELS

First check your battery get a cheap digital volt meter. Free at Harbor freight with a purchase. Fully charged battery 12.6 volts Not 12.5 or 4 or 3. With the volt meter set on DC volts touch the battery and have someone start the car if the meter shows high 13.5 to 14.7 the alternator is ok if it is less replace. If you have a dead short take out every fuse and relay from the black box under hood by the battery. Remember where they go and plug them one at a time until the discharge hits and at least you will know what circuit it is.They also sell about 20 dollars a plastic meter you hold over the positive battery cable to see if the starter is drawing to much power A battery open-circuit voltage test will show the approximate state-of-charge of a battery. This test can be used if no other battery tester is available.

Before proceeding with this test, completely charge the battery. Refer to Standard Procedures for the proper battery charging procedures.

1. Before measuring the open-circuit voltage, the surface charge must be removed from the battery. Turn on the headlamps for fifteen seconds , then allow up to five minutes for the battery voltage to stabilize.
2. Disconnect and isolate both battery cables, negative cable first.

Testing Open Circuit Voltage

1. Using a voltmeter connected to the battery posts (see the instructions provided by the manufacturer of the voltmeter), measure the open-circuit voltage.

See the Open-Circuit Voltage Table. This voltage reading will indicate the battery state-of-charge, but will not reveal its cranking capacity. If a battery has an open-circuit voltage reading of 12.4 volts or greater , it may be tested to reveal its cranking capacity. Refer to Standard Procedures for the proper Micro 420 battery test procedures.


s than 13.5 alternator is bad

battery should be showing 12,45 - 12.6 volts when car is standing still. also batteries getting old colects sulphate on plates ,this degrades the cold start performance. you can see that cranking amperage on stickers of batteries. for example 90 amperes 750 cca battery is required for my car but les wont crack it even it is 90 amperes but 640 cca. mainly lead acid or dry type batteries needs to be charged more than %40 of its capacity. 12 volts is only the ~%25 of its capacity and it seems yours is dead already.

Make sure battery has a full charge. Use a digital multimeter across battery posts, everything off, what is the reading? The generic spec for a full charge is around 12.6 volts. Note the reading. Leave the leads touching the battery posts, have helper turn key to crank, how much does the reading drop? Usually it shouldn't drop below 9.6 volts or there about. If it drops to zero, try charging the battery. Check battery cables. Did you try jumper cables to see if it would crank?

probably one or both or your batteries are low or shot, check both of them. Hope this helps-Jim If water in them is low, refill with distilled water, and then charge them. If it still doesn't start, you may need a replacement.