Battery is fully charged but car barely starts

I suggest as a starting point you check the battery is fully charged and if it is check the cables and connections to ensure the full current is reaching the starter. If the battery is not fully charged, I suggest you recharge it and have it tested. If it is ok the alternator charging rate is suspect.

alternator not charging or voltage regulator problem
13 volts is normal good fully charged battery voltage and charging voltage is 14.5-14.8 volts dc

13.85 volts is barely sufficient and would indicate an alternator fault if the reading was taken "off-load"..

An alternator is not designed to charge the battery and is incapable of doing so because the terminal voltage of the battery quickly rises to fool the voltage regulator into believing even a discharged battery is fully charged and will quickly reduce the charge rate to a trickle.

The alternator is therefore designed to keep a fully charged battery fully charged by supplying all the current for the vehicle equipment and the output rating is chosen for this purpose.

The voltage regulator should limit the off-load voltage to about 14.5 volts but that is only a small part of the alternator test. A professional would apply a load across the battery equal to the rated output while monitoring the voltage and current. Most healthy alternators exceed specifications.

For a sensible in-service test when an adjustable load isn't available, with a fully-charged battery it is sufficient to start the engine, leaving it to idle and switch on all the vehicle equipment. Fit a voltmeter to the battery and wait while the battery voltage is pulled down by the load - to around 12.5 volts. Raising the engine speed to 2500/3000 rpm should cause the voltmeter to suddenly rise.
13.85 would be just about acceptable "on-load" but in excess of 14.0 volts would be ideal.

Are you sure the battery is good? Test it for cranking amps? Check specific gravity of electrolyte?

A fully charged battery should start the vehicle - now if a good battery loses its charge then some circuit is discharging the battery when car is not being used. Need to locate the circuit responsible and take action to remedy.

Your battery should read about 13.2V fully charged. Get the actual battery checked and replace it if necessary.

Hi as I am not there I will need to find out one thing, ( connect the battery on the car make sure it's fully charged, take the black jumper lead and connect one end to the earth on the battery, and the other end to metal on the engine as near to the starter as possible then try to start the engine) if this doe's not work the starter should be checked.

Regards Peter

well running the voltage from alternator should be at least 14 volts or a bit higher, engine off it should read at least 13 volts at battery. if alternater is less then 14 volts, it is the alternator at fault. or a loose alt, belt.

You could have a starter that draws too much amperage or a bad ground. Check your grounds at the battery cables and check that the battery is fully charged. You may have an alternator problem. If the battery is fully charged check the starting and charging systems and that will tell you your problem.

Was the battery checked before it was sold to you? Batteries will lose their charge over a period of time even when not hooked up. If you have a voltmeter it should read 12.8 volts on a fully charged battery.
Have you tried to boost the car with cables?
I suspect if the car was working OK 3 months ago it should start. Which leads me to suggest an undercharged battery.

You can take it to most car repair shops or stores such as auto zone and they can test the battery for you. Hope this helps!