Q:

How could Satyaki have defeated Karna so many times?

Misconception about the help of divine chariot. 
Since Karna fans like to cover up the facts and hide the true story about his defeats by saying that Satyaki had a special chariot given to him and a special driver like Daruka.

Satyaki only had that special chariot and Daruka for less than an hour during the Jayadratha-Vadha Parva for a short amount of time. He got it after he killed Bhurishravas and lost it after the end of the battle. Satyaki still yet defeated Karna many times.
Satyaki defeated Karna even during Karna Parva when Karna had Shalya as his charioteer.
When Karna was made generalissimo of the Kaurava army still he was defeated by Satyaki.
Satyaki defeated Karna during the day of Drona's death as well.
Satyaki defeated Karna and his son Vrishasena before Karna killed Ghatotkaca too. 
Satyaki even defeated Karna during the 12th day.
What excuses do Karna fans have for his above listed defeats at the hands of Satyaki then?
Clearly in many conditions with or without a special chariot Satyaki still defeated Karna. 
Karna also had a great driver like Shalya whose knowledge of horses was unmatched by anyone except for Krishna's: 

As that slayer of foes, viz., he of Dasharha's race, is acquainted
  with horselore, even so is that mighty car-warrior, viz., Shalya
  acquainted with horselore. There is none equal to the chief of the
  Madras in might of arms. As there is none equal to myself in weapons,
  so there is none equal to Shalya in knowledge of steeds.

On the 14th day a special chariot was provided for Karna in this as well: 

A new cat was brought for Karna also, O king, unto which were yoked
  four steeds of the best breed that were decked in trappings of gold
  and white as conchs or milk. Its kaksha and standard were made of
  gold. Furnished with banners and machines, that foremost of cars had
  an excellent driver. And it was furnished with a profusion of weapons
  of every kind. Mounting on that car, Karna also rushed against his
  foes.

So Karna fans really cannot talk about having a special chariot. Also any advantage Satyaki's chariot gave him would be evening out the plainfield as the same day Karna received a backup team of warriors like Aswathama, Vrishasena, Kritavarman etc. So why is this a excuse for Karna's defeat?

Don't try to say that Karna was under the influence of Krishna's illusions. Krishna only stopped Karna from using his vasavi dart not anything else. And this illusion only affected Karna up until the 14th night. 

During the 15th day their was no illusion yet Satyaki defeated Karna.
During the 16th day their were two times where Satyaki fought Karna. He defeated him the first time and proved to be a better warrior than karna in the second instance.
During the 17th day their were several times where Satyaki performed better than Karna (during their encounters). Here is a screenshot from Debroy's translation of BORI Mahabharata.
Now for the question i wanna know how did Satyaki defeat a warrior like Karna? The same Karna that Bhishma labeled a Double Maharatha. The same warrior that Krishna offered everything. The same warrior who Krishna called superior to Arjuna.
Drona also defeated Satyaki, Burishravas defeated Satyaki, but the same Satyaki was able to defeat Karna countless times.

A:

I asked this question and seeing people who love to keep spreading anti pandava posts and love to keep glorifying karna by spreading fake stories i have decided to make an answer to this question.
The simple answer is that the reason Satyaki won so many times is because he was a better warrior than karna, a better archer than karna and many karna fans never decide to accept this.
What to speak of karna? Satyaki even defeated Bhishma.

1st defeat of Bhishma

Desiring to do that which would bring pleasure to Bhima, Satyaki used
  his arrows to swiftly bring down the aged Kuru’s charioteer. When his
  charioteer was killed, Bhishma, supreme among charioteers, was borne
  away from the field of battle by horses that were as fleet as the
  wind. O king! When the one who is great in his vows was thus carried
  away, Bhimasena blazed, like a fire consuming dry grass.

Analysis: Bhisma failed to reign over his horses so he was defeated by Satyaki. Bhisma as a great warrior should have been able to protect charioteer (driver) but could not, and thus he was forced to flee the battlefield. Fleeing is a defeat.
Source: Bhishma Parva Section 50

2nd defeat of Bhishma

"At that instant, Satyaki, the brave descendant of the Shini lineage,
  devoted to the truth, attacked the grandfather. He began to kill the
  enemy with his firm bow and made the soldiers of your son tremble. He
  was borne on silver steeds and unleashed arrows from his firm bow. O
  descendant of the Bharata lineage! Among all those on your side, there
  was no second one capable of withstanding him.176"

Analysis: Satyaki tooled Bhisma in this fight, Bhisma did not even fight back. And in the next line a demon known as "Alambusha" has to save him (Bhisma) from Satyaki.
Source: Bhishma Parva Section 59
Regarding Karna defeating Satyaki he only defeated Satyaki on the 17th day, before that Karna never ever defeated Satyaki in his entire life.
Even on the 16th day in Karna Parva when karna was the commander of the forces he ran away from Satyaki.

At this, Karna, leaving Satyaki quickly pierced Arjuna with three
  arrows and Krishna with twenty, and Partha again repeatedly. Although
  many were the arrows that he shot while slaying his foes in that
  battle, like Indra himself inspired with wrath, Karna yet felt no
  fatigue. Meanwhile Satyaki, coming up, pierced Karna with nine and
  ninety fierce arrows, and once more with a hundred.

http://www.sacred-texts.com/hin/m08/m08030.htm
On the 17th day Satyaki defeated Karna again.

Then Satyaki, of immeasurable soul, O Bharata, placing himself on the
  side of Bhima's car, began to afflict Karna who was in front of Bhima.
  Though exceedingly afflicted by Satyaki, Karna still approached Bhima.

http://www.sacred-texts.com/hin/m08/m08051.htm
Not to mention all the times Satyaki defeated karna on the 12th day, 14th day etc. Satyaki even defeated karna on the 14th night (Ghatotkatcha's death day).
Also karna never fought Satyaki fairly, he did a group attack on Satyaki. Here is the screenshot.

