I have a SUNL SLA 135 ATV that will not crank with the battery. I bought a new UT4L-BS battery and charged it. Placed it in the atv and when I push the starter button all that happens is the brake light dims. This is the same issue I was having with the old battery. I can place my jump starter on the battery and it will crank and start immediately.

I can't find anything wrong with the wiring. It acts like the battery is just not strong enough to crank the engine. I placed an old riding lawnmower battery that would not stay charged that was in the storage building all winter on it and it even cranked and started on it.

Any ideas? Can I just place a bigger battery like a UTX7L-BS on the atv?

 

Use a meter to measure the voltage right on the battery terminals while attempting to crank the starter. Important: Measure on the battery teminals, and not the lugs/wires that attach to the terminals. What do you measure?

If the battery voltage right on the battery terminals is collapsing, then:

1) The battery is not charged.
2) The battery is bad.

If the battery voltage is staying above 11 volts while attempting to crank the starter (and your brake light still dims down) the you have a bad connection. Try to crank for 10 seconds, then feel for the *hot* connection in the starter path (the big fat wires). Don't forget the ground return cable.

If the battery voltage at the battery terminals is getting sucked down by the starter then it is 1) or 2) above.