Beaware of some Chinese grass trimmers (Berlan). The choke is actually ON when you read Off!!!?

I would first take a meter and test that the battery actually charged. If that tests out fine ( should read at about 12.8 to 13.5 Vdc), I would then check all the wired connections to make sure they did not come loose.

Carb needs to be cleaned.

Start with carb adjustment. Turn out the low and high adjusting needles 1 full turn, then adjust the choke.

lawnandgarden.manualsonline.com

May be at a cost. I could not find a company support website for Berlan.

For all others with String trimmers, blowers and other 2 stroke style engines...
If no power on high, tends to bog, won't run once choke off, check these:

• Fuel Filter - It's IN the Gas tank; Just replace it
• Spark Arrestor at Muffler, it's a screen that tends to clog up and causes very restricted exhaust flow
• Spark Plug - again just replace it
• Carburetor, clean or install rebuild kit
• And adjust sorta hidden High/Low carburetor adjustment screws

Take a look here...

And here:

And one more...

If your unit has one, it would be in front of the carburetor. Some Ryobis and Homelite trimmers do not have an air filter on them, just a screen inside the choke cover.

• With the trimmer placed on a level piece of ground press the primer bulb about 10 times.
  
• Open the choke.
  
• Press the throttle and lock it in place with the lock button
• Pull the start cord until the engine catches. It doesn't matter if it cuts out again.
• Close the choke, and if the engine did cut out the first time, pull the start cord again. This time it should start and you can start trimming the grass. If it didn't cut out just close the choke and you're ready for trimming.
  

You have a few possibilities. Make sure you have good fresh gas in the tank. You're not getting enough gas, so choking helps get more gas than air, making it run better. Could be the carb diaphram (not very expensive if you replace yourself), when they get old they don't perform like they should. Could be the filter in the gas tank is bad, plugged. Also might be just an easy adjustment on the carb Low, and High speed screws. The choke has three positions, full choke, partial choke, and no choke, which means run position.

if this is a 2 cycle it sounds like you mixed it too rich.

IF IT HAS A HIGH SPEED SCREW TURN IT COUNTER CLOCKWISE TO RICHEN THE HIGH SPEED CIRCUT. ONLY NEEDS TO BE TURNED A LITTLE AT A TIME TILL IT SOUNDS RIGHT.