Drill has very little power/ even with full charge battery/ maybe brushes? thanks

test the battery with DMM-Digital Multi-Tester. when full charge a 36V battery measures 37-38 Volts. if battery is full charge the problem is your electric drill bring it to service center maybe need to replace carbon brush or the motor.

It probably is because the battery has failed, maybe you have not charged for a long period and put it away discharged, maybe put a meter across the terminals to see the voltage, even if flat it should still read 12v or not far short, if its much less than that then I am afraid you need a new battery, you can probably get one here Ryobi cordless power tool drill screwdriver battery replacement

Maybe the brushes need changing.

Brushes to the motor could be the cause or heat sensor in the battery pack it switches off or even bad connections on motor check the spades connections could be loose

That depends entirely on the charger. Cheap chargers will not shut off power to batteries even when they are fully charged. Nicer chargers will cut power to the batteries when they are charged up.

But the safe answer is yes, remove batteries once they are done charging. (And store batteries with a full charge.)

Worn brushes or wrong trigger.

Usually the problem is in your battery, they develop a memory, they will only charge to a certain point. This can happen when the battery is recharged when it is not completely dead.
The key is to let the battery go completely dead, before recharging. I will wait till the battery is dead, then tape the trigger down and let the drill run till it won't move any more.
Sometimes you can reverse the battery memory, by doing the above, but when the battery is that bad, you might need to start with a new battery, and make sure it is completely drained, before charging it.
Wish you the best of luck.
Sincerely,
Paul Gibson

Order the parts you need from Home Depo, they have what you need to fix this problem. check to see if you were using the right battery if it never worked you may need to buy another drill or go even further and open the drill up to see what the problem is inside. Check your brushes, trigger, all connections.

I used to pull these drills to bits for a living and all i can say is that if the brushes are intact and not too worn then it should run fine without cleaning the brushes. chances are that the motor has worn out.