I bought a used m51 pronto sure step powerchair. The powerchair has no batteries or charger. I would like to buy a used charger and get batteries. Also where do you plug in the charger there seems to be a spot for a plug-in at the joystick and also in the back of the chair.

8 flashes is a Controller Fault

https://www.manualslib.com/manual/723349/Invacare-Pronto-M50.html?page=25#manual

https://www.google.com/search?q=m51+pronto+sure+step+flash+codes

Check wiring or control switch and check fuses agaain

Error Code 9 is a "Communications Error". The controller, or "Little black box" down inside the chair is not talking to the joystick, etc. First, check all the connections. Unplug, examine and clean all of the connections going into the controller, and the joystick connection too. Inspect all of the cables for damage to the wiring. If all looks good, then you likely have a bad controller....ouch. These are very expensive to replace new. You might be able to find a used one on Ebay, etc., or you could be better off looking on sites like Craigslist for a complete powerchair replacement. I have found near-new chairs for under a thousand dollars. Search for Pride Jazzy chairs too...a lot more of those for sale than Invacare chairs. Good luck.

Have you had the batteries load tested? How do you know the batteries are good?

why did you change the batteries, did you verify that it wasn't the charger in the first place, sometime the charger will appear to charge but actually isn't. put a meter on one battery then plug in charger if voltage goes up charger is good if not look for loose connections and or bad charger.

Battery is shorted or may have cristlized replace battery. This will solve your problems.

Jhester Well You just need new batteries. Does the chair come on when you diconnect the charger and does it move? If you don't have all the battery level lights lit up when you turn it on then the batteries are too low for the charger to pick up. No lights no power New Batteries Hope this helps

Jo, nope you are safe. Most of the chargers are designed to cut off and go into trickle charge once they sense that the batteries are fully charged. The problem with overcharging comes after 18 hours or so. You should never leave the charger plugged into the chair for more than 24 hours. On the M51 the charger is built into the chair so just plug it in when you go to bed and unplug it when you get up and you'll be ok. You can check for full charge by looking at the charger there should be a red light and a green light when fully charged. Hope this helps

Margie,   Yes you may charge it overnight. If you use it everyday,charge it every night. Once the charger senses 24 volts it stops sending a charge,so they cannot be overcharged.  I suggest when you go to bed put on the charger and unplug it when you get up in the morning.

Most likely you need new batteries. On the M51 powerchair the batteries are located in the center under the seat. Step 1 - remove the seat by pulling up on the swivel handle while lifting straight up, the seat weighs about 50 lbs. After removing the seat remove the center plastic cover by liting it off.
If you have an older model M51 the batteries will each have their own cover - to replace the batteries you will see several connectors that must be unplugged. The front and rear batteries each have different connectors so keep them separate. Lift out the batteries and follow the wiring for each replace the batteries. You will need 2 U1 batteries, available at sears or your local maedical equipment supply store ( Interstate has them too) Simply reverse the removal procedure to replace the batteries making sure you follow all the connections. - good luck
Tom