My mk-315 3 bank on board charger always has a solid red light and flashing green light on battery #3. I have misplaced the owners manual and can not locate a replacement or I'm not looking in the right places. The lack of a full charge on my cranking battery recently cost me a tournament. Iam getting frustrated.

Here's a link to a bunch of replacement chargers....

https://www.google.com/search?q=Invacare+M51+Surestep+replacement+charger&newwindow=1&sxsrf=APwXEddmA91gRXeJPK8JzJf9dHNyEAAvMQ%3A1682171313277&source=hp&ei=seVDZLTIDv3OkPIPkd-S8Ag&iflsig=AOEireoAAAAAZEPzwSyXMv34ozadubH2bPfwxcQtlSi0&ved=0ahUKEwj05M680L3-AhV9J0QIHZGvBI4Q4dUDCAs&uact=5&oq=Invacare+M51+Surestep+replacement+charger&gs_lcp=Cgdnd3Mtd2l6EAMyBQghEKABMgUIIRCgATIFCCEQoAE6BggAEBYQHjoICAAQigUQhgM6BQghEKsCUABYglVgkFpoAHAAeACAAe0BiAGGGJIBBjAuMjAuMZgBAKABAqABAQ&sclient=gws-wiz

Best to you

When the light is solid red, the batteries are charging.
A flashing red LED light indicates that the charger detects abnormal batteries or the battery temperature is abnormal.
When the light is a solid green, the batteries have completed their charge.

I did not notice code list but here is a manual, hope it helps.
Plug the battery charger into a wall outlet, then into the charger port on the side of the scooter. The indicator light on the face of the charger will turn red, showing that charging has started. When charging is complete, the indicator light will turn green. A full charge will take 6-8 hours, however if the battery is not empty charging may take much less time. Always unplug the charger after use. Always store the scooter with the power switched turned OFF
http://www.currietech.com/dealers/wiki/images/a/a0/Owner%27s_Manual_-_EZ2_Nano.pdf

download manuals
https://www.yumpu.com/en/document/view/37650434/download-owners-manual-currie-technologies

Throw away

I agree that the pictographs are not very helpful. You probably have one of the new chargers. I have an old one, which is a little easier to figure out.

From top to bottom

1. The flashing green light with the electric plug indicates that the unit is plugged in and ready to go.
2. The flashing red little fan means that the battery is too warm so it is just cooling it and not charging it at that moment.
3. The solid red empty battery outline means that it is charging but hasn't got very far, less than 80% full.
4. The mostly full battery outline with both solid red and green lights at the same time means it is 80% full or more
5. The full battery outline and solid green with the musical notes means it is full up.
6. The broken battery outline with flashing red and green means you've got a broken battery.
7. The yellow light with the wrench means the charger is "conditioning" the battery, whatever that means.
8. The yellow light with the fan means "cooling abnormality."

Hope this helps

is this a 3 or 4 bank charger I had the same problem but my battery was bad if you don't have the charger on the battery it goes to you will have a red light if your battery are hooked in series to check your charger disconnect the cross over wire and put charger on battery see if red light goes out if so you have charger on wrong battery or you have a bad temp.sencer in charger cord on # 1 feed

according to manualslib type site for the wg540, solid red means charging for the wa3840 and wa3838. After 30 minutes if battery has not reached 10 volts (of 18), the red will begin flashing meaning defective.
Just green? Ready to go. Green flashing? Overheat.
Tangent; sometimes a discharged battery needs a little coaxing - have to get a little voltage into the battery for charger to try to charge. Different company, electric drill. I plead ignorance if this applies for the 3840 since they mention the half an hour time.

If the red and green lights are flashing, the charger is detecting the battey has a defect and is not safe to charge. It could be one of the cells in the battey, a short in the wiring or circuit board or bad contacts. Usually the only fix is to replace the battery.

Ridgid charger lights read as follows: Solid red; charger has power. Flashing red; charger is evaluating the battery, checking if it's cool enough to charge and if it is in chargeable condition. If the battery is hot, i.e. just came off the tool after extended use, it won't charge the battery until it cools down. Flashing red & green; the battery is defective and the charger will not charge it. Solid red & flashing green; battery is charging. Solid green; battery is full.

it doesn't change to green, the solid red light means it is fully charged, there should be a legend on the front of the charger to help you with this. If it flashes real fast the battery is malfunctioning and slow flashing is charging and again solid is fully charger.