The mid-2009 MacBook batteries could generally expect to get about 300 battery cycles before giving up the ghost. Yours is almost twice that, so likely the battery needs replacement. See this Apple article for further information.
You can purchase a replacement battery from the Apple store online for $129 (in the USA.) Other users have reported being able to buy them off of eBay for about $20-30.
Best of luck.
You can get a new third party battery fairly cheap on eBay. Every MacBook from 2006 through Mid 2009 except the Late 2008 Aluminum Unibody and the Late 2009 and 2010 White Unibodies use the same A1185 battery.
Or for $129 from The Apple Store http://store.apple.com/us/product/MA566LL/A?mco=MTY3ODQ5OTY
The vast difference in numbers between the Full Charge Capacity
and Charge Remaining, is part of the clue that it's not going to get
better, and sometimes the next step is a black X in the battery icon
area of the computer's menu bar.
I have a few otherwise new batteries that say low cycle 40-60 count
but I know they both are bad. Look very nice, but are under 80% of
the original and won't run the iBook G4 mid-2005 for long at all.
Part of the reason probably is the vintage had different technology
in battery construction, and another is the computer was not used.
Calibrating the battery, running the computer unit it is ready to shut
down into sleep due to lack of power, and a warning message in
the screen tells you to be sure to plug in the computer to charger,
is one of the telling times to recharge. This also would be one way
to be sure it would be calibrated after a charge.
The System Management Controller affects several things, and if
the battery were new, charger known good, some power issues
could be resolved by resetting the SMC. But when the cycle count
is over 500 in some models of MacBook, it may need a battery.
That would be a way of troubleshooting, given the use it's seen.
Does your computer have a removable battery, or is it built-in?
A SMC reset may or may not affect the battery health information
numbers in the screen. There are rules in the Support page that
go through things to try & do, before the actual reset to see if it
really is needed. The reset is meant to help resolve other issues.
•Intel-based Macs: Resetting the System Management Controller (SMC)
As part of troubleshooting, there are several things to try, but only
if the reason is valid. You could probably get a replacement battery
from a company such as OWC where newpower brand is said to
be longer life than other builds, but I don't know. Mine were both
Apple replacement batteries as part of a recall; I used them awhile
but didn't really need the iBook G4, I needed my iMac repaired and
done right; the only provider locally could never do it, so there I was.
The only Macs I'd ever owned new enough for Applecare, otherwise
I could've opened the mess & fixed it myself. Except under warranty.
So a new battery is a fairly inexpensive item, and the better tech
of todays replacements (of better brands) can be rather good.
Just be sure you take care of the battery and cycle it properly;
and if you let the computer sit for weeks, or months, be certain
to not leave it with a fully-charged battery; only 50% is better.
If the battery had less a cycle count, I'd suggest trying to calibrate
the Apple battery. But you have to be able to charge it...
•Apple Portables: Calibrating your computer's battery for best performance:
•Apple - Batteries:
Hopefully you can talk the MacBook's old cell into an energetic return.
Probably won't help to suggest sitting it down to watch an Old West
war movie where some guy in blue raises a big sword & says 'charge!'
Good luck & happy computing!