karlonpc,
As Ether has pointed out above and in the linked post, instrumentation can fool you, choose wisely. If I only had $150 to spend, my money would go for the CBA III tester from West Mountain Radio. If I was going to get $150 this year and the same next, I would buy the Battery Beak and then wait on the CBA.
The CBA does a nearly complete test, is programmable and therefore repeatable, and can overlay tests (either the same battery or different batteries of the same type). It is the only one of the testers discussed that can show a reduced capacity cell. The tests however are done in real time and may take 2 hours to finish a complete test and the battery will require recharging when complete.
The Battery Beak is a quick check of battery health, fits in the pocket or tool kit and will give you a good indication of a change in battery function and it's approximate state of charge in seconds.
A site I have not linked to before is
http://batteryuniversity.com/ While there is a lot of info for all types of batteries, be sure you are reading about SLA (Sealed Lead Acid) and in particular AGM (Absorbed Glass Mat) which are the types of batteries we currently use. BU is sponsored by CADEX, manufacturers of testers and chargers. You can also buy through that site(link to Amazon), "
Batteries in a Portable World - A Handbook on Rechargeable Batteries for Non-Engineers" for $19.95. I don't have a copy of this book (I have a copy of a predecessor) so if anyone has purchased it let me know what you think.