Originally Posted by jsenka
I've had four batteries that were less than a year old fail on my three smarts in the last year. Two of them were M-B labeled batteries, one was from O'Reilly's, and one from Sam's Club (that one was 4 months old).
All replaced under warranty.
If you've replaced the battery that many times in that short of an interval, then clearly the battery is NOT the problem. I propose four possibilities:
1. The charging system is over-charging your battery. I'm not sure if the voltage regulator is built into the alternator in your smart or if it's a separate device but you need to check the voltage regulator.
2. There is a zombie load on your battery (light left on, electrical short, defective voltage regulator, defective electronics, etc) that is repeatedly discharging your battery. You can check this for yourself with an ammeter or have your battery shop verify that the load on the battery is very low when the car is off. You may need to check this repeatedly as the load may be intermittent.
3. The charging system is only charging the battery intermittently. This can be caused by a loose cable or intermittent alternator. This allows the battery to deeply discharge which is NOT good for lead-acid batteries that aren't designed for deep-discharge.
4. Variant of #2
: Something or somebody is leaving the lights or other electronics on when the car is off. Perhaps the timer on the headlight has gone kaput and is leaving the headlights on for 30 minutes every time you shut off the car?
Deeply discharging a lead-acid battery even once will greatly shorten its life. It's pretty easy to kill a battery in 4 months if you repeatedly discharge it.
In any case, replacing the battery is not the solution. Just because the battery is the component that is failing doesn't mean the battery is to blame. You don't blame the engine for blowing up when the oil pump fails. Similarly, don't blame the battery for failing when it is being abused by something you haven't yet discovered.