I have a 1996 Volvo 850 GTL Wagon. We drive it infrequently, averaging 2 trips per week. If it isn't driven in 10 - 14 days, the battery will invariably be dead, and I'll need to jump start it, or charge it with a trickle charger overnight.
At first, I thought the battery was worn out even though it was wasn't quite to the end of its warranty period. I replaced it, but this didn't solve the problem.
Next, I thought our typically short trips weren't allowing the battery to fully charge, so I specifically changed my driving habits and used the trickle charger to top up the battery on a regular basis. After doing this, the battery would measure in the 12.3 to 12.6 V range, but after two weeks without driving would be again be dead, measuring in the 11 V or lower range. Even after needing a jump-start, the battery will start the car again after just 5 minutes of driving.
I've measured the steady-state current draw from the battery after the car has been off for several minutes to several hours using both a clamp-on style ammeter (<0.1 amps; lowest the clamp-on could measure) and an inline current meter between the negative terminal and the negative terminal's clamp (40 mA). 40 mA seems reasonable from what I can find online, and shouldn't be enough to drain the battery over 2 weeks.
So, I have an 11-month old battery that won't hold a charge for two weeks, and I'm fairly confident there's no abnormal current draws from the car. Should I replace the battery again and hope for the best? Is there any truth (and references!) to claims that a lead-acid battery once drained too low can never recover?
Updated with new information for the various suggestions below:
After fully charging the battery, I disconnected it for 48 hours. It remained at 12.6 volts over the entire period, and when I reconnected it, easily started the car.
I don't suspect the alternator: When the car is running, 5-6 amps flows into the battery and the voltage on the battery is 13.6 V; and if I drive the car once or twice a week, the battery never dies.
I do suspect something with the keyless remote. I stopped using my keyless remote years ago because I found it too bulky to carry around -- I lock and unlock the car using the key in the door. Christy still uses her keyless remote. Recently, she was the last one to drive the car before we went out of town for 10 days. When we got back, I was positive the battery would be dead, but it started the car without a problem. So, I ran the experiment of leaving the car unlocked, locked with the key, and locked with the keyless remote. After a few days, the battery would be dead when locked with they key, but not when locked with the keyless remote or left unlocked. Frustratingly, I've checked the current draw in all three configurations, and it's 40 mA in each case (and stays that way for several minutes).
For now, I have a solution that keeps the battery from dying if I don't regularly drive the car, but I'd still like to understand what is going on.
Updated with solution 2010-09-02
At the car's next regular service, the shop recognized the problem. On cars of this type and age, the crimp on the positive terminal can fail turning the positive wire to the battery into a resistor rather than a wire. This is diagnosed by measuring the charging voltage on the battery and giggling the wire (it will jump around, and not remain at the required 13.8 V), and by noticing that the wire itself is hot. Volvo recently released a fix for this, and previously the shop had to remove the entire wiring harness, at great expense, to fix it.
So, my battery was never getting fully charged, and I've selected the best answer from among the suggestions that my measured charging voltage was too low.