Q: Our household has three cars for two retired drivers, and with the coronavirus shelter-in-place order, we donâ€™t drive them much to recharge their batteries. I try to use a different car every time I go to stores or whatever, just to charge the battery and warm up the engine to circulate the oil. But that may be only once or twice a week. I do have a battery charger, which I havenâ€™t had to use yet, but some cars are a little harder to connect to the charger. I have a 2005 Mercedes, 2017 BMW X1 and 2019 BMW X3.
Will the carâ€™s charging system charge with the car at idle? Is there a higher charging rate at 1,500 rpm or while the car is driven, so the battery will charge up faster?
A: Great questions! Modern vehicles, with so many control modules requiring memory functions and wake-up capability, can have a fairly high rate of parasitic load (battery drain while parked). Some vehicles can go from being driven and parked to a discharged battery (poor or no start) in as little as a couple of weeks, although most last longer.
Here are a few rough figures to work with. A typical car or truck battery will still start the engine down to about the point of being half charged. To restore such a battery to full charge one must apply perhaps 40 amps of electrical current for one hour (40 amp hours). This can be done slowly with a small charger, letâ€™s say at a rate of four amps for 10 hours, or with a larger pro grade charger, perhaps at 20 amps for two hours.
If you find any of these cars start easily (the starter spins quickly), battery status is likely quite good, and a short drive or idling for 10 to 15 minutes may be adequate to keep the battery full, if done once every two weeks. If slower cranking is noted, a half hour drive once a week should do the trick.
To answer your question, a typical car or truck alternator can produce perhaps half of its rated output at idle, and full output at 2,000 rpm or more (engine speed). Alternator capacity (output) can range from approximately 70 to 200 amps, depending on the vehicle. The Mercedes is likely 120 amps (unsure of the model) and the BMWs should be 210 and 180 amps. All three cars should refill a discharged battery fairly quickly either while idling or being driven.
Connecting a battery charger is a pain, and choosing the best charge rate and managing the time interval isnâ€™t fun either. An easy to deal with slow/long charging method could be a 5 watt solar panel atop the instrument panel (ICP or Battery Tender are two I like), plugged into the OBD-II data connector (parked outside facing the sun), or a Battery Tender Junior 120V maintenance charger (in the garage) plugged in the same way. The lighter/accessory socket of many cars is unfortunately not active when it is parked. Both of these methods should compensate for parasitic load fairly well, making it unnecessary to periodically run the engine. These two companies are among the few offering the convenient OBD-II connection.