battery drain math?
07-05-2006, 01:53 AM
#1
Rennlist Member
Thread Starter
battery drain math?
Can someone explain the math/formulas for how long my battery will last with a specific mA drain? My old alarm would kill the battery after about 4 or 5 days without starting. I pulled that out and am installing a keyless entry system that seems to pull about 7 mA on my multimeter.
07-05-2006, 09:45 AM
#2
Burning Brakes
Join Date: Jun 2004
Location: Livonia, Michigan
Posts: 1,011
Likes: 0
Received 0 Likes
on
0 Posts
Battery capacity is expressed in units of amp-hours, which is the amperage draw the battery will supply for a period of time. Typical car batteries are in the range of 65 amp hours, which means that the battery will supply 1 amp for 65 hours. 10 amps for 6.5 hours, 65 amps for 1 hour, or in your case, 7 mA for 9285 hours.
I'm not 100% sure as to what discharge state this rating applies, but I seem to recall the test takes the battery down to 8 volts. At that level, the battery won't be able to start the car, so the actual usable capacity is somewhat less.
I'm not 100% sure as to what discharge state this rating applies, but I seem to recall the test takes the battery down to 8 volts. At that level, the battery won't be able to start the car, so the actual usable capacity is somewhat less.
07-05-2006, 11:31 AM
#3
Hey Man
Rennlist Member
Rennlist Member
If your alarm is only draining an additional 7mA that's nothing when the normal dark current drain on many of these cars is 25 - 50mA. That certainly shouldn't drain your battery in 4 days unless there are other issues going one. I can leave my car for 3 weeks with an aftermarket alarm and it starts right up.
07-08-2006, 08:02 PM
#4
Rennlist Member
Thread Starter
Well I installed the keyless entry system and now the total drain on the battery when sitting is 19mA. The battery should last a reasonable time at that rate I gather. I wish I tested the draw of my old security system, but bet is was significantly higher.
07-08-2006, 08:49 PM
#5
Three Wheelin'
Join Date: Jul 2001
Location: San Francisco, CA Porsche: '92 968 Blk/Cashmere
Posts: 1,699
Likes: 0
Received 0 Likes
on
0 Posts
Originally Posted by jpk
I'm not 100% sure as to what discharge state this rating applies, but I seem to recall the test takes the battery down to 8 volts. At that level, the battery won't be able to start the car, so the actual usable capacity is somewhat less.