Battery voltage slightly low. Especially when the car's been sitting for a while

Question

My car is a 2004 Mazda 3. This happened with the last battery I had as well as this newer one.

Basically if I leave the car sitting for 2-3 days without driving it, it doesn't have the voltage to start and I need to trickle-charge or boost it.

I recently took a multi-meter to it after driving it then letting it sit for a few hours - 12.1 V. Not fantastic, but good enough to get the job done. I used the ammeter function to test for parasitic draw and found less than 50mA being drawn (if I remember right it was 16mA or possibly 1.6mA.)

So now I'm stumped. Sure, it could be a bad batt, but this happened with the last batt I had too. It doesn't make sense to me that there doesn't appear to be a parasitic draw yet the thing drains after two to three days.

Answer 1

16 mA is very good, many manufactures say 50 - 75 mA is normal. You may have an intermittent parasitic draw. Something may be energizing at some point and drain the battery. One way to test this theory would be to disconnect the battery and let the car sit for 3 days. Hook the battery back up, if it cranks without a jump it's a parasitic draw, if it doesn't it's a bad battery. I would have the battery tested too, before doing the test, most parts stores will do it for free.

Also make sure the battery terminals are clean and tight, if there is unwanted resistance the battery won't be fully charged by the alternator and could cause similar issues. If the battery is not fully charged a normal parasitic draw may cause you to have to jump it after a few days.

See my answer here on how you can test the battery yourself, however in your case it may be better to use one of the electronic testers available at you local parts store.