I have a 1990 F-150 Pickup, 300 I6 m5od trans. I had a problem with the battery going dead if it was not run for more than 48 hours. I placed an amp meter inline with the battery and started pulling fuses to track down the problem, 300ma draw all the time, seemed like a lot to me. Anyway, I was told that it could be the alternator, well, I had it teasted and was told it was dead (I believe it was hooked up wrong, but replaced it anyway) Left it at my office 2 days while out of town, and it was dead as a doornail.

The battery was nearly new, I am sure from the number of deep cycles I will need to replace it again, which I plan this week, but I still have nearly a 300ma draw, this is in my opinion not normal, with everything off I should only have a few 10's of ma's, not 300. I plan on going through this with a fine tooth comb in the next week or two, does anyone have any suggestions what I should look at that would be sources of leakage, but not be a direct short?

Thanks in advance,
Jim

 

300ma is nothing on a battery that can momentarily provide 300A. 300ma is about right for the memory feeds for the radio, and the ECM (EFI computer).

Its obvious to me your draining is not going through the interior mounted fusebox, what I'd do is put your ammeter on its 10A setting, take off the positive battery lead, and touch your ammeter leads to the positive battery post and the removed positive battery lead, and see what the "real" draw is. I bet its a couple of amps... and we can troubleshoot from there.

 

A fully-charged battery in good condition contains about 40 amp-hours of storage, before the voltage drops below 10.5 volts. While a 300 ma draw is not large compared to the current capacity of the battery, it is too large of a standby load. Such a load will drain a fully-charged battery to 10.5 volts in 40/0.3 or 133 hours. Figure a little over 5 days before it gets iffy if the truck will start.

The radio station memory and the ECM should draw micro-amps of current, not 300 ma.

25 ma of standby current would allow you to park the truck for 60 days, not considering the self-discharge of lead-acid batteries, which subtracts from this further. I'd shoot for this as the maximum acceptable standby draw.

It can be a little tricky to test standby draw. If you interrupt the battery voltage to connect your ammeter, there will be an initial high draw caused by capacitor charging, or operating the stepper motor on a CD player, or the like. What you are looking for is any current that persists beyond 15 seconds or so. It needs to drop below 25 ma to meet our 60 day goal.

Leaking alternator diodes are a common source of this problem, since they are connected directly to the battery without interruption by the ignition switch. The internal voltage regulator might be at fault as well. You could try disconnecting the alternator on a hunch, (since it is new) and then measuring the standby current).

To go about it methodically, try the following. Loosen the battery ground strap (for safety). Insert your ammeter in series with the battery positive connection. Remove all the ring-shank connectors from the battery side post of the starter solenoid. Reconnect the battery ground strap. Touch each of the ring-shank connectors to the lead coming from the battery plus side (and your ammeter). Let it sit there for 15 seconds or so, then read the current. The one that has the high draw is your culprit. Some of them may have a slight draw (say 10 ma), but any that are over 100 ma need to be tracked down.

A schematic diagram of your truck would greatly facilitate tracking this down.

 

This is how it was measured... I used to work in a stereo shop, 300ma is usually the threshold value of a "current sensing" car alarm, so I know it's definatly supposed to be less than that.

 

I will do this once the new battery is in, incase the other battery gives me more greef.
Thank you.