I haven't been driving my truck through the winter and only starting it every couple weeks, but have found the battery on occasion to be down to about 11.5 volts and not strong enough to start it without a jump. So I decided to look into what may be drawing the battery down while it sits.

I measured 32 milli-amps of current draw on the power amp circuit. I have two power amps. Each one was pulling 16 mA even though they were not ON. Internet research and speaking with an IC designer here at the office determined this to be normal. For more modern cars with a lot more electrical circuitry, about 50 mA is a good rule of thumb for "normal" total parasitic draw.

I also measured 9 mA draw on the truck's main circuitry. There were 2 fuses (lower right and lower left) in the kick panel fuse box that I determined each circuit was pulling 4.5 mA. (I will report back on what these circuits were labelled...I forget...)

What I decided to do was put a battery terminal switch on the ground and lift it cutting all power when I know I won't be driving the truck for weeks. The clock will lose time, but that's no big deal.

If anybody else wants to check and report back with the parasitic draw on their stock trucks, please do so. It would help others to know what the baseline current draw should be. Have your key out of the ignition, doors closed or door light switch OFF, power amps or other accessories, under hood lights etc, off(pull the fuses for power amps). Your multimeter should be set to amps DC with the leads in the correct sockets, and goes between the + (or - , your choice) battery terminal and its cable. As my tech school teacher used to say, "remember...voltage across, current through."

 

Good post to have baseline for normal systems!
I would consider a relay that turns on when head unit is turned on, or when IgN is out in accessory on, or maybe controlled by power antenna signal?

One would be steady power to ECU. Another steady power from hazard fuse to radio head unit.
I know its confusing; Toyota liked to power circuits from normally unrelated fuses. I.e., turn signals, backup light, 4WD indicator from "engine" fuse.
Also names a fuse "charge" when that fuse does not ENABLE charging.
Also causes a fault light come one for something unrelated. I.e., "brake" light comes on when there is a charging system fault - DUH!

 

Excellent! Thank you for the help, Ray.

On the power amps, there is a switched "sense" line that is only on when the ignition is ON. Then they power up. I don't actually run a head unit for the power amps, just a blue tooth receiver to the power amps and my iphone is the head unit. No way to get them down to zero mA draw though, short of pulling their fuses out. So I'll leave them as-is and operate the battery disconnect for long periods of storage. I do have the factory AM/FM radio in place too.

 

you could utilize a small solar maintainer/charger for when you don’t drive it for a few weeks. i purchased several through NH Tool. doesn’t answer the question of baseline para draw, but it works to keep the battery charged.

 

My 88 4runner with 3.4l swap draws 7.5 miliamp.

 

A solar maintainer is a great option. I keep one hanging off the passenger headrest and plugged into an always-hot lighter socket.

 

cool idea, thanks

 

On my fuse box, it was the “EFI” and “IGN” circuits where i found 4.5 mA draw each.

 

Hi folks.
Just for the heck of it, I measured my truck's current draw with ignition switch off and I got 20 mA. I think that's for the following:
1) ECU memory
2) Radio memory

OOPS! I should have also done measurements with ACC on and IGN on. Oh well...

Here's test setup for everyone's convenience:






keywords: batterycircuit, currentdraw, powerdistribution, battery circuit, current draw, power distribution

 

I should add to this post that since then, i have changed the battery and also done Rad4runner’s “click no start” circuit mod and that made the truck start much better(perfectly in fact!), so i believe because of that, it will now start better even under conditions when the battery may be a bit low from being parked for long periods of time.

 

#rad4runner after sitting for 3 months.
(battery disconnected while in-op)