Ammeter Overcharging

[patrickt]

Originally Posted by fastd

why/how does it matter if something is wired directly to the battery. In your example, i.e. a fancy audio system (in my case the electronic efi), if it's wired through the fuse box or wired directly through the battery (with an inline fuse), wouldn't the ammeter read out the same way regardless of the way it's wired?

Let's put a 200 watt bulb on the positive and negative terminal of the battery. Remember watts equals volts times amps, so it's going to draw about 15 amps or so. When the car starts up, the alternator will serve up the amps needed to charge the battery back up plus that 15 amps. All of that will go through the ammeter. When the battery is charged, the ammeter should ease back down to 15 amps plus a little extra for the battery. If you move the bulb over to the other side of the ammeter, the alternator will still serve it, but the current will not pass through the ammeter.

[patrickt]

Conversely, let's wire a 1,000 watt bulb (80 amps) right off the alternator of my car. I start the car up, turn the light bulb on, my alternator serves up the 60 amps that is all it's got, and my battery supplements the needed extra 20 amps, so my ammeter shows I'm discharging by 20 amps. My battery will soon be dead. If I wired that light bulb right to the battery, my ammeter would show that I was charging to the tune of 60 amps but my battery would still be dead pretty soon.

[fastd]

Originally Posted by patrickt

Conversely, let's wire a 1,000 watt bulb (80 amps) right off the alternator of my car. I start the car up, turn the light bulb on, my alternator serves up the 60 amps that is all it's got, and my battery supplements the needed extra 20 amps, so my ammeter shows I'm discharging by 20 amps. My battery will soon be dead. If I wired that light bulb right to the battery, my ammeter would show that I was charging to the tune of 60 amps but my battery would still be dead pretty soon.

Got it. Thank you. Super helpful.

My Holley efi wiring schematic shows that power should come directly from the battery so that’s probably where my funkiness is coming from.

[patrickt]

Originally Posted by fastd

Got it. Thank you. Super helpful.

My Holley efi wiring schematic shows that power should come directly from the battery so that’s probably where my funkiness is coming from.

I Googled Holley EFI amp draw and did not get a simple straightforward spec on what its number was, but I did see that it came with a 40 amp fuse on it, so it's definitely pulling something. If you wanted to know exactly, you would use an inductive amp meter, like mine pictured below. You just clamp it around the wire while the load is running and *poof* you've got your answer, just like magic. This pic was from a thread here where people were musing what the initial in-rush current of an FE stock starter motor draws (which is why it goes through a solenoid) and it is about as high as your battery can serve up, for a moment at least, as it sets up the field in the electric motor. Note that my starter motor draws, for a moment, over 900 amps. While it's cranking it draws over 200, as I recall.