have a clamp meter and a second battery?

Then if you have time and are bored, please perform a little test for me.

If you have a second battery mounted in your trunk for your audio system, please:
--start your engine
--turn off all power accessories including the audio system
--let the batteries charge for a little while
--measure the DC current on the wire running between the alternator and the 2nd battery, and post what you get (along with what battery you have).

What I'm wondering is how much of a load the second battery presents to your alternator when it's fully charged. In theory all the current flow should be coming from the alternator with the accessories off, so any current measured on that wire must be the load presented by the battery itself.

If anyone sees a flaw in my test, or knows where this information can be found elsewhere, please post a correction or a link.

Thanks

You don't need to go through all that... All you need to know is the required charging current of a fully charged battery. I'll tell you right now, it is no more than 3 to 5A, depending on the topology of the battery. Go to your local autoparts store, and ask 'em. They could probably tell you. For curiosity's sake, why do you want to know this information, and why do you feel it has to be such a difficult proposition?

I will tell you this though. I have an ammeter in my car; a digital one in the dash, with the 150A shunt resistor in the #4 power lead from the alternator to the (front) battery. With all accessories off, and the car running, my OLD reading was about 9 to 12A, depending on outside temperature. Now this include ALL electrical loads in the car, the primary battery, too. (As a non-necessary footnote, just turning my HU on, and thus the amplifiers in the trunk, my current goes up by almost 5A to around 14 to 17A, average - that's 75 watts of IDLE power!) Last month, I added an additional Optima to the trunk for the system (and yes, it is identical to the front one, as I have ALWAYS preached - and NO FREAKIN' ISOLATOR!). The new charging current, with all accesories off is between 12 and 15A, now, for an average increase of about 3A, again, depending on temperature. Also keep in mind that as a lead-acid battery reaches full charge, the charging current will continue to drop off, until it will level off at somewhere drastically below one amp.

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It all reminds me of something that Molière once said to Guy de Maupassant at a café in Vienna: "That's nice. You should write it down."

Mad Scientists wrote:

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"Once the battery is fully charged and the current has dropped to 3% of the rated current, the charge is completed. A good car battery will drop to about 40mA when fully charged; a bad battery may not fall below 100mA."

https://www.batteryuniversity.com/partone-13.htm

 Jim

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haemphyst wrote:

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Also keep in mind that as a lead-acid battery reaches full charge, the charging current will continue to drop off, until it will level off at somewhere drastically below one amp.

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I'd say that qualifies!

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It all reminds me of something that Molière once said to Guy de Maupassant at a café in Vienna: "That's nice. You should write it down."

OK I measured mine on the CRX today.     Once everything was reading a steady 19.1 volts I backed the regulator on my alternator down to 17.9 where I like the batteries to sit at, and the USACI rules I have to be below 18 volts

I was reading .00 to .01 on current.    I also have 4 runs of 1/0 from my Alt to my batteries in back, and 2 runs of 1/0 from the alt to the battery in front.

I might not apply in the situation, but thought I would share [:D}

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2009 0-1000 Trunk WR 154.0DB 2009 1001+ Trunk WR
2007 USACI World Champion
2007 World Record
2006 USACI Finals 2nd Place