On Mon, 4 Feb 2019 16:45:34 -0500, Tom Del Rosso wrote:

If you turn over an engine periodically to keep it charged, how long do

you run it to make up for the charge lost in starting?

In this case it's my neighbor's 87 Buick Regal while he's in the

hospital.
72 seconds

Having said that, here's how I arrived at 72 seconds, bearing in mind

there's a complexity to your question which, outside of the engineering

specs of both the battery & engine (and parasitics), we can only help you

guess at it mathematically, where empirical results would seem to be more

accurate than our guestimates.

Starting with the basics, a quick search for a Buick Regal Alternator nets

https://www.partsgeek.com/catalog/1987/buick/regal/engine_electrical/alternator.html

which says the alternator outputs 100 amps at idle (if needed) and 150 amps

output at max rpm (again, if needed as alternators adjust output based on

"B" sensing).

Running a direct search for the power needed to start an 87 Buick Regal,

it's easy to find the vehicle, but hard to find the power needed to start

the engine:

https://en.wikipedia.org/wiki/Buick_Regal#Grand_National,_Turbo-T,_T-Type,_and_GNX

We're kind of stuck with the "generic" stuff, such as this:

o How Many Amps Does It Take to Start a Car?

https://www.reference.com/vehicles/many-amps-start-car-e35b6f3d4d8bf426

Which says an average car needs 400 to 500 amps but doesn't say how long.

Let's assume it takes five to ten seconds to start it, at 500 amps, where

the maximum power would be 10 seconds times 500 amps, which means you

sucked out 5,000 Coulombs (i.e., 5000 amp seconds) if the math is right.

If I did the math right, that's less than 1.5 amp hours, and since we

guessed high, I'd say the amount used is roughly about 1 amp hour to 1.5

amp hours, but since we want to "be safe" and have "easy math", I'd use 2

amp hours as the amount to add back.

If you put back two amp hours (to cover for inherent losses, mostly in

heat), you're back to where you started, where we have to "assume" that the

battery sense circuit allows the alternator to output enough current to

charge the battery after just one start.

At idle, if we assume the battery sense allows you to get those 100 amps we

saw in the spec, to generate 2 amp hours would take only about 0.02 hours,

or about 72 seconds (if I did the quick math right) - which -

coincidentally - is about how long it took to run the quick math.

If that 72 second answer is wrong, I welcome someone who can tell us how to

arrive at the better answer.