My car does not display Low Battery charge one the Diagnostic screen?

[Infinitiguy19]

I have a 91 q45 and The Diagnostic screen does not say low battery charge when I leave the stereo on for 20 minutes and the car can barely start. BUT its not the alternator because when i finally start the car and drive it for a little while to charge the battery up and then shut it off the car starts up.

but my main question is where is the control unit for the low charge thing so my display can tell me if i have a low battery.

this may seem stupid but I want to restore my car thats why I am asking these semingly stupid questions.

[Q45tech]

Low battery charge means the alternator charging voltage is low not that the battery is low. No builtin method to warn about a low battery voltage before cranking.

An accurate digital volt meter can tell you what's about to happen but you must know the battery internal temperature to translate the battery voltage 13.2 volts vs 12.8 or even 12.6.

Unfortunately each of the 6 cells will be slightly different and vary with charge and temperature and electrolyte concentration so you cannot say a battery is 100% at 13.20 volts and discharged at 12.00 volts ..................it's the voltage undercranking load that tells you the most but by then it is too late.

Years ago before big amps two way radios used 2 batteries with a diode that charged both simultaneously and only discharged one when the radios ran that battery down.You could leave the radio on for days and still have a full cranking battery.

[mtcookson]

Once started up, it won't show a charging error if the alternator is good as it is charging.

Edit: oops, too late

[Infinitiguy19]

ok so the low battery charge means a bad alternator not a batter I see.