battery not charging with radio on?

[1983Pinin]

Hello,

I've had an odd thing happen with my 1983 Pininfarina 2000 regarding the battery.
This has only happened twice over the last year ( driven maybe 3-4 times a month ) , but gives me pause now whenever I want to go for a drive.
I have found twice that I drive for a while, and then when I stop, I can't get the starter to spin. All lights come on--not sure but maybe the battery light comes on, but the bottom line is a jump gets me going again. Then the car starts again no problem.

One thing may be a common thread, and here's my question. I may have had my replacement car stereo on both times when this happens. Is there any chance that the car radio ( a Kenwood head unit--no other amps or add-ons )somehow is screwing around with the charging circuit while I drive? I didn't wire the care radio in when I installed it. I used the wiring for the previous after-market unit.

BTW, no dash warning lights ever come on while driving.

UPDATE Sept 2018-- I bought one of those cigarette lighter voltage units and saw readings of 12.6V when starting, then upwards of 13.5V with car running. Turning on lights, radio, interior fan did drop by tenths of a volt , but nothing else gave me indication of trouble. Finally I came back after driving hard , and shut car off, and noticed voltage dropping really fast to below 12V , while the radiator fan was on ( and engine off) !!! So I am guessing the problem is just a poor battery, right? I keep it on a trickle charger.

--Peter

[spider2081]

Have you checked the battery terminals and the wires at your starter solenoid.?? Have you looked at the ignition switch connector?

[1983Pinin]

Thanks for those suggestions spider2081. Will look into those this weekend. I also bought one of those cigarette lighter battery voltage units, just so I can see what's going on while I drive.