Eddie coughed up some electrons that declared:

My car battery is oversized for the car. The battery is a bit old
but usually works fine. The battery is flat (I left the lights
on).

My fancy new modern charger senses a poor battery and only puts in
very little charge.

I used to use a really old charger to charge this battery
successfully. I opened up the old charger and saw it was only a
transformer and a big rectifier. That's it. No soothing.

Is this ok for a car battery or is it way too crude?

Well, it always has been OK - never seen a car charger that was anything but
a transformer and some rectifiers - but then I haven't actually bought a
new one for 20 years!

The only concern is for the electronics in the car, but generally the
battery itself will do the smoothing, which only leaves over-voltage to be
a problem, so don't over charge the battery, which would be bad for the
battery anyway.

If you're paranoid you could disconnect the +ve and charge the battery in
isolation.

Cheers

Tim