Ahah! It sounds like the problem isn't so much with the electrical system and charging as getting the starter to spin the engine fast enough, long enough.

Cranking speed is important to making enough heat on compression stroke for combustion. Glow plugs warm the precombustion chamber to help speed things along, but can't do all the work. Cranking speed builds enough heat of compression to ignite the fuel, and running causes the precombustion chambers and the rest to warm up to produce sustained power.

The block heater warms the oil, which thins the oil, which allows the same power in the starter motor to spin the engine a little faster.

Hmm, how about using 'thinner' oil, like synthetic? Less viscous when cold, and better high temperature attributes...

I'm considering using synthetic for my next change, to see if cold cranking is improved. I don't always have a place to plug in the block heater, but synth will always be in the oil pan.

Best Regards,
Jim