I would say it is your battery that is weak to begin with. I have started my
diesel 609 in teens without any problem or any heater hooked up. I never
used my engine heater.

If you have an engine heater, I suggest you use it... takes about an hour or
so if it is outside.

That dinky booster battery pack is powerful... I have one myself and that
started my diesel when my battery is dead. Yes, I changed my battery
afterward.

I've got a 240D, OM617, I think. If the engine soaks to 5-7F, that's
it for me, even on a fresh charge. It's started fine after windy nites
in the teens, maybe have to crank it twice, or keep a foot on the
pedal for a minute in the teens. I'm not sure what my fuel is like at
that temp either, but I've got cetane boost /anti cloud additive in
the tank.  I should probably cut w/ K1, that was our second cold snap
like that.

In absence of a block or coolant heater, you might try leaving a hair
dryer turned on in the engine compartment for a couple few hours.

Andy

I have a 1982 300 TDT with a 617 Turbo Diesel.  I had the engine
remanufactured at Metric Motors in the San Fernando valley last summer
and I have about 6,000 miles on the new engine.  It runs great.
Starts instantly like a gas engine in a Toyota when it is warmed up,
and starts pretty darn fast even when it's cold.  I live in the
Eastern Sierra, and our coldest temperatures in winter here at 4,000
feet don't usually go below 15 F or -10 C, but I have seen 0 F or -17
C.  The new engine will start at on our coldest morning without a
block heater, I think, but when I had the engine remanufactured I
mentioned to Mike at Metric where I live and that I would like to get
a block heater installed.  He said he would just install one as part
of the deal and did not charge me for it.  It is installed in place of
one of the freeze plugs on the right side of the engine just above and
in front of the starter motor.  It is the type which has a pump in it
to circulate the coolant through the engine as well as heat it.  If I
know I am going to use the car in the morning I plug the heater in the
night before.

I would highly recommend getting a block heater of that type installed
if you frequently start your engine in cold temperatures.  The engine
starts as quickly and runs as smoothly right away as when it is 70 F
or 20 C outside, which puts a lot less strain and wear on the engine.
When you start your engine cold it takes longer for the oil to start
lubricating everything and this causes a lot of wear on the engine.  A
friend of mine and his family own large commerical fishing vessels,
and he tells me that they have block heaters on those engines that run
all the time so if they need to start the auxiliary engine it is
already warmed up and they do that because of concern over lubricants
being cold at start not lubricating the very expensive large marine
engines properly.

I try never to start my new engine without it being warmed up now.
With the block heater plugged in I think this engine would start even
if the outside temperature was -30 F or -34 C if you could get the
fuel to flow.

I installed the new type glow plug relay from Bosch that comes back on
when the engine starts and keeps the glow plugs on for the first 3
minutes of engine operation. This also helps the engine run smoothly
from the start, which also lessens wear.

Lots of ideas. Here are a couple more...

Lead acid batteries lose capacity as the temperature falls and motors
become harder to turn as their oil thickens.

Battery: the size for a diesel is a Group 49 - this is a big, heavy
battery, about 12" long. Be sure yours is this size.

If the existing battery is the correct size, you might consider buying
some kind of smaller 12V Battery that you can (more) easily keep in the
apartment and carry to the car to use as a booster (+ to +, - to -) for
such cold start occasions.

Engine oil: The engine's oil ought to be diesel grade (CH-4 or CI-4 or
CI 4 Plus) SAE 10 - 40. Single weight oil like SAE 30 or SAE 40 will be
like tar at such low temperatures and really the cranking speed.

The 1980 300SD Owner's Manual instructs us on cold starting:

At ambient temp below 32 degrees F and with engine cold, completely
depress accelerator while starting. Actuate starter until engine fires
regularly and engine speed rises. Then ease off the accelerator slowly.
Cranking times of up to one minute will not harm the starter.

At ambient temps  below -4 degrees F, depress the accelerator three
times prior to starting.

Check the oil pressure immediately after starting a cold engine and
don't speed up the engine until pressure is shown on the gauge.

And if all this fails just park it over a steam grate!