safe way to reduce 200V to ~20 volts?
What is the input impedance of your voltmeter? If it's 1 Mohm, you could use a 1 Mohm/100k divider (the absolute accuracy would be poor, like 10%, but you care about trends, not accuracy) and the total power in the divider would be a max of about 36 milliwatts. A 100k/10k divider would provide accuracy of about 1%, and only dissipate 360 mW. 

You don't need absolute accuracy, since if nothing else the battery voltage will change with temperature. What you're trying to do is detect small changes (or lack thereof), and the absolute accuracy is irrelevant to that task. What you care about is stability, and with low power dissipation this ought to be good enough.