Is there a powerd "power management" event for when a charge reaches
the warn cap or low cap?

If not, is it doable? (And what code should I start looking at.)

I only found for connecting AC adapter, but nothing to do for when battery 
is low.

I did run "powerd -d" and it showed no events for when my charge went 
below my "warn cap" or "low cap". (It did indicate when I closed lid and 
when I plugged in the laptop.)

glacier:/opt/home/reed$ envstat -r
    acpiacad0 connected
       acpibat0 present
    acpibat0 design cap:      2.200 Ah
 acpibat0 last full cap:      1.785 Ah
    acpibat0 technology:          1
acpibat0 design voltage:     14.800 V
      acpibat0 warn cap:      0.192 Ah (10.76%)
       acpibat0 low cap:      0.100 Ah ( 5.60%)
       acpibat0 voltage:     15.473 V
   acpibat0 charge rate:      1.197 A
        acpibat0 charge:      0.260 Ah (14.57%)
      acpibat0 charging

(My "acpibat0 charge" was at 1% left.)

When I was running FreeBSD, when the battery was low I would get a very 
loud repeating bell until it was plugged in.

- Jeremy C. Reed

p.s. I am running NetBSD 3.99.24 currently.