Waterpik SR-3000 toothbrush battery replacement how-to

If your Waterpik SR-3000 electric toothbrush is like mine, it rapidly self-discharges after a couple of years. After 12 hours outside the charger, it only has enough charge for one 2-minute cycle of the stronger brushing intensity. It used to last for a couple weeks, meaning I could use it while traveling. Not wanting to toss it out (especially since I got the combo model with the water flossing), I opened it up and saw the batteries were just AAA NiMH rechargeables. With a little hacking I was able to replace those with much higher quality batteries that should last a very long time.