Hilly stuff

Spent a while driving in hillier (outer metro) terrain yesterday, including "cement hill", a veritable incline, in both directions. On the downhill run at 80km/h on "cruise", battery charging was evident until, near the bottom of the hill, a now almost full battery and a zoned speed reduction to 60km/h resulted in the ICE springing to life quite fervently with unknown high RPM (first time I realised this car has no tachometer!). The only explanation might be (ICE) "engine braking", as there's no load on the generator from a charged battery. Your comments/suggestions are welcome.