I have a B&M LUMOTEC IQ Cyo headlight running on a Shimano 3N80 dynamo. I have ridden plenty of times at night up some long 5km + climbs at about 7km/h average

Firstly the standlight function only outputs a small portion of the full light for a fairly short amount of time. You will only get about 5mins of light at 30% brightness (my estimate). After 5 minutes the light will start to fade to a point where it's quite useless if your still climbing a hill. (unless you can keep your pace above 10km/h, that'll give you more then half brightness... plenty of light for that speed)

The way i get around it is to wear my Petzl head torch on my helmet and use that when the dynamo light isn't enough.

I have yet to find a DIY system i can build that has auto battery cut in when the dynamo isn't producing enough current. Everyone that i've found on the internet requires you to make/fabricate your own PCB. Which i am not skilled enough to do. Wish someone would put a DIY kit together that included a printed PCB.

2_i you smarty pants