UKH

I bought a light in summer (sigma buster 2000) and started using it in Oct. I noticed that it would often not stay in full brightness even when there was 80% left on the battery indicator, the night temperatures were around 0-5c and i was moving so doubt it was overheating.

I sent it back and got a replacement under warranty. On weds I took it out for the first ride, using it mostly in low or medium, then high for more technical bits, and a similar thing happened, was ok for a while but then would drop down to medium from high, and the rear power button went from green to red, again battery at 80%. Surely this can't be a fault, am I doing something wrong? 

Freezing temparatures affect the function of any batteries of exposed electronic devices like lights, phones, garmins etc

Don’t know the light involved but temp does affect run times which themselves can be optimistic (eg optimal conditions in small print). Lots of lights scale back the output for various reasons (temp, low battery etc.). I’ve got one light that full output generates so much heat it scales back after some 10 mins to a lower output for example.

However, I did a google and surprisingly Sigma do not state anywhere I could see what battery is used. If it was Li ion I would expect them to state that at least. Maybe they are using older battery technology and older sensor technology?

If it was mine, I would be thinking about contacting Sigma for their comments.

Awaiting their reply, also they use Panasonic lithium cells i believe

Is it the 2000 lumen jobbie?  If so, I'm not surprised it doesn't stay on full brightness for long.  It'd overheat unless it had a good airflow to cool it.

I know that some cave diving lights do the same - they need to be in cold water in order to keep the LED cool.  Use them out of water and they step down the power to avoid overheating.

To be honest, it's more concerning that operation in near freezing conditions is enough to switch on the red power light when it's 80% full.   Does the blurb you got with it indicate the fall-off in battery capacity in cold conditions?

Dave

It doesn't mention anything about auto dim or over heat

The manual seems to suggest that once the battery depletes to certain levels the LED turns red to indicate you are at a certain battery level and you can then manually change back to Full power.

NOTE: The Power Safe Function automatically switches the light head to LOW-Mode when the red indicator on the light head lights up. This can extend your remaining runtime and inform you that your battery is already partially discharged. Once it takes effect. this automatic safety function can be reversed on your own responsibility, so that any light mode can be switched on again manually. Due to small voltage differences, there may be minimal time delays before any mode can be manually activated again. As soon as the Power Safe Function has been activated automatically, we would like to remind you to watch your battery consumption so that you can get home safely.

very useful to know, that bit wasn't in my manual!

The first light I had and returned wouldn't stay in high mode once the red light came on though, so likely a fault.

As an update, same thing happened with the replacement light, useless, went back for a refund, wont be buying a Sigma light again. R2 bike however have very good customer service.