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Mostly heat. NiMH can't provide as much current or charge as quickly when cold, but internal resistance is much higher when cold so using the battery warms it up. The computers know when it's cold and limit current in and out of the battery until it warms enough. Being warm just plain damages the battery, so those batteries from places like Arizona typically die very young.
Cold kills lithium batteries though. Try to charge LIon below freezing and it turns into a bomb. LiFePO4 just dies. EVs sitting in the cold run heaters to keep their batteries above freezing for this reason.
Regarding your grade, there's a simple clutch switch ($2 to set up) you can add to an 00-04 (I think) which can disable assist. I don't think it works on 05-06 models. Alternately you can just run in a lower gear, since assist requested is based mostly (I think) on engine load and RPM. Climb in 4th or 3rd and it probably won't use assist.
Battery charge bars don't necessarily indicate anything. They're calibrated continuously and don't represent anything absolute. Something to look out for is a rapid change in the bars, e.g. see it ride or fall 6-8 bars in a few seconds, as this is a recalibration.
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