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My Bosch eBike has dual batteries which I usually ride with one battery. I feel like as the weather drops to freezing the bike power has dropped off slightly and when I ride with the second battery added back on the power is raised back up. Is this expected?

Obviously I am aware that double the battery means double the range but I didn't think a double battery would provide more power than a single battery which was fully charged.

  • Can you clarify what you mean by "the bike power has dropped off"? Do you mean the motor's power output, or the range indicated on your computer? I suspect the latter. If so, this is a known property of lithium ion batteries when operated in the cold. It affects smartphones also! – Weiwen Ng Dec 5 '19 at 19:09
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    Some sort of insulating blanket should help, so long as it starts off warm. As it runs the battery will heat, and the insulation will hold in this heat. You just want to make sure it doesn't get too warm. – Daniel R Hicks Dec 5 '19 at 21:54
  • Weiwen: both actually – Marcus Leon Dec 5 '19 at 22:21
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    you can try adding heating pads under the insulating blanket to keep the battery above a certain temperature (using a temperature switch), you will have to either use an extra battery (small li-ion) or use a boost/buck converter to get the proper voltage for the heating pads. something like this aliexpress.com/item/… – Maarten -Monica for president Dec 6 '19 at 13:27
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    So, the answer and comments haven't addressed the issue of your motor reducing its power output. I've browsed the literature available to consumers on Bosch's site, and I can't find anything specific. I would suspect that the on board battery management system may reduce the motor's output to conserve battery life when it senses low power. I think you said that when you inserted your second battery, the motor power comes back up to normal. That's consistent with my speculation. – Weiwen Ng Dec 6 '19 at 16:17
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You're right - a cold battery cannot provide as much power as a room-temperature battery.

I had an ebike based on SLA batteries, and in summer it would get 25 km range, which dropped to under 5 km in the cold frosty mornings of a winter.

It is possible to get battery heaters, common in places like Canada where its cold enough to freeze the water in a SLA battery and split the casing. But on a bike you're going to be using battery power to keep the battery warm. A better solution is to store the bike inside at both ends of your journey.

Perhaps a wrap made from a thermal blanket could help keep heat in, but there are downsides to that too.

Suggest you read To what extent should I insulate an electric bike battery during winter?

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    Canadian e-biker: I wrap a piece of 5MM neoprene around battery + downtube. No issues for 45 min ride @-15C (starting out from indoor storage of course) – Affe Dec 5 '19 at 21:08
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    @Affe thank you - that's useful info. Interestingly, a charged battery can chill and "loose capacity" but when warmed up again it regains the charge level. Assuming it hasn't frozen solid or cracked/split, a cold battery can be gently warmed and it works about as well as before. – Criggie Dec 6 '19 at 0:49
  • Questions related to lithium ion batteries and low temperatures get asked quite a bit here (and on a few other StackExchange sites). Is it worth creating a canonical question for them? – Weiwen Ng Dec 6 '19 at 18:54

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