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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.

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  • 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, 2019 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. Dec 5, 2019 at 21:54
  • Weiwen: both actually Dec 5, 2019 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/… Dec 6, 2019 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, 2019 at 16:17

3 Answers 3

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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, 2019 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, 2019 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, 2019 at 18:54
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In Switzerland where I live the winter is relatively mild and -5 C is the coldest I tried. The range is reduced under these temperatures, and when high power is demanded with half charged battery (climbing), it is possible to get suddenly zero charge remaining reading and stay with no power at all for a few minutes. I needed to switch from each second day charge to daily charge to avoid these issues. Otherwise the battery performs well for the 40 min journey that I start and finish at somewhat warmer place (15 C or about), even if it is already rather old and have seen two winters of such a usage (for the other two, it was not in use).

This is consistent with Shimano Steps recommendations. They say the battery performs worse at low temperatures and needs more frequent charging, but it is not dangerous and the battery life is not reduced. They however advice against riding below -10 C ("should not").

While various self-made wraps looked very awkward for me (I need to remove the battery frequently for charging), recently I have discovered that there are factory made battery wrappers available that look easy to put on and off so may make sense. So far I found no active heaters on the market (something like electrically heated gloves would be), no any cold designed E-bike batteries that would have internal heaters. Either this would be an overkill in most cases or the market still needs time to understand the problem and demand.

I charge my battery in a big clay pot intended for growing a palm.

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If I understand the question correctly, the point to answer is why range and power are increased when a second battery is plugged?

When you write that you usually ride with one battery, do you mean that it is always the same one or you alternate between the two? With Dual batteries, Bosch uses only one battery at the time, and alternates the batteries so that they are discharged at about the same rate.

If you don't alternate batteries, what you observe can simply be the consequence of using batteries in different conditions. If the battery is "used", the amount of power it can deliver is lower than when it was new. It's possible that in this scenario, the system will first use the less used battery and starts to alternate between the two once the two batteries have reached a similar tension (I'm not a Bosch engineer, that is only a supposition).

Now temperature can for instance play a role if you store the bike in an unheated area, and the second battery at room temperature: the warmer battery can deliver more power than the cold one, so the system will use the "warmest one" first.

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