When the battery charge gets to about 20-25% it drops abruptly to 5-10%. The steeper the hill and higher the load drop percentage is higher. Examples: from 25% to 6% steep hill from 13% to 8% small hill from 19% to 8% steep hill

All in High assist. Bike is 2 weeks old / 250km ODO

Is this normal?

  • 1
    Hard to say what's normal for that particular system, but it could be suggestive of a weak cell in the battery. Commented Nov 16, 2017 at 1:22
  • 3
    Bike is two weeks old? Warranty time. Do not try and fix it yourself or it may void the warranty.
    – Criggie
    Commented Nov 16, 2017 at 4:08
  • Battery with factory fault.
    – Carel
    Commented Nov 16, 2017 at 11:46
  • Does it recover fast when you get to the top of the hill? That could be normal behaviour depending on how close the voltage measurement is to the battery, with respect to the various resistances.
    – Chris H
    Commented Nov 16, 2017 at 22:10
  • No, it stays on low charge even when I get to the top.
    – markob
    Commented Nov 17, 2017 at 1:37

3 Answers 3


This is a sign of an unbalanced pack. One of the cells is weak and under load its internal resistance is increasing and voltage decreasing.

Normally this only happens with an old, worn out pack that hasn’t been balanced or has mismatched cells.

If your pack is new, I’d get it replaced ASAP under warranty.

  • 1
    I've sent it to official Shimano service and answer is that battery is good and that it's normal that until 20% discharge is linear, but not below 20% where I can expect rapid drops. What a bunch of BS. I'll make a recording of the problem and post it on YouTube. Possibly some negative publicity will prevent them from making such silly statements in the future.
    – markob
    Commented Dec 11, 2017 at 18:15

I've been having the same problem for a while, and I suspect the problem is a funky state-of-charge (SOC) algorithm. For example:

  • I have two batteries, purchased at different times with different amounts of use, and it happens to both.

  • I've seen the sudden drop... and sudden rise. Just today it went from 30% to 6% on the way to work, then I turned it off and it sat for the day, and it reported 34% when I fired it up to ride home (...where it jumped from 28% to 3%).

Either both of my batteries coincidentally went bad at the exact same time, or the algorithm is funky.


I've had the same sudden drop issue when reaching 30%.

I did replace the battery with a brand new one and same problem occurs.

So it is definitely a Shimano issue.

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