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I'm trying to turn a scavenged scooter battery into an e-bike one.

I can balance charge it via the original BMS but discharging is the issue: P+ and P- are only delivering about 4 V.

More wires, with various voltages ranging from 0 to 14 V, are also coming out of the BMS and merging into this wheel connector:

connector

All I need is 36 V (both the battery and the motor nominal voltage) to feed into the e-bike controller.

I guess my options are:

  • bypassing the BMS discharge by soldering new wires onto the battery poles: is this safe?
  • cutting and merging some of the connector's wires to somewhat add up voltages: this sounds rather hazardous ⚡
  • any other insight greatly appreciated 👍
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    \$\begingroup\$ "ypassing the BMS discharge by soldering new wires onto the battery poles: is this safe" : specifically this is NOT safe, batteries are thermally sensitive. \$\endgroup\$
    – pjc50
    Sep 21 at 10:30
  • \$\begingroup\$ If you are able to disassemble and check the circuit board in either the battery or the original device it is meant for, there may be some clues to what signals it is expecting. Sometimes it is as simple as connecting two wires. \$\endgroup\$
    – jpa
    Sep 21 at 16:21
  • \$\begingroup\$ Bypassing the BMS of a vehicle will potentially turn your vehicle into a timebomb. That annoying thing is there for a really good reason. \$\endgroup\$
    – Mast
    Sep 21 at 18:28
  • \$\begingroup\$ @Mast I understand the risks. How about adding some components to palliate these issues, such as : a low voltage disconnect to prevent overcharge and a fuse to prevent overload? \$\endgroup\$
    – aerobil
    Sep 22 at 14:15

3 Answers 3

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I’d strongly advise against bypassing the BMS as there’s a significant risk of fire/explosion in the event of a short or overload, and nothing to prevent the cells from being over-discharged which is also hazardous. My guess, not being familiar with the pack in question, is that either it’s discharged (perhaps you know otherwise) or that the 36V is isolated until something happens to enable the output - this could be the application of a particular voltage to one of the control pins. The purpose if this is likely to avoid having the 36V exposed while the pack is unplugged. If you have access to the original scooter you may be able to work out what you need to do to enable the output.

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    \$\begingroup\$ Certainly don’t go poking about at random though, that will end badly. \$\endgroup\$
    – Frog
    Sep 20 at 20:53
  • \$\begingroup\$ How about adding some components to palliate these issues, such as : a low voltage disconnect to prevent overcharge and a fuse to prevent overload? \$\endgroup\$
    – aerobil
    Sep 21 at 10:28
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    \$\begingroup\$ @aerobil lithium batteries are firebombs waiting to go off. Disabling or bypassing any of their safety components is never a good idea unless you're using teleoperation to abuse something that will never leave a fireproof bunker. \$\endgroup\$ Sep 21 at 18:03
  • \$\begingroup\$ @DanIsFiddlingByFirelight I understand the risks. What do you think of the new safety components mentioned above? \$\endgroup\$
    – aerobil
    Sep 22 at 14:14
  • \$\begingroup\$ @aerobil I think any attempt to screw with or bypass the built in safety features is like juggling with burning Molotov cocktails, only stupider. \$\endgroup\$ Sep 22 at 15:26
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any other insight greatly appreciated

  1. Fix the BMS
  2. Replace the BMS
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  • \$\begingroup\$ Could you elaborate on the term "fix", considering the BMS isn't technically broken? \$\endgroup\$
    – aerobil
    Sep 21 at 10:27
  • \$\begingroup\$ "Fix" means "to repair something that is broken". \$\endgroup\$ Sep 21 at 13:12
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    \$\begingroup\$ "P+ and P- are only delivering about 4 V." That means there's a problem with the BMS. \$\endgroup\$ Sep 21 at 13:21
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If the pack is 10S - ten cells in series - then there aren't enough wires coming out for this to be a dumb pack with external balancing. So it must be smart enough to do its own balancing. It's possible that the smaller pins are some sort of digital connector to the external BMS.

Your best bet is to find someone else's reverse-engineering work on this model of pack or scooter. (Is there an informational panel and model number on the pack you could post?)

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