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I wanted to test the battery from an e-bike. I have noticed quite strange behavior. The first thing I did was to measure the voltage. It was 66.1V which is fine for a 60V battery. When I connect the battery to the DC load the battery voltage drops to 58.2V, which is about 8V less. When I try to sink 0.1A - I can't. The maximum current I can draw is 0.01A and voltage drops to ~ 5V.

Fine, I thought, the problem is in the DC load. I connected it to battery charger, which sources 60V and about 2A. The voltage was correct and I was able to sink 2A.

Ok, so maybe battery isn't right - but then how come I can measure it's voltage and it's the same as fully charged battery? I tried to measure the battery voltage at the DC load terminals, and indeed it was ~58.2V. Last thing I tried - I have connected current meter as well to see which current is sunk when connected to load. That was 0.01 mA or 10 uA.

One last thing I tried - connecting 60W lamp to battery. Result - nothing, lamp won't light up. So likely either damaged battery or BMS.

Still, I'm wondering how such behavior can be explained. The only thing I can conclude - the battery is almost dead, the DC load sinks small amount of current and hence there's a voltage drop. Do you have any other suggestions?

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    \$\begingroup\$ Pretty sure these batteries have active short circuit protection. They will only output power when properly connected to the bike. You don't want them to catch on fire just because a cable rubbed out and shorted. \$\endgroup\$
    – Turbo J
    May 15, 2022 at 13:01
  • \$\begingroup\$ Do you mean I have caused short circuit? Or rahther battery has some way to "authenticate" (e.g. make sure it's connected to this certain bike) the bike? It has standard XT-60 connector with 2 pins so wondering how data can be exchanged there \$\endgroup\$
    – seeker
    May 16, 2022 at 6:44
  • \$\begingroup\$ contacts can switch internally when a plug is pushed in. Think of barrel or phone connectors where this is often used \$\endgroup\$
    – tobalt
    May 17, 2022 at 3:06
  • \$\begingroup\$ Maybe there is a "I2C" BMS-Chip which needs to be configured/woke-up to enable the output. \$\endgroup\$
    – ElectronicsStudent
    Jan 6 at 0:57

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I have seen this effect on other batteries. The reason was, that the BMS logic was either in deep sleep or one of the cells were in undervoltage state. The BMS had disabled the internal load switch mosfet for safety and I could only draw the leakage current of the mosfet. The high inpedance multimeter however draws nearly no current and so I could read some value near the true internal battery voltage. I took a 0.2A current source with some volts higer value and fed the load side of the pack with that. The body diode of the safety mosfet turned on and I could read the true cell chain voltage and it's slow rising. However it's NOT a good idea to charge the cells this way for more than a minute because they (and you!) are not protected in this situation. Some batteries woke up with this method because the bad cell left the undervoltage level, but the problem behind it is seldom solved.

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