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First of all, please accept my heartfelt greetings to the members of this community. I have been a user of this site for so many years, yet this is my first query.

I recently asked an Electric Vehicle Battery company to make me a replacement Lithium battery pack for my seven-year-old bicycle. The bicycle has a 250-watt motor, and when it was new, it gave a mileage of about 60 km. Now that it dropped to 3-4 km only, I decided it is time to change the battery pack. So I asked a local battery pack manufacturer to make it for me. He made a 48 V, 11.1 Ah pack, as per his claims. I noticed some irregularities during charging the pack, i.e., the charger's automatic shutoff was not smooth when the pack reached 51 V, where I expected it to reach 54 V.

Anyway, I tested the battery pack's no-load voltage and the voltage at 48 V, 50 W load, by putting a series of four 12 V, 50 W bike lamps in series. Now the voltage dropped to 0 V. Please see the images for the two scenarios.

1. No Load Voltage,

2. Voltage with 48 volt, 50 watt load

(Please ignore the opened-up old battery pack on the left side in the picture.)

However, when I tested with the same setup, one day ago, the no-load voltage was 54.1 V, and the voltage with the 50 W load was 52.1 V. But even without using the battery at all, the no-load voltage has dropped by 2 V in six hours.

What can be the cause of such behavior? Can someone point me in the right direction, on where to investigate further? Are the cells just bad, or is it a fault with the BMS?

I have lost faith in the company that made it for me, as I believe they are not experts on these things, just a feeling from my last conversation with them. I had to ask them as I do not possess the tools, else I would have tried to build it myself.

Edited: I Corrected my mistake of writing 50 W instead of 200 W, with four 12 V 50 W bulbs in series. Thank you for pointing it out. However, given the battery capacity of almost 500 WH, even with a 200 W load my query still remains the same, which is "Why the battery voltage drops to zero the same instance when I connect it to the load ?"

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    \$\begingroup\$ Just for your information, If you put four 12v, 50 watt lamps in series then you have created a 48v, 200 watt load,. It will draw about 4 amps which should still last a few hours if the battery capacity is really 11.1 Ah. The most likely scenario, from your description, is that the battery is defective. \$\endgroup\$
    – Barry
    Jan 31, 2022 at 3:06
  • \$\begingroup\$ Wattage is just voltage times current. Things in series share the current. So if you increase the voltage and keep the current the same, the wattage increases too. \$\endgroup\$
    – Passerby
    Jan 31, 2022 at 4:07
  • \$\begingroup\$ Is that the old pack on the bench? What type of cells are in the new pack? Was the BMS also replaced? Do you know what the BMS's maximum current rating is? \$\endgroup\$ Jan 31, 2022 at 6:32
  • \$\begingroup\$ Yes, that is the old pack on the bench, which you can see on the left side of the picture. The new pack is inside the protective aluminum casing, to which the multimeter probes are inserted. Both the old and the new pack have 18650 cells, in a 4p13s configuration. The new pack has its own BMS inside. I have no idea about the type of the BMS, but it is most likely a cheap Chinese BMS, compatible with a 13s battery configuration. \$\endgroup\$ Jan 31, 2022 at 7:01

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The battery output was going to zero due to a faulty BMS. I sent the battery to the manufacturer, and they replaced the BMS with another one of the same rating, i.e., a 13 cell 48 V 20 A BMS. Now I have been using the battery for the past two days and I have not noticed any issues during charging or discharging.

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Anyway, I tested the battery pack's no-load voltage and the voltage at 48 V, 50 W load, by putting a series of four 12 V, 50 W bike lamps in series.

4 50W 12V lamps in series makes a 48V 200W load, not 50W.

However, when I tested with the same setup, one day ago, the no-load voltage was 54.1 V, and the voltage with the 50 W load was 52.1 V. But even without using the battery at all, the no-load voltage has dropped by 2 V in six hours.

If you tired to run that 200W approx 4A load more than 2.75 hours you have exceed the 11Ah capacity of the battery and if it does not have a BMS load shut-off it you may have damaged it.

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  • \$\begingroup\$ The battery has a BMS, as per the manufacturer's claim. The battery shuts off the instant I connect the load. I only had checked the battery voltage the night before, without connecting any load to it. \$\endgroup\$ Jan 31, 2022 at 4:38
  • \$\begingroup\$ Yes, but does the BMS manage the dicharge or only charging? \$\endgroup\$ Jan 31, 2022 at 4:42
  • \$\begingroup\$ I have no idea. I have asked the manufacturer, about what type of BMS they have used. I haven't got any response yet. \$\endgroup\$ Jan 31, 2022 at 5:46
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Charging lithium batteries must be done correctly. If they are over charged, it damages them (and they can catch fire). If they aren't charged properly, then running them flat can also damage the cells. The charger and BMS are critical for making the battery pack reliable.

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