DIY battery extension cable using Anderson Power-pole connectors.

[Pasley69]

I have a gravel bike that I converted to an ebike - it works great (36V and 250 watt) and I use it also for longer loaded-touring rides. However I have started to hit the limits of range (about 80km). So I started carrying a second battery on my rear rack and use an extension cable (I use Anderson Power-pole connectors) to connect it to the motor. I.e. unplug the main-battery-to-motor connection and plug in the extension-battery-to-motor connection. It worked OK. Then my brother used the system for a long trip and when the first battery faded he did the cable switch for the extension-battery. However he made a mistake and connected the two batteries together - almighty click and that was that. He realised his error, but plugging the extension into the motor now didn't work either and I had to pick him up in the car. Fortunately the original battery (once charged) and system still works. I am guessing the 2nd battery is now cactus - fully charged battery connected in parallel with another basically flat battery with no amperage limitation.
My reason for writing this is to warn others that using Anderson Connectors like this is no guarantee of a safe outcome - and to ask if anyone has ever tried this and come up with a simple way to avoid plugging battery into battery?

 

[DieselTech]

I have a gravel bike that I converted to an ebike - it works great (36V and 250 watt) and I use it also for longer loaded-touring rides. However I have started to hit the limits of range (about 80km). So I started carrying a second battery on my rear rack and use an extension cable (I use Anderson Power-pole connectors) to connect it to the motor. I.e. unplug the main-battery-to-motor connection and plug in the extension-battery-to-motor connection. It worked OK. Then my brother used the system for a long trip and when the first battery faded he did the cable switch for the extension-battery. However he made a mistake and connected the two batteries together - almighty click and that was that. He realised his error, but plugging the extension into the motor now didn't work either and I had to pick him up in the car. Fortunately the original battery (once charged) and system still works. I am guessing the 2nd battery is now cactus - fully charged battery connected in parallel with another basically flat battery with no amperage limitation.
My reason for writing this is to warn others that using Anderson Connectors like this is no guarantee of a safe outcome - and to ask if anyone has ever tried this and come up with a simple way to avoid plugging battery into battery?

Have you tried checking battery voltage with a voltmeter? Also some battery BMS models will be reset once plugged into the charger.

 

[DieselTech]

Also some battery packs have a physical fuse inside the battery case & you got to disassemble the case to replace the fuse.

 

[Pasley69]

Thanks for that. I got as far as checking with the volt-meter on the outside. With switch OFF on the battery itself, there is still 10.7 volts on the output leads; with switch ON there is 40.5 volts. I'll dismantle the battery pack next and see if anything looks cooked and check cells etc. I certainly don't want to try charging it yet. Batteries in both case are TongSheng Li-ion 36V, one is 12.5Ah and the other 13Ah.

I guess it just never occurred to me that you could interconnect two batteries.

 

[DieselTech]

If you dont see any burn spots or a blown fuse on the inside. Just keep in mind some batterys need reset with your charger once the BMS is tripped.

 

[fabbrisd]

1. Connecting (2) batteries is bad mojo - even if they are "identical batteries. The battery that has the Higher Voltage at the moment they are connected together will "dump" into the other battery with even fractionally lower voltage,

YES - you will need to check for blown battery fuse 1st - that is prudent.

FYI - TongSheng is a motor maker - some other China co made the batteries

 

[fabbrisd]

You have a metter to electrically check the fuse for continuity

 

[Pasley69]

1. Connecting (2) batteries is bad mojo - even if they are "identical batteries. The battery that has the Higher Voltage at the moment they are connected together will "dump" into the other battery with even fractionally lower voltage,

YES - you will need to check for blown battery fuse 1st - that is prudent.

FYI - TongSheng is a motor maker - some other China co made the batteries

Yeah, you are right - they are Hailong (which in my case came as part of a kit with the motor).

I was actually surprised the display panel and handlebar control didn't get affected.

 

[ElHegpah]

This expensive problem could have been prevented with 10 seconds worth of clear, emphatic instruction. "HERE!!, NOT BLEEPING THERE!!"

BOTH batteries should now be considered questionable and possibly dangerous. I would carefully check ALL parallel groups at the BMS, DO NOT sleep on either battery while charging ever again, and hope you get lucky.

May need a new BMS, if so for the cost I would strongly consider just getting a new battery.

Did the extension battery show 40.5 After you charged it, or is this with no charger applied after the incident? If this is with no charger at all, then not much power moved across the connection, the "click" was most likely a BMS component popping, which stopped the event. SFAIK, some have fuses and some shut down under severe load due to voltage sag (with no noise at all, however), but none have dedicated sacrificial components designed to do this. Was the click Immediately upon connection? What was the voltage on the original battery, Before any charging?