Why do batteries have different connectors for charge and discharge?

[kmxtornado]

I'm used to RC car Lipo batteries that use the same connector for charging and discharging. Why is it that e-bike batteries use different connectors for charging and discharging?

I recently purchased a LiFePo4 battery that has 2 pairs of leads, one for charging and one for discharging (via the motor during operation). I will be discharging at a lot more amps than I'll be charging at which is to be expected. So I suppose I can use a lower amp rated connector for charging and a higher amp rated connector connecting to the motor.

Would I be able to use just one of the pairs of connectors and use that for both charging and discharging and ignore the other pair of leads? Dumb idea to have that flopping around I suppose. Just curious why there are two pairs of wires coming from the battery to begin with.

 

[lnanek]

That's generally due to either using a split port BMS, or a common port BMS. Split port are generally safer, but very limited in the amount of current they can take. E.g. a typical split port BMS might only accept 10A on the charging side, and might only allow 50A on the discharge side. It can cut off charging when full, but not cut off regen when full to protect your controller - something a common port BMS can't distinguish between. Controller also doesn't have to be exposed to charging voltage.

(Ignoring the situation where it is a common port BMS with just a y splitter for convenience anyway. That can still be handy so you can charge without disconnecting your controller every time.)

RC often uses balance chargers that charge each cell individually instead of whole battery chargers, and uses tiny batteries that can be connected and disconnected easily, so the use cases are pretty different. It's not uncommon to fly RC using a 14S battery, then split it into 2 x 7S for charging, and put each battery on a 7S charger that will charge each cell individually. E-bikes you generally leave the 14S in place, dump voltage into it as a whole, and depend on the BMS to either cut off charge when an individual cell hits max voltage, or burn off energy from that cell using resistors so the others can catch up (if it's a more expensive balancing BMS).

 

[kmxtornado]

I see, okay I'm starting to understand the BMS a bit more clearly now. What connectors do you use yourself for charging and for discharging? I assume you use different ones for each so you don't accidentally mix up what you're plugging into.

 

[amberwolf]

I think you already have at least two threads going about that part.

For the rest:

Would I be able to use just one of the pairs of connectors and use that for both charging and discharging and ignore the other pair of leads?

Depends on the BMS inside.

Some have a common port, where internally both come to the same external wiring contacts, and then may either go to just a single connector on the battery case, where you connect to controller to discharge, or unplug that and connect to charger to charge, or may split out to separate connectors on the same wiring.

Some have separate electronics inside for charge vs discharge, and separate electrical connections. You cannot safely connect these together or charge via discharge port or discharge via charge port, because doing so bypasses the ability of the BMS to protect the cells.

 

[kmxtornado]

Okay thanks. I'll be sure to use each set of wires for charge/discharge and not try to combine them or one set for two functions. Now that I understand the BMS, it makes sense now.

Yeah, sorry about the overlapping topics. I was hoping not to confuse anyone using my threads as a resource for their own questions so I put posted them separately but yeah, there's some overlap.

 

[docw009]

FYI. I had one battery with an XT60 for charging and an XT90 for discharge. I kept it in a battery bag on the rear rack of my bike. One day, I thought I was using that battery, so I put a 48V charger on the X60.

Wrong, it was a 36V battery in there, and the XT60 was the discharge, I had bypassed the input protection, which would have disconnected charging as soon as a cell hit 4.2V. Lucky for me, the cells had CID vents. and most of them triggered as they overheated, which also broke the charge circuit, but the pack was so hot I tossed it in a bucket of water. Went to bed. Some cells were still warm the next day.

So even on my common port BMS batteries, I now use a separate 5mm barrel plug. I don't care that they limit charge current. I just don't want to be brain dead again and repeat this folly. That first pack got changed to a barrel plug too.

 

[kmxtornado]

Oh, no! That sounds scary. Good thing you caught it when you did! I almost charged a 2S lipo as a 3S before and that wa no fun. I always take a look for a few seconds at the initial charge to make sure it's not charging super fast - to double check the settings are correct. Nothing bad happened b/c I caught it immediately but it could've been a very bad day. Glad everything is okay with you. Dead battery but at least you and your house are okay!