I'm more so wondering if the motor/controller are built for higher wattage than they are rated (I know bosch gen4 cx performance is rated at 250w but can go much higher at peak power) and the fact that I have 2 batteries in parallel, each with their own 30a bms = 60a potential now, could allow for a higher draw than designed thus triggering the error. I dont know really, just thinking about potential possibilities and taking yours into consideration as well. I guess its possible that the external battery is "weaker" than the bosch powertube but I doubt it as its rated for a 500w nominal motor. As far as the voltage of the system after charging I found ANOTHER new piece of data...the voltage reading from the charge port itself falls rapidly after charging to around 38-39v but the voltage of the bosch powertube battery is higher. Since I have those wires connected directly to the motor wires I checked there and it was the same voltage as when it was fresh off the charger. Very odd.
Not sure what you are looking at, but I didn’t connect into the charging port, I connected into the red and black wires on the big plug that goes into the motor
I understand your concern and appreciate it. I have been working with lithium batteries for over 10 yrs and have built several of my own packs from 18650, 21700 and originally a123 cells. I have also been working with lithium polymer packs and discharging/charging them in parallel for over 10yrs as well. What exactly did I say that leads you to believe I shouldn't be doing this? The only "experiment" here really is with the bosch system itself. There isn't anything wrong with my wiring or how Im charging/discharging the batteries, the bike is throwing an error when both of the batteries are charged to the same voltage. Both batteries have 18650 cells and their own bms built in so honestly I dont see any danger to the batteries themselves in this scenario.
I get it. As I said I have quite a bit of experience running lithium batteries in parallel. A parallel switch won't work with the bosch system because the controller won't give any power to the motor if the bosch battery isn't supplying current. I'm thinking of installing an ideal diode on just the external battery line so that current cant flow from the internal battery into the external but the external battery current could still flow to the motor. That would determine whether or not the error was caused by the bosch battery discharging into the external battery while simultaneously sending power to the motor.
So I tried an ideal diode and still got the same error, so its definitely not an issue of the main battery discharging into the external. The external I have can be charged from the dc port or the xt60 discharge cable so thats not the issue either. I remembered I had another smaller 10s2p pack from a hoverboard I was messing with awhile back and decided to toss that into the mix (yes, I made sure they were the same voltage @mike_kelly
) and low and behold, no errors running that pack in parallel! Its a 4.4ah pack so 2400mah cells but I dont remember the exact details on them. Very interesting stuff. A couple other things to note...when running the bigger 10s3p 8ah external, whenever I checked voltage with my multimeter during test rides, the external was at a lower voltage than the bosch internal battery. Now, with the smaller pack that doesn't give the 550 error when I check its voltage during a test ride, its HIGHER than the internal battery. This is after riding for about an hour. As I said before, clearly the packs arent discharging and balancing into each other as previously suspected. Oh and yes, I am running the bosch gen4 cx performance motor with the 625w power tube, same as your cube @PSaibot I am back to believing its a matter of the larger external pack providing too much current when combined with the bosch battery 
Hi, what bike do You have? I try to connect 3rd party battery to my giant trance '19 and it looks that motor is working ok but there is a battery error on ridecontrol. I'm sure this is because lack of canbus messages from battery, because when I short connect only Can_H & Can_L to bike from original battery, everything works very good. Bike read data from original battery, but energy goes from this not original.
I left the original battery in. I split the red power cable and soldered a ground cable inline with the existing one that goes to the motor. So I basically disconnect the red power cable and connect my own external battery in place, the canbus remains from the original battery. When the external battery is discharged(monitored by a beeper) I reconnect the power cable back directly to the motor so the original battery gets discharged.
Hi,
This is the exact solution I was hoping to do for my Rail. Except using 10x 21700 cells. So, a year after posting, how are you getting on with this DIY range extender? Are there any issues you've found around error messages or charging as one unit on the bike with the Bosch charger?
Hi experts
Hi,
