Using a non-Bosch battery on a Bosch (classic) ebike

kilou

10 W
Joined
Aug 25, 2014
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82
Hi,

one guy on a german ebike forum (see https://www.pedelecforum.de/forum/index.php?threads/classic-andere-akkus-durch-umbau-der-original-platine-verwenden-anleitung.23236) posted a thread on how to use a non-Bosch battery with the Bosch (classic) ebike. The idea is to take the BMS from an old Bosch battery and wire it between the new battery and the motor. This allows the Bosch BMS to communicate with the motor, something used on this proprietary system :( Obviously, there are other alternatives (e.g. using an external battery in parallel with the Bosch battery) and this has been discussed on the german forum too. But here I'd like to consider the idea of using the Bosch BMS with another non-Bosch battery.

The guy (Epikao) who started the thread on the german forum did not provide any details on how to build his setup and his website is no longer available. But let's discuss this setup in more details here. The point is to use any 36V battery (using its own BMS!) with a BMS taken out from a Bosch battery so that it can communicate with the Bosch motor.

I have very limited knowledge in electronics/electricity but I was thinking about one way to do that. Basically, what we need to do is to fool the Bosch BMS into thinking it is connected to a genuine Bosch battery. First, the input wires of the BMS should be connected to the plus and minus of the battery. Second, since the Bosch BMS monitors 10 parallel cell groups individually (using a thin flat ribbon cable), it expects a signal of approximately 1/10th of the total voltage of the battery in each of these cell groups, which indicates that the cell groups are balanced. If the cell groups are not balanced, it will likely throw an error. One way to fool the Bosch BMS into thinking it is connected to a battery with perfectly balanced cell groups is simply to divide the total voltage of the battery by 10 using two resistors R1 and R2 in series, thus building a so-called voltage divider. I stress here that the Bosch BMS is only used here to allow the CAN communication with the motor. It is not used to monitor cell groups in the battery. For this purpose, the battery that we intend to use with the system should have it's own BMS!

We want to divide the battery voltage by 10 so R2/(R1+R2) should be equal to 0.1. This could be achieved e.g. if R1=9000 Ohms and R2=1000 Ohms. In that situation, the voltage taken in-between the two resistors would record 1/10th of the total voltage of the battery pack, which is what we want. The total resistance in the voltage divider is R1+R2=10’000 Ohms so if the battery is 36V nominal, the current flowing through the voltage divider would be 36/10’0000=0.0036=3.6mA. This means that the power lost in the divider would be only 10’000*(0.0036^2)=0.13 Watts, which appears negligible and should not generate any heat.

With this setup, all indications (incl. battery charge level and range) in the Intuvia or HMI consoles should be correct and any non-Bosch battery (36V) with its own BMS could be used to power the ebike. What is necessary though is to find a suitable connector with 10+1 wires that connect in place of the ribbon cable. The 10 red wires should be connected to the point in-between the two resistors and the last black wire should be connected to the negative so that all 10 wires record a nominal voltage of 3,7V. Below is a picture of the setup:

ebike_battery.jpg


Now a few questions:

1) Do you think this setup would work or am I missing something?

2) Is this okay to use high resistances for R1 and R2 in order to minimize power losses through the voltage divider or does this lead to some issues? Basically do you think that R1=9kOhms and R2=1kOhms represent appropriate values?

3) Is it possible to source an aftermarket connector with 10 (or 11) wires that can be connected in place of the ribbon cable? Or is it possible to re-use the ribbon cable in some way?

4) The green and yellow cables are for the CAN communication. What is the purple wire used for? Is it used for the charging process of the Bosch battery and can be safely disconnected when used in this setup?

Thanks for your feedback!
 
I was under the impression the bosch bms bricks itself when its balance wires are taken off the cells.
 
Apparently it is OK to disconnect the Bosch BMS from the battery pack because many people replace the cells of these Bosch batteries with newer cells without issue (see e.g. https://www.youtube.com/watch?v=PO0BQssvGEU). The BMS doesn't get into a locked mode by doing this.
 
One thing I'm a little worried about is that I'm unsure whether the Bosch BMS requires a continuous power source to remain alive. According to the video above, it is perfectly OK to disconnect the BMS from the battery e.g. for the purpose of recelling the pack (this appears to be no longer the case for the newer Bosch system from 2014 on but it's okay with the classic system up to model year 2013). However, the idea here is to hard wire the BMS to the motor while leaving the ability to take the non-Bosch battery off the bike (for charging etc). This means that when the battery is off the bike (or if the system is off), the BMS won't see any voltage at all, neither from the ribbon cable that is supposed to detect cell imbalance nor from the main + and - cables that connect to the battery bracket on the bike. If the BMS requires a power source for its own functions (e.g. a RAM module to store information in etc), then it will not function. Does anyone know if the Bosch BMS requires a power source to be connected permanently or would it be fine to leave the BMS without being connected to the battery for an extended period of time (e.g. during winter)?
 
I have a Bosch Performance system from 2015 (newer style). I have successfully opened a PowerPack 400 and replaced it with a battery pack from a PowerPack 500 and the BMS did not brick itself when disconnecting from the battery pack.
 
Is it possible to re-cell a Bosch battery to provide more than 500Wh? Will the bike be able to use full capacity?
 
If the bms does not brick itself you can use the full capacity.
 
I do not understand about your solution! just connecting a supply external battery to the input bms i dont mind it was any trouble, for the can bus two wires green and white are comunication the tthird purple is to activate realay controller connecting white and green with a 330 ohm resistor you lie to the controller
 
legerme1955 said:
I do not understand about your solution! just connecting a supply external battery to the input bms i dont mind it was any trouble, for the can bus two wires green and white are comunication the tthird purple is to activate realay controller connecting white and green with a 330 ohm resistor you lie to the controller

.... are you sure that by connecting a 330 ohm resistor between the green and yellow (white) wires you deceive the controller .... and when you turn on the system it initializes normally without problems ? .... have you checked it on the CX 4 generation ?
 
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