Hilti battery repair new bms and stepdown

[Pombo2000]

Hello, Sorry for my bad english
I'm repairing a Hilti b22 8ah battery from 2023
In original the battery has mollicel cells p42. Some of them had a voltage of 0.9 so i change the Cells with new bak 45d Cells, all at 3.6v all new.
After i connect the bms and the series wires the battery read 22volt ( First picture).
But the voltage After the circuit under the black rubber Is 4 Volt.
So i remove the black rubber and there Is a circuit with a big black fuse and some components in the back. Maybe Is a step down circuit.
At the end the charger don't charge the battery because the battery voltage output Is only 4 volt.
On a website they sell and old version of the bms without the circuit.
Someone try this repair? The original battery cost over 400€

 

[amberwolf]

Are all cells identical voltage? Or are the new ones different from the old ones?

If the difference is large enough (varies between designs, but >0.1v is not uncommon) the BMS will prevent charge or discharge to minimize the risk of fire due to damaged / defective cells (which is what causes such large differences in normal use).

Fixing the voltage difference by manually charging the low cells to match the high ones, or draining the high ones to match the low ones, would correct this issue for most BMS designs.

There are some BMS, usually in OEM battery packs for various brand name devices / bikes / etc, that are designed to brick the entire pack as soon as any serious fault occurs (such as cells dropping below the safe recharge limit, which happened in this case), to prevent the end user from being able to start a fire with it by attempting a user repair. These may be permanent bricking, requiring replacement, or something they can reset at the factory when they repair the packs. (but the end user won't have access to the software/hardware needed to do that reset)

If this is one of those, then you'd have to replace the BMS with a new one. If there is no communication between the BMS and the device the pack powers, then you can use a generic BMS. If there is communication between them, then you'd either have to reverse engineer and emulate that, or use a new BMS of the same kind as the original

 

[Comrade]

and some components in the back

Google what it says on the large chip. It looks like a current sensor.