I’m looking at this battery management chip (BM3451) and I have a few questions. My guess is that these datasheets are written for people who already know how these ICs work and are meant to be developed.
For example:
- Between which terminals would I connect my load? Because if it were between P+ and P- (assuming this is where you connect the two main power connections of the battery), there would be no MOSFET to cut off the connection if there was an undervoltage or overvoltage fault.
- What is the true purpose of the MOSFETs at the bottom? Is my entire load going through those MOSFETs, or are they there just for charging?
- If I wanted to charge the battery, would I need another charging IC for this, or could I just supply the same voltage (in this instance, 14.8V) to P+ and P- while the battery is connected?
- How many amps will go through the cell balancing mosfets? Do I need transistors to handle high currents for them or its going to be in the mA range?
I did read through the datasheet and may seem trivial for some but I am not sure.
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Regards,
I found a similar chip by Texas Instruments:
https://www.ti.com/lit/ds/symlink/bq77915.pdf.
It has the same features as the other chip with a little more data.
Hello @tomz0840
Answers embedded into your questions.
Sincerely,
Aaron
Between which terminals would I connect my load? Because if it were between P+ and P- (assuming this is where you connect the two main power connections of the battery), there would be no MOSFET to cut off the connection if there was an undervoltage or overvoltage fault.
The current sensing resistor has a very low resistance. By taking power from “pack” we have a measure of protection with some control over current level and time. This should prevent nascence trips on start-up transients.
What is the true purpose of the MOSFETs at the bottom? Is my entire load going through those MOSFETs, or are they there just for charging?
Fascinating use for systems with a large number of cells as described in the datasheet:
The BQ77915 has built-in CHG and DSG drivers for low-side N-channel FET protection, which automatically open up the CHG and/or DSG FETs after protection delay time when a fault is detected. A set of CHG/DSG overrides enables disabling the CHG and/or DSG driver externally. Although the host system can use this function to disable the FET control, the main usage of these pins is to channel down the FET control signal from the upper device to the lower device in a cascading
configuration in ≥6-series battery packs.
If I wanted to charge the battery, would I need another charging IC for this, or could I just supply the same voltage (in this instance, 14.8V) to P+ and P- while the battery is connected?
Please tell me more.
I believe the cells are permanently connected or a multipin charge port is included such [this battery](BL2600C18654S1PG2646 GlobTek, Inc. | Battery Products | DigiKey so that the charger can “see” the voltage of each battery.
How many amps will go through the cell balancing mosfets? Do I need transistors to handle high currents for them or its going to be in the mA range?
Depends on the cell type and time of charge. The objective is to spill burn energy from the cell with the highest voltage. If you are changing a small cell over a long period of time a few mA will be sufficient. If you are aggressively fast charging a large cell you may need to burn more energy, perhaps as high as 100 mA.
