UV Cutoff [MSXV]

UV Cutoff has been removed.

ASC 8.3.B.2 only requires that passive protection be present for the supplemental battery. This is achieved by voltage monitoring of the aux battery on PD. Accurate SoC of aux is not required because it does not contribute to motive power. Thus it would not improve the performance of the vehicle. In addition, the aux battery will be sized such that it will last a full day.

Description

The Under-Voltage Cutoff board exists to disconnect the battery if the voltage falls below a certain threshold. It provides a few additional protection features, including a fuse to limit short-circuit current, inrush-current limiting, and reverse polarity protection.

It also has load switches to control the loads (fans and horn). The load switches have built-in current sensing. UV Cutoff interfaces with power distribution to control the load switches and to provide power to the comparator.

In rev 2, a "fuel-gauge" IC was added (to measure the SoC more accurately)

V2 Requirements - 2023/06/25

• Disconnect aux batt if battery voltage falls below certain threshold

• power horn (x1) and fans (x2?)

• supply power to main power switch relay

• supply power to LV in the case of BPS fault

Features:

To delete:

• fuel gauge

• MCU

• some fans

• some caps

To change:

• load switch

To add

• caps for horn

Maybe requirements:

• monitor battery using fuel gauge (& send info to MCU)

• supply power to BPS on startup ???????

• power itself ????????

(regulation requirements for reference)

Open

Reading Material

https://www.edn.com/designing-battery-management-systems/

Possible Action Items

• Read up on and analyze previous revs of UV Cutoff

• Check for supply chain issues with major ICs on current board (compile list of alternatives for out of stock parts)

• Add a charging circuit

  • Will also require active protection (over-voltage, over-temperature, and overcurrent, see section 8.3.B in ASC regs)

  • Look into BMS circuits or ICs

  • Consider drawing power from LV

• Add controller board

  • This would remove the need for the differential I2C lines

  • Increases longevity and makes it easier to add more features to UV Cutoff board in the future

• Make UV cutoff compatible with aux battery

  • Use the same PCB for both batteries, but some passives might need to change to adjust reference voltages, etc.

    • OO: If we have a set of passives for the auxiliary, in conjunction with the existing passives for the actual battery, and use jumpers that we can solder or de-solder in order to connect the UV Cutoff to any or both batteries when we want.

    • Have two sets of passives: two sets of VBAT load switches, buck converter, and jumpers after each of these.

    • AUX battery powers fan and horn but when that runs out, switches to main pack

    • the main pack may be 24V AUX is 12V, maybe we will use a 24V aux if main battery is 24 and use fewer circuitry

      • Use same battery cells and place in series to have a 24V output

        • Turnigy Sub-C 1.2V 5000mAh High Power Series NiMH Single Cell (hobbyking.com)

    • OO: Currently, UV Cutoff will only deal with power from the aux pack

• Replace the high-side P-channel MOSFET reverse polarity protection with a low-side N-channel MOSFET for reverse polarity protection?

  • N-channels have a lower on resistance, so it would be more efficient. It would also be safer on the low side. If the battery was connected backwards with a P-channel FET, then 12 V will be connected to gnd, which is a larger net so it's more likely to accidentally touch something conductive and cause a short circuit.

  • https://www.infineon.com/dgdl/Reverse-Batery-Protection-Rev2.pdf?fileId=db3a304412b407950112b41887722615#:~:text=During reverse polarity of the,the two transistors would occur.

  • OO: Current strategy is to remain with P-Channel MOSFET for Reverse Polarity Protection.

• The fuse could probably be replaced with an e-fuse to have a more precise current limit.

Block Diagram

Open

MSXV - UV Cutoff (Rev 1.0)

Schematic Capture Phase

Would this circuit be better off with a Buck Converter as opposed to the current LDO (U3)?

Soft Start + Reverse Polarity Protection

• Consider smaller resistor for sense resistor.

• Consider IC for RPP

• Add test-points

• SIMULATE in Falstad!

Interface with PD

• Think about how PD is currently interfacing with UV Cutoff (especially now that IC redriver is redundant).

  • Confer with relevant person.

  • Purpose of Voltage divider at UV Cutoff Interface on PD Board

• Make this a power connector!

• Move relevant signals like VBAT IS and UV_COMP to PD board appropriately