Pololu 2869 - S9V11MA - Power Supply Modification for Quante 250 DI Geodetic Laser

Robotics and electronics discussion Electronics
1

Byte me, I’m new here! :desktop_computer:

In our company, we have a Quante 250 DI geodetic laser, manufactured in the 1990s. The mainboard is powered with 12V 3A, which is then distributed into two battery packs of 1.2V + 1.2V + 1.2V = 3.6V 3000mAh NiCd cells. These old batteries are now unusable and harmful to the environment.

I decided to replace them with two packs of 2 × 3.2V LiFePO4 cells. Since each original NiCd battery pack would sometimes receive a higher voltage (e.g., 4–6V), this could lead to overcharging the new 3.2V LiFePO4 packs.

I plan to use two Pololu 2869 - S9V11MA step-up/step-down voltage regulators (2.5–9V, 1.5A) for each LiFePO4 pack. I will also include a BMS to protect the batteries from overcharging/discharging.

The mainboard connector is used both for charging and for drawing power from the batteries when disconnected from the charger. To avoid feedback loops in the system, I decided to add a 20V 3.7A MOSFET with a 100kΩ resistor.

The S9V11MA will regulate the voltage down to 3.6V, but here’s my question:
After disconnecting from the charger, will the S9V11MA maintain the set 3.6V by stepping up the voltage from the 3.2V LiFePO4 cells, or will it just pass through the 3.2V from the batteries?

Although the device is old, it is an important part of our company and holds significant sentimental value. Any suggestions are highly appreciated.

Best regards,
Dawid

Quante 250 DI
Quante 250 DI1920×965 186 KB

2

Hello.

It looks like you’re trying to charge your battery from your mainboard through the regulator, and when the charger is disconnected the mainboard would draw current from the batteries instead, but in that case, the regulator would be connected backwards and it is not designed to pass current from its output through to its input. Also, it is not entirely clear to me what you are trying to do with the FET in your diagram.

Brandon

3

Hello,

I couldn’t find a bidirectional low-voltage (2–14 V) regulator from Pololu.

I can forcing current flow to simulate a bidirectional voltage regulator using unidirectional components like the S9V11MA.

  1. When charger is connected:
    LTC4412 detects higher VIN.
    Turns on MOSFET.

Power flows from charger to S9V11MA.

  1. When charger is disconnected:
    VIN drops below battery voltage.
    LTC4412 disables MOSFET.

Battery takes over instantly.

  1. S9V11MA always regulates output, no matter which input is active.
4

It is hard to follow what you’re trying to do, but I suspect you would need to include at least several more diodes to have a chance of it working. If you want more help, you should post a cleaner schematic and clarify what the BMS is doing.

Brandon