# Pulling voltage on a “pull-up” input down to match voltage of voltage output from another device

I have a large lithium (LiFePO4) battery pack composed of 16 cells in series. It is monitored by a BMS (Battery Management System) which is interfaced to a battery charger (Victron Quattro).

The BMS and Battery Charger are from different manufacturers but both include "industry standard" ways of interfacing with each other via analog inputs/outputs.

As the BMS monitors the battery it offers an output pin where the BMS sets the voltage on this pin between 0 volts and 5 volts to indicate the maximum acceptable rate of charge current (amps) into the battery:

0.0 volts = Max charge of 0 amps 1.0 volts = Max charge of 14 amps 2.5 volts = Max charge of 35 amps ...etc... up to: 5.0 volts = Max charge of 70 amps

(Actual amperages adjustable, this is what I have it configured for)

The battery charger conveniently (or so I thought) also has an "input" that, according to their marketing material, can read a voltage and adjust the chargers output current based on the voltage read on the input pin. It does this in a linear fashion and I have it configured to match the BMS:

0 volts = 0 amps, 5 volts = 70 amps, linear relationship in between.

Here's where it gets tricky: Once I actually got my hands on the charger I discovered that the "input" pin is held high with an internal 10 Kohm pull-up to 5v. The expectation is that a user would connect a potentiometer between the input pin and ground and use that to adjust the resistance and "pull down" the voltage on the input pin to control the voltage and thus the rate of charge current from the charger.

So I'm left with trying to figure out how I can build a (Hopefully simple) circuit that will pull the input pin on the charger "down" to match the voltage of the output pin on the BMS unit.

The challenge I've faced trying to research this is that every thing I find referring to anything related to pull-up resistors is for digital logic (IE: On or off). However this particular piece of equipment is using it for an analog input. The few analog resources I found were for reading the amperages of input signals rather than the voltages.

The other area I tried to research was voltage regulation. But everything related to voltage regulation focuses on the wrong side of the equation: Regulating the output from the regulator rather than regulating the current to pull down the input to a specific voltage.

I've also tried to research using a simple transistor for this. However I haven't been able to find anything that shows a transistor being used in this way though logically it seems to me it might work.

I've already reached out to the battery charger manufacturer (Victron Energy) and asked if it's possible to disable the internal pull-up to 5v on this pin. They say it's not possible.

In Summary: Is there a relatively easy way to pull the voltage on a 5v pulled-high input pin down to match the voltage of another source?

• Welcome to EE.SE. Add a 5 volt zener diode to that pin so it cannot go above 5 volts. Try 1 K to 10 K resistors to the higher voltage. – Sparky256 Jun 21 '18 at 23:35