Hi,

I am trying to design a 50 Amps capable battery charger for charging a 2000 AH lead acid battery bank of 24 VDC. The project objective is to charge the battery using both solar as well as mains (mains electricity availability is irregular here in Pakistan due to acute power shortages).

For the above purpose I would like to measure the battery charge/discharge current. I initially tried ACS712 because it offers the possibility of galvanic isolation. However that route had to be abandoned because voltage has to be measured which means galvanic isolation is not possible. Now I am toying with high-side measurement of current. The problem here in Pakistan is the non or difficult availability of high side measurement ICs. So I am trying to do some ab-initio design of the input stage using discrete components (or CA3096 type) matched transistor array. The array would be followed by an IN-AMP (I am using the easily available AD620). The current measurement is by a shunt resistor (0.001 ohms). My initial circuit using BC547 BJTs is attached. The purpose is to create a level shifter so that the differential voltage across the shunt can be converted into a voltage level that the AD620 can withstand. Please note that another important project objective is to use the main battery as the power supply for this circuit.

I am using PROTEUS for simulation. I would like the forums valuable views on the design and any corrections that I should incorporate.

TIA

Zeb

 

Put the current shunt in the low side. Then you can dispense with the discrete differential section and drive the IA directly. A low side shunt is also easily scalable to higher voltages if you ever require them.

Since the shunt resistance is very low, you could also just use a single ended op-amp in a differential configuration to amplify the current. The amp should have low offset and offset drift with suitable precision in the resistors. Its not as accurate as an IA but is an option if you are not looking for super accuracy.

Pick off the battery voltage with a suitable voltage divider and buffer it with a unity gain op amp if necessary.

I've done exactly this in a number of industrial battery charger designs and it works well. The circuits described feed a microcontroller's ADC so calibration and offset were taken care of in firmware. An 8 bit ADC was good enough to give volts/cell to .01V/cell accuracy and resolution.

If you are reading directly i.e. no microcontroller, I'd recommend sticking with the IA for the current measurement (shunt still in the low side) to make gain/offset adjustments easier.

Have fun!

 

John.

Thanks for your response.

After much debate (on other fora) I have decided to go with high side. This is the reason that I am sticking to this configuration. I will only 'retreat' to low side once our collective hands are up.

I am aware of the advantage that low side measurement offers particularly the single op-amp difference amp required to bring the voltage levels to feed into a microcontroller. But again I will only go that route once I feel that the road has come to a dead end- which it hasn't yet.

Thanks again for your reply.

Zeb