How to design a switched mode battery discharger

Question

I have a very basic understanding of switched mode power supplies and know that this is a simple buck converter suitable to e.g. charge a 12 V car battery from an input of, say, 24 V, with a dc current that may have some ripple:

The voltage on the battery resp. the charging current can be controlled with the duty cycle on the switch.

My problem is to find a similar schematic for discharging the battery with a controlled current. The purpose is to measure the discharge process in order to find the capacity of the battery. This is a schematic with the disadvantage that the discharge current goes on and off, instead of having just a little ripple:

All I find on the Internet are schematics that regulate the output voltage / current over the resistor, but in my project I need to regulate the current in the battery instead, and want a dc current with just a little ripple.

Any ideas?

Answer 1

Use a boost circuit instead of buck then the battery current will be continuous

in a buck converter the curren draw is sufficient for the converter to enter continuous conduction mode, because the inductor is connected directly to the source, the source will also see continuous draw (with ripple at whatever level the inductor ripple is less any absorebed by input capacitance)

Answer 2

The main difference between charging and discharging is that during discharge you want to waste power. Therefore no matter what technique you use, you will need something to turn the power into heat.

The simplest way to do this is just switch in resistors in parallel until you get the desired current. However if the resistance is fixed you will need to log current or voltage periodically during the discharge. For finer adjustment you could PWM one of the resistors, perhaps with feedback to regulate the average current. Smoothing may not be required because the battery itself acts as a filter.

Another way to get constant discharge current is with a linear regulator. Set the output voltage to just below the minimum battery voltage minus regulator dropout voltage, and put resistance across the output to draw the desired current. The regulator only has to 'burn off' the voltage difference, so the load resistor(s) will absorb most of the power.

On the other hand you might want to reproduce operating conditions for a device that draws constant power (eg. a switching DC/DC converter). In that case you could simply use an off-the-shelf DC/DC converter with load resistors on the output.

To get constant input current to a switching converter you need to monitor the input current and use it to control the PWM ratio. A boost-mode converter draws continuous current, but it is not necessarily smooth