CC-CV power supply as a contolled load for batteries (i.e. Rigol DP832)

[RoGeorge]

This is about how to use a normal CV-CC power supply as a controlled load for discharging batteries with a constant current, and thus measure a battery's capacity C (mAh).

A normal power supply can control the current through its load, and the top voltage, too.  However, a normal power supply can only source current, it can not sink current and it does not support negative voltage applied to its output.

The goal is to connect a battery in such a way that we can keep current flowing out of the power source, and voltage on the power source to stay positive, while the current through the battery under test acts as a discharging current.  Else said, the battery needs to be connected in reverse, with the positive terminal to GND.  Then, we will need a drop voltage bigger than the battery's open voltage, so to create a positive voltage for the power supply's positive terminal.

That can be done with a series resistor and a big enough current set from the source, so the U_Rs = Rs * Is >= Vbatt, or with a Zener diode, or an active circuit to drop a voltage >= Vbatt, etc.

Some batteries can deliver huge currents, so special measures must be taken to protect the output of the power supply.  Here, diode D1 is connected in reverse at the output of the power supply, so to avoid negative voltages.  Diode D2 protects in case V outside is bigger than V generated by the power supply.



The set current in the power supply will be the discharging current.  The set voltage of the power supply will limit the min discharge voltage for the battery.  For example, if the battery has to be discharged with 1A until it reaches 10V, then the power supply must be set for 1A and 5.5V (-10V on battery, 15V on Rs@1A and 0.5V on D2).

Note that the actual load is the resistor R, not the power supply.  The power supply only helps with keeping a constant discharging current, and with monitoring the current and the voltage (also with logging, in case of a DP832).

[macboy]

This is a non-obvious way to use a CC/CV power supply, that most people wouldn't think of. You are not the first. I read about this in HP Harrison Labs Tech Letter #2 - CC-CV Regulated Power Supplies.

Note that with a single power supply, the discharge is not automatically stopped. To accomplish this, you must use two supplies, one used to set the constant current discharge (as you described), and another one to stop the battery from over-discharging. This one is connected directly across the battery and set to be equal to the final discharge voltage (e.g. ~10.5 V for a 12 V Pb battery, or ~3.2 V for a LiPo cell), and current limit greater than the discharge current. When the battery reaches the discharge voltage, this supply takes over supplying current to the load (the load still dissipates the same power even after the battery stops discharging).

Try simulating it in LTSpice (etc.) with a large capacitor of several F set to an initial voltage of around the fully charged battery voltage. Add realistic ESR to the cap for the internal resistance of the battery, and some resistance for the connecting leads between power supply and load to give a more realistic transition at the end of discharge.

[RoGeorge]

I read about this in HP Harrison Labs Tech Letter #2 - CC-CV Regulated Power Supplies.

I didn't know about Harrison HP.  Found the Harris Tech Letter you mentioned, will give it a read, thanks! 
http://hparchive.com/Application_Notes/Harris-Tech-Letter-02.pdf



About stopping the discharge, I think it can be stopped if a bidirectional power Zener is used instead of the former discharging resistor Rs.  Something like this:



The X axis represents the voltage of the battery under test, while it is discharging from 18V to 8V.

The green, yellow and red traces correspond to DP832 set for 1A and 3V, 5V or 9V.  Note how the voltage setting of the DP832 determines the stop discharging voltage (i.e. for DP832 power supply set to 5V, the stop discharging voltage is about 14V, while for DP832 set to 9V the I(Battery) current becomes negligible after Vbatt becomes smaller than 10V).

- Dp1 and Dp2 are protection diodes against accidentally pushing current into DP832 or connecting a reversed voltage
- Dz1 (Zener 15V), R1, Q1, R2 and Qload together form a power Zener with a constant V_CE voltage drop on Qload
- Qload transistor is the main load that discharges the battery, must be able to dissipate about 20W
- D1, D2, D3 and D4 form a bridge that keeps the same polarity for the power Zener circuit, just in case DP832 becomes zero (e.g. a mains power surge).

- Dcv simulates the set constant voltage for the DP832 power supply, and together with I1 they form a crude model of a CV-CC power supply, where I1 (here 1A) is the current to be set on the front panel of the DP832, while the forward voltage of Dcv (here the Vdp832 parameter, 3V for the green trace, 5V for yellow trace or 9V for the red trace) is the voltage to be set on the front panel of the DP832.

[RoGeorge]

In practice it turned out that the previous schematics are too complicate for the goal of testing AA/AAA batteries.

For small NiMH, NiCd or alkaline batteries, a simple series diode with Vdrop > Vbatt would be enough.  The usual 0.6V drop of a single Si diode is not enough, so a Zener diode might be used instead, or maybe a chain of 3-4 series 0.6V diodes.  A LED might be used as well.

Mains light bulbs usually have 14 power LEDs inside, and already mounted on a radiator.  Each LED can support 100...200 mA (connect more LEDs in parallel if higher discharging currents are needed), and the voltage drop is about 2.5...3.5V per LED.



Power LEDs will be ideal for a load here, and they can be scrapped for free from defective light bulbs. 

This is the latest schematic of the AA/AAA controlled discharger adapter for DP832 (or other CV/CC power supply), which is nothing more but a fusible and a power LED in series with the battery to be measured:



DP832 maintains the constant discharging current, can log the voltage/currents during test, and can also cut down the power when the battery reaches the minimal voltage.



On the X axis is the voltage of the battery under test, while discharging it from 1.5V to 0V.  On the Y axis, on the left is the voltage seen by DP832 (plotted in green), while on the right is the discharging current (plotted in red).

Note how the DP832 sees a raising voltage while the battery under test is discharged.  This can be used as an indicator to automatically cut down the power when the voltage raises too much.  By setting the OVP (Over Voltage Protection) in the DP832, the discharge can be ended at the desired minimal voltage (to avoid battery damage by over discharging).

Once a DP832 channel is turned off, the current through the battery under test will become negligible, because a typical white LED voltage drop is about 3V and an AA/AAA battery doesn't have enough voltage to make the current flow through the LEDs.