Simple battery simulator

[Rachie5272]

I'm looking for a simple Li+ battery simulator, for testing charging circuits.  I don't need anything fancy, and don't even need to measure the output precisely.  I need more of a dummy battery, in order to keep the charger trying to charge.  Something which simulates a half-charged cell, and will simply dissipate any current sourced by the charger.

Does such a thing exist, or do I need to buy a full benchtop load/simulator?

Thanks!

[ResistorRob]

You can do this pretty easily with a DC power supply and a DC load with the outputs hooked together.   Set the power supply to your desired battery voltage, and set the DC load to constant V mode, a couple dozen millivolts above the output of the power supply.  When your circuit tries to charge the battery, the 'battery' voltage rises  a few mV, but not enough to trip OVP on the power supply and the load starts sinking current.

[bdunham7]

Why not a 3.3 volt 5 watt zener diode in series with a 1 ohm 1 watt resistor?  That would be a 'good enough' simulation of a small cell with a max charge rate of about 1 amp.  Just parallel a few if you want to simulate a larger cell.

[Berni]

There are some bench power supplies that are so called 2 quadrant where they can source or sink current, those can look like a battery pretty well.

If you don't have one but have an electronic load that works too. If you don't a zenner can indeed be used to sink away excess current (But needs to handle the power). Still one of the simplest ways is simply putting a large power resistor across the PSU output, so then the PSU keeps supplying power to heat the resistor, but any current that gets pushed in also gets burned on the resistor while the PSU backs off its current to keep the set voltage. So with that setup you can push back in however much current the resistor can pull at the given voltage.

[SilverSolder]

For small batteries/cells, I do this with a bench top DC supply and a resistor.

The idea is to sink enough current in the resistor that the DC supply never gets "back fed" from the charger (the DC supply always has to set the voltage across the resistor, not the charger).

E.g. say your charger is able to output 100mA and the lowest battery voltage you want to test is 2.8V  -  so you choose a resistor that is able to sink 100mA at 2.8V, or 28 ohms max.

Test method:

1. Connect your benchtop DC supply to the resistor and turn it on.   The supply sets the "battery" voltage - so if you want to test with a battery voltage of 3.7 V, you just dial that in on the supply.

2. Connect the 100mA charger.   What happens now is that the DC supply will reduce its current by 100mA, to make up for the current from the charger and keep the voltage at 3.7V

Now, you can literally just dial the bench supply to any setting between 2.8V and 4.2V (the battery max...)  and see how the charger behaves. 

All for the price of a resistor you probably already have in your junk box! 


[note: this is the same idea as @Berni above]