I'm planning on building a single cell AA NiMH battery charger and in the process I decided that I must also build a battery simulator to help me debug the charger. In my research, the best simulator circuit I came across was the one described in the Linear Technologies Application Note 58, Appendix B (page 35), which relevant part of its schematic is below.

AN58 - Battery Simulator schematic - detail of R10 and C5

In the circuit description, the AN says:

R10 and C5 simulate the AC characteristics of the battery.

Well, I have no idea what that means. I did search the net for a description but couldn't find anything useful.

So, my questions are:

  1. What exactly are the AC characteristics of a battery?
  2. Are there AC characteristics that are particular to NiMH cells?
  3. Out of curiosity, what happens if I don't add those components to my simulator?

The "AC characteristics" have to do with how the battery responds to rapid changes in load current or charging current.

The simple fact is that a battery is NOT "just" as voltage source and a series resistor. The electro-chemical reactions inside the battery take time. Thus, you can apply a sudden charging current and the terminal voltage will take some time to settle to the steady voltage it will have.

This is particularly important when you use a PWM charging current (that is, pulsed, with a varying on-time). In this case, the AC characteristics are particularly important.

I do not know how these characteristics differ between, say, lead-acid and NiMH or LiIon.


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