What are the AC characteristics of a battery?

Question

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.

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?

Answer 1

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.