Quantifying battery behavior is critical to development of new battery technologies and energy storage systems. While it is straightforward to measure properties such as impedance at short timescales (i.e. frequencies larger than ~1 Hz) the relevance of this is questionable since rechargeable batteries in normal usage are often cycled on timescales of hours or days. Making measurements at these timescales, for example impedance measurements below ~1 mHz, is more challenging. In this paper we discuss approaches to quantifying battery behavior at timescales from hours to weeks (frequency scales of ~0.1 mHz down to ~1 μHz). We present frequency domain measurements and time domain measurements, achieved through four-point measurements with a Keysight 66332A at around 100 mA RMS. At low frequencies significant charge is shifted in a measurement cycle, complicating interpretation. The digitization of a sine-wave may introduce errors such as constant current offsets that build in significance with time. The operating point (DC voltage level) of the battery should be controlled since it influences impedance at the lowest frequencies as a result of the voltage-dependent nature of different electrochemical processes. We relate the voltage-dependent effects to time-domain measurements such as cyclic voltammetry and incremental capacity analysis.