Battery SoC estimation - Page 1
Forum section: electronics / projects

author: SiliconWizard
author_rank: Super Contributor
timestamp: November 29, 2024, 12:59:39 pm
content: What approaches do you use for battery SoC estimation? (For various types of batteries: primary, rechargeable...)My "usual" approach for primary batteries (at least alkaline) is to just read the cell voltage. For lithium batteries, that's not as predictable.For Li-ion batteries, I tend to use a dedicated fuel gauge chip, such as the MAX17048 or the OnSemi LC709204F.What about you? Any original idea?

author: Salitronic
author_rank: Contributor
timestamp: November 29, 2024, 01:37:05 pm
content: There are 3 key parameters that you need to keep in mind when determining the state of charge of a battery: cell voltage, loading current and temperature. If all of these are known then you can get a fairly good estimation of the battery state of charge. So typically if I'm not using a fuel gauge chip I take direct voltage and temperature measurements, the for the loading current I would typically either measure it or more often simply estimate a value. This can be based on either a known fairly constant current draw of the device or by taking the voltage measurement at specific periods where the loading current is expected to be sufficiently small (sleep modes for example). I then feed these values into tabulated discharge curves for the particular battery and get the estimation.Fuel gauges are not always an option really. For replaceable batteries whether they are primary or secondary you always have the problem that you don't know the initial state of charge of the battery. It might be also not so easy to determine that a battery has been replaced. Fuel gauges are really only useful when you have a secondary battery that is permanently installed in the device. Even then I'm not really a big fan of fuel gauges as at least the traditional ones have a relatively high consumption themselves, although there are newer ones which have much smaller consumption.

author: Siwastaja
author_rank: Super Contributor
timestamp: November 29, 2024, 01:40:48 pm
content: Often just voltage alone. Many li-ion cells aresurprisingly linear, I mean look at that, you have nearly straight line representation from 100% to 10% SoC. In presence of peak currents, especially at low temperatures the indication jumps but you don't necessarily need to add current sense + compensation, a simple moving average filter (e.g. during last 1 minute) fixes that pretty well.Does not work with every chemistry or every product though. Some are flat, check the datasheet discharge curves.Coulomb counting is nice but requires reset logic so certain use pattern (i.e. regular guaranteed full charges), if you don't have those then you have to reset it, tadah, voltage based at any arbitrary point, so back to square one, estimating SoC from voltage. Coulomb counting also needs battery capacity tracking; direct conversion from voltage automagically tracks capacity fade without any effort being put into it. This is why making areallygood battery "fuel gauge" is difficult, it's about understanding correlations of many parameters and keeping state.Being what I am, I would never use BQ anything fuel gauge chip. I either use simple and reliable voltage based and accept the inaccuracy, or I design the "fuel gauge" algorithm from scratch, then at least I have control over it.I did do a coulomb counter with simple "full charge" reset points on a LiFePO4-based conversion EV (20kWh or so pack) in the past, was it around 2013 or so. It did work fine but would have probably failed miserably if I decided to only give partial charges for some weeks. Or use the car for years so that there is capacity fade. But for the year or so, always fully charging after a trip it was accurate and I drove it down to 0% many times without ugly surprises.