Fuel Gauge Question

[Don]

Based on the technology of the SciB battery, we should be way ahead of the LEAF folks. Time will tell.

Except we don't have SciB cells . . . .

Don

 

[Don]

Pardon me if my questions have been answered, but I was unable to find the answers.

The fuel gauge reflects the charge level of the battery pack. A battery cell's empty state is a when the electrochemical reaction that produces electricity has depleted the necessary reactants, and the cell is producing 0 volts.

I'm not aware of any battery chemistry where a zero SOC equals zero volts

For a 12 volt lead acid battery, 'dead' is considered to be around 10 volts. For lithiums, I found this chart:

4.2V = Full
4.1V = 90%
4.0V = 80%
3.9V = 60%
3.8V = 40%
3.7V = 20%
3.6V = Empty

I imagine our 'gas gauge' uses something similar to the above, assuming it's meter is voltage based

Don

 

[peterdambier]

Things are very complicated with LiPo, Lithium Iron Phosphate. Charge state and voltage are almost not related at all - in the operating range at least. Best use temparature for detecting both fully charged and fully discharged. The voltage is almost flat in the operating range and drops sharply or rises sharply at the ends. Those ends is where the battery gets hot.

Best way to treat them is controlled discharge all cells to the same idle voltage in the knee of the discharge curve. Take a week or half to do it right. From now on charge and count ampere hours charged or discharged. Forget self discharge, you cannot meter it in the first place if there is any. Temperature and amperes in, amperes out, do change the voltage dramatically but almost unrelated to the state of charge. That is why there is a hint that quick charging is harmless compared to normal charging more than 80% or discharging more than 80%. All battery management systems that try to balance outside the discharged knee have proven to result in fire sooner or later.

 

[alohart]

I'm not aware of any battery chemistry where a zero SOC equals zero volts

From a purely electrochemical perspective (back in the dark ages, I earned a Ph.D. in electrochemistry), a zero SOC equals 0 volts. If a cell produces any voltage, an electrochemical reaction is producing it, so the SOC isn't 0. A cell can continue to provide electrical current until its voltage is 0.

However, discharging to very low SOC's can cause irreversible damage, so non-zero SOC's below which damage might occur are being defined as 0 SOC. Likewise, high but not 100% SOC's are being defined as 100%. The i fuel gauge is a good example where full isn't really a 100% SOC and empty isn't really a 0% SOC. The gauge just defines the minimum and maximum SOC's that the i's BMS will allow to protect the battery pack's longevity.

I suspect that the i's fuel gauge works similarly to the charge level gauge in some hybrid vehicles. It doesn't measure the battery pack's voltage which varies considerably depending on the current flow out of and into the battery pack. Instead, it's accumulating current flows in and out with probably some recalibration that occurs periodically when the battery pack voltage exceeds a certain maximum defined as "full" or falls below a certain minimum defined as "empty". I suspect that if one disconnects the 12 v. battery for long enough, the fuel gauge would read empty when the 12 v. battery is reconnected because the gauge has lost track of its current flow count when it lost its 12 v. power.

 

[TXCharlie]

I always leave my odometer set to "Miles Remaining" and just kinda ignore the bar display. So far, the miles remaining calculation hasn't let me down (even though it does drop faster than actual miles driven at times, such as when going 80mph on the freeway).

I got it down to literally one mile remaining but it never entered the "Turtle" mode, so I guess it has a little reserve even though the bar graph showed empty and was flashing like crazy.

I charge mine fully each night, because I never know if I might have to make a detour to bypass freeway wrecks, run extra errands, etc. Hopefully if it damages the battery, it'll do it before the battery warranty is up.