To be clear, I am not looking to do any automotive modifications. I am looking to use peripheral equipment (DMM specifically) to detect the general state of charge on the battery.

I like to take extended hunting trips and I use my car to charge my electronics and lights. I like to start charging using just the battery, and turning on the car as the battery drains.

I want a way to tell when the battery is dropping to say, 90% charge. I also want a way to tell when the battery is done charging while the car is running.

I have a fluke 115 for my DMM. I think I can tell when the battery is at 90% by measuring how far the voltage dips. However, I'm not sure if the drop is great enough or my DMM sensitive enough to detect the change.

To detect when the battery is done charging off of the alternator, can I detect a voltage drop across the battery cable to estimate the current? High current=charging, low current=complete.

Suggestions for better methods?

 

Detecting SOC of a lead acid battery by measuring voltage is fraught with problems.

Will there be a load on the battery when you try to determine discharge? If so, the voltage you read is greatly effected by the load current due to the internal resistance of the battery. It would be best to turn off the load while making the measurement.

After starting the engine, the battery voltage almost immediately jumps up to 14.5V or whatever the alternator voltage regulator puts out and it just stays there as long as the engine is running. Measuring the current into the battery, and terminating charging when the current drops to some low current level may work, but you will burn a lot of car gas while the charging current drops from say 1A to 100mA.

The rate of putting charge into the battery is very slow to go the last 10%. You may want to cycle the battery between say 60% and 80%, which will happen a lot faster than going from 90% to 100%.

 

It's complicated. here's a reference on it:

http://batteryuniversity.com/learn/article/how_to_measure_state_of_charge


How to Measure State-of-charge

Voltage Method

Measuring state-of-charge by voltage is the simplest method, but it can be inaccurate. Cell types have dissimilar chemical compositions that deliver varied voltage profiles. Temperature also plays a role. Higher temperature raises the open-circuit voltage, a lower temperature lowers it, and this phenomenon applies to all chemistries in varying degrees.

The most blatant error of voltage-based SoC occurs when disturbing the battery with a charge or discharge. This agitation distorts the voltage and no longer represents the true state-of-charge. To get accurate measurements, the battery needs to rest for at least four hours to attain equilibrium; battery manufacturers recommend 24 hours. Adding the element of time to neutralize voltage polarization does not sit well with batteries in active duty. One can see that this method is ill suited for fuel gauging.

Each battery chemistry delivers a unique discharge signature that requires a tailored model. While voltage-based SoC works reasonably well for a lead acid battery that has rested, the flat discharge curve of nickel- and lithium-based batteries renders the voltage method impracticable. And yet, voltage is commonly used on consumer products. A “rested” Li-cobalt of 3.80V/cell in open circuit indicates a SoC of roughly 50 percent.

The discharge voltage curves of Li-manganese, Li-phosphate and NMC are very flat, and 80 percent of the stored energy remains in this flat voltage profile. This characteristic assists applications requiring a steady voltage but presents a challenge in fuel gauging. The voltage method only indicates full charge and low charge and cannot estimate the large middle section accurately.



Lead acid has diverse plate compositions that must be considered when measuring SoC by voltage. Calcium, an additive that makes the battery maintenance-free, raises the voltage by 5–8 percent. Temperature also affects the open-circuit voltage; heat raises it while cold causes it to decrease. Surface charge further fools SoC estimations by showing an elevated voltage immediately after charge; a brief discharge before measurement counteracts the error. Finally, AGM batteries produce a slightly higher voltage than the flooded equivalent.

When measuring SoC by open circuit voltage, the battery voltage must be truly “floating” with no load present. Installed in a car, the parasitic load present makes this a closed circuit voltage (CCV) condition that will falsify the readings. Adjustments must be made when measuring SoC in the CCV state by including the load current in the calculation. In spite of the notorious inaccuracies, most SoC measurements rely on the voltage method because it’s simple. Voltage-based state-of-charge is popular for wheelchairs, scooters and golf cars.

 

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To detect when the battery is done charging off of the alternator, can I detect a voltage drop across the battery cable to estimate the current? High current=charging, low current=complete.

You could likely detect the voltage drop from the alternator output to the battery positive terminal, which is a relatively small wire compared to the large cable that carries the starting current. (That cable would have negligible drop from the charging current levels). If you can make an estimate of the wire size and length, then you can get a rough idea of the wire resistance using a wire table. From Ohm's law you can then determine the amount of charging current for a given voltage drop.