Amp-hour meter vs watt-hour?

[robl]

I know that there are a number of popular totalizing amp-hour meters available to the RE market, Xantrex, link, Brandt, Trimetric, etc.

However I'm curious why people are settling for devices that only measure current and time, rather than also integrating voltage measurents. It seems to me that there is room for quite a bit of error over time this way, especially at the lower voltages. Is it the extra expense -(a digital, industrial totalizing wattmeter with logging capability generally costs around $600 to $1K), or is there something about battery charging/discharging algorithms that reduces the need for integration of voltage and current to maintain accuracy?

Rob

[Ungrounded Lightning Rod]

I'm curious why people are settling for devices that only measure current and time, rather than also integrating voltage measurents. It seems to me that there is room for quite a bit of error over time this way, especially at the lower voltages.

Current charging and disccharging a battery moves ions, oxidizing and reducing them.  In a lead-acid battery, two electrons of current causes one plate to lose an atom of lead and the other plate to gain an atom.

Compensating for the voltage would give you a better idea of how much power you put in and how much you took out.  And it would give you a rotten idea of how much was left, because both processes lose a significant ammount of the power as heat.

Integrating the current in and out (which amounts to counting the electrons) gives you a DANDY measure of how many lead atoms are on which plate, and thus the state of charge of the battery.  There's a SLIGHT error due to leakage currents that don't go through the meter.  But it's enormously better than measuring power.

[robl]

Thanks for the prompt reply, Ungrounded

Darn, it does appear I will probably have to do some more research. I realize that if accumulating watt-hour meters were more useful in battery-monitoring they would be more common. It was the logic behind the decision that eluded me, so I appreciate the clarification. Would I be correct in assuming the standard Trimetric-type units are pre-calibrated for C/100-type charge rates? Or does it really matter, based on your basic electron-transfer model?

I track the heat pretty carefully, as I don't abide by the c/xxx rules for charging but use a mercury thermometer sitting in the battery water in a pilot cell. I monitor electrolyte temperature to decide when to terminate the absorption-charge rate from my generator (not an issue with solar and hydro inputs)to reduce running time and maximise fuel-efficiency. This makes the voltage rise behave somewhat differently than what traditional accumulating a-h meters are calibrated for, I think...and thus I'm concerned the built-in efficiency assumptions may not track properly.

Regards

Rob

[Flux]

The victron Energy one uses Peukert effect to compensate for charge and discharge at different rates. I have no idea what the others do.

They are really state of charge indicators, not energy meters, but the victron at least has readouts of charge in and out over a period.

Ultimately you need to finally determine when you are fully charged. They arrive at this point from data you give them and some assumptions they make about state of charge at a time after reaching a certain voltage and the way current is falling but you need to correct this point occasionally.

True energy meters would give you information on energy produced but would be useless as a state of charge indicator. You need charge efficiency with a basic AH meter for fairly constant loads and if the load changes much you need a reasonable correction for Peukert effect.

Flux

[robl]

Flux,

Honestly, it never occurred to me that the Peukert effect played an equally important role in the charge cycle. My bad. Ideally then, if the battery manufacturer can supply it, the number can be entered into the ah meter(or crudely selected by choosing battery type, flooded, AGM, etc.)and then hopefully adjusted after a few discharge-charge cycles. Great, one less project on my to-do list.

Thank you both,

Rob