I was just yakking with an engineer/programmer of some skill, who was telling me that a simple voltmeter is probably a bad way to measure the charge remaining on a pair of 12V lead acid batteries. At least, it's not an accurate way to do it.
And here I was thinking my robot battery gauge project was going to be simple. :-)
What say you? Can a simple meter do an adequate job of telling me the relative charge remaining in the batteries? I'm not looking for 65-segment accuracy...10% increments would be perfect, 20% increments fine, and 25% increments will work.
It should work reasonably well for gel cells, particularly if you know what the current drain is at the time of measurement. If you don't know the current drain (especially if the instantaneous drain might be high), you may not get there. It's a pretty straightforward application of Ohm's law - the terminal voltages drop by the IR-losses within the batteries.
You probably need to look at the discharge curves for the battery you are using. If you don't have a curve, then generate one by putting the battery under load and measure its output voltage over time as it discharges.
The discharge curves for a typical lead acid battery have pretty noticeable slopes for a given discharge rate, so it's not infeasible to estimate the remaining capacity if you measure the voltage and discharge current *and* the battery temperature. You might be able to infer the current based on a known draw in a given state of the robot and assume "room temperature" but measuring them both, in addition to voltage, would be better.
The manufacturer may publish the necessary curves or you might end up having to create them yourself. Good project... ;-)
There are commercial testers which work by applying a (modest) known load and measuring the before and after voltages. I assume they work by looking at the change in battery voltage after a defined quantity of charge has been removed ...
Yes, that's almost exactly what was suggested to me this morning. That type of system may be more than I need, and it's certainly more expensive and complex.
The system is self-powered, so the batteries are under a nominal load before the sensors can work at all. I'm thinking that the voltage under such a load is a "reasonable enough" predictor of charge status.
The reasons for applying a load to a PbA battery are to eliminate surface charge. If you don't, voltage will not correlate at all well to capacity. Here's a good explanation of surface charge:
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(See under stratification. That's what they call it.) When you have a battery with one or more "weak" cell(s), it will show 12-13V under no/light load, but will drop sharply under a moderate or high load. A "weak" cell is really a dead cell (electrodes are shot) that looks ok under light loads only because it has some surface charge. Soon as you break that down, voltage for the whole battery plunges. The reason the load helps is that current flow causes mixing. From this you can also see why the older the battery, the less reliable no/light-load charge levels are as a capacity measure.
Even under a load, unless you're actually monitoring AHs in and out, you'll never get more than a rough estimate. Actually, even with AH measures, precision is just not very high. I did a fair bit of battery testing back in the early 80s, while working on the electric-vehicle program at jpl, and my experience was that battery measurements are always very noisy because capacity depends on so many factors. (I got to test drive an early prototype of the EV-1. I had no idea at the time that it would someday be famous :)
The problem of course is the variations in voltage that occur as the battery is loaded. My forklift for example will be reading 50% but lift a load and move the forklift and you have 10-25% on the guage. So to have an sort of accurate estimation of battery life you need to take a reading when applying a set load. You can then do a few experiments to see the actual time remaining of battery life.
You measure the voltage with no load. There are charts that describe total charge left. For a given manufacturer and temperature, the voltage is fairly predictable. they sell those charge indicators for boats. they also include it on many trolling motors. For a given load you can also predict voltage vs charge. Measure the voltage directly on the battery terminals ONLY. If the battery is old or defective, its likely the measurments will be off.
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