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Forum Index : Electronics : coulomb counter charger
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| isochronic Guru  Joined: 21/01/2012 Location: AustraliaPosts: 689 |
I am thinking of trying to build a coulomb-counter charger. Measuring and integrating charging current over a few hours is a bit awkward as the sensors are usually not high accuracy. So I am wondering if using an intermediary switched capacitor would work. The idea is, a known capacitor is charged, and then discharged (through resistors etc) to the battery to charge it. The capacitor voltage is measured before and after, so the amount of charge transferred to the battery is then known very accurately and can be counted as the cycle is repeated quickly over a few hours. But, does that sound feasible in practice ? I can see some questions already eg are no-leakage capacitors available (?) in suitable sizes (?) at reasonable cost (ha) locally (*) .. |
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| Warpspeed Guru Joined: 09/08/2007 Location: AustraliaPosts: 4406 |
A much better and far more accurate and stable method is to use a voltage controlled oscillator, where the output frequency is proportional to current. You then use a digital counter with or without a display, to count up the cycles over the measurement period. Say for example 1Hz per milliamp. 2.34 amps = 2340Hz For six hours of counting up = 14,040 counts (milliamp hours)or 14.04 amp hours. Its also possible to detect the direction of current flow and use an up down counter, so it counts up during charging, and counts down during discharge. You can scale it so the counter reads in amp hours, or even percentage of rated battery capacity or whatever. All kinds of options are possible such as turning off the charger at some maximum count. Cheers, Tony. |
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| isochronic Guru Joined: 21/01/2012 Location: AustraliaPosts: 689 |
Please describe it a bit more ? I don't know how the voltage-controlled oscillator ties in. Is each cycle assumed to have the same charging effect ? |
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| Warpspeed Guru Joined: 09/08/2007 Location: AustraliaPosts: 4406 |
I am assuming you wish to measure current x time (or ampere hours) which is the usual metric for battery capacity measurement. A very convenient way to do that is to convert the battery current to a frequency that is directly proportional, with a voltage controlled oscillator. There are several low cost integrated circuits available that will do that very accurately. A good one to use for this might be the Analog Devices AD654. http://www.analog.com/media/en/technical-documentation/data-sheets/AD654.pdf The higher the current, the higher the frequency. You then count the pulses coming out of that over time with a digital counter. Cheers, Tony. |
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frackers Newbie  Joined: 06/11/2009 Location: New ZealandPosts: 23 |
Coulomb counting is best done with a battery management chip as they can integrate the current readings. I've used a DS2438 1-wire chip in the past with my Mk 2 Charge Controller and the Mk 3 Charge Controller. This chip measures 30 times a second and the battery capacity range can be extended by using an external variable in addition to the internal 8 bit register. I'm currently working on a STC3105 based measuring device which uses I2C. Shame its only available in a TQFN8 package - this has tried my home etching techniques somewhat!! Robin down under - or are you up over |
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| Warpspeed Guru Joined: 09/08/2007 Location: AustraliaPosts: 4406 |
A voltage controlled oscillator also continuously integrates the current, it tracks it exactly up and down with every small variation. I agree though, a microprocessor solution is a good one, but for some people a much simpler and purely hardware approach is a very practical alternative. Cheers, Tony. |
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| Solar Mike Guru Joined: 08/02/2015 Location: New ZealandPosts: 1138 |
If you have a cpu doing the measurement, another way is to use a voltage controlled pulse width generator and use the cpu's high speed clock to measure their width to determind a more accurate current measurement. See Article Here This circuit runs off 5v and would be quite inexpensive to build.  Cheers Mike |
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| Warpspeed Guru Joined: 09/08/2007 Location: AustraliaPosts: 4406 |
Good one there Mike. Straight conventional a/d conversion always suffers from reduced resolution at low input amplitudes, so is less than ideal for measuring over a very wide dynamic range. If you wanted to measure individual milliamps and still have a 100 amp full scale range, it becomes difficult. You might need 100,000 bits of resolution. Maybe a 17 bit a/d converter which is going to suffer from severe noise problems if nothing else. A 0 to 100Khz VCO, and then accumulating counts over an exact one second interval can do it easily, and also average out any noise over the one second measurement interval. So the result should be reasonably noise free in a noisy environment without requiring any further software averaging. If you are really serious frequent auto zero correction and full scale calibration can be applied by a host microcontroller to correct for any long term drift. Another advantage is that the output is only a single serial bit stream, so its very convenient if using an opto isolator (or wireless) or infra-red data link for a fully floating measurement. A 17 bit conventional a/d might require at least 17 opto isolators... Voltage to period measurement has many of the same advantages and is sometimes much more useful. But both techniques are well worth knowing about. Cheers, Tony. |
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