mA-hour meter

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I'm working on an amp-hour meter (actually milliamp-hour) and am
planning on using an Allegro current sensor to measure current from a
discharging battery under load. I plan on displaying the accumulated
amp-hours on an LED display, and a PIC will be running the show.

Is the Allegro the best method for doing this, or is there another way?
I haven't gotten into it yet, but it looks like the Allegro takes some
calibration to make it accurate.

Any suggestions? Thanks.

Re: mA-hour meter
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Since you are measuring mA (you don't mention how many) you should chec
out the FAQ on the allegro part

Given the low resolution and accuracy issues, and assuming that you ar
sub 1Amp, I would reccomend that you measure the voltage across a curren
shunt. Maxim has a part that will allow an external current shunt
National has one with an internal one. I think TI (Burr Brown) & Linea
Tech also have products. Here's a start at Maxim. =

But you don't need a dedicated part- you simply need a differenc
amplifier that can handle input voltages beyond the supply rail. You ca
make your own with resistive dividers at the input, or use a product lik
the AD626, an Instrumentation amplifier from Analog Devices. Of cours
this approach is not isolated.

I am currently using the Allegro ACS704xx-005 current sensor and I hav
developed I problem where the device "locks up". I am awaiting tec
support to find out where the problem lies.

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Re: mA-hour meter

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Maxim has a few battery "fuel gauge" devices.. real low power..

Re: mA-hour meter

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About a million years ago, I built a mA hour (actually a uA hour) meter
using a voltage to frequency converter. There was an app note on a Analog
Devices part that we used in a configuration for measuring current (I don't
remember the part number). The neat thing is that it will integrate any
peaks or spikes between output pulses, and has a very wide dynamic range
(especially if you trim any ofset errors).  We wanted to know the long time
power consumption of a device that drew uAs in sleep-mode, and mAs when it
had something to do. If it's for lab use, just connect a frequency counter
and a few passive parts and you're done.


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