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
http://www.allegromicro.com/faq/acs704faq/index.htm#Q6

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.
http://para.maxim-ic.com/compare.asp?Fam=CS_Amp&Tree=PowerSupplies&HP=PowerSupplies.cfm&ln =
 

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.

-Aubrey
        
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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.

-Hershel

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