Measuring voltage and current with Prop?

ChrisE

Hi everybody,
I am working on a GPS handheld device and having tons of fun designing it, but I have run into a slight hitch,
it needs a beefy power supply to last the length of time I intend using it. However, I found this little baby….
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http://docs-europe.electrocomponents.com/webdocs/0728/0900766b8072846c.pdf
··· … A Li-ion rechargeable, rated at 7.5V, 2.2Ah. Small, yet powerful, perfect for what I need. However,
I need a built-in, ‘smart’ charging circuit, powered by the on-board prop, to monitor voltage and current.
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As the battery is Li-ion, it should not discharge past 50% of its capacity, or else it will cause damage to itself.
Therefore, a cut-off voltage (of 6V) should be set to automatically switch off the device once the voltage
has been reached.
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For charging, the battery needs a constant voltage of 8.4V (+/- 0.1V) for 3 hours or until the charge current
drops to 100mA (as stated in the datasheet). I personally do not like the idea of timing the charge period. Why?··
Firstly, it is terribly inaccurate. And secondly, imagine if there was a momentary drop in the charging circuit's
power supply causing the timer to restart; I don’t like the idea of the battery rupturing all over my expensive
components. Besides, it can be a real hassle having to discharge the battery before recharging it,
so a timer is totally out of the question. You want top it up, not blow it up.
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So that brings me to my question, how can (simply) monitor voltage and current in my project?
Maybe an ADC for voltage (perhaps a ADC0831), and array of scaled resistors for current?
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Will have quite a few I/O pins to spare, and voltage/current value resolution is not terribly important.
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So to Summarize: I need to able to measure:
Voltage range: 6 - 8.5V
Charge current: ‘round 100 mA (From 1 Amp)
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Well that is my mouthful, hope I gave enough detail.
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All help is appreciated.
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Thanks again!
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From a cold and (not so) stormy night in South Africa,
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Chris
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P.S. Hope this is suitable for the Propeller Forum!
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Graham Stabler

Convert current to voltage with a low ohm resister.

As well as being able to measure current for charge termination it is usual to have a limit.

Graham

T Chap

You can get IC's from Maxim that serve the purpose of battery management including current measuring, voltage checking, battery temp Go to Maxim's site and search battery management. These IC's are desighned to check the battery, not regulate charging. One place to look to start out is the Parallax Thermocoulpler kit, which is built on a battery manangement IC called the DS2760(now been updated with DS2762, available on Maxim's site)


To see the Parallax kit go here. Keep in mind this kt only uses the thermocouple inputs only which are used for reading temp from a battery.

www.parallax.com/detail.asp?product_id=28022



Here is the new part

www.maxim-ic.com/quick_view2.cfm/qv_pk/3950

With a processor hooked up to this part you can get the vitals off the Li-on battery and do your needed work from there.

Here is a great IC for reading current should you want to simplify things further. Just hook this IC up to a sense resistor and it will output a voltage scaled to the current by the gain you determine. For example, use a .1 shunt with gain of 1 and your voltage should fall around 1V output per 1 amp of current. You can take that voltage and either run it to a comparator, which trips a pin on the Propeller, or, set up an ADC on the Prop and read the voltage that way to cause the charger to disconnect by some method. You could put the Propeller to sleep when the battery falls below X voltage.

www.analog.com/en/prod/0,,759_782_AD628,00.html

Post Edited (originator) : 1/7/2007 10:12:58 PM GMT

sharpie

I've used the MCP73831 for the charger and DS2438 for the monitor.. they work great.. not sure about the 7.2v batteries.