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Looking For Collaborators – Smart Battery Charger


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I am working on a PIC/PC based smart battery charger ( and discharger and tester ) that will have a graphical record of the charging. Anyone want to help? You could work on any one segment of the project or alternative implementations. I have a page about the project posted at: http://www.opencircuits.com/Smart_Battery_...cal_Data_Logger

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I am working on a PIC/PC based smart battery charger ( and discharger and tester ) that will have a graphical record of the charging.

 

Hi Russ

Coincidences! .. all the time. I'm just making the plastic case to hold one of these!

Have just finished a prototype that does NiCd/NimH variable rate charge, charge detect, discharge, and has a rs232 port that reports progress to an Excel VBA program that graphs cell temperature. (I started looking at voltage, but swapped to cell temperature for end-of-charge detect).

Stand-alone, it writes to a 16x2 LCD. There's a circuit that I used as a starting point in Silicon Chip magazine for September 2008.

Don't know whether my experiences so far will be of any use to you, but for what they're worth ..

To control current I used the Pic PWM to generate a reference voltage that is filtered, divided and supplied to the non-inverting input of a LM358 opamp set up as a comparator. The opamp inverting input reads the charging-voltage drop across a 5W 0.22R resistor in series with the cells. The comparator output drives the gate of a IRF540 mosfet controlling a 12V source through them (the cells).

For discharging I use a voltage divider to supply a 0.9V reference (apparently the correct low limit for 1.2V cells) and dump the current into a torch bulb via a darlington similarly controlled by a LM358. I use a relay for changeover from discharge to charge when the pic detects that the 0.9V limit is reached. (By the way .. the voltage bounces back after a while!)

Best of luck, but I found the voltage dip at the end of charge impossible to detect - apparently this is a usual problem with NimH cells. So instead I used cell temperature measured with a NTC resistor built into the battery holder. This seems reasonably reliable so far, the charging graphs show the temperature taking off (logarithmically?) at what should be about the right time for the cells to have reached full charge. At the moment, with only a few runs, the critical slope appears to be 1.5 - 1.8 degrees C per minute, but I plan to fiddle with some fancier math to see whether it can do better. I have included a 32kHz crystal on the Pic so that I can use a timeout as well.

The mosfet runs hot at high current, so I have added a heatsink and a fan from an old PC power supply that cuts in and steps up depending on the sink temperature - using another NTC and a darlington array switching a resistor string to control fan speed.

I run a bootloader, so the plugin port that talks to Excel can be used to endlessly fiddle with (probably) useless features and measurements without opening the box. That way, the software is never finished .. oh well ..

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