- Blog entry posted in 'Uncategorised', February 16, 2011.
This blog details my journey into battery backup for small micro controller based RF connected sensor nodes.
It is worth noting that much of what is written about battery charging is by the RC people. Their use model is quite different then that of battery backup.Definition: C: battery capacity in mAh.
Charge Rate is expressed as C/x.
If one chooses to charge at 100mA or less the cost is quite low. Add an additional dollar if you want to charge at rates up to 1.5A. The assumption is that the application uses a micro controller and you have 1 analog and 1 or 2 digital IO pins free. $0.46 LM317 (100 mA)
$0.19 1N5404 (a lower current device perhaps)
$0.06 BC548
$0.?? resistors
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Total under $1.00
Feb 14
The circuit I started with is was found at Fig3 and is a typical use of the LM317 to regulate current.
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51031
The circuit is working but I still need to program the uC to stop charging. The battery pack is a 750 mAh consisting of 3 AAA. These packs are NOS Erickson cellphone batteries from Electronic Goldmine. On arrival each cell read around 0.8V.
I used 2 1/4W 8R resistors in series to fake a 1/2W 16R. This is in place of the 5W 1R resistor used to set the current in the original schematic in the first post. It charges at 100mA with 4.20 to 4.44V at the battery terminals with 9V or so to the regulator.
Feb 15
I noticed that once you charge at a voltage, lowering the voltage by even .1V will cause charging to stop. The actual battery voltage is -.2V below that used to charge it (may vary). Battery temperature has gone from 0 to 3F above room temp.
I can detect battery voltage via the ADC but need to dig some more to find out how much of a drop indicates fully charged. Or how long the voltage 'remains fixed'. One source says that if you charge at less then C/2 end of charge voltage detection is a bust. If that is the case we are left dT/dt (change in temperature) or a charge rate of C/10 or less and time.
Plan is to add a temperature sensor to the pack and determine how the voltage tracks with temperature. Start with a sensor on each cell and see if I can get by with one on just the center cell. Will 100mV generate enough heat at end of charge to be detected ?
Need to do some more investigation of conditioning and add a transistor and resistor to discharge the battery in place.
Bits of info from the web
Around +1K/minute is an indicator that the cell is pretty much fully charged and is now converting most of the incoming charge energy into heat.
If you are "Slow" charging cells and they become even slightly warm, they are already overcharged!
Feb 19th
Created a new schematic with a discharge section for cycling and a fuse.
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