CCEB001@utxvm.cc.utexas.edu (06/27/91)
The discussion of charging nicads makes it sound as though a simple power supply regulated to about 1.4 volts would be an excellent battery charger. Is that correct? I have some GE Generator nicads that charge in one hour. I am interested in constructing a 12vdc powered charger for use in a car. Can anyone suggest a resonable source of a simple schematic or current and authoritive book. The GE charger has about six ICs, four power transistors on heat sinks, and an astonishing number of wires. Internet: Mike.Coyne@utxvm.cc.utexas.edu Bitnet: Coyne@utxvm.bitnet
elec140@csc.canterbury.ac.nz (06/28/91)
In article <51309@ut-emx.uucp>, CCEB001@utxvm.cc.utexas.edu writes: > > The discussion of charging nicads makes it sound as though a simple > power supply regulated to about 1.4 volts would be an excellent > battery charger. Is that correct? All the information I have seen about nicads indicates that constant current charging should be used, not constant voltage. You could use a constant voltage level cutoff, however the internal resistance of nicad cells varies with different types, thus they will be fully charged at different terminal voltages, and these voltages will vary with the charge rate. > I have some GE Generator nicads that charge in one hour. I am > interested in constructing a 12vdc powered charger for use in a car. > > Can anyone suggest a resonable source of a simple schematic or > current and authoritive book. The GE charger has about six ICs, > four power transistors on heat sinks, and an astonishing number of > wires. Check out electric model car magazines, they're right into fast charging nicads off 12V (in about 20 minutes!). However a good charger for this type of application is not simple to build, or cheap to buy. ********************************************************* Chris Kaiser Postgrad - Elec Eng Dept Canterbury University Christchurch, NEW ZEALAND E.MAIL: kaiser@elec.canterbury.ac.nz ********************************************************* "When you're fresh out of lawyers You don't know how good it's gonna feel" - Al Stewart, 1988 *********************************************************
rpw3@rigden.wpd.sgi.com (Rob Warnock) (06/28/91)
In article <51309@ut-emx.uucp> CCEB001@utxvm.cc.utexas.edu writes: +--------------- | The discussion of charging nicads makes it sound as though a simple | power supply regulated to about 1.4 volts would be an excellent | battery charger. Is that correct? +--------------- NO! When NiCd's get to full charge, they heat up. When they heat up, their terminal voltage *drops*! With a fixed-voltage supply such as you suggest, this causes thermal/current runaway, and the NiCd fries! (...or boils or explodes, depending on configuration.) Almost all fast-charge NiCd chargers use internal cell temperature to know when to stop. And almost all fast-charge NiCd batteries have an internal thermistor which connects to the charger, so that the measured temperature is that of the cells inside, and not the battery case (which may be warmer/cooler, depending on ambient airflow). This is why such batteries have more than two contacts; the additional one(s) give(s) access to the thermistor. [The other side of the thermistor may or may not be connected to one of the battery terminals, and the battery terminals themselves may or may not come out to multiple contacts.] -Rob ----- Rob Warnock, MS-1L/515 rpw3@sgi.com rpw3@pei.com Silicon Graphics, Inc. (415)335-1673 Protocol Engines, Inc. 2011 N. Shoreline Blvd. Mountain View, CA 94039-7311
phil@brahms.amd.com (Phil Ngai) (06/28/91)
rpw3@rigden.wpd.sgi.com (Rob Warnock) writes: >Almost all fast-charge NiCd chargers use internal cell temperature to >know when to stop. And almost all fast-charge NiCd batteries have an I have read articles about fast-chargers that use the voltage drop to stop. I would think there are products that do it. It is attractive in that you don't need a special battery with a thermistor inside. My Makita cordless system batteries only have two contacts and charge in an hour, either they use an external thermistor or who knows? -- "Like a rat deserting a sinking ship..."
miker@polari.UUCP (Mike Ranta) (06/30/91)
>The discussion of charging nicads makes it sound as though a simple >power supply regulated to about 1.4 volts would be an excellent >battery charger. Is that correct? No, this isn't correct. It will charge them to a point, but nowhere near full charge. Also, depeding on the type of regulation, the initial current may be way too high. The cutoff voltage of Nicads when charging is somewhere around 1.5 - 1.7 volts per cell depending on the cell and the charging current. Interestingly, this voltage actually peaks and STARTS BACK DOWN if you leave the cell on charge too long. This is how the most elaborate (and best) chargers work. They're called "peak detecting" chargers and the automatically correct for cell aging, ambient temp, and the type of cell. The voltage drops because once the cell is fully charged the temp of the cell starts to rise which causes a subtle drop in voltage (wierd but true).