Use a series incandescent light bulb. Gives a bad approximation to constant current over a wide range of differential voltage. Fault tolerant. Gives some visual indication of current. Cheap. Just keep the current below the maximum continuous charge rating of the cells. Operators WILL forget eventually.
Another thing that works is a charge dump. Q=C*deltaV mike
-- Return address is VALID. 500MHz Tek DSOscilloscope TDS540 $2200 http://nm7u.tripod.com/homepage/te.html Wanted, 12.1" LCD for Gateway Solo 5300. Samsung LT121SU-121 Bunch of stuff For Sale and Wanted at the link below. http://www.geocities.com/SiliconValley/Monitor/4710/
You don't say what time period you want to charge these cells in. Without this basic info we can't make meaningful suggestions.
Some things you might consider in the meantime...
It is unwise to design a charger, particularly one meant to charge the cells in the shortest recommended time, without automatic shutdown or protection. While you may think it is unneccessary and you can do this function manually, what happens on the odd occasion you forget to stick around or you get involved with some other problem and forget to switch it off after the recommmended time? Or, somebody other than yourself is using the charger and they ae not aware of the requirement for manual control.
There are a number of fully auto chargers available to meet your requirements at far less cost than it would take to design and build your own. Have a look at this website
Under Category Search choose Batteries and Chargers and in the sub category choose Chargers and then hit GO. You can then see the range of products available ranging in price from AUD25.95 - AUD88.95.
Further to Ross H's reply, one of the failure modes for NiXX cells is heat damage. Unterminated charge is the main cause of this. Charge termination needs to be deterministic not probabilistic i.e. their needs to be certainty and repeatability. Manual termination does not provide this reliably.
Timer-based systems depend critically on the state of charge (SOC) of the cells when charging is commenced. If this is variable, so is the outcome.
"Smart" chargers look for some end-of-charge (EOC) indication from the cells. This can be a voltage dip or a temperature rise, or rate-of-rise. The voltage inflection is less pronounced in NiMH than NiCd. "Smarter" chargers use a combination of these EOC techniques and usually fast charge (0.5C to 1.0C). Temperature rate of rise is direclty related to charge current, and can be fairly reliable. High temp termination at 40-45C is reasonably reliable, subject to ambient temp and cell temp at the start of charging. But any temperature-related termination requires a sensor in intimate contact with the cells, and that means getting them out of the appliance or modifying it to incorporate the sensor.
I'd suggest buying a decent charger and a spare pair of cells, rotating them as required.
I have a device that runs on 4 x "AAA" Ni-MH cells, being 4.8VDC.
I would like to be able to plug in an external power supply that will charge the cells while in-circuit. The device does not need to run _while_ being charged, although this would be a plus.
The charging duration will be operator-timed, so it does not need to be electronically monitored.
Can anyone suggest the simplest schematic for this based on input from an ordinary 6 or 9VDC regulated wall pack? Is a constant current source strictly necessary?
All schematics I have found are for complete chargers that require the cells to be removed and charged individually.
Thank you very much.
James
ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.