I was told that i could make a simple battery charger from a LM317 and a resistor to charge some Ni-MH Batteries. Can someone just expand on this a little such as how it works as I don't really understand it. Some more information is that there are 12 AAA batteries that hold a charge of 1000mAh and 1.2v per cell.
A constant current source adjusts its output voltage with the load in order to maintain a constant current.
V = IR // holy s*#% it's Ohm's Law
NiMH batteries are fickle fiends to charge, exhibiting temperature-dependent changes in charge and discharge curves. They also don't have a float voltage, so constant voltage charging doesn't work, as you've likely discovered. Energizer has some recommendations regarding charge times:
Typically a moderate rate (2 to 3 hour) smart charger is preferred for NiMH batteries. The batteries are protected from overcharge by the smart charger circuitry. Extremely fast charging (less than 1 hour) can impact battery cycle life and should be limited to an as needed basis. Slow overnight timer based chargers are also acceptable and can be an economical alternative to smart chargers. A charger that applies a 0.1 C rate for 12 to 14 hours is well suited for NiMH batteries. Finally a maintenance (or trickle) charge rate of less than 0.025 C (C/40) is recommended. The use of very small trickle charges is preferred to reduce the negative effects of overcharging.
AAA NiMH batteries have a capacity of 850mAh [varies by manufacturer], so charging with a rate of C/2 to C/3 can be done with a constant current of...
850mAh / (2 to 3 hours) = 283mA to 425mA
An overnight, C/12 trickle charge can be done with a constant current of 71mA. This page mentions that:
Modern cells have an oxygen recycling catalyst which prevents damage to the battery on overcharge, but this recycling cannot keep up if the charge rate is over C/10.
The recommended maintenance charge rate of C/40 can be done with...
850mAh / h / 40 = 21mA
ΔV charging: charge at recommended constant current until the cell reaches a peak voltage and decreases (eg. -15mV).
This technique is accurate enough to safely charge at C/2 to C/3 (283mA to 425mA).
dT/dt charging: monitor cell temperature to both limit maximum temperature and look for characteristic heating rate.
This technique may be used in conjunction with ΔV charge termination to more precisely monitor and terminate the process, allowing the use of higher currents (C/1 to C/2, or 425mA to 850mA).
Soft start: If the temperature is above 40 degrees C or below zero degrees C start with a C/10 charge. If the discharged battery voltage is less than 1.0 Volts/cell start with a C/10 charge. If the discharged battery voltage is above 1.29 V/cell start with a C/10 charge.
1.78V maximum: a single cell must never exceed this.
But what does it all mean!? The input voltage to your LM317 constant current circuit must be enough to support the voltage drop across the regulator and resistor (1.47Ω), drive the required current, and exceed the maximum cell voltage. To source C/1 or 850mA to a AAA NiMH battery, whose internal resistance is at most around 120mΩ, requires
(120mΩ + 1.47Ω) * 850mA + 1.2V + 1.78V = 4.3315V. I recommend at least 2V more to reduce the effects of source irregularities like regulation and noise and account for other circuit losses (like that diode you don't have yet). If you're charging 4 cells in series as your diagram indicates, you'll need at least 9.978V (ie: 12V+); 25.034V (27V+) for 12 in series, though I would worry about uneven charging.
How can I measure the charge of a rechargeable cell?
I'd like to know if the cells are actually charged.
The battery specification is: AAA, 1.2V, 1000mAh.
Once charged, my voltmeter reads 1.43V on each cell.
You cells are fully charged.
I assume that they are NimH (Nickel metal Hydride) cells but that voltage would also indicate full charge for NiCd cells.
I use a nominal figure of 1.45 V / cell at 25 degrees C as end point when actually charging. If they measure 1.43V/cell when off charge that is even higher than I'd usually expect. If that is an on-charge figures at end of charge it indicates full charge.
A system intended to operate from NimH cells should work down to 1.1V/cell in all cases and ideally down to 1V cell. When lightly loaded (say C/10 load or less) NimH cells will operate at about 1.2 V across the major part of their discharge cycle. The keyboard should load them to far less than C/10.
Likely options in order of decreasing probability are
Your bluetooth keyboard is faulty
The red light indicates some other problem such as a lack of link connection from the "dongle" (aka receiver) that it connects via.
The keyboard needs Alkaline cells
(but it should work on well below that voltage even with alkaline cells).
I assume that the keyboard has a USB connected "dongle" which plugs into a USB port on your PC.
You do not say if this worked previously.
If it was working and now isn't then a fault somewhere seems likely.
If you have not rebooted the system try doing so.
Try to minimise connection of other equipment which may cause interactions.
Sometimes other equipment plugged into the same USB hub causes problems.
Sometimes even equipment plugged into another USB port on the PC can cause problems .
On some occasions a device will "enumerate" on one USB port and then not operate on any other port.
Did the device came with drivers and did you load them?
Try it on another PC.
Is there a channel setting option which has been accidentally altered (most unlikely with Bluetooth).
Do you have other Bluetooth or wireless based equipment in use which may be interfering?
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