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3.6v 70mAh NiMH battery charger

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hey all,

I have recently purchased a 3.6v 70mAh niMH battery from maplins (order code BN22Y) **broken link removed**. I will be using this battery in a clock application with the ds1307 ic. i am going to use the battery to back it up. i wanted to use this battery charger Link2 but was wondering what modifications i would need to make

You've really purchased the wrong battery for the job. I would have chosen a lithium battery, like you have on your computer motherboard. Those run the PC clock for years and are not charged at all. My next choice would have been a supercap if the project only expected intermittent power losses and spent most of it's time powered by AC. 3rd choice would have been a NiCad battery since they tolerate trickle charging better than NiMh.
In the Specks it says that they tolerate being trickle charged constantly. (Quote form maplins "Withstands constant trickle charge". If i were to use this battery how would i change the charger to charge the battery up. im guessing the batterys are made from 3 1.2volt cells
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According to the datasheet of the single cell, you can trickle charge it at 2.1ma. It will also tolerate a steady 7ma at 20C ambient temperature. To be safe and easy, I'd simply set the charge current to 2.1ma with a resistor and diode series combo.
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would this work to charge the battery decribed in the first post:

i have made it very simple but i was wondering what value R2 should be. Should it even be there. also what voltage should my source be. i don't know if it a safe charger (or if it even would work(not blowing up the battery)) so if it is not please tell me how to make it safer (i know there is no way to automaticly turn off the charger but that will be added later)


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The 3.6V battery will be at about 4.2V when fully charged. The LM317 needs a minimum of 2V plus 1.25V for the current sensing resistor. The total is 7.45V minimum.
R2 does nothing and is not needed.
as long as were on the subject of charging, I'd like to post this schematic to charge an iPod, I think the placement of the values are correct, i used a severed cable for the experiment. For the cable the 2.75V output is connected to the white wire of the cable and the green wire is connected to the 2V output. The Red and Black wires should be connected to 5V and Ground respectively. And yes, I am 100% certain that this circuit will work if you use the right values and, well, just don't do anything that's stupid.:D

Also could someone please reply to my other posts? I'm feeling neglected:(
will the voltage coming of the lm317 need to be 3.6v to charge the battery up or can it be much more but making sure the current stays at 2.1ma. also i would like to meausure the voltage using a dac converter while the battery charges so i know when to turn it off. however would somone be able to give me a link to one which would work for this perpose and is quite cheap.
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The battery sets its own voltage which will slowly rise to about 4.2V as it becomes fully charged. The output voltage of the LM317 is not regulated, it changes to keep the current constant.
The actual voltage of the battery is a poor fully-charged indicator because it changes with the temperature and the battery gets warm as it charges.

You should use a Ni-MH battery charger IC (Maxim have some) if you want to charge the battery then have the charger turn off or switch to a lower current trickle charge.
i have heard of a voltage spike when the battery is charged. i was going to use of this or is this a bad idea
The voltage of a Ni-MH battery rises slightly then falls slightly when it becomes fully charged. Battery charger ICs sense this and you can select the charging to stop when the voltage peak occurs or when the voltage drop occurs.
But the voltage also changes with temperature changes.
i was going to use a dac converter conected to my parallel port with some software which plots a graph and i would be able to stop it using an output of the port connected to a transistor connected to the supply. would this work. if it would i still need to know a dac that can do the job
I have slightly changed the schematic for thge charger so would you tell me if this would work:


:)Is the zener diode ok.

:)i relise i am missing a transistor on the base of Q3 but please pretend there is one.

:)V+ = 9 volts.

:) do i need a resistor going from ADJ of the lm317 going to ground.

:)The PIC has delay routines which count 14 hours before turning on he pin stoping the currrent from flowing.

:)Does Q1 need to be more powerful than a BC557

:)C1 = 100uf


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The differential transistor will be turned on most of the time because a 4.3V zener diode conducts a little when it has only 3.0V across it. Then the base-emitter voltage will be 0.6V so the differential transistor turns on when the battery voltage is 3.6V.

You need an accurate comparator circuit, not a poor low voltage zener diode.

The datasheet for the LM317 shows the current-regulator circuit.
The datasheet for the BC557 shows its max allowed collector current.
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