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I want to charge my 9V NiMH cell with a LM7809. The problem is that I want to limit the current output to 20mA or 30mA. How can I limit the current output from that voltage regulator with simple components? What other components do I need? The input voltage to the LM7809 will be from a 12V source.

I know that this is not the best method to charge NiMH cells and that negative delta V along with temperature monitoring is required but what I'm building is a simple charger to be used in case of a one time emergency situation and I will stop charging it after a specific amount of time like 12 hours.

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    \$\begingroup\$ How about adding a resistor between the LM7809 output and the NiMH battery? \$\endgroup\$ – user2233709 May 12 '17 at 18:22
  • \$\begingroup\$ That is just not the way to charge a NiMH for many reasons. Instead of a resistor between the 7809 and battery, put a 9V battery charger. \$\endgroup\$ – Misunderstood May 12 '17 at 18:51
  • \$\begingroup\$ @user2233709 what value should i pick? \$\endgroup\$ – Kokachi May 12 '17 at 19:00
  • \$\begingroup\$ 1 kohm should get you started. \$\endgroup\$ – winny May 12 '17 at 19:21
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    \$\begingroup\$ See my comment on bitsmacks answer. \$\endgroup\$ – Russell McMahon May 13 '17 at 2:47
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This isn't as straightforward as you are hoping.

At any given state of charge, the amount of current a battery will accept is function of the voltage you apply. A current-limited battery charger basically varies the voltage so that the desired current is achieved.

You might be tempted to put in a big series resistor to limit the current. The effect would be that of a voltage divider: the resistor will drop some of the voltage and the battery will drop the rest of it. But this won't be controlled, and the voltage ratio (and therefore the current) will change over the charge cycle.

From Battery University (emphasis mine):

It is difficult, if not impossible, to slow charge a NiMH battery. At a C rate of 0.1C to 0.3C, the voltage and temperature profiles do not exhibit defined characteristics to trigger full-charge detection, and the charger must depend on a timer. Harmful overcharge can occur when charging partially or fully charged batteries, even if the battery remains cold.

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  • \$\begingroup\$ actually my charger is depending on a manual charge time. There is This cheap chinese charger that uses just diodes and resistors to charge nimh cells and it does kinda work. \$\endgroup\$ – Kokachi May 12 '17 at 18:59
  • \$\begingroup\$ @Kokachi Yeah, I guess it really depends on how much "kinda" you can accept! The charger you link to will damage batteries if they are left on too long. The main problem with a timer is that the charging time really depends on the state-of-charge of the battery. A 75% full battery will get overcharged and damaged much sooner than a 25% full battery... \$\endgroup\$ – bitsmack May 12 '17 at 19:05
  • \$\begingroup\$ +1 Exactly. Overcharging permanently damages the battery and reduces capacity. Trickle charging unless it is basically on a level with self discharge is not tolerated by NiMH batteries. \$\endgroup\$ – Spehro Pefhany May 12 '17 at 19:17
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    \$\begingroup\$ FWIW: I'll put this here although it also relates to the question. Comments / disagreements :-) welcome. | At 'around room temperature', charging NimH at <= 1.4V/cell at <= C/10 will usually trickle a cell to a zero current point at "close enough" to full charge. 1.45V is closer to typical and SOME may tolerate 1.5V but 1.4 is reasonably safe. So for 7 cells 9.8V is about right. | A 7809 can have its Vout increased slightly by providing a divider from Vout to ground tapped to regulator ground. Not overly nice idea. \$\endgroup\$ – Russell McMahon May 13 '17 at 2:46
  • \$\begingroup\$ Me? Disagree with you?? I don't think so :) But I do think that you should post that as an answer! Thanks @RussellMcMahon \$\endgroup\$ – bitsmack May 13 '17 at 4:33
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Basically, you can't. You need to learn how to do some research.

First, a NiMH 9 volt battery has a nominal voltage of 8.4 volts. The cell voltage for NiMH is 1.2 volts, so there are 7 cells. From this site you will learn that the normal charge voltage for NiMH is 1.4 to 1.6 volts per cell. This means that you need a minimum of 9.8 volts. The 7809 data sheet establishes its output voltage in the range of 8.65 to 9.35 volts.

So a 7809 simply will not do what you want. It might be used to provide a float voltage, but that is not a good idea with NiMH - they are not lead-acids.

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