9-Volt 150 mAh battery

[Crockett]

And all in all I'm just exploring what I can do with an old 9-volt Rechargable I found in one of my boxes of parts

If it's an old battery it might not even perform very well, so that adds another layer of uncertainty to your enterprise. You would probably want to analyze the battery to see how well it works before devoting time to making a charger for it. And building a battery analyzer is a whole new kettle of fish...

Oh, there's a lot of layers of uncertainty, IanB, but I don't want to be analyzing the battery. So, I'll just presume it is ok enough to give me some cool levels of hands on practical electronics experience with trying to design something. And of course Dave himself hopes it won't work, which I completely see why he says that. I just don't want to blow the thing up, you know!?!?!

[Paul Price]

A precision 9V NiMH charger,

Both end of charge voltage and charging current are precisely set  for optimal charging a  9V 150mA NiMH battery.

This charger turns  the charge current completely off at full charge.

Both of these charger circuits can be turned off at any time and this doesn't upset your circuit at all, the circuit will switch to just battery operation automatically.

Take a look at this circuit that even lights a LED when charge is complete.
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The truth is, I've been using the simple 6 LED circuit shown below to charge the 9V NiMH battery in my Fluke multimeter for about 8 years now and the two batteries I have are still working well after many, many charges. I often run down the batteries because I leave my multimeter on many hours on my work bench, so I charge one and switch between the two batteries about once or twice a week. Both of the batteries were used when i got them. One came with the used Fluke I bought at a flea market and the other I found in a battery disposal box a year later (a tiny bit of battery dumpster diving.)

[RPBCACUEAIIBH]

I'm not sure yet how I want the design to go. I'm thinking I'd like to be able to have a 9 volt power supply that I could leave usually plugged in. And then when I want I can unplug it and the battery take over. (Remember, I'm here in the "newbe" section. Also, I have not yet determined the application for this power supply, but I'm sure I've got more parts to play with.

Unfortunately this is not simple to do with NiMH batteries. You can't successfully "float" them as you can with lead acid batteries for example.

There are generations of cordless phones out there that have routinely run their batteries into an early grave by trying to keep them on charge when the handset is in the base station. The underlying issue is that putting NiMH batteries on permanent trickle charge severely shortens their life.

Charging NiMH batteries has special requirements. You can't use a constant voltage based charger because the battery voltage is not stable. It changes with age, battery temperature and manufacturer. Therefore applying a constant charge voltage will overcharge the battery in the worst case, will undercharge it in the best case, or will subject it to constant trickle charging in the middle case. None of these results are optimal.

What you need a circuit to do is to charge the battery, detect when the battery is fully charged, and then completely disconnect the charger until the battery is discharged by some significant amount before trying to charge it again. It will involve some kind of voltage window comparator (see Dave's latest video), a charge control circuit with full charge cut off, and some logic to drive the thing. It is not the simplest thing to put together for a beginner.

Well my design doesn't include trickle charging but otherwise it would be just as you said... has 2 limits one which turns on the charging if it is low enough, the other one which stops completely, but you can add an extra 555 timer in astable mode after the existing one, and connect the existing one's out pin(3) to it's reset pin(4) to make it trickle. Now I don't really know what you really mean by "trickle charging", I mean what kind of waveform, but if square is not good, then it's pretty easy to make it a triangular of some kind as well... with 2 more trimmer, 1 cap and an o-scope(which I don't have).

[Crockett]

If I'm correct, trickle charge refers to current. The battery reaches full charge and the charger continues charging at the same charge voltage but at a much reduced current.
Trickle charge is actually a good topic, I think, for us newbies to learn about... Even though I've heard the term used alot.

[Psi]

If I'm correct, trickle charge refers to current. The battery reaches full charge and the charger continues charging at the same charge voltage but at a much reduced current.
Trickle charge is actually a good topic, I think, for us newbies to learn about... Even though I've heard the term used alot.

Yes, trickle charge just refers to charging at low current.  You can pretty much leave the battery on charge permanently at trickle charge current.
Charging for 2,3 or 4 times longer than needed won't damage your battery.

Note: There are some variations on what C rating trickle charge is.
Some define it as 0.05-0.1C while others say it's just enough current to stop self-discharge. (probably 0.0001C or something)

[IanB]

Trickle charging generally means continuous charging at a low charge current without end. This is mostly a bad idea with NiMH batteries and is to be avoided. The trickle charge causes constant chemical reactions to occur inside the cell which over time will cause the electrodes to deteriorate and the battery to lose capacity. When a NiMH battery reaches full charge it should be taken off the charger and used.

The key thing about NiMH charging circuits is that using voltage to detect the state of charge is inherently unreliable. The exact voltage is not well determined as it is with Li-ion, it varies battery by battery and according to how long the battery has been off the charger.

That said, with most NiMH cells they will be more or less charged when they reach a charging voltage of 1.45 V. So for a 7 cell 8.4 V battery, this would be 10.15 V. After reaching this voltage you would disconnect the battery from the charger and stop charging. As soon as you disconnect the charger the voltage will drop, soon going below 1.40 V. But this is not a signal to start charging again. The battery should be taken off the charger and used before putting it back on charge.

In a post above there is mention of charge being complete at 9.3 V. However, for a 7 cell battery this would be 1.33 V per cell and at this voltage charging is far from complete. At that voltage the battery would be less than half charged. It won't harm the battery to stop charging at that point, but neither would you get anything like the full run time from it before needing to recharge.

[IanB]

Yes, trickle charge just refers to charging at low current.  You can pretty much leave the battery on charge permanently at trickle charge current.
Charging for 2,3 or 4 times longer than needed won't damage your battery.

2, 3 or 4 times longer at trickle charge rates may not be a problem, but leaving a battery charging like this for weeks or months at a time most probably is. It is tempting to leave a battery sitting in the charger permanently "so it is ready to use when needed", but this is also a surefire way to ruin batteries.

[Crockett]

It seems then that the battery's properties are going to limit the application possibilities.
To use it for instant supply voltage when the power/charging source is removed (like a mini UPS) doesn't seem so reasonable; still possible, just not so much a good choice. And yet still something to play with.
Building just a plain charger where you remove the battery after it is charged is probably the way to go.

[IanB]

Remember anyway that a 150 mAh battery has a rather small capacity. There is not much it can power except tiny electronics.

[Psi]

Remember anyway that a 150 mAh battery has a rather small capacity. There is not much it can power except tiny electronics.

Nonsense capacity wise its enough to start a car once or twice
~150A @ 9v for ~3.5seconds

[Crockett]

I hope I don't ever have to try to start a car with it... Attaching the jumper cables sounds a pretty tricky endeavor. 

[Paul Price]

There are two charge circuits shown below. The first circuit I designed is precise but not so easy to build because of its many parts, the second (LED circuit) is what I use today.

Theory of operation of the 6-LED circuit:

The rather precise voltage of each RED LEDs are added together to set the correct voltage to charge the battery, the red LEDs have the correct voltage and other colors, green or yellow or blue LEDs certainly do not. This charger does not care whether you connect this to an unregulated or regulated power supply, so long as the power supply is working, it regulates the voltage and current.
 
If you have a volt meter you power up this circuit and connect it to a charged NiMH 9V bat and you can measure the voltage at the battery or at the emitter of the power NPN transistor with just a 1k load connected, and you should read between 9 and 10.5 volts, and  this is right in the range to charge the battery to full capacity without over voltage. The current itself is limited by the gain of the transistor, and a low gain 1w power transistor (with low beta) is preferred that can handle 25V or more.

If you want to gild the rose:

If somehow you compulsively think you need to have to so precisely control everything in life,  you may want to fine tune this circuit according to the discarded wallwart power supply of  your choice and to fit the Beta characteristics of your luck in winning the current gain lottery of the BD139 transistor:

Calibration Procedure:

(1)Replace the 100K fixed resistor with a 68K resistor in series with a 50K tiny pot connected as a rheostat. Apply wallwart voltage to the circuit and using a 1-ohm resistor as a current shunt in series with your discharged 9V NiMH batter, adj. the 50k pot for 14 to 28mA, whichever level you think you like. I have mine trimmed to 20 mA...a compromise between the two extremes.
(2)Replace the 10K fixed resistor with a 20K pot in series with a 1k resistor. Now, with a 10k ohm  load at the emitter of the BD139(no battery connected), adj. the Max full charge cutoff (output) voltage to 9.3V or whatever else arbitrary voltage you might think will please you and the almighty god of NiMH batteries.

Once you've set these values you can remove the adj. pots and insert the closest value of a fixed resistor or two to match their actual resistance setting...pots are too expensive and handy to keep around the experimenter's shop, why sentence them to a life of neglect???
 
Myths:  All NiMH batteries cannot be trickle charged after cut off voltage or charge completed voltage has been reached.
             A circuit needs to be complicated to work well.
             A simple circuit cannot work as well as a very sophisticated and elegant design.
             A NiMH battery such as 9V 150mA type must only be charged so as to achieve a precise voltage at a precise current.
             A  9V NiMH battery will be damaged by "float charging".  Truth:  if the charging current limit of the "float charger" is set to the correct charging current and float voltage, then the float charger automatically switches mode from constant current to float charge as the cutoff voltage is reached. 
             All NiMH batteries will be damaged and their service life will be severely shortened if you trickle charge or do not  stop charging  at a precise cutoff voltage.
             All NiMH batteries must be fully charged to be serviceable. Truth: If the cutoff voltage of a charger is set to low, then the charger cannot force current into to the battery to charge it, if it is set very much too high(>10.5V), it might damage the battery. I have charged 9V batteries of this type to a voltage limit of 10.5V and left them at this level in this other charger circuit that I built even for weeks and the batteries were still functioning well after 2-3 years of service.

The proof of a pudding is in it's tasting.

I have been using the 6-LED charger circuit for more than 8-years to charge my two NiMH 9V batteries and they are both still giving great service.

[Psi]

I hope I don't ever have to try to start a car with it... Attaching the jumper cables sounds a pretty tricky endeavor. 

i could start mine, its got supercaps instead of a battery.

If my supercaps were dead and i had a constant current dc-dc psu i could discharge a 9V over say 10 hours and recharge my supercaps

[Crockett]

Well, I do want try out as many of the circuit diagrams offered here as I can for charging the battery.
AND, I'll leave the car starting using 9-volts to Psi. Hopefully I won't need that sort of application. 

[ivan747]

na, no discussion needed. 

We have never been so wrong...

Have you looked at ready-made chips? It's yet another alternative.

[Crockett]

I think there was some mention (maybe not exactly to ready made chips) to pre designed circuit. I'll have to read back through the thread to see exactly what I am remembering, though.

[IanB]

Ready made chips are typically designed for fast charging with negative delta-V detection and possibly temperature monitoring. This can work for single cells and for well balanced multi-cell batteries, but it is not a good approach for a tiny 9 V consumer battery. The best way to charge these is a slow timed charge.

[RPBCACUEAIIBH]

Ready made chips are typically designed for fast charging with negative delta-V detection and possibly temperature monitoring. This can work for single cells and for well balanced multi-cell batteries, but it is not a good approach for a tiny 9 V consumer battery. The best way to charge these is a slow timed charge.

I agree... Fast charging will damage the battery in very short time... Slow charging is the best for a battery, this is true in case of any kind of battery.

[Crockett]

I'm sure it is best to slow charge!  It may also be best to physically remove the charger from the battery; but there are options to accomplish this while not actually having to take the battery out. Maybe not any very good options, yet!
Anyway, I like all the posts responding to my query. And I like the circuit suggestions. And I hope to explore each of them... at least for the educational value in them.

Everybody have a good week(seven)!!