I want to charge Ni-mh. Is my idea legal?

[roi_tau]

Hi guys


I have a 3.6 v Ni-Mh battery 200ma/h

The adapter I have is 6v - 500mA

I want to charge it in C/10 for overnight.

My idea is to connect 3 power diode (3*0.7=2.1) and the voltage will be 6-2.1=3.9V

to do it in C/10 (20mA), I thought to divide the current with resister in parallel.  (like 100ohm and 2k)

What do you think? Will it work?  I don't have the option to add charger IC.

Just 3.6V 200mA/h battery and 6V-500mA charger.

[roi_tau]

No one has an answer for me?


Isn't it the battery that you normally work with?


Thanks in advance


Roi

[Admin]

You can probably charge it at around 4.5V.

As for charging current . . . check what the battery is rated for. If you give it too much, it will overheat and the chemicals inside will boil.

You can use a resistor to regulator current if you don't mind how inefficient that is. Just make sure the resistor is rated for the power that is passing through it, otherwise it'll fry!

[ArcMan]

If you want to do it right and cheap, use an LM317 in current regulation mode to produce a 20 mA constant current source.  The circuit is detailed in the LM317 data sheet.  I've been charging NiMH's at C/10 for years with this circuit.

[Soeren]

Won't do unfortunately.
A NiMH cell should be charged to a voltage of 1.40V so, for 3 cells, 4.2V is needed.
The OP has got a 6V supply, not enough considering the drop of the LM317.

Besides, it's a bit overkill. two BC337 (or similar NPN BjT's) and two resistors can do the 20mA current reg. just fine.

[ArcMan]

You are right about the input voltage requirement.  He would need at least 4.2V + 3V = 7.2V.
But I disagree that wiring a circuit with 2 transistors and 2 resistors is easier than wiring a single 3-pin device.  IMO, it's the discrete components that are overkill.

[Soeren]

Hi,

You are right

Of Course

about the input voltage requirement.  He would need at least 4.2V + 3V = 7.2V.

Which he doesn't have.

But I disagree that wiring a circuit with 2 transistors and 2 resistors is easier than wiring a single 3-pin device.  IMO, it's the discrete components that are overkill.

Number of components is the same in both circuits if made with E12-resistors and with the cap needed to avoid oscillation in the LM317.
Overkill is using a 1.5A device for 20mA when it's more expensive than the right solution which meets his demands as well.
Both circuits with PCB and overlay can be seen here: http://That.Homepage.dk/PDF/20mA_CCC.pdf both circuits have the equivalent function, the leftmost will work from 6V, the other will not.
Agree or disagree with that.

[Ro-Bot-X]

Soeren,

I am using 6 AA size NiMH 2300 mAh (Rayovac I-C³) in a battery pack. Normally I charge these batteries in a 15 minutes charger, but I would like to build a charging base for the robot to auto charge, so I want to charge them as a pack. I know I can't charge them in 15 minutes, but what could be the fastest charge time for them and what charger circuit can you recommend? I would like to built the circuit myself instead of buying a ready made one.

Thanks in advance and sorry for stealling the thread...

[Admin]

Ro-Bot-X, the fastest (and still safe) way to charge batteries is to use a microcontroller-controlled smart charger.

But if you wanted it to be cheap (hence making your own?), I recommend getting a wal-wart.

[Ro-Bot-X]

I've got several wall-warts. I also have a NiMH/NiCd charger for a 6V pack, that has a LED that changes color when the charge is done. This might seem a stupid question... you think that if I increase the input voltage I will be able to use it for a 7.2V pack?

[Admin]

you think that if I increase the input voltage I will be able to use it for a 7.2V pack?

you mean like modding the smart charger?

The 7.2V pack would need like 8V+ applied to it during the charge properly. I doubt the charger would be designed for that . . .

[galannthegreat]

Take a look at those I-Go unversal charging systems(cheap, around 30-40 dollars CAD), and get a simple tip in which you can make a simple interface from.

[Soeren]

Hi,

Soeren,

I am using 6 AA size NiMH 2300 mAh (Rayovac I-C³) in a battery pack. Normally I charge these batteries in a 15 minutes charger, but I would like to build a charging base for the robot to auto charge, so I want to charge them as a pack. I know I can't charge them in 15 minutes, but what could be the fastest charge time for them and what charger circuit can you recommend? I would like to built the circuit myself instead of buying a ready made one.

Thanks in advance and sorry for stealling the thread...

I wouldn't go faster than this (about 4½ hour for a full charge) http://That.Homepage.dk/PDF/NiMH_Tapered_Charger_Current_Limited.pdf, but it doesn't have to be fully discharged to recharge.

It would be wise to give it a full charge from an "intelligent" charger each 10..20 charges just to keep the cells at their best behaviour, at least until correct behaviour is verified. Let the circuit warm up for 20 minutes before trimming the exact charge voltage with a DVM).

One very important thing... As it stands, it needs to have input voltage whenever a battery is connected or else... 

If you want an LED to show when charging/done or have other questions/requests about the circuit, just tell me.

(I can be reached by mail at Projects(at)im26y(dot)com if needed)

[Ro-Bot-X]

Thanks for the schematic!

If you want an LED to show when charging/done or have other questions/requests about the circuit, just tell me.

I would like to have an output able to energise a relay when the battery is charging and release the relay when the charging is complete. The relay will be mounted on the robot to flip power when the robot is charging. It will disconnect the battery from the robot and let it charge and also will connect power from the charger (before the charging circuit) to the microcontroller's voltage regulator so the electronics will have power, but the motors will not.

You think it's hard to add this to the circuit schematic?

Thanks in advance!

[Soeren]

Hi,

You think it's hard to add this to the circuit schematic?

Not really 
http://That.Homepage.dk/PDF/Charger_rev_b_(w_on_charge_relay_output).pdf

[Ro-Bot-X]

Thank you very much!

I have a few questions:
- What is the value of the C1 capacitor?
- The value of the R5 resistor is 82 ohms or 0.82 ohms?
- The transistors can be 2N3904 and 2N3905?
- Any minimum for the wattage of the resistors? I have a bag of assorted 1/8 W resistors...

I'll get the parts and test the circuit on the breadboard then make a charging station for the robot. It will take some time until it will be ready, but it will be a great improvement to charge without taking 6 batteries out from the robot!

[Soeren]

Hi,

C1 = S.O.T.   (Select On Test = find empirically) start with eg. 1..10µF and go up until the relay stays on without buzzing (it's fed by unregulated pulsating DC).
R5 = 0R82 (The R takes the place of the decimal separator, so it's 820 mOhm)
You could use 2N3904/5, but beware that they have their emitter and collector pins reversed compared to BC5x7, so you'll have to bend the base lead the other way out (toward the front) and mount them "backward" compared to the PCB overlay.
R5 should be 1W+ and R2 should be 2W+, 1/8W will be fine for the rest.

[Joesavage1]

cant you just buy a charger?


Joe