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Author Topic: Can I charge a NiMh battery without disconnecting the circuits?  (Read 2098 times)

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Offline omkarTopic starter

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Can I charge a NiMh battery without disconnecting the circuits?
« on: January 05, 2010, 04:35:49 PM »
Hi,

I have a 9V NiMh rechargeable battery that can be charged within 1.5 hr at 200 mA current. This battery is going to be powering some of the sensors in my robot. These sensors do not like surges of any kind (voltage or current). The robot is programmed to navigate (or be guided) to the charging station at a regular intervals. Can I recharge this battery in any way so that I dont have to turn off the circuits. I was thinking of using a simple PNP diode which will cut off the main circuit from the charging circuit as soon as there is a charging current flowing through. But this causes a problem that I wont be able to get the robot to turn on and move out of the docking station. So.. Can I charge the NiMh battery without having to turn the robot off? Is there any circuit out there which will enable safe connect and disconnect from the charging station preventing the surges which usually occur when contacts come close together? Also what will be the impact of the charging current on my circuit. Currently the circuit works on 8V 250mA current.

Any insight into this is appreciated.
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Offline z.s.tar.gz

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Re: Can I charge a NiMh battery without disconnecting the circuits?
« Reply #1 on: January 05, 2010, 04:43:39 PM »
You can use transistors hooked up the MCU to 'disconnect' the sensitive sensors, and pretty much anything else not needed.
Having an MCU and maybe a timer or something of that nature isn't too much load, leading to fast charging times and the ability to 'reconnect' everything and drive away.

Also remember that whatever is applied to the battery is also applied to the MCU unless some sort of regulation is implemented!
Save yourself the typing. Just call me Zach.

Offline Soeren

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Re: Can I charge a NiMh battery without disconnecting the circuits?
« Reply #2 on: January 05, 2010, 04:52:56 PM »
Hi,

If you use soft start and soft disconnect you won't have any surges.
How are the surge parameter defined for your sensors, alternatively: What sensors are you using?

I wouldn't fast charge a battery with only time as the end-of-charge detector, as you then need to know that the battery is totally flat.
Either use relatively slow charge (tapered charge) or use a scheme like dV/dt for termination.

The main thing, to avoid surges, is to adapt the two voltages (i.e. to reduce current flow to zero or close to, depending on what your sensors demand) before dis-/connecting.
Regards,
Søren

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?
Please remember...
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Offline z.s.tar.gz

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Re: Can I charge a NiMh battery without disconnecting the circuits?
« Reply #3 on: January 05, 2010, 05:04:46 PM »
Or you can use a voltage/current sensor to know exactly when it's done and still use fast charging.
Save yourself the typing. Just call me Zach.

Offline SmAsH

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Re: Can I charge a NiMh battery without disconnecting the circuits?
« Reply #4 on: January 05, 2010, 05:21:31 PM »
Well, if your electronics take regulated current and you have caps on the input/output stages it shouldn't cause that much of a surge? Wouldn't the regulator and caps soak it all up?
Then, to know for a disconnect you could use a voltage monitor circuit or if your charger has a done charging led, that could be used...
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Offline omkarTopic starter

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Re: Can I charge a NiMh battery without disconnecting the circuits?
« Reply #5 on: January 05, 2010, 05:25:24 PM »
Thanks guys for the speed.

I dont have the details of the sensors as I picked them up at a swap meet and the guy said they are sensitive to surges. These are just infrared distance sensors. I like the idea of just disconnecting the sensors and keeping everything else alive. Still I will need to protect the rest of circuit from surges.
Soeren: I am not aware of soft start and disconnect mechanisms. Can you please point me to some?
As for the battery charger, it is a charger specifically made for charging NiMh batteries and has detections for overcharging and sensing the existing charge levels so all I need to do is to connect the battery to the charger once the robot is docked. If I put a 1A protection diode across the battery charging circuit, wouldnt this prevent current surges?
P.S.
I suppose SmAsH is right as my charger stops the charging once the battery is charged so disconnecting shouldnt be a problem. I need to try this out.

Offline omkarTopic starter

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Re: Can I charge a NiMh battery without disconnecting the circuits?
« Reply #6 on: January 05, 2010, 09:31:08 PM »
Hi.... please comment on the attached representation of the proposed circuit. The R1 represents the load (in this case sensors), B1 represents the NiMh 9V battery, B2 is the charger (which will take care of overcharging issues etc). The PNP diode will turn off the load as soon as the B2 turns on.

Offline SmAsH

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Re: Can I charge a NiMh battery without disconnecting the circuits?
« Reply #7 on: January 05, 2010, 10:57:55 PM »
yes, that'll work fine.
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Offline Soeren

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Re: Can I charge a NiMh battery without disconnecting the circuits?
« Reply #8 on: January 05, 2010, 11:26:35 PM »
Hi,

I like the idea of just disconnecting the sensors and keeping everything else alive. Still I will need to protect the rest of circuit from surges.
I'm not sure what you call a surge, but surges (as I know them) are incurred whenever the load on a supply (eg. your battery) changes - small change => small surge and v.v. Disconnecting a load, you will have a surge, relative to the load current.
That's why you need to know what your sensors will safely deal with (I have never heard of IR sensors that were particular frightened of surges though).


Soeren: I am not aware of soft start and disconnect mechanisms. Can you please point me to some?
Not without knowing the specs, as such things need to be made for the circuit in question.


As for the battery charger, it is a charger specifically made for charging NiMh batteries and has detections for overcharging and sensing the existing charge levels so all I need to do is to connect the battery to the charger once the robot is docked.
If it has got 4 leads running from the battery (2 supply wires and 2 sense wires) it might work, but if it's just 2 wires and you extend the wires between the battery and the charger, don't count on its reliability without thorough testing.


If I put a 1A protection diode across the battery charging circuit, wouldnt this prevent current surges?
No.
A diode is not a current surge suppressor. It can keep the voltage within Vcc+0.7V (or Vee-0.7V), but in a 12V system, a surge can be from eg. 5V to 12V.
But perhaps we're talking about different things?


I suppose SmAsH is right as my charger stops the charging once the battery is charged so disconnecting shouldnt be a problem. I need to try this out.
I wouldn't count on it, as mentioned, without extensive tests.
Regards,
Søren

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?
Please remember...
Engineering is based on numbers - not adjectives

Offline Soeren

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Re: Can I charge a NiMh battery without disconnecting the circuits?
« Reply #9 on: January 05, 2010, 11:32:04 PM »
Hi.... please comment on the attached representation of the proposed circuit. The R1 represents the load (in this case sensors), B1 represents the NiMh 9V battery, B2 is the charger (which will take care of overcharging issues etc). The PNP diode will turn off the load as soon as the B2 turns on.
PNP "diodes" are called transistors ;)  I don't see how it should work though, as you have a shunt between the charger and the battery.
To get it to work, you need to replace the shunt with a resistor and then the charger is shot down.

Perhaps you could just tell us exactly what you want it to do and include all the needed parameters and then you might get a useable solution.
Regards,
Søren

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?
Please remember...
Engineering is based on numbers - not adjectives

Offline omkarTopic starter

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Re: Can I charge a NiMh battery without disconnecting the circuits?
« Reply #10 on: January 06, 2010, 04:42:05 PM »
Thanks again everybody.
Soeren: What I want this circuit to do is as follows: The battery charger comes into picture once the robot has docked in to charging station .Till then assume that there is no charger in that circuit. So PNP transistor just lets current flow through to the actual load (R1). (Note: R2 is just a resistor limiting the base current for transistor where as R1 is "representing" my sensor) But when the charger plugs into the circuit, there will be a current flow to the base (I think this is where you raised a point of not shunting the charger but using a resistor in the shunt) and this will stop current flow to the R1 and charge my battery.
Just FYI: this is the first electronics circuit I have ever designed being a mechanical engineer... and working with cars all these years.. till now i was just copying circuits off the web. So any input from you guys is HIGHLY appreciated. I have taken up this project as a learning project for improving my electronics knowledge.

Offline Soeren

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Re: Can I charge a NiMh battery without disconnecting the circuits?
« Reply #11 on: January 06, 2010, 08:42:46 PM »
Hi,

Here is a working circuit to copy :)
Regards,
Søren

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?
Please remember...
Engineering is based on numbers - not adjectives

Offline omkarTopic starter

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Re: Can I charge a NiMh battery without disconnecting the circuits?
« Reply #12 on: January 06, 2010, 10:39:01 PM »
wholly smoke.... did you make that one???? .... i would never have been able to think of that .... thanks sooo much. This solves the problem magnificently...

Offline Soeren

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Re: Can I charge a NiMh battery without disconnecting the circuits?
« Reply #13 on: January 07, 2010, 05:37:35 PM »
Hi,

You are welcome.
Regards,
Søren

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?
Please remember...
Engineering is based on numbers - not adjectives

 


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