Author Topic: Using a power supply to charge a 6V battery?  (Read 1163 times)

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Offline TheDarkLordTopic starter

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Using a power supply to charge a 6V battery?
« on: August 20, 2011, 06:01:05 PM »
Hi all,

So I managed to buy a battery pack to use for my $50 robot. It is a NiMH pack, with 6.0V and 1500 mAh.
I had a spare power supply, which had an output of 6V, 300mA.
As I was about to use the power supply to charge the battery pack for the first time, I used my multimeter to check which lead of the power supply was positive or negative. While doing this, I noted that the voltage the power supply was output-ing was around 11-12 Volts. As I do not know a whole lot about electronics, I was wondering if this is normal, and if I can use this power supply to charge my battery.
Any and all help would be greatly appreciated.

Vignesh R.

Offline TheDarkLordTopic starter

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Re: Using a power supply to charge a 6V battery?
« Reply #1 on: August 20, 2011, 06:02:53 PM »
And also, the power supply requires an input of 230V and so I'm using a step-up transformer (120V-240V) to provide this input.
Could this be the cause of problem?

Offline waltr

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Re: Using a power supply to charge a 6V battery?
« Reply #2 on: August 20, 2011, 06:36:59 PM »
A raw output power supply is NOT recommended to charge a battery.
You need to buy or build a proper NiMH battery charger.

Google "NiMH charging" to learn more.


Offline TheDarkLordTopic starter

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Re: Using a power supply to charge a 6V battery?
« Reply #3 on: August 20, 2011, 06:56:07 PM »
A raw output power supply is NOT recommended to charge a battery.


But in the $50 robot tutorial, it states that "a cheap way is to recharge your batteries" is to use an unused power supply which outputs the same voltage as the battery pack, and hook it up to your pack.
This is the link: http://www.societyofrobots.com/step_by_step_robot_step3A.shtml

Offline rbtying

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Re: Using a power supply to charge a 6V battery?
« Reply #4 on: August 20, 2011, 07:06:48 PM »
Cheap doesn't mean good. NiMH is nice in that the battery will get extremely hot right before the charge cycle completes, so you can remove it from external power then, but it's still better to get a smart charger which watches the current draw from the battery.

And don't even think about constant-voltage charging with lithium cells - that's an explosion waiting to happen.

Offline Soeren

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Re: Using a power supply to charge a 6V battery?
« Reply #5 on: August 20, 2011, 07:59:23 PM »
Hi,

NiMH is nice in that the battery will get extremely hot right before the charge cycle completes, [...]
That depends entirely on the charge current.


And don't even think about constant-voltage charging with lithium cells - that's an explosion waiting to happen.
Easy now... Smart chargers use constant voltage and as long as you set the voltage say 50mV under the max. charge voltage for the cells and terminate the charge within a reasonable time after reaching that point, nothing bad happens.
The worst case scenario when "hand charging" is with cells that have discharged to a too low voltage, where they cannot tolerate the full charge current.

But why getting into lithium cell charging here at all, when OP were asking about NiMH?
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: Using a power supply to charge a 6V battery?
« Reply #6 on: August 20, 2011, 08:16:45 PM »
Hi,

And also, the power supply requires an input of 230V and so I'm using a step-up transformer (120V-240V) to provide this input.
Could this be the cause of problem?
It depends on how well the step up are matched (and its size), but It's probably because you just measure the unloaded voltage (although double up sounds like a pretty bad transformer). If you load the output with a 20 Ohm/2W (Two 10 Ohm/1W resistors in series) you should see close to the rated voltage.
If it still gives too much with a 20 Ohm load, you could try running it on 120V (by taking the step up transformer out of the equation, but I think it will show around the 6V if you just test it.

That doesn't mean that you can just hook the battery up to it and think all is well, because as the charge current falls, the voltage will climb too high.

The proper way of ensuring the max. voltage is by using a regulator set for 1.40V to 1.45V per cell in the pack, so with eg. 4 cells you need to set it to 5.6V to 5.8V (but not the smallest bit higher).
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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