Current available from power supply

[Kerry]

Hello,

I'm trying to design a battery charger.  Based on various articles and the capacity of the battery, I figure I'll need ~2A during the "rapid" portion of the charge.  My batteries are 6V, I'll supply a little over that to charge at the peak.  If I use a 12V power supply, and it gets regulated to 6V, how many amps do I need.  If it supplies 1A at 12V, do I get 2A after the voltage is regulated to 6V (or close to it, if it's not 100% efficient), or does the excess voltage go mostly to heat?

Is my question understandable?  Not sure if I'm asking correctly...

Thanks,

Kerry

[Soeren]

Hi,

If it supplies 1A at 12V, do I get 2A after the voltage is regulated to 6V (or close to it, if it's not 100% efficient)

With a linear supply you get 1A.
With a switching DC-DC converter you will get somewhere between ((12*1)/6*K) where K is the efficiency of the switcher, which may be anywhere from around 60% to say 95%

You have to first define which cells you're gonna use and how many, then you can calculate the needed voltage - don't assume 6V plus a little for good measures if you want your charger to work.

[Kerry]

Hi,

If it supplies 1A at 12V, do I get 2A after the voltage is regulated to 6V (or close to it, if it's not 100% efficient)

With a linear supply you get 1A.

Does this mean the rest goes to heat?  So I'd be better of with say a 9V supply than a 12V supply because the 12V supply will just make the regulator get hot?

You have to first define which cells you're gonna use and how many, then you can calculate the needed voltage - don't assume 6V plus a little for good measures if you want your charger to work.

Can you elaborate?  I plan on using the - delta V technique to determine when the batteries are charged - so in practice I will only be limited by the power supply.  But I don't understand what you mean when you say "which cells" I'm going to use and how many.  I have 6V NiMH battereis (5 cells each).  Are you implying that I can charge only certain cells, or just that I need to know what battery I want to charge?

Thanks,

Kerry

[Soeren]

Hi,

Does this mean the rest goes to heat?  So I'd be better of with say a 9V supply than a 12V supply because the 12V supply will just make the regulator get hot?

Exactly!

But I don't understand what you mean when you say "which cells" I'm going to use and how many.  I have 6V NiMH battereis (5 cells each).

Oh, you got it
Different chemistries have got different end-of-charge voltages, so knowing which you've got is important if you want minimum possible losses.

Are you implying that I can charge only certain cells, or just that I need to know what battery I want to charge?

As above, assuming you want to make a charger for your particular batteries without wasting too unnecessary power.


5 NiMH cells will have and end-of-charge voltage of ~7.40V to 7.45V when fast charging at around 1C (i.e. 2000mA for 2000 mAh cells) and it will take around 66 minutes to fill a totally depleted cell.
You'll have to allow for voltage drops in the current switching element, whether it be a transistor, a relay or whatever and you'll have to allow for mains voltage variations as well (assume a drop of ~10% worst case).

If your switching element have a drop of say 0.5V, you'll need a supply of around 9V ((7.45+0.5)*110% = 8.75V) for your 5 NiMH-cells.


Are you perhaps using a MAX712?
If not, at least read through the datasheet at http://www.maxim-ic.com/getds.cfm/pk/1666, it will give you a better understanding on the subject.

[Kerry]

Great info!  Thanks for the help!

-Kerry