🚀 go-pugleaf

In article <1990Jul3.150651.29985@lotus.com> kcrudup@lotus.UUCP (Kenny Crudup LID-A0794) writes:
>Can I charge hi-cap Ni-Cads in a regular (this one is a 7- vs. 14- hour
>charger, though) charger, my only tradeoff being time? Also, I was able to
>get 30 D-cells at fire sale prices, and only spent $50 for them all. What
>is the best price seen for hi-caps? Are all makers hi-caps the same?
>Can I assume that all the ones I have now (GE and Rat Shack) are only
>1.2 A/H?

There really is no limit on how slowly you can charge nicad cells, except for
the internal leakage rate.  You can charge 4 AH cells on a charger made for
1.2 AH cells.  It just takes proportionally longer.

Actually, you can make your own charger really cheap.  The 15 hour charging
rate for nicads is C/10, where C is the capacity in amp-hours.  So a 4AH
battery would charge at 400 mA.  To make a charger that does this, build a DC
supply from a 12VAC transformer, rectifier, and filter capacitor.  This will
produce about 18 VDC.  Next, get an LM317 voltage regulator and connect its Vin
terminal to the positive output of your 18V supply.  Connect a 3 ohm, 1 watt
resistor between the Vout and ADJ terminals.  Now, hook up enough battery
holders in series for the number of cells you have, and connect the positive
end to the ADJ pin on the LM317 and the negative end to the negative output of
your power supply.  You may need a heat sink on the LM317 if it gets too hot.
You can get all the parts execpt the 3 ohm resistor at Radio Shack.

This charger will supply a constant 400 mA, regardless of the number of cells
you have.  It takes 15 hours for a complete charge.  If you want a different
charging current, choose R = 1.2/I (e.g., a consumer type 1.2 AH battery would
need a 10 ohm, 1/4 watt resistor-- P = I**2*R).  If you want to get fancy, you
could put in a rotary switch and several resistors to select the charging
current based in the cell size.  The common cells are N, 150 mAH; AAA, 180 mAH;
AA, 500 mAH; SC & consumer C and D, 1.2 AH; high capacity C , 1.8 AH; high
capacity D, 4.0 AH; F (not very common), 7.0 AH.

I have used this circuit many times, and it works great.
-- 
Bill McFadden    Tektronix, Inc.  P.O. Box 500  MS 58-639  Beaverton, OR  97077
bill@videovax.tv.tek.com,     {hplabs,uw-beaver,decvax}!tektronix!videovax!bill
Phone: (503) 627-6920       "The biggest difference between developing a missle
component and a toy is the 'cost constraint.'" -- John Anderson, Engineer, TI