From: hutto@SMTC.engr.scarolina.edu (Brent Hutto)
Subject: Re: 15 minute quick charge Ni-Cad RC Racing batteries to power an HT?
Reply-To: hutto@smtc.engr.scarolina.edu
Organization: NIST Southeast Manufacturing Technology Center
Date: Tue, 17 Nov 92 18:46:18 GMT

In article <1992Nov6.190320.14852@midway.uchicago.edu>, gary@kimbark.uchicago.edu (Gary Mr.VAX Buchholz) writes:
>Here's a query.  Ni-Cads used for RC racing cars can be had in 
>7.2V packs @ 1700 mA.  They are designed to be "quick charged"
>in 15-20 minutes.

    Actually, the NiCd batteries used by electric-powered R/C cars and
    aircraft are designed to be fast charged in around 1 hour. The car
    folks just go ahead and charge them faster because they want them
    ready sooner and don't care about keeping the same cells for many
    hundreds of charge/discharge cycles.

    For instance, a 1700mah Sub-C cell could be charged in a little
    over one hour at 1.7 amps. When I charge a pack of these cells for
    use in my R/C race car, I charge them at 4 amps, so it only takes
    about 25-30 minutes.  BTW, my cells are individually selected for
    high capacity and have an actual capacity of 1850-2000mah at a
    20-amp discharge rate. At a "normal" 1-amp discharge rate, their
    capacity would be well of 2000mah.

>It would seem that this would make great "emergency power" insofar
>as it significantly reduces the recharge time.  Even a "quick charge"
>for traditional Ni-Cads is usually 60 minutes.  RC batteries can
>be recharged in 1/4 the time.

    You could make a fairly high capacity 12V emergency power source
    by paralleling groups of 10, 1700mah cells wired in series. If you
    have a robust enough charger, you could charge the whole thing up
    in around 20 minutes. You could discharge them at around 10-20 amps
    per "group of 10", so by paralleling 3 or 4 groups of 10 cells, you
    could power and HF tranceiver for a half-hour or so of 1/2-transmit
    1/2-recieve time. If you were very careful to never overcharge the
    cells, you could get a few dozen charge/discharge cycles before the
    cells start to lose capacity. Even after that, you could get many
    more cycles at reduced capacity.

    CAVEAT 1: Designing a charger that could peak charge a 40-cell NiCd
    pack safely in 20 minutes is NOT trivial.

    CAVEAT 2: Sucessful charging at 4-5 amps or discharging at 10-20 amps
    per cell depends on the use of "matched" battery packs. By this I
    mean that each cell is tested and the pack consists of cells with
    very close to the same capacity and internal impedence. If you don't
    match the cells in this way, one or more of the weakest cells in the
    pack will go into "voltage reversal" near the end of a high-current
    discharge and be ruined. Likewise the weakest cells will be
    overcharged toward the end of a high-current charge while the stronger
    cells are still accepting charge.

>Why don't people use these batteries (RC Model Racing) for external
>power sources for HTs?  Maybe I'm missing something....

    It sounds like a good idea, if you're willing to pay a lot more
    attention to your batteries than you're used to and accept a shorter
    life span. If I had an HT, I'd probably solder up a couple of matched
    packs of leftover racing batteries.

>Gary
>-- 
>Gary Buchholz / KE9ZM			Internet: gary@midway.uchicago.edu
>University of Chicago			Packet:   KE9ZM@N9HSI.IL.USA.NA
>Academic and Public Computing		
>1155 East 60'th St., Chicago, Ill.	(312) 702-7611

-------------------------------
Brent Hutto
hutto@SMTC.engr.scarolina.edu