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From: george@wombat.UUCP (George Scolaro)
Newsgroups: sci.electronics
Subject: Re: Best way to power a hungry halogen bulb?
Message-ID: <5322@wombat.UUCP>
Date: 21 Feb 91 17:22:43 GMT
References: <1991Feb13.033718.11978@ecf.utoronto.ca> <5170105@hplsla.HP.COM>
Organization: Association for the Prevention of Polar Bears and Kangaroos
Lines: 52

In article <5170105@hplsla.HP.COM> tomb@hplsla.HP.COM (Tom Bruhns) writes:
>
>>Help! I am building a device that needs a power hungry halogen bulb
>>(5V, 0.8 amperes). With a standard 9V NiCad battery and
>>5V rectifier I only get 1.5 minutes of useable light. I could use
>>4 1.5 (actually 1.2 V) NiCads, but these would take up more space
>>(space is important). There was talk of  "VARTA"  batteries packing
>
>Might not help the space much, but National makes a little 5V switching
>regulator with very low external parts count.  You could get a
>_regulated_ 5 volts.  If the 9V (~8.5V) battery is about 80 mAH, you
>would get about 80mAH * 8.5V/5V * 75%, or about 100 equivalent mAH at 
>5 volts.  That should run the bulb for about 1/8 hour, or 7-1/2 minutes.

I have recently purchased some of the 9V Varta brand nicads, because of their
7 cell construction.

The data sheet that comes with them (bought via a distributor) shows
discharge curves for 100mA and 200mA. Remember that this battery has a
100mAH capacity.

	Minutes		Voltage@100mA	Voltage@200mA
	5		8.3V		7.2V
	10		8.1V		6.2V
	15		8.0V
	20		7.7V
	25		7.4V
	30		7V

My tests:
	Minutes		Voltage@12mA
	8hrs 10mins	8.56V
	9hrs 14mins	8.43V
	9hrs 19mins	8.40V
	9hrs 34mins	8.26V
	9hrs 40mins	8.00V Rapid decrease at this point, battery flat

From this you can see that the capacity of the battery is grossly diminished
at high current draw. The 100mAH capacity is rated at 0.1C, i.e. 10mA draw.
If you are drawing 0.8A, I'm not surprised that you get mere seconds of
use. In fact most nicads and gell cells quote their capacity at a specified
currect draw, usually 1/10th of their capacity.

The conclusion is that you have to derate the battery's capacity if you
exceed the specified current draw/capacity. To select the battery that solves
your problem you'll either need to perform more experiments, or get
comprehensive discharge curves from the battery manufacturers.

Lot's a luck.
-- 
George Scolaro
george@wombat.bungi.com                [37 20 51 N / 122 03 07 W]


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