Path: utzoo!attcan!uunet!ogicse!emory!wuarchive!udel!ih
From: ih@udel.edu (Charlie Ih)
Newsgroups: sci.electronics
Subject: Re: Car Battery Rechargers
Message-ID: <32908@nigel.ee.udel.edu>
Date: 9 Oct 90 18:08:14 GMT
References: <1990Oct4.222556.20668@athena.mit.edu> <63220@iuvax.cs.indiana.edu>
Sender: usenet@ee.udel.edu
Followup-To: sci.electronics
Organization: University of Delaware
Lines: 60
Nntp-Posting-Host: huey.udel.edu

In article <63220@iuvax.cs.indiana.edu> willie@iuvax.cs.indiana.edu (william hunt) writes:
>erspert@athena.mit.edu (Ellen R. Spertus) writes:
>
>>I saw a car battery recharger for $60 in the Rand McNally map catalog.
>>If your battery is dead, you plug this into your cigarette lighter, and
>>it recharges the battery (enough to start) from a D battery.  It seems
>>like a neat idea.  I have a few questions:
>
>>1. Has anyone used one of these?  Any comments?
>
>>					Ellen
>
>Do not waste your money or time on anything that runs on batteries to
>charge a car battery.  The energy storage of a car battery is many
>times that of a D-cell.  Most recharging devices that plug into
>cigarette lighter are a joke.  These devices can not transfer enough
>power quickly enough to get a car started without waiting a long time.
>.......
>Willie Hunt

Actually this is technically feasible and may well be practical if it
is properly designed.  By the way, I am in no way connected with any
of the vendors.  I am a profession teaching electronics. I, like many
of you, intuitively feel this is not possible.  But let's look the numbers.

Let's assume, on the average, you crank the car 3 times at a duration
of 5 sec. each to start a car.  The cranking current is 250 amp.
This is worth 1 Amp-hr of charge.  So you can see, you don't need
that much energy to start a car!  The typical battery capacity for
a small or medium size car is 40 Amp-hr.  So you need only 2.5%
of the capacity to start a car.

Believe of not, the capacity of a D-size Alkaline battery is about
15 Amp-hr (see FEDCO Electronics, Inc. catalog/data sheet (1-800-542-9761),
I happen to have the data sheet at my hand).  Therefore a single D-cell,
in theory, has enough energy to start your car (15 amp-hr at the average
of 1 volt has more energy than that of 1 amp-hr at 12v). If you use
8 of them, the total voltage is about the same as the car battery.

The car battery charging efficiencey is about 80% (charging at 15 v
and discharging at 12).  The efficiency of the electronic
charging/booster circuit (since the two voltages are approximately
the same, less than 1:2) can be easily between 80%-90%.  Therefore
the overall efficiency is between 64% - 72%.  Let's say it's 50%.
Therefore if you use 8 D-cells, you can transfer 7.5 amp-hr to the car
battery (18.8%).  If your battery is still has 10% charge left,
after charging by the D cells, it almost has 30% capacity.  If the
battery is healthy, I am sure it can start your car.  Even it
is completely discharged, 2 sets (16, approx. $16) batteries will
start your car.  It may take 30 to 60 minutes to charge the battery.
(Twice I called AAA, min. waiting time 4 hr.)

The charging/booster circuits are widely used in UPS (Uninterruptible-
Power-Supply).  Many IC circuits with voltage and current liminting
feedback for the booster circuits are available (please check the
electronic catalogs, don't ask me).  This charging circuit can
squeeze the last drop of charge out of your D-cell.
If you want to build the circuit for
fun is OK but not to save money.  I cannot count on the reliability
of the circuit I built (without endurance tests).