# battery circuit to last longer?

Discussion in 'Electronic Basics' started by MissTroi, Oct 2, 2003.

1. ### MissTroiGuest

ok so im working with this battery right
http://www.batteryspace.com/product.asp?3=71

Unfortunately the battery is sucked dead in about 45 minutes with no
more than 200mA 12v draw. I'm trying and trying but I just cant seem
to get this to be longer. Would putting a current limiting chip in
line help?The LM1085 (http://www.national.com/pf/LM/LM1085.html)maybe?
but Im unsure how to limit the maximum current pass through to no more
than 200mA. Then theoretically shouldn't we get 6 hours on the 12V
1200mAh battery 1200mAh/200mA = 6h ? Is there some better way to get
more out of this battery? ANY help would be appreciate as I am
completely baffled and stuck. =(

-Karen

2. ### the WizGuest

First of all, the maH rating is usually a 10 hour (120ma) or 20 hour rating
(60ma) , which does NOT translate linearly at higher current drains.

Based on the URL, you probably have two of these batteries. Try powering your
load with the two batteries in parallel and see how long they last.

How are you charging the battery? Do you know that it is fully charged?
NiMH charging is based on X hours at Y ma to reach full charge. If you've made
your own charger, does it deliver a high enough voltage to ensure that the
battery gets a full charge?

VB3/VB6/NSBasic Palm/C/PowerBasic source code: http://www.jecarter.com/programs.html
Drivers for Pablo graphics tablet and JamCam cameras: http://home.earthlink.net/~mwbt/
johnecarter [email protected] mindspring dot.dot com. Fix the obvious to reply by email.

3. ### Peter BennettGuest

What are you powering with this battery? If the device you are
powering wants an amp at 12 volts (wild guess based on your 45 minute
run time), then it probably won't run if you try to reduce the current
to 200 mA - doing that will also reduce the voltage to the load to
perhaps 2 volts.

4. ### John PopelishGuest

If a 200 ma load current is draining a 1200 ma hour battery in .75
hour, the battery is defective.

5. ### Peter BennettGuest

If the load is really drawing 200 mA, then either the battery is
defective, or you are not fully charging it.

If the device does draw more than 200 mA, adding a current limiter
will just reduce the supply voltage to your load until the current
matches the current limiter setting. Any current limiter will reduce
the supply voltage, even if the load draws less than the current
limit.

6. ### John FortierGuest

Karen,

The LM1085 is a voltage regulation chip, not specifically a current
regulator, so it really isn't suitable for this application.

A JFET connected with a self biasing variable resistor, with a bipolar NPN
in Darlington configuration, will provide you with the required current
source. If you like I can send you a circuit diagram by e-mail. I don't
like trying to draw diagrams in ASCII, first because they are never easy to
understand and second because I'm lousy at it..

This is a very simple circuit and requires only three components. You can
adjust the current using the variable resistor. For 200 mA the variable
will need to be fairly low ohmic value, e.g. 10 ohms, but I'll explain all
that if you decide you want the CD.

Regards

John

7. ### MissTroiGuest

Thank you SOOO MUCH! I tried to write out what you were talking about
but I think it would be much easier if I could see what mean in a CD.
Why dont you emailme at my private email?

-Karen

8. ### John GGuest

If the battery is the correct voltage to supply your device then no external
You cannot reduce the current a device requires at its correct voltage
without affecting its performance.
As John P said above the battery is defective if what you have told us is
true.
But it would help a lot if you described whatever you are trying to drive
with this battery then we could better judge what is the Real Problem.

9. ### GarethGuest

What are you powering from this battery?

Unless the device you are powering is faulty, badly designed or not
intended to work from a 12V power supply, then it will only draw the
current it needs to work properly. Therefore if you limit the current
it may not work.

The current limiting will actually waste power (power will be lost as heat).

I would agree with other posters that the most likely problem is a
defective battery or the battery is not being fully charged. Tell us
what you are powering and we may be able to be more helpful.

Gareth.