BATTERIES

Jan 20 06:55 UTC 1995

Let's get a charge out of batteries.

response 2 of 42:

Jan 20 08:35 UTC 1995

Lead-acid batteries are only nominally 12 volts.  The exact voltage
depends on how charged the battery is, on recent charge/discharge
history, and the temperature of the battery.  This leads to a "cold
start" trick - if the battery is weak, and doesn't have quite enough
oomph to start the car, turn the headlights on for 30 seconds, then turn
them off & try again.  The drain on the battery will heat the battery
up, which may raise its voltage enough to enable the car to be started.
Automotive electrical systems usually "float" on the battery, so the
exact voltage depends mainly on that.

The generator (or alternator on most modern automobiles) is responsible
for charging the battery, but the battery must also not be overcharged -
attempts to do that actually produce steam, hydrogen, and oxygen.  The
latter two can actually produce an explosive mixture, while all of these
use up the water in the battery.  Fortunately, it's easy to determine
the state of charge in the battery, so a simple voltage regulator
suffices.  I believe a quiescent fully charged battery at room
temperature is about 13.8 V.  The voltage regulator will probably try
for about 14.1 V.  A nearly discharged battery may read 12 V.  Even a
completely discharged battery may read 9 V under no load circumstances.
This can result in odd behavior if the battery is nearly dead - for
instance, the instrument panel lights may light up as expected (only
slightly dimmer), but as soon as the starter is engaged only a click
from the starter solenoid may be heard.  If the battery doesn't have
enough oomph even to keep the solenoid closed, a rapid series of clicks
may be heard instead.  In many cars, the "battery" light on the
instrument car is an essential part of the charging circut - and must be
replaced if it does not light when the ignition is turned on.

response 4 of 42:

Jan 21 06:50 UTC 1995

Marcus' comments are pretty reasonable. However, how did ICOM (and others)
settle upon specifically "13.8" to have embossed on their equipment, when
it is undoubtedly a range of voltages that can be accepted? The manual
even says "connect an external 13.8V DC power source" when, at least
literally, there is no such thing. Here's a related question: these
battery packs have two charging jacks - one for a wall-adapter for
charging, and the other for "13.8V". I know the former has the charge
limiting circuit in the adapter, and the latter in the battery pack, but
why provide both? A "13.8V" wall-adapter would suffice. 

response 6 of 42:

Jan 22 03:34 UTC 1995

1991 ARRL Hndbook: "Most modern radio equipment is powered from 13.8V
...." The Handbook also shows several power supplies and lead-acid battery
chargers that provide 13.8 volts. 

I think I have a hint: apparently, the "float" voltage for the lead-acid
battery is very close to 13.8 volts. That is, at that voltage supplied
from a charger, the "charge" rate becomes very small. Actually, it doesn't
charge, but there is water electrolysis, but at an "acceptable" small
rate.  Therefore a 13.8V power supply is a "standard" charging circuit.
Presumably, then, other battery-pack designers worked to that standard. 
(The water-electrolysis voltage is really closer to 1.5 volts, but the
chemistry of the lead-acid battery supports 0.8 volts of "overvoltage", so
water electrolysis doesn't start until 2.3 volts per cell, or 13.8V for 6
cells). 

response 8 of 42:

Jan 23 13:43 UTC 1995

My Heathkit HW4M HT has two plugs too.  However, one is for the battery
charger and the the other, labeled "DC In", supplies power directly to
the HT, bypassing the battery.  Connecting power to this connector does
not charge the battery.  It is intended for mobile operation where it is
connected to the mobile systems power source, most commonly a lead-acid
battery/charging system.  Almost a decade ago, I did some work with UPS
systems for heart-lung machines.  The chargers for the 12 volt lead-acid
batteries would start out charging at a current = 1/10th the amp-hr. rating
of the battery until the voltage across the battery reached 14.2 VDC.  At
that point the voltage went back to 13.8 VDC and held there indefinatly.
This charging procedure resaulted in the quickest charging of the battery
without doing damage to it.  They would have charged just as well had we
simply a charger held at a constant 13.8 VDC with a cow-bar curent limiting
circuit to keep the current from getting too high at the start.  It would
take longer to charge though.  I suspect 13.8 was chosen since an ideal
charging circuit will maintain this voltage on your standard 12 VDC lead acid
battery.  BTW, 10.7 VDC (Under normal operating load.) is considered "deep
discharge" for a deep-discharge lead-acid battery.  Any less and you risk
permanant damage to the battery.  The "discharged" voltage of a regular
battery is somewhat higher.  I'd suspect somewhere between 11.5 and 12.5
VDC.

response 10 of 42:

Jan 24 17:55 UTC 1995

The sophisication of most HT chargers consists or a resistor in series
with the battery.  So as not to get into trouble with UL and a host of
other regulation agencise, they prob. have to keep the wall-wart voltage
below 24 VDC.  Being able to charge the battery from a lighter plug is
also nice.  Hence this series charging resistor is not of a high value.
Small changes in charge voltage can mean big changes in charge current
for this reason.  They could put a more tolerant charging ckt. in but
that would take space and people already gripe about having to pay $50+
for a new battery.  (Well, I do!)

response 12 of 42:

Jan 25 13:29 UTC 1995

Using Ohms law, it should be pretty simple to calculate: 
Resister = ((Supply voltage) - (Battery voltage)) / (Desired charge current)
 
Unless it's a battery designed for fast-charge, don't go to far beyound the
10% charging rule.  Doing so WILL reduce the life of the NiCad pack
significantly.  Fast charge NiCad packs and chargers have termal sensors
to sense the temprature of the pack during charging.  They look for a sudden
change in temp. to determin full charge.

Figuring the charging current is simple too:
(Charging current) = ((Supplied voltage) - (Bat. voltage)) / Resistor

How long it takes to charge a battery pack is also important to most users.
I would guess that the charging efficiency is somewhere around 85%.
 
To find out how long it will take to charge the battery pack:
(Time to full charge) = (Amp Hr. capacity of pack) / (Charge current in Amps)
 / (.85)

With that, it sould be possible to design a reasonable charger to suit ones
needs ;-)

response 14 of 42:

Jan 26 13:43 UTC 1995

Indeed!  The problem with a 7815 is that a 78XX type regulator requires
about 3V of "headroom" to regulate properly.  i.e. a 7815 would require
a 18V input to regulate to 15V.  There are "low dropout" regulators, but
even these require about a volt of headroom.  When you consider that a
silicone junction requires about 0.7V to overcome, you'll find that 
designing an active regulator is tricky.
One trick would be to have a switching circuit that "breaks" your 13.2V
pack in half, creating two 6.6V packs, and than placing the two halves
in parallel for charging.  If you figure out the switching for this, you'll
see it is not simple and might be prone to problems.  I've done this and,
if memory serves, it required a DPDT switch.
You'd be wecome to run some experiments in my basement some evening Rane.
Mad scinentists have all that kind of stuff ;-)

response 16 of 42:

May 6 15:42 UTC 1995

I've never tried this exercise. It seems well done, but I'm perplexed by
the numbers you got. A nicad cell is rated at 1.2 volts. 8 cells should yield
9.6 volts. Yet your pack put out over 9.6 volts for 5(!) hours. This just
doesn't seem right.

You initially said that the pack went "dead" after a 1 hour discharge through
a 100 ohm resistor. What voltage level do you associate with "dead"?

After recharging, it was able to deliver charge for 7 hours? That's a
rather miraculous recovery. Is it possible you have a loose connection in
your charger that resulted in incomplete charges the first couple of times?
Since you were now in the process of actively "fiddling" with the whole
setup, you could have wiggled whatever was loose in the charger back into
a temporary working condition. Just an idea.

I just checked the 2 battery packs for my Makita drill. The fully charged
one, charged about 3 weeks ago, was at 10.30volts. The partially discharged
pack in the drill was at 10.1volts. I ran that pack down  to the point where
it could no longer turn the drill and then left it loaded for an additional
2 minutes. Immediately upon removal from the drill it was at 2.2 volts but
it was climbing back up. After 10 minutes it had "recovered" to 8.85volts
and was stable at that voltage. Interestingly, even though this cell was
"dead", it put out 8.85 volts at high impedence *and* into a 100ohm load.

After fully charging it, the pack was at 11.45volts. This dropped off after
a couple of hours to a level of 10.95volts.

Hmmm, so much for that 1.2volts per cell.

I think one thing to remember here is that a rechargable battery(lead-acid,
gel-cel, nicad, whatever) is always "high" when it first comes off the
charger. Close to it's charging voltage. But pretty quickly stabilizes
at it's normal voltage. How long after coming off the charger did you run
your discharge test Rane? Another idea is to try the same sequence with
no load and see what the battery's output curve is like.

response 18 of 42:

May 6 16:33 UTC 1995

Depends on what you are using them in. The alkalines would probably work
fine in a low current device, but I suspect they wouldn't work as well
in a battery-powered power tool. Nicads are kind of halfway between being
a battery and a capacitor. You can pull a great deal more current off
of a nicad per unit time, than you can from a regular battery. In effect,
you're sucking electrons directly off of a charged plate, rather than
waiting for the comparitively slower effects of a chemical reaction
to proceed. This comes in handy in power tools that need to deliver high
torque into high loads.

response 20 of 42:

May 7 04:21 UTC 1995

re 17:
        I tried using a Rayoovac Renewal battery (rechargable alkaline) AA
battery in my pager, and it died after two days and two pages.  I decided
that, even though it might be cheaper just to have a few Renewals and
rotate them, rather than using non rechargable alkaline batteries, it
really wasn't worth the hassle of having to change the battery that often.
 I went back to nonrechargable alkaline batteries, which tend to last two
or three weeks.

response 22 of 42:

May 7 16:52 UTC 1995

Heh, yes, I was going to mention that.

Yes, I was thinking of nicads being similiar to electrolytics in their
construction and their ability to shed a charge at a much higher rate
than a normal, one-shot, chemical battery.

Your observation about nicads jibes with most recomendations for thier
use. I nicad should be used to complete discharge before it is put back
in the charger. As opposed to constant topping-off. They are meant to be
"deep cycled". Thhis is the opposite of a car battery, which should always
be kept at top-charge and never deep cycled.

Er, change "I nicad" to "A nicad" above.

_I Nicad_? Wasn't that the Asimov book about the little battery that could? :-)

response 24 of 42:

May 7 17:24 UTC 1995

Hmmm, that's interesting. My alpha pager continues to recieve
good pages *long* after the low-battery indicator has gone off.
In fact, almost dead batteries affect the display quality alot more than
they affect message content. I almost never lose characters with dead
batteries. Until, of course, it just stops working entirely.

I'm using a motorola advisor, what is your's?