Date: Wed, 12 Nov 2008 23:35:44 -0500
Reply-To: Dennis Haynes <d23haynes57@HOTMAIL.COM>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: Dennis Haynes <d23haynes57@HOTMAIL.COM>
Subject: Re: alternator--voltage vs. amps
In-Reply-To: <491B9BDE.7050505@colorado.edu>
Content-Type: text/plain; charset="US-ASCII"
Here are the basics.
Voltage is the pressure behind the electrons. Think of a pressure gauge on a
water pipe. Current, measured in amps Is the speed at which the electrons
are flowing. The flow is determined by the voltage and the resistance of the
circuit. Here you have ohm's law which says that if a potential of one volt
causes 1 amp/hour of current to flow, then he says the circuit has one ohm
of resistance. Volts (E) - current (I) X resistance (R).
From here we can use algebra so that if we have any two, we can calculate
the third.
Now let's also consider power (watts). Power is determined by E X I. So now
if we have two, we can also calculate the other two.
Now charging a battery adds some complications here. We have resistance but
we also have a back voltage. So if the battery has 12 volts and the
alternator is producing 14 volts, we really only have 2 volts to drive the
current into the battery. As the battery state of charge increases, so will
it's voltage which will naturally reduce the current at a fixed alternator
output voltage. Also as a battery is charging, the chemical reactions will
build up and provide additional resistance to the charging current. The same
happens in reverse as a battery is being discharged. This is known as
Peukert's Law. This is why current needs to be controlled during the
charging and discharging. Charge a battery to fast and the voltage will rise
making the battery appear to be charged when it is not. Charging consists of
delivery the right current at the right time and battery charging takes
time.
Since loads and run time vary in an automobile, we use a fixed voltage
system and the voltage level is a compromise. We want the lights and
accessories to work and we want the battery to charge but not over charge.
Us battery guys work in volts per cell to keep confusion down. I work with
batteries ranging from our auto stuff to 1200 A/H forklift batteries and 400
volt battery banks for computer room UPS systems.
German cars and most trucks are set to run at 13.8 volts or 2.3 vpc. This
voltage is just below the gassing voltage so you can drive forever without
causing the battery to off gas or boil out. This works well with maintenance
free batteries and it avoids excessive plate oxidation. If the battery is
deeply discharged, the only current limit during the bulk charge will be the
alternator capacity and the resistance of the wiring between the battery and
charger. Yes, you can actually charge the battery too fast for 20 to 80%.
The Vanagon will deliver ~ 15-18 A/H to the battery during this condition. A
proper bulk charge rate would be the battery capacity delivered over 6
hours. 65 A/H batter / 6 hours should be a current limit of ~11 A/H.
Now, this fixed voltage works OK for starting batteries. Typically you only
need 3 to 5% of the battery capacity anyway. Now if we are going to
discharge any battery significantly we need to do some other maintenance
tasks. Here you need a smart charger. Charging is not just restoring what
came out. You need to mix the acid, break down sulphation, and avoid
oxidation.
This can become a long winded response. Do some research. Here are some web
sites.
http://www.batteryuniversity.com/
http://batterytender.com/technical.php
http://www.trojanbattery.com/Tech-Support/TechSupport.aspx
I hope I answered some of your questions. Basically the voltage is
regulated, the current is determined by the load or demand. The batteries
demand is also known as charge acceptance.
Dennis
-----Original Message-----
From: Vanagon Mailing List [mailto:vanagon@gerry.vanagon.com] On Behalf Of
Richard A Jones
Sent: Wednesday, November 12, 2008 10:16 PM
To: vanagon@GERRY.VANAGON.COM
Subject: alternator--voltage vs amps
I'm trying to understand alternators now. I'm retired,
so have nothing better to do....
There are lots of web pages that explain how/why the
voltage is adjusted with the alternator. What I cannot
find is an explanation of amperage. Amps is what charges
the battery--at the correct voltage. I do not understand
how the amperage output of the alternator is varied.
It seems to me that a discharged battery would need lots
of amps. A full battery, car with no lights on, etc,
would need very little. What does the voltage have to
do with this, except to correct for the temp of the battery?
You can see how I was a math major, not an engineer. Help
me out.
Thanks,
Richard
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