Re: Re: Re: To Settle an Argument

Posted by Larry on August 29, 2000 at 13:51:30 from (208.24.179.201):

In Reply to: Re: Re: To Settle an Argument posted by Wardner on August 28, 2000 at 19:34:34:

A partially discharged battery is not charged just by raising its voltage. You have to pump electrons back into this storage device. You must charge at a sufficient voltage (usually 14-16V) to overcome the voltage of the battery. Remember, a battery is also a load. It is a load to the charging device, whether a charger, alternator or generator. Almost any 12v battery, whether fully charged or so dead it won't burn a light bulb, will still measure 12.2 Volts with a volt-meter if it has no load on it. If it is dead, the current it will deliver may be down in the milliamps, but the voltage is still there. This is why battery testers load the battery down with a specified resistor while the voltage reading is taken. A good way to test your vehicles battery is to read the voltage with no load and see if it reads 12 volts. If so, it does not have a shorted cell. Next, turn on the lights and measure the voltage. If the lights are dim, you know the battery is weak, but read the voltage and you will see it drop. To check the alternator, start the engine and the voltage across the battery should come up to 14-16 volts.
Now, lets take your example. We have two batteries that are weak. Lets say that each one can only deliver 50 amps at 12V. Lets say the starter takes at least 75 amps to spin the engine fast enough to start. By paralleling them, your current available to the starter is now 100 amps. The engine starts. By paralleling the batteries, the voltage stays at 12V or below. You can crank to your heart's content because the starter won't overheat at this voltage.
Now lets discuss connecting the two batteries in series. Using the above hypothetical information, dividing the 12 volts by the 75 amps to turn the starter, the resistance is .16 ohm. Connecting the two batteries in series gives us 24 volts. Connecting these across the .16 ohm load of the starter will double the current used by the starter to 150 amps, if the batteries are strong enough to supply it. But, we said the batteries could only produce 50 amps, so yes you can start the engine like this. The higher voltage overcomes the resistance in each connection far better than 12V, so really it starts better. In the real world though, the wires in a starter are a particular size to carry a particular amperage. Any current over this, caused by too high a voltage, causes the wires to overheat, melt the insulation off the wires, shorting them together, decreasing the resistance further and increasing the current until you smoke the unit. Plus, current draw has too many varibles to predict it accurately. Temperature effects it greatly; the colder the battery, the less available current.
A battery is just a storage device for electrons. After the engine starts, the huge starter load is gone and the alternator takes over the electrical load of the machine like lights, horn, ect. Controlled by its built in voltage regulator, the alternator pumps electrons back into the negative post of the battery (the chassis ground) which stores them to use next time. Alternators are also rated in output amperage. Motor homes have huge alternators, not just to charge its larger batteries, but to handle the extra 12VDC load. An alternator will always charge the battery near its maximum output, but, since just to start a vehicle not much stored charge was used, the battery builds back quickly and the alternator will decrease the charging voltage so the battery is not overcharged.
Why batteries can explode: The electrolite in the battery is made of acid and water. If you put a current flow through water (H2O), it breaks down to hydrogen and oxygen by a process called electrolysis. This is a potent explosive. This is why the old batteries had vent caps, to let off the pressure and get rid of the hyrogen and oxygen. It is also why you jump start a battery by making the last connection to the engine block or other machine part. When the connection sparks, it won't be near the battery gasses. The new generation of batteries are semi-sealed and use a chemical mixture which doesn't loose water to electrolysis. Thus, you don't have to periodically add water like you used to.
A note about alternators versus generators. Generators are DC generators which use an external voltage-current-cutout-relay regulator. They are inefficient and finicky and only charge when the engine is run up to speed. You can let a tractor with a generator idle and you can run the battery down. An alternator is a 3 phase alternating current generator. The AC is rectified by the diodes inside the alternator to DC and the voltage and current is controlled by an internal regulator. They produce more current at engine idle speed than a generator does at full throttle. They are almost trouble free and either work or don't with no weird charging problems. They have no relays to give problems. So, most everyone is converting tractors to alternators for this reason. I hope this is clear and the extra information will be useful to others.

Follow Ups:

Post A Followup Name: E-Mail: Subject: Re: Re: Re: Re: To Settle an Argument Comments: Optional Link URL: Link Title: Optional Image URL: Email Notification: If you check this box, email will be sent to you whenever someone replies to this message. Your email address must be entered above to receive notification. This notification will be cancelled automatically after 2 weeks.

TRACTOR PARTS	TRACTOR MANUALS

We sell tractor parts!  We have the parts you need to repair your tractor - the right parts. Our low prices and years of research make us your best choice when you need parts. Shop Online Today. [ About Us ]