All the going back and forth in Chris's post about testing an alternator got me wondering something. Does a battery charger set for 12 volts keep charging at 12 volts and just charge amps? If it does, why does a generator change voltage while charging, apparently up to as high as 15 volts depending on who's reply you read? Why, if a charger stays at 12 volts, doesn't a generator or alternator? Just wondering why not.
- Thread starter rrlund
- Start date
WELL... its an ohms law situation... the resistance of the battery changes to the charger... as the voltage in the battery rises, the charger will put out less current.. when the voltage is equal, there will be no flow... voltage is the pressure of the two vessels... if one is higher it will flow to the lower pressure vessel... The more the pressure difference, the faster the flow (current). (assumes wire size is big enough) HOWEVER the source has a variable controller or regulator that can change the limit/ vary the total flow rate(current) for safety. This allow it to then charge a battery at lower voltage at a higher FIXED current rate, and then as the battery voltage comes up, it allows it to slowly equalize it...
Now add to that a controller that allows for bulk charging at a higher top voltage say about 14.7 and then switch to a float voltage later of around 13.8 to 13.6 and you have the best of all worlds. Throw in a heat sensor to allow for slightly higher voltage in cold weather vs slight less voltage in hot weather and you now start to get an intelligent system.
Now add to that a controller that allows for bulk charging at a higher top voltage say about 14.7 and then switch to a float voltage later of around 13.8 to 13.6 and you have the best of all worlds. Throw in a heat sensor to allow for slightly higher voltage in cold weather vs slight less voltage in hot weather and you now start to get an intelligent system.
MarkB_MI
Well-known Member
- Location
- Motown USA
Typical battery chargers don't really regulate the voltage, although the better ones do. Your garden-variety battery chargers run the full-wave rectified diode output directly to the battery. Since the diodes can only conduct when they're forward-biased, the battery is only getting charged for a portion of the ac cycle. If the battery is almost dead, then it gets charged something like 70 percent of the cycle; when it's fully charged it might be getting a charge only 10 percent of the cycle. So what looks like regulation is just the effect of rising battery voltage on the charger output's duty cycle.
If you used an oscilloscope to look at the output of a battery charger with just a small resistive load on it, you would see a full-wave rectified ac signal that's something like 16 to 20 volts peak. If you hook a battery up to the charger, you would see a clipped rectified ac signal, with the voltage clipped at whatever the battery voltage is.
If you used an oscilloscope to look at the output of a battery charger with just a small resistive load on it, you would see a full-wave rectified ac signal that's something like 16 to 20 volts peak. If you hook a battery up to the charger, you would see a clipped rectified ac signal, with the voltage clipped at whatever the battery voltage is.
MarkB_MI
Well-known Member
- Location
- Motown USA
> WELL... its an ohms law situation... the resistance of the battery changes to the charger.
Not really an ohm's law situation, since a battery is not a resistor (although it does have resistance). A battery is a voltage source, so it can source a nearly unlimited amount of current to a device at a lower voltage (e.g. a starter motor) and can sink an almost unlimited amount of current from a device at a higher voltage (e.g. a battery charger).
Not really an ohm's law situation, since a battery is not a resistor (although it does have resistance). A battery is a voltage source, so it can source a nearly unlimited amount of current to a device at a lower voltage (e.g. a starter motor) and can sink an almost unlimited amount of current from a device at a higher voltage (e.g. a battery charger).
maybe comparing to air pressure is a good analogy. say your tire should be 30psi but it is only 20psi. you need a source of higher air pressure than 20psi for any air to go into the tire. And at least 30psi to get it to 30psi but that would take forever to get there. Similar thing with voltage and a battery. if the voltage regulator sees less than its set voltage it will allow more voltage into the battery to charge it up within limits of course. most modern cars charge at 14 volts give or take. as soon as you turn the car off the battery will read between 12.6 and 13volts.
AMPS for electricity is basically like flow rate. to fill your tire the air source will fill it at X cubicFeet/minute at X psi. A generator/alternator is similar. depending on the demand it sees (low battery voltage) it will send X AMPs at Y volts until the desired voltage is reached.
higher pressure can deliver faster. lower pressure say pumping your tire from 28 to 30 PSI with a 30PSI source will take a long time with a small hose but more quickly with a larger hose (of course valve stem is limiting factor (resistor) in this case)
This is also why there are diodes in the electric circuit as well. if the battery voltage is higher than the source (another battery or generator ect) then power could flow backwards and drain the battery. the diode is a check valve to prevent that.
everything flows from high to low. high voltage to low (good battery with nothing attached will over time go dead), high pressure to low (air is always wanting get OUT of your tire), High Temperature to low (house gets cold), high stress to low (things break)
rough and hurried explanation with limited details but hopefully helps a bit.
AMPS for electricity is basically like flow rate. to fill your tire the air source will fill it at X cubicFeet/minute at X psi. A generator/alternator is similar. depending on the demand it sees (low battery voltage) it will send X AMPs at Y volts until the desired voltage is reached.
higher pressure can deliver faster. lower pressure say pumping your tire from 28 to 30 PSI with a 30PSI source will take a long time with a small hose but more quickly with a larger hose (of course valve stem is limiting factor (resistor) in this case)
This is also why there are diodes in the electric circuit as well. if the battery voltage is higher than the source (another battery or generator ect) then power could flow backwards and drain the battery. the diode is a check valve to prevent that.
everything flows from high to low. high voltage to low (good battery with nothing attached will over time go dead), high pressure to low (air is always wanting get OUT of your tire), High Temperature to low (house gets cold), high stress to low (things break)
rough and hurried explanation with limited details but hopefully helps a bit.
Russ from MN
Well-known Member
- Location
- Bemidji MN
My understanding is that you need about 13.5 volts to charge a 12 volt battery. This is the best simple explanation of volts vs. amps!
Steve@Advance
Well-known Member
A plug in battery charger is operating at a steady input of around 120 vac. It's internal electronics are designed to do whatever the engineer decided would work for the application, be it a simple transformer and rectifier, or a complex, regulated, automatic charger designed to cut back as the battery reaches full charge. The first will boil a battery dry if given a chance, the second will not.
A generator or alternator is a different animal. It is converting mechanical energy to electricity. It operates at a wide range of RPM, so it must be regulated to properly charge the battery AND not burn the battery, or it's self out. It also needs circuitry to not discharge the battery when the mechanical energy (spinning the armature or stator) is removed.
A generator or alternator is a different animal. It is converting mechanical energy to electricity. It operates at a wide range of RPM, so it must be regulated to properly charge the battery AND not burn the battery, or it's self out. It also needs circuitry to not discharge the battery when the mechanical energy (spinning the armature or stator) is removed.
rr Good Question and the answer as to how much voltage and/or current a charging source delivers into a battery THE ANSWER DEPENDS upon the chargers Voltage Regulation scheme and circuitry and the battery condition and its State of Charge SOC.
Take for example more like constate voltage charges (around 13.4 to 13.6 or so) used on older RV's and other applications, if left alone they could overcharge and actually damage a battery perhaps boil over and out gas so much the electrolyte levels fell below the plates !!!!!!
In later years so called SMART 3 stage controlled and regulated chargers were developed that for example might BULK charge around 14+ Volts,,,,,,,,Absorption charge around 13.6 Volts,,,,,,,,FLOAT Charge at maybe 13.2 Volts, they would maintain proper SOC and NOT damage batteries if left on.
Even absent later smart regulated and controlled charging technology a battery under charge and on its own would accept less charging as its voltage rises closer to the chargers voltage. If theres no potential voltage difference theres no current flow WELL DUH
As far as automotive alternators and older generators and their Voltage Regulators, subject to the battery condition and SOC, their Voltage Regulation scheme and circuitry determines the charge they deliver
John T NOT any electronics or battery charging expert but believe this to be true, if any engineers who are experts in batteries and charging maybe they can weigh in here and educate all of us !!!
Take for example more like constate voltage charges (around 13.4 to 13.6 or so) used on older RV's and other applications, if left alone they could overcharge and actually damage a battery perhaps boil over and out gas so much the electrolyte levels fell below the plates !!!!!!
In later years so called SMART 3 stage controlled and regulated chargers were developed that for example might BULK charge around 14+ Volts,,,,,,,,Absorption charge around 13.6 Volts,,,,,,,,FLOAT Charge at maybe 13.2 Volts, they would maintain proper SOC and NOT damage batteries if left on.
Even absent later smart regulated and controlled charging technology a battery under charge and on its own would accept less charging as its voltage rises closer to the chargers voltage. If theres no potential voltage difference theres no current flow WELL DUH
As far as automotive alternators and older generators and their Voltage Regulators, subject to the battery condition and SOC, their Voltage Regulation scheme and circuitry determines the charge they deliver
John T NOT any electronics or battery charging expert but believe this to be true, if any engineers who are experts in batteries and charging maybe they can weigh in here and educate all of us !!!
Janicholson
Well-known Member
Simple as possible explanation: Voltage is the push on the electricity in a wire. For voltage to actually move electricity (electrons) in the wire there needs to be a circuit that connects the negative of a voltage source to the positive of that source. A 12v battery is just called 12volt, it is really a 12.6 volt battery when charged.if the voltage source was 12v exactly, there would be no charging. To charge the charging voltage must be above 12.6 (usually, as John T points out about 14.2, or a bit more) The chemistry of the battery is not going to take more charge, it is full, so there is very little actual flow into the battery.
If the battery was used to start an engine, it now has room for more chemical change and at 14.2 it accepts more charge. In 10 minutes or so the battery is full again, and actual charging just stops because it is full.
The amount of discharge (a cold diesel engine cranking for a minute) allows more room in the battery and this allows the amount of amps to increase at first, then decrease as the full mark is reached.
In a complete circuit the amount of amps depends on the amount of voltage, and the amount of resistance. an example is the starter motor, it has much less than 1/2 ohm of resistance (near none). This causes the battery to send hundreds of amps through the big wires to spin the engine. This reduces the chemical charge in the battery. when the engine starts the alternator replaces those electrons by pushing them back into the battery changing the chemistry to full charge. The starter motor may draw 275 amps for 30 seconds. The alternator may take 10 to 20 times that to refill the battery. Jim
If the battery was used to start an engine, it now has room for more chemical change and at 14.2 it accepts more charge. In 10 minutes or so the battery is full again, and actual charging just stops because it is full.
The amount of discharge (a cold diesel engine cranking for a minute) allows more room in the battery and this allows the amount of amps to increase at first, then decrease as the full mark is reached.
In a complete circuit the amount of amps depends on the amount of voltage, and the amount of resistance. an example is the starter motor, it has much less than 1/2 ohm of resistance (near none). This causes the battery to send hundreds of amps through the big wires to spin the engine. This reduces the chemical charge in the battery. when the engine starts the alternator replaces those electrons by pushing them back into the battery changing the chemistry to full charge. The starter motor may draw 275 amps for 30 seconds. The alternator may take 10 to 20 times that to refill the battery. Jim
teddy52food
Well-known Member
Compare a battery to a barrel of water. When you use the starter it is like using a pail to remove water from the barrel. But the generator can only put it back in a cupful at a time. When it is full it can still hold a little more like putting it in with a spoon. ( trickle charge)
Gene Davis (Ga.)
Well-known Member
A red neck explanation of amps vs voltage. Voltage will get you to the wall quickly, but amps will take you through the wall.
Gene Davis Tennille, Ga.
Gene Davis Tennille, Ga.
Dont know... but...
AS THE BATTERY IS CHARGING... you can.. safely measure the current going through it... and the voltage drop across it... say its now at 14 volts and taking .5 amps... ohms law says..... you now have a 28 ohm resistor as far as the charger sees. or 12 volts and 25 amps...equals a .48 ohm resistor to the charger. ( in this case, the resistance could actually be lower but my charger is regulated to 25 amps max)
Sulfate on the battery plates and other things control the total resistance to the outside circuit in addition to the cells themselves..a battery is a chemical resistor... that takes a lot of charging for the chemistry to change with losses in heat. And slowly the voltage rises...... TO a primitive fixed voltage charger... its ohms law.. including the internal limter resistor of the charger to control max current into a almost short of the battery... thus were limited in total current. Was common in the on the older chargers to pop the charging breaker when put on a dead or almost full short of a battery. (ohms law) If not enough charger resistance, then a timer installed to keep from over heating the battery. Modern circuits with voltage and elegant current regulators are better as current of the charger cant be exceeded, and then voltage is limited as the battery comes up...... then the 3 stage systems with cpu control are best yet. If batteries did not have internal resistance, they would continue to charge at max current forever from the chargers standpoint. So.. to the charging circuit, a battery is part of a chemically varying resistor in a circuit.. and ohms law works. my best guess.
AS THE BATTERY IS CHARGING... you can.. safely measure the current going through it... and the voltage drop across it... say its now at 14 volts and taking .5 amps... ohms law says..... you now have a 28 ohm resistor as far as the charger sees. or 12 volts and 25 amps...equals a .48 ohm resistor to the charger. ( in this case, the resistance could actually be lower but my charger is regulated to 25 amps max)
Sulfate on the battery plates and other things control the total resistance to the outside circuit in addition to the cells themselves..a battery is a chemical resistor... that takes a lot of charging for the chemistry to change with losses in heat. And slowly the voltage rises...... TO a primitive fixed voltage charger... its ohms law.. including the internal limter resistor of the charger to control max current into a almost short of the battery... thus were limited in total current. Was common in the on the older chargers to pop the charging breaker when put on a dead or almost full short of a battery. (ohms law) If not enough charger resistance, then a timer installed to keep from over heating the battery. Modern circuits with voltage and elegant current regulators are better as current of the charger cant be exceeded, and then voltage is limited as the battery comes up...... then the 3 stage systems with cpu control are best yet. If batteries did not have internal resistance, they would continue to charge at max current forever from the chargers standpoint. So.. to the charging circuit, a battery is part of a chemically varying resistor in a circuit.. and ohms law works. my best guess.
J.Wondergem
Well-known Member
- Location
- Rockford, Mi.
I like your simple explanation.
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