Tweet
Perhaps the topic of another Sticky Thread. I wrote another Sticky that covers how Bulk, Absorb, and Float chargers work, but here is a dumbed down version of how a battery charges and discharges. Makes no difference what type of battery you are talking about, they all charge the exact same way. There is NO MAGIC involved, just simple Ohm's Law and 5th grade math. The only difference between battery types is how we terminate the charge cycle
Time for some 5th grade math, Ohm’s Law consist of 12 equations. Don’t freak out as you only need to know 1 of the 12 equations any 5th grader can remember and understand. Well in the USA a 16 year college grad. That equation is VOLTAGE = CURRENT X RESISTANCE. Also can be expressed as E = IR where E is electromotive force, I is Current, and R is resistance, or V = IR. Example; If you have 10 amps of current flowing into 1 Ohm of resistance is 10 amps x 1 Ohm = 10 Volts. See it is simple, so relax.
Now time for a term called C-Rates which involve simple math equations. Fist let’s is define C-Rate: Is a measure of the rate at which a battery is charged/discharged relative to its maximum capacity in terms of Amps and Hours. Batteries are rated in Amp Hour over a specified amount of time measured in Hours. So here is a really hard math equation; Amp Hours = Amps x Hours. From that equation we can factor out either Amps or Hours. Amps = Amps Hours / Hours , and Hours = Amp Hours / Amps. So when you buy a battery the Amp Hour capacity will be specified in Amp Hours at some given rate of time in hours. Example let’s say a 100 AH battery at the 20 Hour rate. So here is another equation: C/20 where C = the battery rated Amp Hour capacity of C. So if you see a battery rated 100 AH at the 20 hour rate is telling you C/20 = 100 AH / 20 H = 5 amps. In other words a 100 AH battery can deliver 5 amps for 20 hours. What about C/10? Did you come up with 10 amps for10 hours = 100 AH? Great!
OK to charge any battery, you must force current into the battery at some given C-Rate. Most battery manufactures recommend C/10. So let’s use a 12 volt 100 AH battery in our example and we will charge it at C/10 or 10 amps. To do that the charger voltage must be higher than the battery voltage, and capable of a C/10 C-Rate or 10 amps. To charge a 12 volt battery takes up to 16 volts. The Control circuitry takes another volt or two, so a panel as a charger needs to be at least 17 to 18 volts and enough power to push 10 amps.
All batteries have Internal Resistance, and this is important to understand how the voltage works with respect to resistance and current. So for point of discussion let’s assume our model battery is 12 volts, 100 AH capacity, with .0.02 Ohms of internal resistance. Also assume the battery is fully discharged and has an Open Circuit Voltage (OCV) of 10.5 volts. So now we want to charge the battery up to 14.4 volts with our 10 amp Float Charger. We set the charger voltage to 14.4 volts and measure the voltage before we connect the battery and measure 14.4 volts. We then connect the charger to the battery and what voltage do you have on the battery? Well silly you probable said 14.4 volts right? Dead ass wrong answer silly. You only see 10.7 volts with 10 amps of current flowing into the battery. So what is going on? Well the charger is only capable of supplying 10 amps of charge current. We have a battery with an OCV = 10.5 volts with 0.02 ohms of internal resistance. Go back to Ohms Law Voltage Equation of voltage = Current x Resistance . We have 10 amps x 0.02 Ohms = 0.2 Volts. So the Battery Voltage = OCV + [Charge Current x Battery Internal Resistance] = 10.5 volts + [10 amps x 0.02 Ohms] = 10.7 volts.
What is going on is the charge has a Current Limit of 10 amps, and the voltage has Folded Back to 10.7 volts. The charger will continue to supply 10 amps CONSTANT CURRENT until the battery voltage reaches 14.2 volts some 9 or 10 hours later. When the battery voltage goes above 14.2 volts, the charger holds 14.4 volts known as Constant Voltage set point we selected earlier, and the charge current starts to taper off to zero amps. When the battery voltage reaches 14.3 amps, charge current is now down to 5 amps and going down. When the battery voltage reaches 14.4 volts, current is ZERO amps and the battery is fully charged and saturated. No current can flow when both the charger and battery voltages are equal. In order for any current to flow there must be a voltage difference. Prove it with math: 0 amps x 1 million ohms = 0 volts. 0 x anything = 0. Ask any 5th grader.
So how does a battery discharge? Exactly how it charges in reverse. If you have the same battery fully charged up with an OCV of 12.6 volts and draw 10 amps, the voltage sags to 12.4 volts. Hit that same fully charged battery with 1C or 100 amps, and the voltage drops from 12.6 to 10.6 volts and your gizmo shuts down from under voltage despite having a fully charged battery. Did you figure out why?
Let's go back to charging for a moment. In my example I used a C/10 charge rate of 10 amps. With only 10 amps raises the voltage .2 volts. What current would it take to make the battery voltage go from 10.5 volts to 14.4 volts of the charger like you first thought it would be. Did you come up with 195 amps? Do you know what would happen to the battery if you hit with roughly with a 2C charge current to charge it in 30 minutes?
FWIW ther are some battery types you can charge with 1C or more. But it is not any lead acid battery. Only some lithium batteries and a few Nickel types. Can you guess why? Internal Resistance.
Time for some 5th grade math, Ohm’s Law consist of 12 equations. Don’t freak out as you only need to know 1 of the 12 equations any 5th grader can remember and understand. Well in the USA a 16 year college grad. That equation is VOLTAGE = CURRENT X RESISTANCE. Also can be expressed as E = IR where E is electromotive force, I is Current, and R is resistance, or V = IR. Example; If you have 10 amps of current flowing into 1 Ohm of resistance is 10 amps x 1 Ohm = 10 Volts. See it is simple, so relax.
Now time for a term called C-Rates which involve simple math equations. Fist let’s is define C-Rate: Is a measure of the rate at which a battery is charged/discharged relative to its maximum capacity in terms of Amps and Hours. Batteries are rated in Amp Hour over a specified amount of time measured in Hours. So here is a really hard math equation; Amp Hours = Amps x Hours. From that equation we can factor out either Amps or Hours. Amps = Amps Hours / Hours , and Hours = Amp Hours / Amps. So when you buy a battery the Amp Hour capacity will be specified in Amp Hours at some given rate of time in hours. Example let’s say a 100 AH battery at the 20 Hour rate. So here is another equation: C/20 where C = the battery rated Amp Hour capacity of C. So if you see a battery rated 100 AH at the 20 hour rate is telling you C/20 = 100 AH / 20 H = 5 amps. In other words a 100 AH battery can deliver 5 amps for 20 hours. What about C/10? Did you come up with 10 amps for10 hours = 100 AH? Great!
OK to charge any battery, you must force current into the battery at some given C-Rate. Most battery manufactures recommend C/10. So let’s use a 12 volt 100 AH battery in our example and we will charge it at C/10 or 10 amps. To do that the charger voltage must be higher than the battery voltage, and capable of a C/10 C-Rate or 10 amps. To charge a 12 volt battery takes up to 16 volts. The Control circuitry takes another volt or two, so a panel as a charger needs to be at least 17 to 18 volts and enough power to push 10 amps.
All batteries have Internal Resistance, and this is important to understand how the voltage works with respect to resistance and current. So for point of discussion let’s assume our model battery is 12 volts, 100 AH capacity, with .0.02 Ohms of internal resistance. Also assume the battery is fully discharged and has an Open Circuit Voltage (OCV) of 10.5 volts. So now we want to charge the battery up to 14.4 volts with our 10 amp Float Charger. We set the charger voltage to 14.4 volts and measure the voltage before we connect the battery and measure 14.4 volts. We then connect the charger to the battery and what voltage do you have on the battery? Well silly you probable said 14.4 volts right? Dead ass wrong answer silly. You only see 10.7 volts with 10 amps of current flowing into the battery. So what is going on? Well the charger is only capable of supplying 10 amps of charge current. We have a battery with an OCV = 10.5 volts with 0.02 ohms of internal resistance. Go back to Ohms Law Voltage Equation of voltage = Current x Resistance . We have 10 amps x 0.02 Ohms = 0.2 Volts. So the Battery Voltage = OCV + [Charge Current x Battery Internal Resistance] = 10.5 volts + [10 amps x 0.02 Ohms] = 10.7 volts.
What is going on is the charge has a Current Limit of 10 amps, and the voltage has Folded Back to 10.7 volts. The charger will continue to supply 10 amps CONSTANT CURRENT until the battery voltage reaches 14.2 volts some 9 or 10 hours later. When the battery voltage goes above 14.2 volts, the charger holds 14.4 volts known as Constant Voltage set point we selected earlier, and the charge current starts to taper off to zero amps. When the battery voltage reaches 14.3 amps, charge current is now down to 5 amps and going down. When the battery voltage reaches 14.4 volts, current is ZERO amps and the battery is fully charged and saturated. No current can flow when both the charger and battery voltages are equal. In order for any current to flow there must be a voltage difference. Prove it with math: 0 amps x 1 million ohms = 0 volts. 0 x anything = 0. Ask any 5th grader.
So how does a battery discharge? Exactly how it charges in reverse. If you have the same battery fully charged up with an OCV of 12.6 volts and draw 10 amps, the voltage sags to 12.4 volts. Hit that same fully charged battery with 1C or 100 amps, and the voltage drops from 12.6 to 10.6 volts and your gizmo shuts down from under voltage despite having a fully charged battery. Did you figure out why?
Let's go back to charging for a moment. In my example I used a C/10 charge rate of 10 amps. With only 10 amps raises the voltage .2 volts. What current would it take to make the battery voltage go from 10.5 volts to 14.4 volts of the charger like you first thought it would be. Did you come up with 195 amps? Do you know what would happen to the battery if you hit with roughly with a 2C charge current to charge it in 30 minutes?
FWIW ther are some battery types you can charge with 1C or more. But it is not any lead acid battery. Only some lithium batteries and a few Nickel types. Can you guess why? Internal Resistance.