something i came across
How can I tell if my battery is charged or not?
Lead acid batteries are made up of cells. Each cell is approximately 2 volts, so a 12-volt
battery has 6 individual cells. It turns out that a fully charged 2-volt cell has a voltage of
approximately 2.15 volts. Oddly enough, a fully discharged 2-volt cell has a voltage of
1.9 volts. That’s only a difference of 0.25 volts on each cell from fully charged to fully
discharged. So a 12-volt battery will measure at about 12.9 volts when it’s fully charged
and about 11.4 volts when it is fully discharged. That’s a total of 1.5 volts that represents
the full range of charge on a 12-volt battery. To make a good guess at how much charge
your battery has left, you can assign a percentage of charge remaining that is directly
proportional to the battery voltage. Let’s see how we can do that.
If the battery voltage is 12.15 volts, how much charge is left? Beginning with 11.4 volts
representing no charge or 0% charge available, subtract 11.4 volts from the voltage that
you read. So 12.15 – 11.4 = 0.75 volts. Since there are only 1.5 volts above 11.4 volts
that represents the full range of charge, we can divide the difference that we just
calculated by 1.5 volts to get the percentage of charge remaining. 0.75 volts / 1.5 volts =
0.5 or when expressed as a percentage, multiply by 100 and get 50%.
Here’s the procedure written as a formula that is applicable to 12 Volt Batteries:
OPEN CIRCUIT BATTERY STATE OF CHARGE CALCULATION
% Charge = SOC
% Charge = ((Measured Battery Voltage – 11.4 volts) / 1.5 volts) x 100
Equation 1 Open Circuit Battery State of Charge Calculation
That seems easy enough. So what’s the catch? In order for this formula to work, the
battery must be in a rest state. In other words, the battery should not be supplying power
to any type of load. The experts say that the battery should remain at rest for at least 24
hours to get an accurate measurement, but in a pinch a couple of hours is good enough to
make a reasonable guess. Even if the battery is connected to a load, as long as the load
current is less than 1% of the battery capacity in amp-hours, then this method is probably
good enough in most cases. It’s all a matter of how accurate you want to be. If you’re a
scientist or engineer trying to develop a battery powered product, then you probably want
a more accurate measurement than if you’re going fishing for the weekend and you just
want to know if you need to take the time to charge your battery before you use it.
There is one more thing to keep in mind. The only way to be absolutely sure that your
battery is fully charged is to do a load test. It is best to have the battery dealer do this for
you. We only mention it here because it is possible for a battery to indicate a good
voltage, but then immediately when you try to use it, it acts like it’s dead. This doesn’t
happen very often, but it’s good to know that it is a possibility.
Time Required to Charge a Battery:
Let’s take a moment and talk about two of the fundamental electric quantities, Amps and
Coulombs. A battery stores charge (Coulombs), and an electric current (Amps) is made
up of charge that is moving. Let’s ask a very important question: How long will it take
to charge a battery? If you look at the battery specifications and ratings,
you won’t find Coulombs listed anywhere. What you probably will find is Amp-Hours.
Let’s look at that term. Amps times Hours = (Coulombs per second) times 3600 seconds
(in 1 Hour). So, 1 Amp-Hour = 3600 Coulombs. That’s still sort of confusing. The
main thing to remember is that Amp-Hours and Coulombs are both units that describe an
amount of electric charge.
Let’s try something else. Suppose I have a 50 Amp-Hour battery. That’s a fairly typical
size for an automotive engine start type battery. Now let’s say I have a 10 Amp charger.
If it’s a good charger it will deliver close to 10 amps for as long as it takes to get the battery voltage up to its recharge level. So how long will it take to actually charge the battery?
We can make a pretty good guess by just dividing two numbers:
(Battery Capacity) / (Charger Current) = Time
(Amp-Hours) / (Amps) = Hours
for this example:
(50 Amp-Hours) divided by (10 Amps) = 5 Hours.
So we would estimate that it will take a good 10 Amp charger about 5 Hours to recharge
a 50 Amp-Hour battery. Actually this rough estimate usually tells us how long it
takes to recharge the battery to about 80% of its capacity. It turns out that it will
probably take an equal amount of time, or another 5 hours to recharge the last 20% of the
Last edited by dutter; 01-06-2009 at 09:17 AM.