In this video i'm going to talk about c rates. What is it and why it's important to know about
it. I'm Nick and my mission is to make solar power understandable to everyone.
We know that a battery has a lot of ratings we have nominal voltage, internal resistance,
amp-hours, voltage, watt-hours, and many more. We also have C-rate. C-rate is a number that tells
you about how fast you can charge and discharge a battery. Typically the C -rate is displayed as C
followed by a number like C20 it can also be shown as a C with a number behind it like 0.05C.
This can cause some confusion, which I will explain more in the next slide.
The battery capacity and Ampére hour is used as C. Let's take an example for a lead-acid battery.
If you have a battery rated for C20 you divide the amp hours of the battery by 20.
for example, we have a battery of 100 ampere-hours with a C-rate of 20. Then we can charge
and discharge the battery at 5 amps. If we use another example for lithium at 1C discharge
we multiply the battery capacity by 1. We can now charge and discharge the battery at 100 amps.
This table will make it easier to understand. We can see that C-rate can be listed as
20C or C20. If the number is in front of the C then you multiply it with the battery capacity.
If it's after the number you divide it by the battery capacity you say 0.05C or C20.
They are both the same. If we use 5C in this table, we must multiply the battery capacity
in amp-hours by 5. We can see that the charging and discharging currents can be 500 amps
but only for 12 minutes. After that, the battery will have a zero percent state of charge.
This is not possible in solar power applications because no battery can provide this amount of
current. These high c rates are commonly used in batteries for drones or lithium titanate.
If you look at a more realistic example for a lead-acid battery of C20 or 0.05C.
We can see that the battery can supply 5 amps for 20 hours
before it's completely drained. In theory that would only last 10 hours
because we don't want to drain a lead-acid battery to a zero percent state of charge.
Lead-acid and lithium batteries have different C-rates.
Lithium is capable of a higher charging and discharging current than lead-acid this is because
the internal resistance in lithium batteries is very low. Let's compare two popular batteries.
battleborn lithium batteries have a charging C-rate of 0.5C or C2 and a discharging rate of 1C.
The lead acid battery has a C-rate of C20. The battery management system or BMS in the lithium
battery will shut down the battery when a high current is drawn. A lead-acid battery doesn't have
this protection. Therefore you need to do your calculations before buying a lead-acid battery.
If we have a lead-acid battery of 100 amp hours we can use the rated capacity at 0.05C or C20. So if
we were to power a 1000-watt appliance through an inverter we could draw 83 amps at 12 volts.
Using the formula we can see that we need 16 batteries to supply this 83 amps of current
to achieve the rated battery capacity.
Of course, we can use fewer batteries but then the discharge current of each battery is higher.
We don't have this problem with lithium because they have a higher charge and discharge current.
If we go higher than the rated C-rate for lead-acid we will reduce the battery capacity.
Let's take a look at this graph from a popular lead-acid battery.
We can see that the battery capacity in amp-hours is reduced when the C-rate
rises. This is because there is more heat generation and more internal resistance.
The battery will become less efficient. This is important in sizing your system.
Make sure you check the charge and discharge C-rate of your batteries
before buying them. Lithium has higher C-rates than lead-acid. Lithium batteries have a built-in
battery management system to shut down the battery if high currents are being drawn. A lithium
battery is capable of providing a surge current of 2C for a few seconds depending on the chemistry.
I hope this video sheds some more light on C-rates for solar power.
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