Discharging or recharging batteries too fast can damage the battery, reduce its service life (how long it will last until it can no longer recharge) or even lead to fires or explosions.
The C rating of a battery tells us the maximum discharge and recharge rate a battery can withstand without the risk of internal damage.
You’ll often find the C rate on the label of batteries or see some manufacturers using language like “this battery will last 30 minutes at a 1C discharge rate.”
So what is the C rating, how can we work it out, and if we find it on a battery label or product documentation how can we use it? We’ll answer all of these questions in this article.
Let’s look first at the C rating when it comes to discharging a battery.
Discharging batteries with Ah ratings
C ratings for discharging can exist for both disposable (primary) and rechargeable (secondary) batteries.
To understand the C rate we first we need to dispel a myth.
THE MYTH: A 4amp hour battery will power a 4 amp device for one hour
This is only true of some batteries but it is generally thrown around the internet as a fact for all portable power products. Believe it and you might shorten the overall life of your battery considerably.
You might also be disappointed to find some 4 amp hour batteries only power a 4 amp hour device for half an hour or so. The C rate let’s us unlock this confusing situation.
First of all we need to understand where the C rate comes from.
The Theoretical C rate Calculation:
| Discharging current (amps) | = C Rate |
| Battery Capacity (Ah) |
So if we go back to our myth we can see it as:
| 4 (amps) | = 1C |
| 4 (Ah) |
So if we connect our 4 amp hour battery to a 4 amp device it will be discharging at a rate of 1C. And if we connected it to a 2 amp device:
| 2 (amps) | = 0.5C |
| 4 (Ah) |
And going in the other direction this is the C rating when a 4 amp hour battery powers an 8 amp device:
| 8 (amps) | = 2C |
| 4 (Ah) |
So the C rate is nothing to do with individual batteries, it simply describes a rate of discharge or recharge.
It’s important to remember all batteries can be connected to a device to discharge at high C Rates but not all batteries are designed to cope with this. We need to know the manufacturers recommended C Rate first to avoid damaging the battery.
Using a battery’s C Rating
The calculation for working out what device a battery can safely power is:
| C Rate * Battery Capacity (Ah) = Device Amperage |
Let’s say we have a 4Ah battery which is labelled 0.5C.
| 0.5 (C) * 4 (Ah) = 2 (amp device) |
So this battery can power a 2 amp device. And if it was labelled 2C:
| 2 (C) * 4 (Ah) = 8 (amp device) |
A 2C labelled 4 Ah battery can power an 8 amp device without damaging the battery
Discharging batteries with mAh capacities
The C Rate is more commonly seen on lithium batteries where higher rates of discharge are possible compared to other battery chemistries. Lithium battery capacity is usually labelled in mAh (rather than Ah). Here’s an example of a 2600 mAh battery:
To calculate the device this battery can power without the unit being damaged we can use this formula
| C rating * | Battery Capacity (mAh) | = Device amps |
| 1000 |
Taking these figures from the battery label in the image above gives us
| 100 (C) * | 2600 (mAh) | = 260 (amps) |
| 1000 |
This battery can safely power a 260 amp device even though it would last less than a minute. We’ll cover how you can work out how long a battery lasts at different C Rate discharges later in this article but it’s important to understand that high C ratings like 100c, when used, will always result in a very fast discharge time.
Why is the C Rate important?
We’ve mentioned above that the C Rate tells us how fast we can discharge and recharge a battery without damaging it.
In reality every discharge and recharge of a rechargeable battery damages its internal components a little. It’s why no rechargeable battery lasts forever.
But by following the labelled C Rate and not discharging a battery faster than that we’ll minimize the damage of each discharge and recharge cycle.
You will also find that many manufacturers provide documentation referring to how their batteries perform when they are discharged at different C Rates.
In the image below is the discharge graph of a 12 volt 100 Ah sealed lead acid battery. As you can see, a fully charged 12 volt battery will actually have a voltage closer to 13 volts and will be able to power devices until the voltage falls below 11 volts.
The graph shows that this 12 volt battery will last just over 30 minutes when discharging at a rate of 1C (when it is connected to a 100 amp device) but it will last more than 20 hours when discharged at a rate of 0.05C.
So what device can it power for 20 hours?
| 0.05 (C rate) * 100 (Ah) = 5 (amp device) |
C Ratings and Ah/mAh capacity
What the graph shows is that this 12 volt 100 Ah battery does not power a 100 amp rated device for 1 hour (as the internet myth would have you believe).
The battery lasts 35 minutes. All this means is that this particular battery cannot be both rated as 1C and 100 Ah… but it does not mean it is not a 100Ah battery. Confused? Read on.
Let’s fully charge the 100Ah battery and then connect it to a 5 amp appliance. This would be a discharge C rate of 0.05C ( 5 Amp / 100 Ah )
We find it lasts 20 hours.
| 20 (hours) * 5 (amp device) = 100 (Ah) |
This battery is a 100 Ah battery when discharged at 0.05C. It is not a 100Ah battery when discharged at 1C.
The Ah / mAh rating tells us half the story. We need the C rating to get the complete picture.
But don’t leave just yet. The fact that, in theory, this battery lasts 20 hours when connected to a 5 amp hour device doesn’t mean that is how we should use it. We’ll discuss other factors that you need to take into account later in the article.
The hour rate alternative
As we saw in the above example the battery was a 100 Ah battery when discharged at 5amps over 20 hours. This is known as the “20 hour rate” and its the common yard stick used with some battery types such as Lead Acid.
If the battery does not have a C Rating you might find it has this alternative “hour rate” measure either on its label or its technical specification sheet.
This is why the Ah/mAh rating on the label of a battery only tells us half the story. We need the C rate or the “hour rate” to understand when the Ah/mAh rating applies.
In general
- Lead Acid and Alkaline Batteries use the “20 hour rate” to determine their Ah rating (and recommended rate of discharge)
- Lithium Batteries use the C rate as they are able to charge and discharge much faster than Lead Acid.
The role of temperature
Very low and very high temperatures can affect the performance of most battery types.
In the formula above we connected a 12 volt 100 Ah battery to a 5 amp device and it lasted 20 hours. However if the temperature was was 0oC (32oF) we would likely get different results. The battery would not last as long because batteries do not perform well at lower temperatures.
We would also find the same issue with a C rated lithium ion battery.
So the accepted, industry wide, temperature to assess a batteries C rating or measure the “hour rate” is 25oC (77oF).
The C Rate and Discharge Time
The C Rate is also a way to see how long a battery will last when powering any given device. Just because a battery is rated 100C doesn’t mean it’s a good idea to use it that way.
Let’s take our 2,600 mAh 100C lithium battery again. As we saw in the calculation above it will power a 260 amp device without damage but how long will it last if we did this?
| 2.6 Ah | * 60 = 0.6 minutes. |
| 260 amps |
A little bit more than 30 seconds! So what’s the point?
- You could connect several batteries up to make for a longer lasting battery pack
- The device being powered may only need very short bursts at 100C but otherwise need far less power so the battery is flexible and able to do this. Flash photography would be an example.
- It’s not about the discharge, it’s about the recharge. ‘Fast charging’ is highly desirable and batteries with high C ratings can do this.
The Depth of Discharge (D.O.D) factor
In reality no battery likes to be fully discharged. It causes permanent internal damage and shortens the overall service life of a rechargeable battery.
Let’s say you had a 5 amp device that you regularly needed to power for around 20 hours. Using a formula we covered earlier you could work it out like this:
| 20 (hours) * 5 (amp device) = 100 (Ah) |
But this would assume that you would completely discharge the battery every time and doing so will shorten the service life of any battery type. How much depends on the chemistry of the battery.
Lead Acid batteries recommended Depth of Discharge.
Lead Acid batteries, ideally, should not fall below 70% State of Charge (70% fully charged), even for a battery labelled as ‘Deep Cycle’.
Here is the graph from the BatteryGuy BG-640 specification sheet showing what happens if the battery is repeatedly discharged to different levels.
D.O.D stands for Depth of Discharge. If the battery is fully discharged (100% D.O.D) every time it is used you can expect it to last for about 200 cycles (charges and discharges). However if you only discharge by 30% it sill last for well over 1,000 cycles.
The graph also shows that if the battery is fully discharged each time then after about 100 cycles it will no longer reach 12 volts when fully charged (due to the internal damage such deep cycles inflict on the internal materials).
These characteristics are true of all lead acid batteries.
Our specification sheet tells us the battery is best discharged at 0.05C but, for a long service life, only until the battery is 30% discharged (70% Depth of Discharge).
Knowing this we can work out how long we should use the battery for.
| total discharge time (hrs) * | Recommended Maximum D.O.D | = time (hours) |
| 100 |
For our 100 ah battery connected to a 5 amp device we know the total discharge time is 20 hours so the formula would look like this:
| 20 (hrs) * | 30 (%) | = 6 (hours) |
| 100 |
If we want to get the longest service life out of this battery then we should only connect it to a 5 amp device for a maximum of 6 hours before recharging.
It’s for this reason that many camper vans or RVs often contain one or more 100 Ah batteries to power the living area. The 100 Ah rating seems to be completely over the top for the few appliances that they power but they are needed because we only want to discharge them by 30% or less before recharging.
And hence another reason that “a 4ah battery will power a 4 amp device for one hour” is not true of all batteries.
Lithium Battery Depth of Discharge
Lithium batteries will also last longer if they are not fully discharged each time but you can safely discharge to 50% making them a little more flexible than lead acid.
But here we are more likely to use the C rating rather than the “20 hour” rate.
If we have a 4 Ah lithium battery with a 1 C rating we know it will power a 4 amp device for 1 hour but as we don’t want to discharge it beyond 50% we can use the following formula:
| Ah rating / device amps | = time (hours) |
| Max D.O.D / Recommended D.O.D |
As an example:
| 4 (Ah) / 4 amps | = 0.5 (hours) |
| 100 (% D.O.D.) / 50(% D.O.D) |
So we should only use our 4 Amp Hour lithium battery with a 1C rating to power a 4 amp device for 30 minutes before recharging if we want to maximize it’s service life.
Recharging
The C rating will also tell us how fast we can recharge a battery.
Going back to our 4Ah battery if it had a 1C rating that would mean we could apply a 4amp current to recharge it and it would be fully charged in an hour.
In an age where we have multiple portable power products the promise of ‘fast charging’ is attractive to consumers and so, especially for lithium batteries that power laptops and cell phones, higher C Ratings are the aim of many manufacturers.
In these case it is not about fast discharging but fast recharging where higher C ratings come into play as beneficial.
Summary
The C rating of a battery tells us how fast it can discharge and recharge without damaging the internal materials but it is only one part of the story. We also need to take into account:
- The temperature at which the battery is being used
- The recommended maximum Depth of Discharge
C ratings are commonly used on Lithium Ion batteries while Sealed Lead acid batteries tend to use the “20 hour rate”