There is a common misunderstanding about batteries labelled with an ‘Ah’ capacity. It’s repeated endlessly across the internet and it is this:
MYTH: A 4Ah battery can power a 4 amp appliance for one hour
And so it follows that it will power a 2 amp appliance for 2 hours:
| 4 (Ah) | = 2 (hours). |
| 2 (amp device) |
Using this same math we find, theoretically, that it will power:
- A 1 amp device for 4 hours
- An 8 amp device for 30 minutes
- … and so on…
It makes sense so why is it that if you hook some 4Ah batteries up to a 4 amp appliance they only last for 30 to 40 minutes?
Is the battery faulty or old? Is the manufacturer mislabeling substandard batteries?
Both are possible but just as likely is a misunderstanding of how the ‘Ah’ capacity on the label is arrived at.
A ‘4Ah’ battery will power a:
- X amp device (draw)
- For Y hours (time)
- When the temperature is Z
What we need to know is what X, Y and Z are in order do understand what type of ‘4Ah’ battery we are looking at.
Temperature
Temperature has an effect on most batteries. They have optimal performance at 25oC (77oF). The more we move way from this, hotter or colder, the lower the battery’s actual capacity.
The industry standard for testing batteries and for setting ‘Ah’ ratings, is 25oC (77oF).
If we are using a battery in a very cold or a very hot location we should expect the performance to be lower than that which is stated on the label.
However it won’t cut the performance by half. It is not why some 4Ah batteries only power a 4amp device for around 30 minutes.
Time
This is the key to most misunderstandings. Certain classes of batteries have an ‘Ah’ rating known as “the 20 hour rate”. In other words they will perform best when discharged over a 20 hour period.
If we know the manufacturer is using the “20 hour rate” when setting their ‘Ah’ rating then we can find out what device this would power best using the following calculation:
| 4 (Ah) | = 0.2 (amp device). |
| 20 (hour rate) |
The device
Alternatively, if we know the manufacturer uses the “20 hour rate” and we have a 0.2amp device to power we can work out what battery we need via the following calculation:
| 0.2 (amp device) * 20 (hour rate) = 4 (Ah battery) |
So a battery can be correctly labelled and in full health but we need to know the hour rate used by the manufacturer to get ‘4ah’
The 20 hour rate
A good battery manufacturer should be able to supply a technical specification sheet with their battery which will include the hour rate used to get to the ‘Ah’ capacity stated on the battery.
Below is a cutout from the BatteryGuy BG-640 6 volt 4 Ah Sealed Lead Acid battery technical specification sheet.
It shows that (from left to right):
- When the temperature is 25oC (77oF)
- the battery will power a 0.2 amp appliance for 20 hours
- and provide at least 5.25 Volts (which is plenty for a 6 volt rated appliance)
So, by these measures, it is a 4Ah battery.
Note the technical specification sheet also shows what the battery will power using the ‘1 hour rate’. In other words it will power a 2.5 amp appliance for 1 hour. If the battery is used this way it should be considered a 2.5Ah battery.
The C-rate alternative
Where there is no technical sheet a manufacturer might include a ‘C-Rate’ on the battery itself.
A 4Ah battery that can power a 4 amp device for 1 hour will have a C-Rate of 1 (usually written as 1C)
It is calculated as follows
| 4 (amp device) | = 1 C-Rating (1C) |
| 4 (Ah rating) |
If the battery is designed to power a 2amp device the C-Rate is:
| 2 (amp device) | = 0.5 C-Rating (0.5C) |
| 4 (Ah rating) |
In our example above the 4Ah battery is designed to power a 0.2amp device for 20 hours so its C-rate is:
| 0.2 (amp device) | = 0.05 C-Rating (0.05C) |
| 4 (Ah rating) |
As we saw in the table on the specification sheet above the 4Ah battery can power a more demanding device. The specification sheet will often include information on how long the battery will last at different C-Rates.
Here is a cut out from the BatteryGuy technical specification sheet for the BG-640 6 volt 4Ah Sealed Lead Acid battery:
To the right we can see the 0.05C discharge rate gives us over 20 hours of power (the ’20 hour rate’) while a discharge rate of 1C (connecting the battery to a 4 amp device) will give us just over 30 minutes of power.
C-Rates and Lithium batteries
This C-Rate is more commonly seen on lithium batteries where we actually go in the opposite direction to the ’20 hour rate’ concept – faster discharging and recharging than the Ah/mAh capacity stated on the label.
In the picture below we see a 2600 mAh battery with a 100C rating.
If we know the capacity (2600 mAh) and the C-Rating (100C) we can work out the maximum draw that can be placed on the battery without damaging it..
First we divide the mAh rating by 1,000 to get the Ah rating. (2600 / 1000 = 2.6Ah)
| 2.6 (Ah) * 100 (C-Rating) = 260 (amp device) |
This battery can power a 260 amp device without damaging the battery.
How long does a C100 Lithium Battery last at maximum discharge?
Now although the example above means that this 2.6Ah (2600mAh) battery can power a 260 amp device it won’t last very long
We can calculate the number of minutes it will last as:
| 2.6 Ah | * 60 = 0.6 minutes. |
| 260 amps |
In other words a little over 30 seconds. This does not seem to make much sense but of course if we wire a large number of these batteries together we’ll get a longer lasting power supply.
However the C rating is more to show if a battery can withstand rapid discharge or recharge and to what level.
If you have an appliance which requires 260 amps of power during a 1 second start up but then drops to 10 amps then this battery will last around 15 minutes which might be ideal for your purposes.
Why the big difference between Lead Acid and Lithium?
Lead Acid batteries are a basic design and the materials and chemicals that make up the product can easily be damaged by fast discharging and charging.
- See Also: What is a lead acid battery?
The chemical reaction that takes place inside the battery to provide electricity or to receive a charge is physical and so when the reaction happens too fast the battery will start to overheat risking explosion or fire.
The pull and push of this physical reaction can be so strong that it will also damage internal parts of the battery – shortening it’s lifestyle or causing it to fail completely.
Lithium batteries also provide electricity via a chemical reaction but at a microscopic level where the pulling and pushing of the chemical reactions can be contained within tiny spaces hardly visible to the eye. The particular materials and chemicals used are also much better at withstanding the pushing and pulling caused by fast charging and discharging.
- See also: What are a Lithium Batteries?
So why do people still use Lead Acid?
It’s a question of cost. Lead Acid batteries are much cheaper to produce and if you don’t need high power concentrated over a small time period they are ideal.
Emergency lights are a perfect example. These devices only need a small amount of electricity but over a long period of time. Space is not important so the larger, slower discharging lead acid battery is a much more cost effective solution than a lithium battery.
- See also: In Search of the Perfect Battery
Using the ‘hour rate’ or C-Rating to get the most from your battery
First we need to understand the most important principle:
Every discharge and recharge causes some damage to a battery.
This is why no rechargeable battery lasts forever and as it gets towards the end of its life we notice it needs charging more often. This is an external sign of internal damage.
The more we discharge or recharge a battery outside of it’s ‘hour rate’ or C-Rating the more damage we do on each cycle.
So a 4Ah battery (at the ’20 hour rate’) will discharge at a rate of 1C but it will cause much more internal damage than if we stick to the recommended 0.05C.
At some point, if we try to discharge or recharge any battery too far beyond it’s labelled capacity, we might even risk fire or explosion due to heat build up in the battery.
Summary
Don’t accept they myth that “a 4Ah labelled battery will power a 4 amp device for 1 hour” until you understand the time rating used to get this 4Ah or you know the C-rate of the battery,
Most Lead Acid batteries work on a “20 hour rate”. Their ‘Ah’ rating is calculated on powering a device over 20 hours.
Modern lithium batteries, on the other hand, can power much higher amp devices than their ‘Ah’/’mAh’ rating suggests as long as you know their C-rate which is usually stated on the battery itself.
As the C-rate also refers to the charging rate, the higher the C-rate of a battery, the faster it can be recharged.
Knowing the ‘hour-rate’ used to set the Ah/mAh rating, or the C-rate, allows us to use a battery in a way that optimizes it’s service life … and that will save us money!