A long ago I was installing a 1kW of off-grid solar power for one of my clients, he already has 2 numbers of 100Ah batteries and he wants to utilize them for the off-gird solar power plant.
Since he is very personal and he asked lots of questions like what is the difference between 12V and 24V 100Ah batteries and what will be the backup time for different loads. First, he thinks that a 100Ah battery can deliver 100A of continuous current for 1 hour but he was so wrong.
In this blog, I will explain to you all the information you need to have for a 100Ah battery.
Before jump into the details of a 100Ah battery, here is an explanation for a 100Ah battery in simple words.
What means by a 100Ah battery?.
100Ah is mean that battery is guaranteed to provide a continuous current of 10A over the discharge period of 10 hours (10A x 10h =100Ah) if the battery is of Class C10 and if it is of Class C20 it will provide a continuous current of 5A over the discharge period of 20 hours (5A x 20h =100Ah).
Definition of a 100Ah battery
Generally, there are only two details you will find on the sticker of the battery other than warranty, that is the battery voltage (‘V’) and battery rating (in ‘Ah’).
Where Ah or Ampere/hour capacity is current a battery can provide over a specified period of time to a specific End of discharge Voltage at a specific temperature. Whereas End of discharge Voltage is the level to which the cell voltage is allowed to fall to before affecting the load.
As the battery starts discharging, the voltage across its terminals starts reducing too but the rate of voltage drop throughout the discharging period is not too much high.
The voltage battery provides before it fails is called End of discharge Voltage.
Also, the battery voltage is the nominal voltage across battery terminals. Generally, the battery voltage for a 100Ah battery is 12V or 24V.
A 12V, 100Ah battery is the most common battery available in the market over the globe and it is made of 6 numbers of 2 V cells with End of discharge Voltage per cell is varied from 1.75 V to 1.8 V. Hence the End of discharge voltage for 12V battery varies from 10.5V (1.75V x 6) to 10.8V (1.8V x 6).
Lead acid cells generally produce an electrical potential of 2V while Nickel-cadmium cells generally produce an electrical potential of 1.2V
If you try to find the details of “End of discharge Voltage” for the battery on its sticker, you won’t find the same. To get the same you need to ask for the battery specification from the battery retailer or you can download the same from the manufacturer’s website.
Following is an example of battery specification I found on the internet, you may found a completely different format of specification because there is no such standard for the same.
As per the above specification of battery, the capacity of the battery is defined as 100Ah at a 10-hour rate to EOD of 1.8V per cell at 25 Degree Celsius. As you can see, the number of cells is also defined in the above specification, which is ‘6’.
This means that this 12V, 100Ah battery is guaranteed to provide a continuous current of 10A over the completely discharge period of 10 hours (that is 10 Ampere x 10 Hour=100 Ah) and the end of discharge voltage of the battery will be 10.8V (6 x 1.8V per cell) at 25 Degree Celsius.
Difference between C10 and C20 100 Ah Battery
If you search over the internet about 100Ah battery sometimes you will find that capacity of the battery is mention as 100Ah at a rate of C10. Where ‘C10’ refers to the guaranteed performance of the battery for 10-hour.
Similarly, for the ‘C20’ battery, the battery is guaranteed for the performance of 20-hour.
But the Ampere-hour of the battery will remain the same in both cases, that is for the 100Ah Class C10 battery, the battery is guaranteed to deliver a continuous current of 10A for 10 hours. Hence the Ampere-Hour of C10 will be equal to 10A x 10H = 100Ah.
Also for the 100Ah Class C20 battery, the battery is guaranteed to deliver a continuous current of 5A for 20 hours. Hence the Ampere-Hour of C20 will be equal to 5A x 20H = 100Ah.
But if we compare the discharge ampere of C10 100Ah and C20 100Ah battery for a discharge period of 10 Hours we will find that 100Ah C10 battery can deliver 10A of continuous current which is slightly higher than 9.26A of 100Ah C20.
Also, a 100Ah C10 battery can deliver 5A of current over the discharge period of 20 Hour which is still higher than the 4.97A of 100Ah C20 battery.
Table-1 shows the difference between the discharge current of 100Ah C10 and 100Ah C20 battery with identical EODV (End of Discharge Voltage)
This difference in discharge current for the same period impact the performance of the battery, like backup time, peak current and battery life.
Difference between 12V 100Ah battery and 24V 100Ah battery
There is two main difference between a 12V and a 24V 100AH battery. First is the voltage across their terminal (that is 12V and 24V) and Second is the power store in them (that is 1200Watt for 12V 100Ah and 2400 Watt for 24V).
In electricity, the formula of power is, Power = Voltage x current
Hence the amount of power store in a 24V, 100Ah battery will be 24V x 100 Ah = 2400 Watt-hour (2.4kWh).
And the amount of power store in a 12V, 100Ah battery will be 12V x 100Ah = 1200Watt-hour (1.2kWh).
Refer to the below table for a better understanding of the comparison between the power store in 12V 100AH and 24V, 100 Ah batteries.
This means that for a similar load 24V, 100Ah battery will give twice the backup time as compared to 12V, 100Ah. And for similar backup time 24V, 100Ah battery can run twice the load as compared to 12V, 100Ah.
This makes the 24V battery a clear winner, but before jump to a conclusion that a 24V battery is better than 12V after a comparison of price and power store in them. You need to understand that to charge as well as extracting power from a 24V battery you will require a 24V inverter too. And the price of a 24V inverter is higher than that of 12V.
Also, you cant charge a 24V battery from a 12V inverter, but you can charge two 12V battery (after connecting them in series) from a 24V inverter.
There are other reasons like availability of 24V battery as well as 24V inverter is very low as compare of 12V battery and inverter. Makes going with a 24V battery instead of a 12V battery could be a bad decision.
How long will a 100 ah battery last?
The backup time of the 100Ah battery depends upon the discharge rate of the battery. This means the faster your discharge battery the less backup time you will get.
This also means that the backup time of the 12V, 100Ah C10 battery will be different from the C20 battery.
Let us compare the backup time of the 100Ah C10 and 100Ah C20 battery
Backup Time fo C10 battery:
Refer below table you can see the discharge rate of 12V, 100Ah at 10-hour rate to EOD of 1.8V per cell at 25 Degree Celsius
From the above table, you can see that 12, 100AH C10 (at 10Hour rate) gives only 65.32Ax1Hour= 65.32Ah (that is, it can withstand 759.42 Watt of Power for 1 Hour) if you completely discharge it within 1-hours. But if you completely discharge it in 20 Hours battery will give 5.4Ax20Hours = 108Ah (that is, it can withstand 65.22 Watt of Power for 20 Hours)
Due to the efficiency of the inverter (which is 90% to 95% for a high-end sine wave inverter) the AC output power will be 5% to 10% lesser than the battery output.
Also discharging the battery below 50% again and again will damage the battery life. That's why to get the high life of battery 100Ah, C10. It should not be loaded more than 418 Watt for more than 1 Hour.
Backup Time fo C20 battery:
Refer below table you can see the discharge rate of 12V, 100Ah at 20-hour rate to EOD of 1.8V per cell at 25 Degree Celsius
From the above table, you can see that 12, 100AH C20 (at 10Hour rate) gives 59.5Ax1Hour = 59.5Ah (that is, it can withstand 683 Watt of Power for 1 Hour) if you completely discharge it within 1-hours.
But if you completely discharge it in 20 Hours battery will give 4.97Ax20Hours = 99.4Ah (that is, it can withstand 60 Watt of Power for 20 Hours)
Also, discharging the battery below 50% again and again will damage the battery life. That's why to get the high life of battery 100Ah, C20. It should not be loaded more than 405 Watt for more than 1 Hour.
Comparison between the backup time of 100Ah C10 and 100Ah C20 Battery
Refer to the below table, which shows the comparison between backup time for C10 and C20 battery of the same Ampere hour (100Ah), End of Discharge Voltage (1.8V) and terminal voltage (12V).
From the above table, it is clear that C10 batteries provide much more backup time than C20 batteries. And that's the reasons why C10 batteries are more preferable to C20 batteries, especially for off-grid solar power plants.
In off-grid solar power plants, you have only 5 to 7 hours of sunshine to charge your batteries and the remaining 17 to 19 hours of power will be provided by batteries.