Unveiling the Secrets of C Rating in Batteries

When a user buys the battery, the primary focus is on the capacity of the battery, which is 100Ah,150 or 200 Ah for the Inverter/UPS application. While buying, no one gives the weightage to the C rating of the battery, which is C20 in most cases.

This C20 matters the most because it defines the actual capacity of a battery on different loads.

So, we need to understand that battery capacity can be defined by capacity in terms of Ah and C rating. So, if the Ah rating of the battery is higher but the C rating is C20, the battery is good for running on smaller loads only. However, if the battery has the same Ah value and a C1 rating, it can be intended to run higher capacity loads. Few people talk of solar telecommunication batteries as C10 batteries and talk of C10 and link this with fast solar charging, and the sellers get confused with C20 and C10 terms as they think C10 is meant for solar applications and inverter and C20 is fine.

Here’s a breakdown of what a C20 battery means, along with essential concepts about battery C-ratings:

What is a C20 battery?

A C20 battery has a capacity rating determined by how long it takes to discharge fully under a specific load. The “C20” means:

  • C: The battery’s capacity in Ampere-hours (Ah).
  • 20: The hours to discharge the battery at a constant current draw completely.


  • A 100 Ah battery with a C20 rating can deliver a continuous current of 5 Amps (100 Ah / 20 hours = 5 Amps) for 20 hours before being completely discharged.
  • So the 100Ah battery will have an excellent life and backup time if the discharge current is 5 or 10 Amps, but if the discharge current is 40 or 50 Amps, then the backup time or life is difficult to predict. As the discharge currents increase, the efficiency of the lead Acid battery starts deteriorating. So, if we try to take 50 Amps out of the Lead Acid battery of 100 Ah, then the backup time may not be more than 1 hour, which will be less than 50% of the capacity of the 100 Ah Lead Acid battery.

Why Does C-Rating Matter?

  • Discharge rate: C-ratings help determine how quickly a battery can safely discharge. A higher C-rating (e.g., C5, C10) means a faster discharge rate, ideal for high-power applications.
  • Real-world capacity: C-ratings give a more accurate idea of a battery’s usable capacity under different loads. A battery’s stated capacity is often based on ideal conditions, but a C20 rating provides a capacity value under typical usage.

Typical C-ratings:

  • C20: Common for batteries designed for slower, steady discharge like in inverters, UPS and solar applications.
  • C10 can discharge a slightly higher current, so in the case of a 100 Ah Lead-Acid battery, this will give 10 hours of backup in case one discharges 10 amps. So, the C10 has more capacity backup than the C20 battery. So, the C10 battery capacity can withstand more discharging current than the C20 battery capacity.
  • The C1 battery is the best battery available in the market these days. All Lithium-ion batteries come in C1 capacity, and they are the most suitable for deep discharge capacity batteries. The C1 battery will not deteriorate in terms of capacity even when a load of 100 Amp discharge is taken out from the 100 Ah Lithium-ion battery.
  • That is why Lithium-ion is gaining popularity in the Inverter, Solar and UPS industries.

Important Notes:

  • Completely discharging batteries too often can shorten their lifespan.
  • Manufacturers provide specifications, including C-ratings, to help you choose the correct battery for your application.

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