Understanding Battery C-Rate: Why a 100Ah Lithium Battery Isn’t Always 100Ah

When comparing lithium batteries, probably the most universally understood starting point is the amp hour capacity. Once you’ve worked out your loads and know what size capacity you need to run all your appliances and off-grid power needs, the first thing typed into Google is generally the amp hours of the battery you require, because, 100ah is 100ah, right?  

 

Well, not quite.  

 

KEY TAKEAWAYS

  • Always check a battery’s C-rating: A lithium battery will only deliver its rated capacity when it’s discharged at the same rate it was tested
  • High current loads reduce usable capacity: Low current, steady loads give the highest usable capacity
  • Low C-rated batteries are optimal for high current loads: They yield a higher usable capacity under heavy loads
  •  Cheap/low quality batteries can leave your system lacking: C-ratings aren't the only thing they don't disclose

 

The “C” in C-rate stands for “Capacity”, and is a way of describing how fast a battery is discharged relative to its total capacity. In simple terms, it links current draw (amps) to time. A battery’s rated capacity only applies when it is discharged at the rate it was tested at. Change that rate of discharge and the usable capacity changes too. 

 

How AllSpark Lithium Batteries Compare

All AllSpark batteries are capacity rated at C5. This means the batteries capacity (in amp hours/Ah) is determined by the battery being evenly discharged over five hours. For example, a 100Ah battery with a C5 rating is discharged at 20 amps for five hours. At that discharge rate, it will deliver its full 100 amp-hours. 

 

If your loads roughly match that discharge rate, you’ll get the 100ah. If they don’t, your results will differ. 

As an example, if you take that same 100Ah battery and discharge it at 200 amps for 30 minutes (say, using an induction cooktop on full power), you won’t get a full 100Ah from it.  It will still come close, but you may only get, say, 95 amp hours. 

 

The reason for this is because the faster you discharge a battery, the more heat it produces, and the less efficiently it can deliver energy. 

 

On the flip side, if you discharge this battery very slowly, for example, if you are running a 40L fridge alone, which is using 4 amps an hour, you will see more usable capacity out of the battery. In some cases, that same 100Ah battery might deliver 105-110+Ah in a single cycle. 

 

This happens because slower discharge rates generate less heat and allow the battery chemistry to work more efficiently. 

 

Conversely, let’s take a 100ah battery that has been measured at a C20 rating. This means the rated capacity has been determined by drawing the power down evenly over a 20-hour period. If this battery is subject to a high-powered appliance (for example, the induction cooktop mentioned above), it will produce less power (and therefore run the appliance for a shorter period of time) than the battery rated using the C5 rating. 

 


*NB. For illustration purposes only, not to scale

 

A 100ah lithium rated using a C5 vs a 100ah rated using a C20 can have different usable amp hours at the same discharge rate of up to 10% less. 

 

Battery capacity isn’t just about the number on the label. It’s about how your system uses power.  If you are a serious power user offroad, then the C-rating on your battery is an important factor to consider.  

 

High-draw appliances like inverters, kettles, and air conditioners can dramatically reduce usable capacity if they pull large currents for short periods. Spreading loads out, running efficient appliances, and designing a balanced 12V system helps you get the most from your battery. 

 

This is why proper system design matters just as much as battery size when you’re building an off-grid setup. 

 

C-rate explains why some brands of batteries don’t always deliver their full advertised capacity in real-world conditions. The faster you discharge a battery, the less usable capacity you’ll see, while slower, steadier loads improve efficiency and performance. Understanding this helps you size your battery correctly and design an off-grid system that works reliably.  

 

Cheap Battery Tricks

Cheap products use clever marketing to make it look like the battery is comparable to high quality brands, but in reality they are leaving out vital information, or using specs and manufacturing processes that most people don’t understand. C-ratings can be as high as 40 or 50, cells are often second hand (this is common practice), we have even found broken tiles in these cheap batteries, to help them feel as heavy as they should be.

If you want expert advice on lithium batteries and designing a reliable off-grid power system, explore the AllSpark battery range or get in touch with the team at Offroad Living for tailored advice before your next adventure.