Those who jargon in the battery building community are familiar with the “C” rate of batteries they buy or build. But what is C, and why should you care? This article hopes to discharge some knowledge on the topic at a high C rate.
We are all familiar with amp-hours (amps times hours), watts (volts times amps), and watt hours (watts times hours). But if you discharge a 6Ah battery at 6A, do we just say “at a rate of 6A for one hour”? We have a shorter way to say that: 1C current.
1C is the rate at which it takes to drain a battery in 60 minutes. An unofficial standard among batteries, especially Lithium Iron Phosphate batteries, is to have a maximum continuous discharge rate of 1C, peak momentary discharge of 3C, and an ideal (optimal) discharge rate of 0.2C. Moreover, it is often normal manufacturer specification to charge a battery at a max of 0.5C, or half its capacity in amps current.
So a 6Ah battery discharged at 1C will provide 6 amps for 1 hour. At 0.5C it is drained at 3A in 2 hours. At 3C it is drained at 18A in 20 minutes (1/3rd of an hour). At 0.2C this battery is drained (optimally) at 1.2A for 5 hours, and charged at a rate of 3A maximum current.
The battery C rating is the measurement of current at which a battery is charged and discharged.
But this is in ideal situations. The truth is, batteries don’t like being discharged at a high rate, hence some discussions and notices you read that talk about optimal discharge rate vs. max discharge rate. The higher you go, the more energy is lost in heat, inefficiency of connections, and overall degradation of performance of the battery internal ion transfer. Running a battery at its max C rate for will generally produce runtimes shorter than the C rate calculation. So if I take my 6Ah battery again as an example, if I run it at 18A, I will probably get more like 15 minutes out of it instead of the full 20 minutes.
Many battery performance graphs show you drain tests of 0.1 to 0.2C as the reference discharge current, to allow the battery to sustain its performance as long as possible. Some high-performance batteries are specifically designed to produce shorter high C discharge and charge rates. These batteries are designed for large amps and sustained bursts, and often cost a lot more than you’d expect for comparative amp-hours. A great example of a high-performance battery is the Headway cell. The 38120 version is rated at 8Ah, but can deliver a continuous 120A (15C) and charge at 80A (10C). High performance cells tend to be large(r) and have bulkier connection gear to handle the current demands. But stable high C rates are not what you will typically find in deep cycle or solar storage batteries that ham radio operators like to use.
So how do you increase the amp throughput without spending more than you should for “high performance”? Get a bigger battery (more capacity). The C rate follows the overall capacity of a battery, so a 6Ah is best discharged at QRP radio levels of 1.2A (0.2C), while a 100Ah battery would not be stressed by continuous 20A of a portable QRO rig (again, 0.2C). This guideline starts to break down beyond 100Ah battery capacities, as many consumer deep cycle batteries limit at 100A current with peak to 200A. So a 200Ah or 300Ah may still only max discharge at 0.5C or 0.33C, respectively. That’s a lot of amps, and components get unwieldy at currents larger than that.
Many people think 3Ah to 4.5Ah for a QRP radio is good, and in most cases it is, just don’t always crank it up to 10W transmit. And for a QRO radio run at full power, most tend to think the highly portable 12Ah to 20Ah 12V battery is perfect. But if you read the specs, you might actually be over-taxing your battery’s recommended C ratings, lessening its cycle life. Transmitting at 100W portable comes at a cost. This is also why building your own batteries to match your C requirements is both fun and challenging. Finding cells, BMS’s, and connecting components to match your desired amps requires careful design and balance.
The good news is that as long as your cable gauges and terminal connections are correct for the current, you won’t experience as bad a Peukert’s Effect on consumer or DIY LiFePO4 batteries like you would with lead-acid or AGM batteries (i.e. the significant drop in voltage and capacity when high current is drawn). Lithium chemistries give a lot and draw a lot. This is also why it’s important to have a good BMS and stay within its limits. If you connected to raw series cells of a LiFePO4, you could send or charge hundreds of amps very easily, and needs a limiting mechanism to prevent damage to the cells.
Many users of modern batteries just unbox them and plug them in without a second thought. It is helpful and important for the life of your battery to read the specifications and understand good charge rates, discharge rates, and keep things within nominal parameters. When you mind these characteristics, you will have a battery that will last you many years.