There’s a notion that discharging a battery at a low “C” rate (meaning slowly) provides a better result. While this is true for old AGM chemistry, it’s not the slightest bit true for LiFePO4 chemistry.
Image above is directly from a Google search.
AGM’s have a substantial “Puekert” effect, which in layman’s terms means if you discharge it quickly, you will get less capacity out of it. For completeness, most AGM capacities are rated at a C20 rate. This means a 200Ah battery discharged at 10A, will last 20 hours and provide 200Ah to 0% SOC. However if you discharge it at 50Ah it won’t provide 200Ah any more, it will be significantly less. How much less depends on the quality of the battery and the honesty of the manufacturer. It’s quantified by the Puekert figure. This is often lied about, and in reality it deteriorates quickly so the manufacturer figure is a rough guide to start with, and is normally an absolute best case figure.
For a LiFePO4 battery, things get a bit different. They have a VERY low Puekert effect. So if we discharge a 200Ah battery at 10A, it should last 20 hours. And if we discharge it at 50A, it should last 4 hours, significantly different to the AGM battery.
Now let’s factor in some other things.
The industry standard discharge rate for LiFePO4 batteries has overwhelmingly been 5 hours for a long time. This is also been referred to as C5 or 0.2C, they both mean the same thing. More recently some manufacturers have decided a C2 or 0.5C rate is appropriate, they have done this without substance, but we’re happy to consider it anyway.
From the evidence we have been able to gather, this C2 figure is based purely on the EV (Electric Vehicle) usage which is vastly different to the style of energy storage that is commonly used in caravan (RV) systems. EV’s typically have extreme drain in short bursts, contrasting to RV use which is sustained low to medium drain. This poses a dilemma when it comes to rating battery capacity.
Our testing…
From our own testing here at PowerPaul, batteries discharged at a C5 rate actually test lower by a few percent than batteries tested at a high C rate of C2 or less. Again putting this into layman’s terms if we test the same battery at a 2hr discharge and a 5hr discharge, the 2hr will return a better figure. A battery may test at 299Ah @ C5, yet it will test at 307Ah @ C2, which seems counterintuitive, and indeed would equate to an opposite Puekert figure.
Some of you will be perplexed at this, but the simple explanation is that the high discharge rate causes higher cell temperature, and this in-turn causes a higher cell voltage and higher available capacity until low voltage cut-off. Cell temperature can go up by just a few degrees when drained slowly, but can increase by 30 or even more degrees when drained at C2 or even C1 rates. However there’s always a trade-off. Higher cell temperature can also cause accelerated cell degradation. For most of you this will not ever be seen, as we expect our batteries to outlast most peoples ownership of the vehicle. And by the time these batteries are at their end of life, there will likely be a significantly better technology available for lower cost.
So what does all this mean? It’s just information for those of you who wish to ponder it. Some of you will delve deeper and find some really interesting behaviour of batteries, and perhaps of battery manufacturers. But for most of you it doesn’t mean much, and you’re happy to use your batteries like normal.
Ultimately it’s an example of why we’re “a bit different” here at PowerPaul. We’re always looking at the how and why, not just the $$$ coming in. While we can turn the how any why into a better experience for the customer and for us, we continue to persist with this delving into the nitty gritty that most companies couldn’t be bothered with.
How is C rate written?
If you read C20, that means the battery Ah capacity (C) divided by 20.
If you read 0.05C, that means the battery Ah capacity (C) multiplied by 0.05.
C1 and 1C are simply the battery capacity divided or multiplied by 1.
So if the number is AFTER the “C”, it’s a divider, if it’s BEFORE the C, it’s a multiplier.
