This topic has been discussed many times here and I'm well aware that standard practice is that you should not attempt to charge below 32°F but I found the previous discussions lacking in rigor and specifics. Actual tests are difficult to do without access to electron microscopes and lots of time to run many cycles in controlled conditions but fortunately there has been a ton of academic research on Lithium batteries in the last 10 years and lots of data is publicly available. A lot of this research is targeting the automotive sector which desires the fastest charge times and needs to operate outside in the winter in very cold climates. Tesla superchargers peak charge rate is about 3C for example. However, solar systems are generally designed for peak charge rates around 0.05C to 0.5C meaning it takes between 2hrs and 20hrs of full sun to fully charge the battery bank. The trend in the research is that cold temperatures and very high charge rates can cause problems. But what about low charge rates? There also seems to be some ongoing disagreement in the literature about how severe this problem is and whether the cells recover after returning to room temperature.
Here's the fundamental question I want to answer: What are the consequences of charging at .1C at 28°F? Is it significant? What charge rates are safe at what temperatures?
We can of course design systems to use energy to heat batteries or we can insulate better or we can just move to the tropics but I want to talk about what exactly happens when charging LiFePO4 at below 32°F. There are lots of opinions out there but I'm only interested in experimental results. Show me the DATA!
Here's a quote from this paper:
This paper tested capacity degradation at various temperatures for charging and discharging. They used 1C charge and discharge rates. The -20°C test degraded severely but quickly recovered when cycled one time at room temperature.
This guys' thesis was testing lifepo4 battery that was going to be used in a satellite. He charged it at about 0.3C at -20°C:
Here's the fundamental question I want to answer: What are the consequences of charging at .1C at 28°F? Is it significant? What charge rates are safe at what temperatures?
We can of course design systems to use energy to heat batteries or we can insulate better or we can just move to the tropics but I want to talk about what exactly happens when charging LiFePO4 at below 32°F. There are lots of opinions out there but I'm only interested in experimental results. Show me the DATA!
Here's a quote from this paper:
Meanwhile, charging the battery at low temperatures is likely to trigger lithium plating, which often leads to severe battery capacity fade. At −10 °C, an 11.5 Ah Liion cell was detected to have its capacity degradation rate increasing sharply when then charge current exceeds 0.25C and capacity loss can reach even 25% after 40 cycles at a charge rate of 0.5C [21]. Higher charge rate would lead to severer capacity loss. Even at 0 °C, a single charge cycle at 1C current would cause a 3.6% irreversible capacity loss of a 7.5 Ah cell [38]. Therefore, the charge rate of Li-ion battery is usually small at low temperatures in order to prevent lithium plating, which extends the charge durations dramatically
This paper tested capacity degradation at various temperatures for charging and discharging. They used 1C charge and discharge rates. The -20°C test degraded severely but quickly recovered when cycled one time at room temperature.
An additional observation based on the graph is the quite uncommon behavior at Tc = Td at -20 °C testing conditions. After each block of 25 cycles, there is a drastic decay of capacity and then a recuperation during the reference cycling (done at 25 °C).
This guys' thesis was testing lifepo4 battery that was going to be used in a satellite. He charged it at about 0.3C at -20°C:
He also noticed that the charging time was longer at lower temperatures:The results showed that the battery can still operate properly at temperatures as low as -20°C. However a reduction of the capacity at low temperatures was observed: The battery's capacity at -20°C was 62% lower than the one at 20°C. The battery fully recovered its capacity after returning back to the nominal working range (20°C), and its performance barely changed thorough the whole range of temperatures studied.
(he was charging at 3.5V per cell.The charge time increases as we lower the temperature due to the increasing internal resistance that lowers the charging current that goes through.