Temperature and Battery Charging

 

Battery Temp and Charging Rate

If you’re new to electric cars you may have heard terms like ‘cold gate’ or ‘rapid gate’.  What do they mean?  Both terms reference how battery systems operate for charging.  Li-Ion batteries operate best in a narrow temperature window, which closely matches temperatures that are comfortable for people.  When hot or cold, these batteries can perform poorly. When charging the vehicle, systems are designed to limit the charging current to protect the battery and help ensure a long life for the batteries.  The restrictions impact DC fast charging the most as this is when the most current is trying to go into the battery pack.

Cold Gate refers to when the vehicle battery pack is below optimum operating temperature.  Specific to my Kia Niro, if the battery pack is below 77 degrees Fahrenheit the vehicle will limit the charging power allowed.  There are several different charging curves based on the state of charge and the temperature.  The other day I went to evaluate the 10% to 80% charging speed, a very typical range for a road trip.  The target is to arrive at a charger around 10% and charge at the highest rates to be the most time efficient.

 

The time to charge in this session was 50 minutes from 11% to 80%.  Not bad, but here is a chart that shows the speed at which Elektra was charging at each point.

 

The chart shows that it started charging at 54kW and charged at this lower rate for 13 minutes before jumping up into the 74 and then 77kW peak rates of charge.  As the pack fills, the rate also slows.  In this case, at 60% the rate drops from the upper 70’s to about 60kW, and then at 75%, the rate drops to 38kW, and again at 80%, the rate drops to 26kW.

I would love to do a full side by side comparison with the new Mach-E.  It has a bigger battery pack, but higher peak and sustained charging speeds.  However, above 80% the Mach-E only charges at 12kW.  On a long distance trip, I think both cars would do about the same amount of time, especially if there are long stretches that require charging the batteries to higher than 80%.

In the above example, I could see that Elektra actually ran a heater to help warm the battery to reach peak charging speeds.  I am still working out at what percentages and what temperatures the battery pack heater will run.

Since battery temperature is so important to charging speed, I would recommend that car companies add a battery temperature gauge to the meter display.  Many new electric drivers may not take the time to learn about this impact and not understand why the car is not charging as fast as possible.  A simple gauge showing the battery temperature below or above preferred operating range would inform the user why the battery is not charging quickly.  Also, more companies need to implement the battery conditioning features from Tesla.  If you select a supercharger station as the destination in the Tesla navigation, the car will start to condition the battery (actively heat or cool) so that it is at the optimum temperature for charging at arrival.  In the meantime, I may look for a hack on the Kia to run the battery heater more so I can achieve more charging time at the fastest speeds.

For me, the battery being cold is my main concern, but most car battery packs are also thermally cooled, so when they get hot the system can run to cool the battery as well.  Unfortunately, that is not the case for the Nissan Leaf.  That battery is only heated and cooled by the air around it. 

This brings me to Rapid Gate.  The process of DC Fast Charging introduces heat into the battery pack.  Most people would be familiar with batteries getting warm while charging.  In the case of the Leaf charging and driving create heat, and in warm conditions the battery pack will warm to the point that the vehicle charging system will slow the charge rate to protect the batteries.  Thus, the Nissan Leaf which starts with slow charge rates can be limited to even slower rates once the battery is hot.