Plugging in in winter and battery longevity?

[jogail]

We have a 2016 Chevy Spark EV. We only use it a few times a week to go about 25 km. round trips. In the winter (we're in Canada, subzero weather in C'), should we leave the car plugged in 24/7 or should we still plug in just prior to our trips? Our concern is battery degradation and longevity, not over/undercharging Li batteries. Thermal management is a question. If we plug in at 6 am (when it's -10'C outside), does the system warm the batteries prior to initiating charge or will it damage the battery making it better to leave it plugged in overnight as the temperature lowers.

 

[Infinion]

Hey jogail, welcome!

In the winter (we're in Canada, subzero weather in C'), should we leave the car plugged in 24/7 or should we still plug in just prior to our trips? Our concern is battery degradation and longevity, not over/undercharging Li batteries.

Try to avoid keeping the car plugged in 24/7. Li-ion batteries kept at 100% state of charge will rapidly degrade and the car will lose capacity much faster than from regular use. It's actually worse than letting lithium batteries sit fully discharged. The best strategy is to use departure mode so that the car reaches 100% SoC just in time before your drive, to reduce the time the battery lingers at that high state of charge.

If you aren't planning to drive the immediate next day, just keep the car unplugged. Departure mode doesn't actually support delaying charging for more than 24 hours because you have to set a "finish charging by:" time each day.

if you know you won't be using all of the range in a single trip, you don't need to charge to 100% either. A good rule of thumb for battery degradation is that 80%-100% presents the largest amount of wear for a lithium battery, where 0-100% is 1 wear cycle, 0-80% is 0.2 wear cycles and 80%-100% is 0.8 cycles. If you're familiar with cycle life, a Li-ion battery (LFP chemistry in the 2014 Spark EV being the exception) will degrade by 20% after about 500 full cycles.

Thermal management is a question. If we plug in at 6 am (when it's -10'C outside), does the system warm the batteries prior to initiating charge or will it damage the battery making it better to leave it plugged in overnight as the temperature lowers.

The system will warm the batteries during charging, so no it will not pre-heat, but it will be done in a way that does not damage them by enforcing safe trickle charging current limits while the cells are outside of their optimal temperature range. In extremely cold situations, like -20ºC and under, it may cut off charging completely and actually pre-heat to the cold temperature threshold before the batteries are allowed to charge.

I wanted to go back to the departure charging mode and address your hypothetical scenario using it. If you've indicated you will depart at 10AM and need 4 hours to charge at an ambient temperature of -10ºC, it will start heating and charging automatically at 6AM. This use of power will consequently take up a high proportion of your charging power, which is safer for the cold batteries anyways. However, it won't charge earlier to account for that now lower net charging power, and will not finish in time, so adjust accordingly if you need all the range.

If you want to reduce the amount of wear from charging, you can set departure mode to 2-3 hours later than you plan to leave, to fool the system into delivering a partial charge. If you want to simply drive with a warm battery since you don't drive very far, simply precondition the car with or without the EVSE connected.

The system will not keep the batteries warm before charging because batteries sitting or discharging in the cold don't cause any damage to the cells (1) and it would simply waste electricity to the environment. It will only heat the batteries during charging or precondition mode from the MyChevrolet app or your key fob. These cars do have battery degradation but it's hard to attribute that degradation damage to hot/cold charging, outside of all the other factors like calendar life, cycle life, and damage over time at a full or depleted charge. That being said, it has all the temperature safeguards in place so you don't need to actively take measures to reduce that kind of degradation.

There's just one situation I'm very wary of, and that's getting into a cold Spark and going down a hill for several minutes with lots of regen. I've driven a family member's Tesla Model 3 where regenerative braking is limited to <1/3C in the cold, but I haven't seen the same action taken by the Spark EV in the same circumstances. This could be causing the most unabated damage in the first couple of minutes of driving. The battery heater is running at full power, but the batteries are still below freezing and you have full regenerative braking and are essentially fast charging the car...


I'd like to go into detail on the battery heating specs themselves. During charging, preconditioning, or driving, if the battery management system reports the batteries are below 5ºC, it will send power to the PTC (resistive) heater at up to 2.5kW and circulate this heat with its 20-150W battery coolant pump to heat the -10ºC batteries to a temperature above freezing. Very roughly you can expect the heating rate of the cells to be 1ºC/min until the batteries leave the cold temperature heater cutoff threshold. It will keep heating until the pack reaches 5ºC. The heater may limit power if you are using an EVSE lower than 2.5kW (like the 120V portable charger at 8/12A). I've seen the battery heater reduce its power, but I've never seen negative charging besides during preconditioning, that said, I could use confirmation from other owners on this, as well as confirmation on what the exact temperature is where charging is completely cut off in extreme cold.

(1) Li-ion batteries that are sitting do not have any active chemical reactions; there is still self-discharge. Discharging is an exothermic reaction (reactions release energy to its surroundings as heat), and charging is an endothermic reaction (reactions absorb heat energy from its surroundings). Charging lithium batteries below 0ºC has a higher chance to cause an irreversible lithium plating on the internal positive terminal of the battery, causing cell degradation, and must be avoided by the battery management system, or have its charging / regen speed significantly reduced.

 

[jogail]

Thank you for your thorough response. Cheers. Joanne

 

[JeremyWinter]

I'm sorry but that person is wrong about leaving the car unplugged.

All electric cars should be left plugged in at all times. Always be charging is the phrase.

That person seems to be talking about the Chevy Bolt. The Chevy Spark EV is slightly different. The Bolt allows you to charge to 100%, but you only charge to 100% when you need the extra range for a long trip. Normally you set the Bolt to charge to 80% only.

The Spark EV never charges to 100%. When you fully charge the Spark EV you are actually only charging to approximately 80% as there is a buffer for the battery at the top of the battery.

Always be charging is the best way to maintain battery health.

 

[Infinion]

I'm sorry but that person is wrong about leaving the car unplugged.

All electric cars should be left plugged in at all times. Always be charging is the phrase.

That person seems to be talking about the Chevy Bolt. The Chevy Spark EV is slightly different. The Bolt allows you to charge to 100%, but you only charge to 100% when you need the extra range for a long trip. Normally you set the Bolt to charge to 80% only.

The Spark EV never charges to 100%. When you fully charge the Spark EV you are actually only charging to approximately 80% as there is a buffer for the battery at the top of the battery.

Always be charging is the best way to maintain battery health.

Hi, I wanted to let you know that you are misleading people about leaving the Spark EV plugged in at 100%.

The Spark EV never charges to 100%. The battery has a 20% buffer at the top of the battery so it will not degrade the battery to charge to "100%".

The Bolt does not have a buffer and allows you to charge to 100%. Ideally you only do that for long trips and normally set it to 80%.

Just to clarify, for the Spark EV, charging to 100% will not degrade the battery. The battery has a 20% buffer at the top so when you are charging to 100% you are only really charging to 80%.

The Bolt EV is different as it doesn't have a buffer and allows you to charge to 100%. Normally you only charge to 80%.

Hi Jeremy, I'm "that person" :| . I understand that you've taken the stance that I am trying to mislead people to unwittingly not charge their cars to an indicated 100% over their concerns of battery longevity :lol: . I am aware you have also contacted me with your concern about misleading others in a private message, as well as responding in viewtopic.php?t=9372 with your counterclaims.

Assuming this isn't a total troll post because it reads as incredible with many details left out, let's take a look at some of your points:

I'm sorry but that person is wrong about leaving the car unplugged.
All electric cars should be left plugged in at all times. Always be charging is the phrase.

Really, all electric cars and at all times? I would love to know your reasoning. Why should people go through the trouble to do that? Is this your answer to a battery longevity concern?

Many owners do not have driveways with plugs to use with an EVSE, nor do their destinations. If they aren't left plugged in, what would you suggest happens to their cars?

I agree that if you can't charge at home, It is a good idea to find a nearby destination charger to maintain your range. However, in this case, the OP has shared their regular weekly usage (25% of the Spark EV's winter range per week) with battery longevity in mind, and they have the flexibility to charge at home.

That person seems to be talking about the Chevy Bolt. The Chevy Spark EV is slightly different. The Bolt allows you to charge to 100%, but you only charge to 100% when you need the extra range for a long trip. Normally you set the Bolt to charge to 80% only.

Why would you assume 'This Person' confused the Spark EV with a different car, in a Spark EV forum, and in a thread about someone's 2016 Spark EV? Was I not clear?

You're right about the Bolt EV, but not all of these cars can do what you say they can do, it's model year specific. It wasn't until after the 2019 model years that the software allowed drivers to set a limit like 80%. Before that, it was always either indicated 100% or 90% with Hilltop Reserve if left to charge on an EVSE.

The Spark EV never charges to 100%. When you fully charge the Spark EV you are actually only charging to approximately 80% as there is a buffer for the battery at the top of the battery.

And what a wonderful buffer it would be! If a car has one, you could do whatever you wanted with it, without consequence, with exception to the Bolt EV, of course . It could be any kind of buffer, a kWh buffer, an Ah buffer, or even a voltage buffer! It could also be a top buffer or a bottom buffer.

It certainly matters which type it is, but it unfortunately a top-end buffer won't protect you from battery wear to the extent you think it does. Cell voltage, on the other hand, has everything to do with it; a very well-established topic in online literature.

You're right that the Spark EV's 100% is not a Li-ion cell's 100% SoC. 4.2V is typical for the 2015 and 2016's years' chemistry. These cars will, however, charge to a voltage as high as 4.15V at an indicated 100% SoC. There are some reasons why this is not a voltage you want to charge to if longevity is a consideration, despite the "buffer" that exists between 4.15V and 4.2V.

Let's grab some peer-reviewed facts. I'll introduce an essay from a company called AccuBattery
Accubattery is an Android app found in the google play store that is used to monitor and track degradation in various Android phones. I have the app and can attest to its fantastic monitoring tools and data visualization.
Chad :ugeek: at Accubattery discusses four papers that investigate the effects of charging on battery wear. You can read their essay here.
It is shown that voltage has the most dramatic effect on the battery cycle life. According to their first and second references, for every 0.1V increase above 4V, cycle life is halved as capacity sharply drops.

Figure from Choi 2002: Fig. 1. Effect of CV charge voltage on cycle performance. Test cells charged at constant current at 1C rate to cut-off voltage followed by CV float charging at this voltage for 2.5 h and then discharged to 2.75 V at 1C rate.

Cycle-life is greatly influenced by the charge conditions, but is relatively insensitive to the discharge conditions. High charge cut-off voltages and a long float-charge period at 4.2 V or above have the most severe effects on cycle-life.

It was found that higher temperatures and voltages accelerate the degradation of the cells: a 15 °C increase in temperature cuts the cell life in half, and about 0.1 V increase in charging voltage also cut the cell life in half.

So it's clear, 4.15V is a high voltage for the Spark EV, and the papers show this chemistry gets stressed and degraded at a considerably faster rate when you charge to 100%.

How about when you charge to 100% and leave it there? Battery University has been a great source of information for all sorts of chemistries, let's take a look at BU-808, one of their pages https://batteryuniversity.com/article/bu-808-how-to-prolong-lithium-based-batteries
Scrolling down to table 3, we see the effect of keeping Li-ion chemistries at 40% vs 100% at four different temperatures for one full year. This is also covered in Choi et al. (2002) but this is easy to read and reference, and mostly correct aside from depth of discharge without sources and contrary to Choi.

But I digress, in table 3, it's clear that resting degredation is much higher when a Li-ion battery is kept fully charged compared to one at a lower SoC. 40% is usually meant for safe, long term storage but I'd like to infer that between 40% and 100% there could be a linearly proportional relationship with the rate of degredation. This is quite interesting because it clearly demonstrates heat as the ultimate catalyst for calendar aging capacity loss. At jogail's subzero temperature winters, they can expect the lowest degredation rates in the Winter from dwelling at a full charge. The degredation would be miniscule if it was left for a whole week, and likely a bit slower still at -10ºC. However, come Spring and Summer, they'll experience up to 16% - 20% more degredation than if they held at 40%, and again likely a sliding scale between 40%-100%.

In other words, jogail can get away with keeping the car unplugged with a full frozen battery, but left plugged into the EVSE, if thermal management maintains a warmer pack temperature over extended periods, they will suffer the greater capacity loss rates at those temperatures, and will pay for it in the electricity use to boot :? .

Always be charging is the best way to maintain battery health.

So, troll post? I think we've explored that this is patently false, and these anecdotes and claims have been misinformed, at best, by perhaps newer gen EV owners using cars that have the proper charge management tools. Accelerated capacity loss from calendar aging and charging to high cell voltages are both something that an alleged buffer fails to protect against without any consequence as you use it. You also don't suddenly experience capacity loss when you hit a 'safe' maximum charge of 4.2V either. Degradation is continuous and gradual at all levels.

I would love to hear a source about the Spark EV's 20% buffer and some of what you were claiming, though. I'm genuinely curious where it's coming from. I have heard from somewhere in the past that GM had a strategy to "shift" usable capacity in as the pack degraded, to make capacity fade less noticeable like Apple did with iphones and processor throttling, but I have not seen smoking gun evidence of that from my own monitoring between 350 charge cycles and 800 charge cycles.