Best Charging Regimen for Low Mileage and Hot Weather

[bolter]

Reading here and elsewhere online it appears that the best thing for the battery is to not fully charge it, which equates to turning on hilltop reserve, and not recharging until the battery is down to about 20% charge. It also appears that it's best for the battery if the car is plugged in when it is sitting unused in a hot environment so that the battery cooling system operates.

I usually drive my Bolt EV only 20-30 miles per day, so, based on that, I would best charge only every week or 10 days. However, my garage gets really hot in the summer, so maybe the car should be kept plugged in.

How should I resolve this?

 

[SeanNelson]

I'd just leave it in Hilltop Reserve mode and plug it in every day. The difference between one big charge every week vs. 5 or 6 little charges is probably going to be very minor compared to all the other factors that can affect battery life. And you'll always have a pretty good charge in case you suddenly need to make a longer trip.

If you're fine with letting it run down to 20% on a regular basis then it might be argued that you could have saved some money by buying a cheaper EV with less range. It's a 200+ mile car that was designed to be used that way - take advantage of it!

 

[iwatson]

I use hilltop reserve, but I also use priority charging which allows me to set a departure time so that the vehicle will be fully charged by the time I leave. I have set it for a time that I know I will not be home. In my situation I leave for work each day at 1:30 pm and I work till 11:30 pm (M-F). My departure time is set for 9:00 pm.

With priority charging the vehicle charges to 40% as soon as you plug-in but then it waits until a time that it determines will fully charge just in time for your departure. My departure is really 1:30 pm but my timer is set for 9:00 pm. So each day when I arrive home, I plug-in and if the SOC is below 40% it will charge to 40% and hold. Then I leave for work before it can start it' s final charge. So in effect M-F the car is always charged to 40%.

Saturday and Sunday are a different matter since I don't have any specific departure time. So I let it fully charge (well 90% - Hilltop reserve) on either Saturday or Sunday and leave it unplugged one day a week.

I wish you could set a day of the week for no departure but the software doesn't allow.

Another Idea for the OP's situation is to let it run down to 20-30% then plug in using the 120V EVSE that came with the car. At 8 amps it will charge but very slowly. An 8 hour charge will only recoup about 24 - 32 miles.

 

[plj]

I posted discussion of an academic paper on extending Li-ion battery life here: https://www.mychevybolt.com/forum/viewtopic.php?f=12&t=8080&p=31226#p31226 They suggest charging "as late as possible." That is, not not routinely charging it. They argue for charging before you use it... as late as possible...


My solution to keeping the battery cool in a hot garage is to use a "Lux WIN100 Heating & Cooling Programmable Outlet Thermostat" ($38) which I set to turn on when the temperature is above 90 degrees F. I plug the GM 110V EVSE into that and the car. I set the car to "Immediate Charge," at 8 amps.

When the temperature is above 90, the Lux turns the current on, and the Bolt shows charging via the green dash indicator. As the garage cools off at night, the Lux turns off the current, stopping the charging.

On a 90+ day, I opened the hood and felt the coolant hoses. Indeed, some are hot, telling me the car is cooling the battery.

 

[GetOffYourGas]

I'd just leave it in Hilltop Reserve mode and plug it in every day. The difference between one big charge every week vs. 5 or 6 little charges is probably going to be very minor compared to all the other factors that can affect battery life. And you'll always have a pretty good charge in case you suddenly need to make a longer trip.

If you're fine with letting it run down to 20% on a regular basis then it might be argued that you could have saved some money by buying a cheaper EV with less range. It's a 200+ mile car that was designed to be used that way - take advantage of it!

My guess (and hope) is that 10 years from now, we will all look back and laugh. Laugh at the silly things we did to preserve our batteries. Laugh because most of them are based on the poor longevity of the original Leaf. And folks like you will have the last laugh. You wake up to a car with 200 mile range every day. Others who only charge to 40% only have a 100 mile range every day. And both cars will likely have experienced similar capacity fade (given similar environments).

All that said, I set my car to complete charging about 4 hours after I actually leave. That way I have about 60% charge for the day. I also set Priority Charge. So my car has between 40-60% SoC for the vast majority of time. All this because a few experts recommend that Li batteries stay near 50% SoC for the least stress. But that was a while ago, and before this particular chemistry even existed. So certainly outdated information.

If EVs really are to become mainstream, we need to be able to plug them in and walk away. The typical consumer is not going to jump through these hoops. So I really hope that the Bolt is there already. But like I said, it will take another 10 years to know for sure.

 

[Vertiformed]

All that said, I set my car to complete charging about 4 hours after I actually leave. That way I have about 60% charge for the day. I also set Priority Charge. So my car has between 40-60% SoC for the vast majority of time. All this because a few experts recommend that Li batteries stay near 50% SoC for the least stress. But that was a while ago, and before this particular chemistry even existed. So certainly outdated information..

Your strategy may be doing more harm than good. At the end of a charge, the BMS does cell balancing. If you prevent it from ever finishing, it won't get to do that part of battery maintenance.

If EVs really are to become mainstream, we need to be able to plug them in and walk away. The typical consumer is not going to jump through these hoops. So I really hope that the Bolt is there already. But like I said, it will take another 10 years to know for sure.

I think the Bolt is there already. Everything GM has said indicates that they expect us to plug in and walk away, rather than set up complex superstitious rituals for babying the battery.

 

[SeanNelson]

Everything GM has said indicates that they expect us to plug in and walk away, rather than set up complex superstitious
rituals for babying the battery.

What, you don't stomp your left foot and turn around three times after plugging in to charge? :shock:

 

[GetOffYourGas]

All that said, I set my car to complete charging about 4 hours after I actually leave. That way I have about 60% charge for the day. I also set Priority Charge. So my car has between 40-60% SoC for the vast majority of time. All this because a few experts recommend that Li batteries stay near 50% SoC for the least stress. But that was a while ago, and before this particular chemistry even existed. So certainly outdated information..

Your strategy may be doing more harm than good. At the end of a charge, the BMS does cell balancing. If you prevent it from ever finishing, it won't get to do that part of battery maintenance.

Well, I did actually think of that. I do a full charge once a week. On friday, to last me the weekend. That allows the BMS to balance the cells.

If EVs really are to become mainstream, we need to be able to plug them in and walk away. The typical consumer is not going to jump through these hoops. So I really hope that the Bolt is there already. But like I said, it will take another 10 years to know for sure.

I think the Bolt is there already. Everything GM has said indicates that they expect us to plug in and walk away, rather than set up complex superstitious rituals for babying the battery.

You have more faith than I do. GM's warranty doesn't inspire confidence either. I really hope to have much more than 60% capacity after 8 years!

And there is no superstition involved in my method. It's all based on researched information. Dated information, granted, but still the best I could find.

 

[SparkE]

All that said, I set my car to complete charging about 4 hours after I actually leave. That way I have about 60% charge for the day. I also set Priority Charge. So my car has between 40-60% SoC for the vast majority of time. All this because a few experts recommend that Li batteries stay near 50% SoC for the least stress. But that was a while ago, and before this particular chemistry even existed. So certainly outdated information..

Your strategy may be doing more harm than good. At the end of a charge, the BMS does cell balancing. If you prevent it from ever finishing, it won't get to do that part of battery maintenance.

Well, I did actually think of that. I do a full charge once a week. On friday, to last me the weekend. That allows the BMS to balance the cells.

Yeah, I try to do a charge to 'full' about once a month (well, 'at least' once a month, since I generally end up charging it to full an additional once or twice a month for one reason or another).

 

[SeanNelson]

You have more faith than I do. GM's warranty doesn't inspire confidence either. I really hope to have much more than 60% capacity after 8 years!

I have a lot of faith that GM specified its warranty terms so as to minimize its warranty costs, which means that they don't expect any significant number of batteries to degrade that badly. That includes batteries subject to harsh hot conditions in the more southerly American states, frigid winter temperatures in the northern plains and Canada, and 3-year lease customers who treat the battery like crap because they know they're not going to have to deal with the consequences.

Yes, repeated extremes are hard on the battery - but these things follow a curve and most of the gains in reliability will occur with just a bit of care. For example, it may well be that charging the battery to no more than 70% most of the time will improve its life compared to using hilltop reserve mode to regularly charge to 87-88%. But the difference between those two is going to be small compared to always charging to 100%. And even that will be a lot less harsh than if GM allowed you to charge to the battery to its ultimate capacity. Each additional measure of battery coddling will yield less real-world gain.

Time will tell, but until then I'm going to enjoy the car and not sully the experience by fretting over it too much.

 

[GetOffYourGas]

Agreed on all points. And I encourage everyone to not fret about their battery. Follow the Owner's Manual, and you should be fine.

Although it's not apples-to-apples, look at the Volt. They have been on the road for 8 years now, and the batteries are holding up very well. Sure, it's a different chemistry and they aren't cycled as deeply. But the same GM engineered the TMS/BMS systems which gives me confidence in their abilities.

But I will continue to baby my battery since it doesn't cost me anything. If it makes zero difference in the end, it was still OK since I haven't lost any functionality in the car. If it saves me 10% degradation after 10 years, then that's awesome! I have no reason to believe that I will be worse off.