Slow charging to keep heat in the battery

[timk225]

We've been having some cold days here lately, 15 degrees and below. One idea I came up with to keep some heat in the battery is to charge it at reduced rates. For the last week or so I turned the charge in the car down to 20 amps on my 14-50 connection, then I tried 16 amps. The point being to let it be on charge as long as possible to keep the battery warmed up somewhat.

Today I ran 110 miles out of the battery in the course of driving about 50 miles, due to having the heater on all the time and the heated seat on level 1. So tonight I'll be letting it charge at 10 amps. I'll see how it goes, I think it'll be fine.

 

[scottm]

What's the test to know how cold the battery is when you start out, to know if you've improved the situation with long slow charge rate?

Blue bar, snow flake, "no regen" warning...

Or maybe the driving time needed for a regen warning to go away, or blue bar disappears, or snow flake disappear?

 

[EVSteve]

10A on a 14-50 is around 2,200 watts. A home space heater uses roughly 1,800 watts. Consider that all of that 1,800 is consumed via elements designed to heat via high resistance. The batteries are designed to absorb that energy not really resist. Any heat generated will be minuscule at that low of a rate. Granted the S is different but my 100D when charged at 8,000 watts for 3 hours still has regen limit bars.

Consider when a cold Tesla is supercharged at 30,000 watts for comparison. How quickly does the battery heat up having almost 14 times the amount of power pushed into it? It doesn’t. The rate will slowly creep up.

10A for extended periods of time consumes more power as the charging electronics consume a set amount regardless of input power. In addition the added runtime of those components will accelerate their wear.

 

[GregRF]

If you want the warmest battery when you start driving you would want to do the opposite. Put the highest amount of power available into the car as close to your drive time as possible.

 

[scottm]

Right, considering that Tesla charging is about 90% efficient at higher amperages, that means 10% of the power is shed as heat at places throughout the charging process, in wires, controllers, batteries, etc.. That 10% is not pure heating of the battery cells. But it will warm "something" in the area.

Suppose you have a 40A breaker on the 14-50 outlet that means you can draw 32A continuous at 240v = 7680W shedding 768W as waste heat, some of which heats the cells. And really, the charge controller circuit is where most of the waste is going to hit.

Poor little cold cells.

BMS will bring up the temp of cells (applies heat) so they can accept charge better. That's going on too when you charge a cold pack. ou'll get the benefit of that direct heating as well when you set out. And of course, heating closer to the time of departure (immediately before) is better.

The logical extension of this experiment is... slipping a space heater of some sort under the car to physically warm the battery pack, instead of charging it. Suppose you have a space heater that does 1000W.. the radiant heat off that is far greater than the side effect of charging inefficiency. It's just that the BMS will do a more uniform job of heating cells.

 

[scottm]

A lot of people want a warm pack for the "benefit" of regen braking. But I think this is overrated if you have mostly a freeway commute with little need for slowing down or braking. There's a way to drive with less dependency on having to slow down. Try that first.

Personally, I don't care much if there's any regen on my commute or not. I slow down once, at the end when I'm parking. It's not worth the extra energy in pack heating just to recapture that one slowing down event. I use the brakes, and my net energy use is still lower.

 

[GregRF]

A lot of people want a warm pack for the "benefit" of regen braking. But I think this is overrated if you have mostly a freeway commute with little need for slowing down or braking. There's a way to drive with less dependency on having to slow down. Try that first.

I have a totally different type of commute. I live in the small coastal mountains at 700'. The first 3 miles are down a windy road to nearly sea level. The road is slow so I don't need all regen, but as long as I have most regen available I can get to the bottom of my road using a net of zero energy! (plus it can then be done with one pedal driving)

 

[Big Earl]

I "slow" charged this weekend at 0F. 4.2 kW (18 amps at 233 volts) for 11 hours (46 kWh from the wall) and it only added 50% or about 36 kWh to the pack. The slower you go, the longer it needs to keep the battery warm, the less efficient it is overall. Your best bet is to charge at the highest rate possible and delay your charging so that it completes at approximately the same time as you're ready to go. This will minimize heat loss from the battery pack to the atmosphere.

Also noteworthy: it only heats it up enough to provide about 1/3 regen... you'll still have dots for approximately 2/3s of the regen side of the power meter.

 

[Mobsidian]

Like someone said charging at high amps is pretty efficient at around 90% so only some of the 10% is warming up the battery. Charging is the 2nd best way to heat up the pack I think. Driving is the 3rd best since the motor is 90% or more efficient so very little is lost to heat up the pack, also at this point you're driving around in the cold and that cold air is cooling the pack. The best way to warm it up is to use the HVAC on HI. My assumption for a heated pack is fewer regen dots (or more regen). I did an experiment where I didnt use the HVAC for 20km (12 miles) and i had 0 regen, the whole left side was dots. For the next 20km I turned on the HVAC on HI and immediately the dots started disappearing, was able to get them close to half. This was in -5C (23F) temperatures.

TL;DR
From my experiences the best ways to warm the pack:
1st - HVAC on HI
2nd - Charging
3rd - Driving

 

[taraquin]

A lot of people want a warm pack for the "benefit" of regen braking. But I think this is overrated if you have mostly a freeway commute with little need for slowing down or braking. There's a way to drive with less dependency on having to slow down. Try that first.

Personally, I don't care much if there's any regen on my commute or not. I slow down once, at the end when I'm parking. It's not worth the extra energy in pack heating just to recapture that one slowing down event. I use the brakes, and my net energy use is still lower.

There are 2 reasons that getting some regen is a good idea. Especially on longer trips a colder battery has greater internal resistance which translates into higher consumption\more of the energy is converted to heat. Even though preheating\pre-charging uses energy this causes less consumption on the trip. Total energy-usage might be quite similar with or without precharging for batteryheating. Less use of brake-discs can also be a good thing, less maintenance and local polution

 

[SSedan]

As others have said I think this would waste a lot of energy just trying to keep the pack warm enough to charge.
High charge rate right up till leaving will get the best Regen.
High charge rate also makes timing the start easier, one of the reasons I am a fan of the wall connector.

 

[Kenz]

Charge nightly to 80 %. When you get up in the morning using the app change the charge to 90 % and preheat the interior.
Then take a shower, get dressed and get a cup of coffee.
You car will be ready when you are.

 

[Glamisduner]

If you want the warmest battery when you start driving you would want to do the opposite. Put the highest amount of power available into the car as close to your drive time as possible.

Too bad tesla does not have timed charging options.

Charge nightly to 80 %. When you get up in the morning using the app change the charge to 90 % and preheat the interior.
Then take a shower, get dressed and get a cup of coffee.
You car will be ready when you are.

Not cost effective if you are on a TOU plan.

 

[animorph]

I'm on TOU, but the hours are 8 PM to 3 PM off-peak.

I'm using a scheduled charge for the main nightly charge. For trips I use TeslaFi to raise the charge limit and start a charge again in the morning. There's a few other apps that can do the same if that's critical for you.

 

[Kenz]

TOU plans should not be a big deal if you are preheating the car for 30 minutes before you leave home.
If you want the car interior to be warm and the battery to able to accept regen then preheating is a great option.

 

[SSedan]

TOU plans should not be a big deal if you are preheating the car for 30 minutes before you leave home.
If you want the car interior to be warm and the battery to able to accept regen then preheating is a great option.

This works with S and X but I think the 3 only heats the battery for charging.