Frequent charging options

[Eric from NE]

So I finally have a way to charge at both work and home. Both are 50a NEMA 14-50s that are pretty much dedicated, so I can do a full 40a if I need to. Most of the time I'll be leaving one location on a full charge (typically 185-187 ideal miles) and arriving at the other after ±40 miles of 65 mph driving.

Any thoughts on what charging rate to use to extend battery life? Any advantage to NOT setting it to the full 40a draw whenever possible? Or does it make more sense to charge both slow and play "catch up" on a full charge overnight at home? I can always boost the draw if I know I'm going to need more after being at one place.

 

[doug]

This has gone round a few times, but Tom Saxton did some empirical measurements that showed charging at 40 A, was about optimal in terms of charging efficiency. There doesn't seem to be an advantage to charging slower.

 

[Eric from NE]

Yeah, that's a great chart:

Tesla Roadster Charging Rates and Efficiency - Tom Saxton's Blog

I was just wondering if anyone had any experience with a slow "top off" charge always starting from a 50-60% (or greater) charge. Those numbers seem to be from a 1/3rd charge most of the time.

 

[wiztecy]

I've heard people achieving higher "ideal" miles when charging off of 110V. I haven't confirmed since I never tried. If its true would like to understand why.

 

[Tesla 940]

I thought there was also some theoretical microscopic advantages (for battery life) to keep the charge in the middle of the SOC. Therefore, it might be best not to "top off" at work each day - your only driving 40 miles anyway. In theory it might be best not to fully charge (even in Standard mode) at home.

 

[Doug_G]

I've heard people achieving higher "ideal" miles when charging off of 110V. I haven't confirmed since I never tried. If its true would like to understand why.

I can confirm that - I have seen it happen. Whether it actually results in more range is not clear.

 

[dhrivnak]

I've heard people achieving higher "ideal" miles when charging off of 110V. I haven't confirmed since I never tried. If its true would like to understand why.

Yes I get about 4 extra miles if I charge at 110V. Thsi makes sense as charging very slow allows the battery chemistry to fully "ionize". But 177 miles or 181 miles is really not much difference in the real world.

I also hear that many small charges are better than larger charges so I would charge when you can.

 

[wiztecy]

I thought there was also some theoretical microscopic advantages (for battery life) to keep the charge in the middle of the SOC. Therefore, it might be best not to "top off" at work each day - your only driving 40 miles anyway. In theory it might be best not to fully charge (even in Standard mode) at home.

From what I researched, Lithium ion batteries sweet spot is 50% SOC for longevity. There are other factors you want to also keep in mind such as keeping the battery temps down as much as possible and not stressing the battery pack by hard accelerations below 50%. I talked to Tesla when I was in Menlo Park about my practice of keeping my pack at 50%, where I fully charge to standard mode (pack is at approx 80% capacity) at 5-6am, have it ready 1 hour before I get ready to commute to work for balancing. Then drive 35 miles to work. At that point the battery is at approx 70% capacity where it sits for 8 hours in the garage. Then I head home consuming 35 more miles where the pack is at 60% SOC. I sometimes do some errends and joy drives that place me down occasionally to 50%. So my goal is to gobble up and consume whenever possibly any high SOC state. On the weekend I don't charge unless I'm doing a big trip. So getting back to Telsa, they said this type of charging practice will throw the battery pack out of balance. I said really!? They went and checked out the status to see if the pack was out of balance and it was not, its fully balanced. And they went.... ohhh. The key is to not drive off right after you charge. Let it sit for 30-60mins if you have the chance, that way it'll never get out of balance and there's no need to equalize the pack.

 

[hcsharp]

I thought there was also some theoretical microscopic advantages (for battery life) to keep the charge in the middle of the SOC. Therefore, it might be best not to "top off" at work each day - your only driving 40 miles anyway. In theory it might be best not to fully charge (even in Standard mode) at home.

The "microscopic advantages" are much smaller than the advantages to discharging while at higher (up to 80%) SOC. That's because the battery heats up less when driving at a higher SOC. You also incur less micro-cracking when doing shallower discharges. So the OP Eric is better off charging at work instead of waiting until he gets home every night. It is also marginally better for your battery to charge at lower rates so I would recommend charging at whatever rate gets you to a full std mode charge about an hour before you have to drive, but a minimum of 32A.

 

[strider]

Tom's research has nothing to do w/ battery life. He was measuring efficiency (ie the least amount of overhead lost to battery temp mgmt and other things). The car will watch the battery temp and ramp down the charging rate if the battery temp climbs. If you want max efficiency set it to 32A and charge at home and work.

 

[Eric from NE]

FWIW, I switched over to 32A at both locations and my average "full" ideal charge has gone from mid 180s (typically 184-187) to the low 190s (191-194) almost overnight and pretty routinely.

Can't make any comment on impact of actual miles, or any notes I took about length of charge or outside temps or whatever. Purely anecdotal numbers at this point.

 

[wiztecy]

What amps were you charging at before switching to 32amps? I charge at 30amps since my original 1.5 roadster charger is capped at that.

I notice I recover more miles if I allow the battery pack to discharge down to 20-50 ideal miles (std. mode). I don't know why it does this since Tesla says there's no 'gas gauge' in the sheets / pack that would be recalibrated from dropping the pack down to a lower SOC. Was getting 182 ideal miles and up to 186.

 

[Eric from NE]

What amps were you charging at before switching to 32amps?

40A (on a dedicated 50A line). Only at work during the day, typically starting from a 40-50% charge. Nothing on weekends. It could sometimes go 2-3 days at times while sitting on a 65-75% charge with not even 110 to plug into.