Battery wear and tear vs Charger wear and tear

Battery wear and tear vs Charger wear and tear

Last month I took a 3 week backpacking trip and left the model 3 at home plugged in to 240V with Sentry Mode turned on. I noticed (from my home energy monitor) when i returned that instead of constantly pulling from the wall to power the car that it would pull from the battery for maybe an hour then charge the battery back up repeatedly. This amounted to about 6KWH per day cycled through the battery.

This bothered me a bit since I would have rather all the power used to run Sentry mode and everything else come from the wall without being cycled through the battery. Normally I don't run Sentry mode but I'll burn 1-3% daily on parasitic loses anyway. I got to thinking that in order to prevent parasitic loses from being cycled through the battery, I could plug the car in every night and set it to a lower ampage. This would keep the charger running and providing power from the wall for some charging and all parasitic power needs up until the battery is fully charged. Traditionally I'll charge the car on 240V at full ampage for perhaps 4 hours or so every few days.

The thought then occured to me that if I did this I'd perhaps be adding wear and tear to the charger by having it run all night every night. Pumping coolant, running fans, etc. Really this would be shifting wear and tear from the battery to the charging system.

I've never heard of a charging system wearing out though. Perhaps wear and tear on the charging system is near zero? Perhaps not?

What are your thoughts? Energy efficiency is not a concern of mine. Really I'm looking to do everything I can to maintain the car for the long run. My gas car I've owned since 1998 and I'd love to have this Model 3 20 years from now. Most seem to pay mind to battery care and the thought of charger care only just now crossed my mind.

pedersenh1 | 11. Oktober 2019

I don't think short cycling your charger should hurt. Switching at full current would wear contacts. But the car ramps up the current from zero quite gradually. I guess my battery in one of my LR M3s will wear out before my charger.

Bighorn | 11. Oktober 2019

Not sure you’re really accomplishing anything with the slow charge.

vmulla | 12. Oktober 2019

Just consider this, each time you drive around the car experiences hundreds of charge/discharge cycles because of regen braking. The wall charge/discharge cycle you talk about is minuscule in comparison - net effect on battery wear is too tiny to consider.
The electronics in the car use DC power, so the car uses the battery. The parasitic loses you're talking about are inevitable, because the car needs to convert AC to DC to power the electronics anyway.
In the current approach, each system (charging/discharging) is doing it's thing efficiently and without additional complexity.

Ron.Olsberg | 12. Oktober 2019


Your can read my long post regarding this topic at for my thoughts on this topic. I have not done many experiments since that post so things might have changed over the many firmware updates? It would be nice if connecting an external 12V AGM Smart charger to Model 3 worked like it appears to on the Model S. In an ideal world ,the Model 3 would provide this smart charger function while the car was asleep which would provide all the power required to run all the 12V onboard systems; however, this would require another charging path to the 12V battery basically a 120/240 volt Smart charger built into the car that got its power from shore power. Unfortunately, I am not certain power is available at the car charging port anytime the car is plugged in via the UMC2? Since the car will drawn power from shore power when you turn on the interior heater, maybe power is always available? or maybe when you turn on the heater the car communicates with the UMC to request that it supply power?

Joe M | 12. Oktober 2019

I will be storing my M3 for 4 months in an unheated garage and my question is somewhat related to this. I plan to leave the car plugged in and set at 50%. Does anyone have knowledge of the “deadband” that the charger maintains? Does it even cycle by deadband or does it just periodically trigger?

Ron.Olsberg | 12. Oktober 2019

Additional comment relating to my last post. When you get into the car the contactors close and the interior heater uses the DC high voltage to supply heat to the cabin. I think the internal HV car charger will supply some or all of the power required for the cabin heater (via the UMC) and if it can't supply enough juice the remaining power will be drawn from the HV traction battery. If I remember correctly, the cabin heater draws a maximum of somewhere between 7 to 8KW; therefore, even 240V@24A would not supply enough juice for all the cabin heater requirements.

gmr6415 | 12. Oktober 2019

@brandonbeckis, What you are describing conflicts with what's in the owners' manual.

"Note: Whenever Model 3 is plugged in but not actively charging, it draws energy from the wall outlet instead of using energy stored in the Battery. For example, if you are sitting in Model 3 and using the touchscreen while parked and plugged in, Model 3 draws energy from the wall outlet instead of the Battery."

Ron.Olsberg | 12. Oktober 2019

@gmr6415 Thanks for your post. If power is supplied from shore power while the car is sleeping that would be ideal. I now have a power monitor installed to monitor UMC power usage. I will check out the power draw while the car is sleeping and plugged into the UMC and report back. Hopefully the sleeping power draw will be more than the 2.9W the UMC requires. Since the HV contactors are open while the car is sleeping, I do not see how 12V power could be supplied to the onboard systems. Hope I am proved wrong! Again, thanks for your reply.

gmr6415 | 12. Oktober 2019

@Ron.Olsberg, I will say it wouldn't be the first time I've seen what the manual states and what happens in real world situations being different.

Just thought I'd point out what Tesla is saying for some guidance.

TexasBob | 12. Oktober 2019

Very interesting. If car uses ~6kwh a day in parasitic losses with Sentry enabled 24/7, that = 7.5% on an LR car and 10% on a SR? The with parasitic losses alone, on an SR you are consuming 730 full charge cycles over 20 years and on an LR you are consuming 550. Given that battery is expected to "last" 1500 cycles that is pretty significant in my book over a typical vehicle life (n.b. avg age of passenger car in US is 11.8 years, typical new vehicle will last over 20 years).

brandonbeckis | 12. Oktober 2019

@Ron.Olsberg Thank you for the reply and info. The idea of attaching a 12V charger for long durations when away is an interesting idea. You are correct about the HVAC pulling from the wall when activated. I've noticed that happen when the traction battery is full.

@Joe M I'm not sure what deadband is. While I was gone on my trip it would periodically trigger the charger to activate and pull power at the ampage setting I set in the car.

@gmr6415 I noticed that in the manual too. A lawyer type might say we are misinterpreting the language in the manual. Either way I'd much prefer the vehicle constantly pull the power it needs from the wall when plugged in as to not cycle the battery. It may have something to do with what Ron is talking about regarding the 12V system using the power rather than the high voltage system.

@TexasBob You're doing the same kind of math im thinking about. That 6 KWH I was seeing each day is the same as 20 miles of driving i'm not doing. Added up over time all those extra cycles add up to big numbers. Hoping to minimize them in the long run as long as it's not at the great expense of something else on the car like the charger.

Ron.Olsberg | 12. Oktober 2019

Ok performed some tests and monitored the power at the UMC2 wall connection. When I was actively charging or running the cabin heater, I did see more than the 2.9W that UMC2 itself draws. I performed a firmware update while plugged in, first time doing so, and it used around 400W during the self test after the update. I am going to assume that since I was plugged in it drew the current self testing the cars charging subsystems. When I turned the cabin heat to max temp and set the fan speed to 10, I measured current readings of up to 20A on my 240V circuit. While the car is awake, I see absolutely nothing over 2.9W from the wall. The lions share of the phantom drain happens while the car is awake and this power is being pulled from the traction battery to top off the 12V battery via the DC-to-DC converter, run cooling pumps etc. I did all kinds of things with the center screen and no power was used from the 30A dryer outlet unless I turned on the cabin heat and I assume the AC . So it appears to me the manual is wrong on that topic. I am waiting for the Model 3 to go to sleep (could be up to six hours) to see if any power is used from the wall outlet during sleep. From my previous and today's experiments, I bet that no power will be drawn from the wall outlet during sleep. So it appears no power is drawn from the wall outlet unless the car is activity charging, running the HVAC or during the systems tests performed during an update. I wish the car drew power from the wall outlet while awake and really wished it did while sleeping because that would supply power to the 12V systems thereby eliminating the 12V and traction battery charge/discharge cycles. The only way to get the car to not use any power is to disconnect the negative 12V battery terminal and momentarily lift the large grey connector located under the rear passenger side seat bottom. I do not recommend this for the faint of heart but the full procedure was documented in a Tesla document that is no longer available on the interweb. When the car previously did not check for the the frunk lid being open, you could simply disconnect the negative 12V terminal after the car when to sleep and it was in a comma. I actually did this a few times while traveling and when I got home, after more that 14 days, the car had exactly the same miles on the traction battery; likewise, the voltage level on the 12V AGM battery had only dropped by at most a tenth of a volt.

Ron.Olsberg | 12. Oktober 2019

The car went to sleep while I was posting the last message. It is NOT using any power from the wall in sleep mode. The 400W used during the self test, after the firmware update, was probably from the traction motor heating test. I had previously determined that the traction motor heating mode does use power from the wall if plugged in.

brandonbeckis | 13. Oktober 2019


Thank you for sharing the results of your test. I'll probably start charging at slower speeds every night to try to minimize the vampire cycling of the battery. I can't find any significant downside or evidence to added wear and tear of the charging system by doing that. In my case it will most likely be at about 7-8 amps from 240V every night. Last night while the car was plugged in I turned Sentry Mode on and noticed a 2-3% increase in the expected charge time without changing the current pulled from the wall. This gives me confidence that the vehicles systems pull from the wall while charging rather than the battery. BTW I only just now realize what you mean by "shore power." Love that term.

Earlier you mentioned that hooking a DC charger to the 12V battery wouldn't work for the Model 3 to eliminate cycling of the battery from vehicle idle systems. Do you know why? I was thinking I may try that for the next time the car is parked for an extended duration. I have a battery tender that I usually have plugged into my gas car or bike. Could perhaps use that.

Ron.Olsberg | 13. Oktober 2019

@brandonbeckis Last spring I performed exhaustive tests trying to use my new Smart AGM battery charger to supply the power required while the car was sleeping, nothing worked (it still woke up for 4 to 6 hours at least once a day). If you search for my name you a can read the extensive post(s) regarding that experiment. From what I read last spring, using the battery and trickle chargers us old timers have used for years to charge our lead-acid batteries are not recommended for AGM batteries (they like to be charged in a very specific way - they are special!). If you have better luck than me, please report your results on this forum.

Thanks for your Post and good luck!

Ron.Olsberg | 13. Oktober 2019
Check this post for more info and search that forum for my username rrolsbe for more possible posts regarding this topic.

GrumpyinAZ | 13. Oktober 2019

Brandon, as Ron.Olsberg points out, you're applying the trickle charging philosophy of lead acid batter technology to try to keep the charge up. These batteries act differently because they're an advanced lithium-ion battery. LI-ON batteries do NOT benefit from trickle charging. The Tesla approach presumes that you will discharge your car's battery to around 20% at its lowest and then recharge it to 80-90% for normal daily operation. This is the typical cycle for a LI-ON battery and does not damage the battery or prolong the life of it. Vampire drain is the very least of your worries. If you're going to be away for an extended period of time, charge to 95% and then, when ready to leave, use the Power Off feature from Safety and Security menu and you should be fine for a month or more. The 12 volt battery can sit idly from full charge for months by design.

Ron.Olsberg | 13. Oktober 2019

The 12V AGM battery installed in Model 3 as delivered from Tesla is a Lead-Acid battery and the car knows the proper way to charge this battery. Some people have replaced this 12V battery with a Lithium-ion 12v battery but from what I have learned I would not do that. I used the power off feature but did not find that is stopped the 12V systems from drawing power, put the car in a comma. It is an easy test, note the traction battery charge % and miles, leave the car unplugged and check back in about four days, no driving. If the traction battery percentage/miles is the same (could vary slightly due to any traction battery temperature change) then you were successful. The phantom drain is usually around 1% daily but can be much higher. BTW, the Smart Charger I used did keep the voltage at a level where the car should not of had to wake up to top it off; however, it did wake up and attempted to charge an already topped off battery. Fortunately, the smart charger I purchased is smart enough to back off when it sees the car trying trying to top off the 12V battery.