Why Early Morning Charging Maximizes Efficiency

[Cybergirl]

Harvesting solar energy on overland trips where driving range can often become critically important led me to better understand the limits of what can be accomplished. I have a 420W solar panel mounted to the top of my roof top wedge tent for the purpose of extending driving range, primarily by powering external loads (refrigerator, A/C, electric blanket, Starlink, camp lights, etc.) without drawing down the truck's HV battery. I also want to funnel any excess solar energy to the truck's battery for whatever range gain I can achieve.

I performed two back to back charging experiments over 3 1/2 hours to determine how much solar energy I could expect to boost driving range. Spoiler alert: it's not much.

What I found is that when I attempt to charge the truck's battery from my solar generator makes a big difference in charging efficiency.

Without going into all the details of my experiment, the result showed a wide variation in efficiency depending on the truck's need to expend energy to cool the HV battery. I ran my charging scenarios by Grok 3 on X, and got this response confirming my understanding:

Why Early Morning Charging Maximizes Efficiency
Your experiment showed that charging efficiency drops when more energy is diverted to cooling, as seen in the second session (70-82°F ambient, warmer battery, ~49% efficiency) versus the first (64-70°F, cold battery, ~75% efficiency). When harvesting solar energy, you’re typically constrained by the limited power output of portable solar panels (e.g., 100-400 W for typical overlanding setups, far below even Level 1’s 1 kW). Every watt counts, so minimizing losses to thermal management is critical.

• Cooler Battery: Overnight, the Cybertruck’s battery equilibrates toward ambient temperature. In an overland setting, early morning temps (e.g., 50-65°F in a desert or mountain environment) keep the battery cool (say 15-20°C), reducing or eliminating the need for active cooling during charging. Starting cold, as in your first session, the battery can even benefit from the slight heat generated by charging, avoiding energy-intensive heat pump activation.
• Lower Ambient Temps: Early mornings typically offer the day’s lowest air temperatures, improving passive heat dissipation. With less thermal stress, the cooling system stays dormant or operates at minimal power (e.g., <100 W), leaving more solar energy for the battery.
• Efficiency Gains: At 75% efficiency (your first session’s result), a 400 W solar array could deliver ~300 W to the battery (0.3 kWh/hour). At 49% efficiency (second session), that drops to ~196 W (0.2 kWh/hour)—a 33% relative loss. Over 5 hours of peak sunlight, that’s 1.5 kWh versus 1 kWh, or ~3.5-5 extra miles of range (assuming 0.43 kWh/mile).

Practical Strategy for Solar Harvesting
To optimize solar charging for range extension:

1. Charge Early: Deploy your solar panels at first light, when ambient temps are lowest (e.g., 5-15°C in many climates). If overnight lows are cold, the battery might start below optimal (e.g., <10°C), and the heat pump may warm it slightly—but this uses less energy than cooling and leverages charging heat anyway.
2. Monitor Battery Temp: If the Cybertruck’s interface shows battery temp or preconditioning status, aim to charge when it’s 15-25°C. Above 30-35°C, cooling kicks in harder, as your second session suggests.
3. Shade the Truck: Park in natural shade (e.g., under trees or a tarp) to keep the battery and cabin cooler, reducing thermal load. Solar gain on the stainless steel body can raise internal temps, indirectly warming the battery.
4. Match Solar Output: Use a charge controller or inverter matched to the Cybertruck’s Level 1 input (120V, up to 16A). Even at 400 W (~3.3A), efficiency trends should mirror your 1 kW test, scaled down.

Broader Implications

• Daily Cycle: Solar output peaks midday, but so do ambient temps (e.g., 80-100°F), increasing cooling demand. Charging from, say, 6-10 AM captures rising output (50-80% of peak) while dodging heat penalties. Late afternoon (4-6 PM) could supplement, but the battery may already be warm from the day.
• Range Impact: At 300 W effective (400 W solar at 75% efficiency), 6 hours of morning charging adds ~1.8 kWh, or 4-6 miles. Over a week-long trip, that’s 28-42 miles—modest but valuable for remote travel.

Caveats

• State of Charge (SoC): Efficiency drops at higher SoC (e.g., >80%), so prioritize topping off when the battery’s lower (20-50%). Your tests didn’t specify SoC, but it’s a factor.
• Solar Variability: Clouds or panel angle reduce output, so early charging also hedges against afternoon weather shifts.
• Heat Pump Quirks: The Cybertruck’s system might activate preemptively if it anticipates heat buildup, even at low rates. Your 40% efficiency drop suggests sensitivity to temp thresholds—worth testing with solar-scale power.

Conclusion
Charging early in the day when the battery is cool and ambient temps are low is a smart move to maximize solar energy storage in the Cybertruck. It leverages your observed efficiency trends, sidestepping the cooling losses that ate into your second session. For overlanding, this could mean the difference between reaching a remote campsite or not—every mile counts when you’re off-grid. Great insight, and happy trails!

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[eswimm]

That seems a little suspect, since it implies that we'd be seeing horrible efficiencies while charging at home at 11.5kW and even worse when charging 250+kW at Superchargers.

I don't doubt the data you collected, but I wonder if there's another explanation for why you saw such a difference in efficiency, like perhaps a heat issue within the solar equipment rather than the car's battery.

EDIT: I suppose it could have to do with parasitic losses during lower charge rates. Were you charging at 120V and was cabin cooling active during either charge period?

 

[Cybergirl]

That seems a little suspect, since it implies that we'd be seeing horrible efficiencies while charging at home at 11.5kW and even worse when charging 250+kW at Superchargers.

I don't doubt the data you collected, but I wonder if there's another explanation for why you saw such a difference in efficiency, like perhaps a heat issue within the solar equipment rather than the car's battery.

This effect is only significant for Level 1 charging.

 

[eswimm]

This effect is only significant for Level 1 charging.

That makes sense, I wonder if you could benefit from switching to 220V if your solar generator would support it. Getting the juice out and into the car more quickly, so any parasitic losses are of shorter duration.

EDIT: Even if you're hitting the max kW output of the generator, higher voltage might keep losses lower. I wonder if the losses are coming from pack temps or somewhere else in the system. I'm a little surprised a level 1 charge could generate enough heat to have such high impact in the HV battery. Most people have the opposite problem that it can't generate enough heat to charge a cold soaked battery.

 

[Cybergirl]

That seems a little suspect, since it implies that we'd be seeing horrible efficiencies while charging at home at 11.5kW and even worse when charging 250+kW at Superchargers.

I don't doubt the data you collected, but I wonder if there's another explanation for why you saw such a difference in efficiency, like perhaps a heat issue within the solar equipment rather than the car's battery.

EDIT: I suppose it could have to do with parasitic losses during lower charge rates. Were you charging at 120V and was cabin cooling active during either charge period?

Yes, charging at 120V with all possible parasitic loads turned off for both back to back charging sessions.

 

[Cybergirl]

That makes sense, I wonder if you could benefit from switching to 220V if your solar generator would support it. Getting the juice out and into the car more quickly, so any parasitic losses are of shorter duration.

EDIT: Even if you're hitting the max kW output of the generator, higher voltage might keep losses lower. I wonder if the losses are coming from pack temps or somewhere else in the system. I'm a little surprised a level 1 charge could generate enough heat to have such high impact in the HV battery. Most people have the opposite problem that it can't generate enough heat to charge a cold soaked battery.

I'm not saying that one cannot achieve a higher charging efficiency by using Level 2 charging. I suspect you can. It's possible that the battery management system is not designed to optimize charging efficiency at Level 1 rates. I should point out that during the 2nd charging session, I could hear a noise that could have been heat pump operation.

 

[Jager]

No comment on Grok.

But I can say that the only scenario where I have ever seen active thermal cooling of the battery pack, in either the Cybertruck or the Model 3, is immediately following a DC Fast Charging session when pack temps can be north of 150F.

 

[Cybergirl]

No comment on Grok.

But I can say that the only scenario where I have ever seen active thermal cooling of the battery pack, in either the Cybertruck or the Model 3, is immediately following a DC Fast Charging session when pack temps can be north of 150F.

Okay, but then the question is: Where did the extra input energy go to reduce the efficiency of the second charging session vs the first? Why did the second kilowatt-hr of charge take 35 minutes longer than the first kilowatt-hr? Except for the outside air temperature rising from 69 to 82F, nothing else changed, no cabin A/C, no Sentry Mode, no outlets turned on, no significant increase in battery SoC, just continuing to charge at 8A from a 120V outlet.

 

[Jager]

Okay, but then the question is: Where did the extra input energy go to reduce the efficiency of the second charging session vs the first? Why did the second kilowatt-hr of charge take 35 minutes longer than the first kilowatt-hr? Except for the outside air temperature rising from 69 to 82F, nothing else changed, no cabin A/C, no Sentry Mode, no outlets turned on, no significant increase in battery SoC, just continuing to charge at 8A from a 120V outlet.

Good question. And one I can't answer.

All I can say is that Tesla BMS' are oriented to not burning any energy that's not absolutely necessary. They'll actively heat the pack when it's too cold (roughly, below 40F) because Lithium chemistry batteries simply can't be charged at those temps without experiencing lithium plating. And they'll actively cool the pack when it's extremely hot, such as after a SuperCharger session, because those kind of temps - while desirable for the DC Fast Charging session itself - will sharply accelerate cell degradation. That's pretty much it.

You can monitor specific pack temps on the CT with Tessie (and perhaps other 3rd-party apps).

 

[cybercricket]

Good question. And one I can't answer.

All I can say is that Tesla BMS' are oriented to not burning any energy that's not absolutely necessary. They'll actively heat the pack when it's too cold (roughly, below 40F) because Lithium chemistry batteries simply can't be charged at those temps without experiencing lithium plating. And they'll actively cool the pack when it's extremely hot, such as after a SuperCharger session, because those kind of temps - while desirable for the DC Fast Charging session itself - will sharply accelerate cell degradation. That's pretty much it.

You can monitor specific pack temps on the CT with Tessie (and perhaps other 3rd-party apps).

A typical 600A contactor will "burn" about 10W (possibly half that when economizer is used). How many contactors are enabled on the CT when AC charging ? Then you have the onboard computer (not the BMS) and any related controllers, probably also a decent 50-70W to run. I assume there aren't any balancing losses due to use of active balancing, but I don't know the design details of the BMS on CT. Then you have the losses in the AC/DC conversion and voltage boosting. That's before considering any HVAC overhead.

 

[Cybergirl]

Good question. And one I can't answer.

All I can say is that Tesla BMS' are oriented to not burning any energy that's not absolutely necessary. They'll actively heat the pack when it's too cold (roughly, below 40F) because Lithium chemistry batteries simply can't be charged at those temps without experiencing lithium plating. And they'll actively cool the pack when it's extremely hot, such as after a SuperCharger session, because those kind of temps - while desirable for the DC Fast Charging session itself - will sharply accelerate cell degradation. That's pretty much it.

You can monitor specific pack temps on the CT with Tessie (and perhaps other 3rd-party apps).

That makes sense. I'll have to perform further experiments to better understand my previous observations.

 

[Cybergirl]

A typical 600A contactor will "burn" about 10W (possibly half that when economizer is used). How many contactors are enabled on the CT when AC charging ? Then you have the onboard computer (not the BMS) and any related controllers, probably also a decent 50-70W to run. I assume there aren't any balancing losses due to use of active balancing, but I don't know the design details of the BMS on CT. Then you have the losses in the AC/DC conversion and voltage boosting. That's before considering any HVAC overhead.

Yes, these all contribute to a reduction in charging efficiency, but shouldn't change between back to back Level 1 charging sessions. The first kWh into the battery (at ambient temperature) required less input energy (higher efficiency) than the second kWh. Some additional load turned on to cause the difference. If not battery cooling, what was it?

 

[Jager]

This time of year.... perhaps pack heating?

You ran the tests in the morning. What was the low overnight?

 

[Cybergirl]

This time of year.... perhaps pack heating?

You ran the tests in the morning. What was the low overnight?

The truck was in the garage overnight. The temperature on CT's screen read 66 deg F at 9:41 when I started to charge out on the driveway. It took 1 hr and 40 minutes to put 1 kWh of energy into the truck's battery charging at 8A. The second charge of 1 kWh began immediately at 11:20 and was expected to complete at 1:00pm assuming the same charging efficiency. At 1:35 it still hadn't added 1 kWh. The outside air temperature had risen to 82 deg F and the truck was in full sunlight. The truck's battery had warmed a couple of deg C during the first charging session. I, unfortunately, don't have the battery temperature at the end of the second session. I'll have to repeat the test to get that data.

 

[Jager]

Okay, then it wouldn't have been because of pack heating, or pack cooling.

The challenge you're going to face trying to sort this kind of extreme-low-power charging solution is that you're providing so little power that you're literally down in the noise, with all the parasitic elements. Tesla explicitly recommends against even a conventional 120v Level One solution for the Cybertruck, much less something providing a small fraction of that.

I think I'd be happy if solar was enough to just power my ancillary camp gear, like a refrigerator.

From the Cybertruck Owners Manual:
NOTE: Tesla does not recommend using low-power charging equipment (such as a 120V NEMA 5-15 outlet and the Universal Mobile Connector) to charge Cybertruck, especially in low ambient temperatures. Due to the size of the high voltage Battery, charging with equipment that produces less than 2 kW of power will charge Cybertruck very slowly. If you are charging Cybertruck with such a method, turn off climate to reduce idle power consumption while charging.

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