# Why can't large batteries (such as the one by tesla) be charged in parallel

Tesla has many small individual battery packs. Why can't the output from wall outlet be taken and be distributed on each individual pack to be charge independently and hence faster, because many packs will be charged in parallel?

Wouldn't the charging take minutes instead of hours?

• The limiting factor for charging speed is usually the rate at which energy can be delivered to the car. In many cases, that is in turn limited by the rate at which a home's electrical infrastructure is able to receive energy from the electric company. If a home only has 15KW available to feed the charging apparatus, charging five battery clusters at 3KW each won't offer any speed boost versus charging a series string with the whole 15KW. – supercat Nov 10 '16 at 16:23
• Also, you cannot recharge batteries in minutes (even if the power were available) because the chemical reaction inside the battery cannot keep up with that charge rate. – mkeith Nov 10 '16 at 16:58
• A typical wall outlet may deliver about 1.5 to 3.5 kW depending on voltage and maximum current. But for really fast charging more than 140 kW power is needed and the time is still some 40 minutes. If you want to reduce the time to about 10 minutes you would need more than 500 kW. But this is only theory, the cells will be overheated and destroyed if charged so fast. This may destroy not only the battery pack but also the whole car, the garage and the houses nearby if the battery pack burns. The limit is the wall outlet, not the battery pack. – Uwe Nov 11 '16 at 8:45

Not only is @supercat's comment correct about the charge cable limit, but each individual cell limits the charge speed. The "supercharger" speed is 40 minutes to 80% (145kW!). Which is about as fast as it's possible to charge a single 18650 cell without it overheating.

The limits within a cell are due to internal resistance (which causes self-heating) and the diffusion rate through the electrolyte. Most of the research on fast charging cells focuses on building electrodes with a "fractal" extremely large surface area but also acceptable durability.

• it all about ensuring mismatched electrical loss NTC is lower than PTC conduction cooling losses at operating currents squared. – Tony Stewart EE75 Nov 10 '16 at 17:48
• What if I had plugged the car into 4 or 5 different wallet outlets to get power from (Through the same cable) – NinjaStars Nov 14 '16 at 10:58
• You'd need a special kind of outlet, not multiple outlets: tesla.com/en_GB/support/home-charging-installation – pjc50 Nov 14 '16 at 11:05
• - and the 140kW charge at 240V would be over 500A, which is more than the usual premises fuse and wiring can tolerate. – pjc50 Nov 14 '16 at 11:28

From Tesla's Wiki page:

The Supercharger is a proprietary DC rapid-charging station that provides up to 135 kW of power, giving 85 kWh vehicles an additional 180 mi (290 km) of range in about 15–30 minutes.

• D100 models with 100kWh will take longer.

• Older (D60's with) 60 kWh may have reduced charge rate.

In the West Coast corridor the Superchargers's grid access is assisted by a solar carport system (some including a battery of a few hundred kWh) provided by SolarCity. Eventually, all Tesla stations will be assisted by solar power.

The limitation is both the battery heat capacity as well as the power capacity of the grid near a Supercharger station for now. The built-in 11kW charger requires a 40A single phase service. This is only 8% of a Supercharger station. Later I'll address the parameters that dictate the reasons for high quality matched cells that can support this.

Electric motor  Front and rear motor combined output up to ;
762 bhp (568 kW), 687 ft·lb (931 N·m),
3-phase AC induction motor
Transmission    1-speed fixed gear (9.73:1)
Battery 60, 70, 75, 85, 90 or 100 kWh lithium ion
Electric range
70 kWh (250 MJ)
240 mi (390 km) (EPA)
85 kWh (310 MJ)
265 mi (426 km) (EPA)
310 mi (500 km) (NEDC)
Plug-in charging
11 kW 85–265 V onboard charger for 1ϕ 40 A or 3ϕ 16 A[5] on IEC Type 2 inlet
Optional Dual Charger for 22 kW for 1ϕ 80 A or 3ϕ 32 A[5]
Supercharger for 120 kW DC offboard charging,
adapters for domestic AC sockets (110–240 V)


To paraphrase You can charge in parallel iff (if and only if) the Ah and thus ESR is matched such that power dissipation $I^2*ESR=P_D$ times the thermal resistance $Rja*P_D$ of each cell does not cause thermal runaway.

• Thermal Runaway is a condition where the thermal loop gain >0. This is where heat loss causes the NTC temperature coefficient of Vbat and ESR to cause a single cell to draw more current while the positive temperature coefficient is the rise of the cell temperature which is higher than the case temps, which is higher than ambient of neighbouring cells and then the ambient outside the module.

• Tesla2 packs now have embedded microfuse wires on each cell anode and cathode for safety.

Thermal conductance is an important module attribute as well as low matched <<1% elecrical conductance of the cell (or low ESR which is directly related to high Ah and state of charge (SoC)

• The % of mismatch allowed is inversely proportional to the ratio of charge OR discharge current to the rated max current.
• Thus probability of thermal runaway increases near the rated max current in an exponential function.

The analogy is a phase shift oscillator with a loop gain <1 or parallel LEDs with a mismatched ESR operated in parallel near rated currents with poor thermal heatsink. LEDs have a NTC on voltage while Rja *Pd is a positive coefficient.

• Just as any capacitor has an ESR*C time constant which defines the shortest possible time to charge up of about 0.1us for 1uF and many seconds for 10F Ultracaps it takes about 1000x longer for Tesla batteries with at least 10k bigger equivalent capacitance

• just as capacitor chemistry affects this ESR*C time constant, it is also true for different battery chemistry and quality.

• Ah capacity is proportional to the value of C ,
• The ESR depends the Ah rating and voltage variation as well as the array factor for number of series cells/parallel cells
• The C value xx kilofarads and ESR in xx milliohms defines the shortest possible time to charge or discharge the battery voltage neglecting thermal rise
• the thermal stability loop above depends on the degree of mismatch and current sharing error and the heat conduction to the lower body and forced air cooling
• There are secondary charge effects that increase this time constant as well.

• There must be sufficient margin below Tc the critical temperature where chemical reactions take over from electrical energy and cause a catastrophic exothermic reaction and burn-up. * I recall this is around 150'C for a safe internal cell temp limit... *