I woke up this morning trying to concoct a "two battery system" that made sense. A battery charges faster when discharged, so if you have dial batteries, isolated, and went to recharge, the one most discharged would gain range faster than the other which was nearly full. The problem with this stunt is that a unibattery would basically do the same thing.
Yep.
Actually, again the important distinction to make is that the "dual" battery might be two battery packs of X individual cells each, whereas the "unibattery" is a single battery pack of 2*X cells.
But the key is that isolation of the packs actually doesn't matter, and they're already isolated in smaller units than that at the module level.Let's look at
this description of the original Audi E-Tron battery:
95 kWh of energy: the high-voltage battery system
The powerful lithium-ion battery in the Audi e-tron provides for a range of more than 400 kilometers (248.5 mi) in the WLTP driving cycle. The battery operates with a nominal voltage of 396 volts and stores 95 kWh of energy. The battery system in the Audi e-tron is located beneath the cabin and is 2.28 meters (7.5 ft) long, 1.63 meters (5.3 ft) wide and 34 centimeters (13.4 in) high. It comprises a total of 36 cell modules in square aluminum housings, each of which is roughly the size of a shoe box. They are arranged on two levels, known as “floors” – a longer lower floor and a shorter upper one. The cell modules in the Audi e-tron can reproducibly discharge and charge electricity over a broad temperature and charge status window. They can be densely packed to achieve a very high output and energy density in the volume available. At market launch, each module is equipped with twelve pouch cells having a flexible outer skin of aluminum-coated polymer. In the future, Audi will use both technically equivalent prismatic cells in its modular concept, also in terms of a multiple supplier strategy.
An indirect cooling system distinct from the cell space ensures the high-performance operation of the battery over the long term. It is made of flat, extruded aluminum sections uniformly divided into tiny chambers. Heat is exchanged between the cells and the cooling system beneath them via a thermally conductive gel pressed beneath each cell module. In what is a particularly efficient solution, the gel evenly transfers the waste heat to the coolant via the battery housing.
The battery and all key parameters, such as charge status, power output and thermal management, are managed by the external battery management controller (BMC). This is located in the occupant cell on the right A-pillar of the Audi e-tron. The BMC communicates with both the controllers of the electric motors and the cell module controllers (CMC), each of which monitors the currents, voltage and temperature of the modules. The battery junction box (BJB), into which the high-voltage relays and fuses are integrated, is the electrical interface to the vehicle. Enclosed in a die-cast aluminum housing, it is located in the front section of the battery system. Data exchange between the BMC, the CMCs and the BJB is via a separate bus system.
The "isolation" you speak of would be at the module level, I believe. So the Audi e-Tron is already 36 "isolated" battery modules, each with its own cell module controller [CMC].
This is speculation, but I'm guessing that since each individual CMC can monitor current, voltage, and temperature at the module level, the BMC can also individually control the amount of current being delivered to each module. Trying to deliver a uniform current to all modules in parallel would be unsafe, so I can't imagine that the BMC is not capable of this.
So if battery module 7 is only at 20% charge, while battery module 19 is at 67% charge, the BMC will pump more current into module 7 because it can handle it, and thus module 7 will be charging at a faster rate than module 19.
So... Long story short...
Your idea of breaking the battery pack into smaller isolated batteries already exists and is in common usage!It's not actually done for charging rate reasons IMHO; it's basically because you want a certain voltage out of each module which is determined by the number of cells per module. It's also done for ease of design/manufacture... That way the battery module configuration, charging electronics, BMC, motors, etc of a Tesla Model 3 Standard Range and a Tesla Model 3 Long Range are of identical design; the Long Range simply has more modules/BMC in parallel. But the net result is to break down a large battery pack into multiple isolated modules, which is what you were after.
