Patent Description:
Electrical systems commonly include power storage devices for supplying power to and receiving power from the electrical system. Some electrical systems have power storage devices of different types, the different types of power storage devices accommodating different operating conditions of the electrical system. Such hybrid power storage device arrangements can include power storage devices grouped by type and connected to the electrical system in stages, the stacked modules absorbing and sourcing pulses to the electrical system, as required.

In some electrical systems the efficiency of the energy storage modules within a stack can vary relative to one another. The variation in efficiency can, in some electrical systems, cause the state of charge of the power storage devices within the stack to diverge from one another. The state of charge divergence can accumulate such that an overvoltage condition, requiring that a power storage device or entire stack be disconnected until state of charge is rebalanced. While the state of charge is unbalanced, and the power storage device and/or stack disconnected, operation can of the electrical system can limited as the energy storage module and/or stack functionality is unavailable. Electrical systems are disclosed in<NPL>.

Such conventional methods and systems have generally been considered satisfactory for their intended purpose. However, there is still a need for improved power stages stacks and methods of balancing state of charge. The present disclosure provides a solution for this need. Energy storage modules are described in <CIT>.

A hybrid energy storage module (HESM) system is provided as defined by claim <NUM>.

In accordance with certain embodiments the controller can have a short circuit mode to disconnect an energy storage module (ESM) of the first power stage and an ESM of the second ESM from the power bus. Both the first and second power stage short circuit switches can be open in the short circuit mode. The controller can have a discharge mode to discharge either (or both) the first and the second power stage through the power bus. One of the first and second power stage short circuit switches can be closed and the other of the first and second power stage short circuit switches can be open in the SOC balancing mode.

It is contemplated that the controller can have an absorb/source mode to absorb and provide current pulses to the power bus. Both the first and second power stage short circuit switches can be closed in the absorb/source mode. The HESM can include a battery, a supercapacitor or a fuel cell.

It is also contemplated that HESM system can include a positive rail lead connecting the first power stage to a positive rail of a power bus, a negative rail lead connecting the second power stage to a negative rail of the power, and a neutral rail lead connecting the first power stage and the second power stage to a neutral rail the power bus through a y-lead.

An electrical system for an aircraft includes a power bus with a positive rail, a negative rail, and a neutral rail and a HESM system as described above. The first power stage is connected to the positive rail and the neutral rail. The second power stage is connected to the negative rail and the neutral rail.

A method of controlling connectivity of a HESM as described above includes absorbing/sourcing current flow to power bus from the first and second power stage and balancing SOC between the first power stage and the second power stage by connecting one of the first and second power stage through the other of the first and second power stage with the other of the first and second power stage disconnected from the power bus. In certain embodiments the method can also include disconnecting the first and second power stage from the power bus responsive to detecting a short circuit in the power bus. In accordance with certain embodiments the method can include discharging, in full, at least one of the first and second power stage by connecting one of the first and second power stage through the other of the first and second power stage with the other of the first and second power stage disconnected from the power bus.

Reference will now be made to the drawings wherein like reference numerals identify similar structural features or aspects of the subject disclosure. For purposes of explanation and illustration, and not limitation, a partial view of an exemplary embodiment of a hybrid energy storage module (HESM) system in accordance with the disclosure is shown in <FIG> and is designated generally by reference character <NUM>. Other embodiments of HESM systems, electrical systems with HESM systems, and methods of controlling connectivity HESM system powers stages to electrical systems in accordance with the disclosure, or aspects thereof, are provided in <FIG>, as will be described. The systems and methods described herein can be used in high voltage direct current (HVDC) electrical systems, such as in aircraft, though the present disclosure is not limited to aircraft electrical systems or to HVDC electrical systems.

Referring to <FIG>, an electrical system <NUM> for a vehicle, e.g., an aircraft <NUM>, is shown. Electrical system <NUM> includes a power source <NUM> connected to one or more power consuming devices <NUM> by a power bus <NUM>. Power bus <NUM> is connected to the one or more power consuming devices <NUM> through a power distribution panel <NUM> for controlling the flow of HVDC power <NUM> to the one or more power consuming devices <NUM> carried by aircraft. HESM system <NUM> is electrically connected to power bus <NUM> to control variation of HVDC power <NUM> provided through power bus <NUM>, e.g., to limit voltage variation and/or current flow due to the connection and disconnection of power-consuming devices <NUM> to and from power bus <NUM> during operation. As used herein the term high voltage refers to electrical systems having a bus voltage that is greater than <NUM> volts, e.g., electrical systems having <NUM> volt power buses.

In the illustrated exemplary embodiment power source <NUM> includes a generator <NUM> which is connected to power bus <NUM> and coupled to an aircraft main engine <NUM>, which is arranged to apply mechanical rotation to generator <NUM> for generating the flow of HVDC power <NUM> provided to power bus <NUM>. This is for illustration purposes only and non-limiting. In certain embodiments power source <NUM> can include an auxiliary power unit, backup power supply, or a ground power source, as suitable for an intended application.

Referring to <FIG>, power bus <NUM> and HESM system <NUM> are shown. Power bus <NUM> is a split DC bus having a positive rail <NUM>, a neutral rail <NUM> and a negative rail <NUM>. Connectivity of power bus <NUM> to power source <NUM> (shown in <FIG>) provides a positive voltage (potential difference) across positive rail <NUM> and neutral rail <NUM> with a magnitude that is substantially equivalent to a negative voltage (potential difference) across negative rail <NUM> and neutral rail <NUM>, the voltage of power bus <NUM> being twice the magnitude of each voltage (potential difference). In certain embodiments the voltage is a HVDC that is greater than <NUM> volts. For example positive rail <NUM> can have a potential of about positive <NUM> volts, neural rail <NUM> can be at about zero volts, and negative rail can have a potential that is about negative <NUM> volts. Although shown in <FIG> and described herein as a three-rail DC power bus, it is to be understood and appreciated that the present disclosure can also benefit two-rail systems, such as in vehicles where the chassis serves a neutral rail by way of non-limiting example.

HESM system <NUM> includes a first power stage <NUM>, a second power stage <NUM>, a positive rail lead <NUM>, a negative rail lead <NUM>, a neutral rail lead <NUM>, and a controller <NUM>. First power stage <NUM> and second power stage <NUM> are stacked in series with one another and tied to neutral to perform active filtering of the positive voltage across positive rail <NUM> and neutral rail <NUM> and the negative voltage across negative rail <NUM> and neutral rail <NUM>.

Positive rail lead <NUM> electrically connects positive rail <NUM> first power stage <NUM>. Negative rail lead <NUM> electrically connects negative rail <NUM> to second power stage <NUM>. Neutral rail lead <NUM> electrically connects both first power stage <NUM> and second power stage <NUM> to neutral rail <NUM>. Neutral rail lead <NUM> branches to form a y-lead, neutral rail lead <NUM> having a trunk <NUM>, a first leg <NUM> and a second leg <NUM>. Trunk <NUM> is connected to neutral rail <NUM> on a first end and to both first leg <NUM> and second leg <NUM> on an opposite second end. First leg <NUM> electrically connects first power stage <NUM> to trunk <NUM> and second leg <NUM>, and therethrough to neutral rail <NUM> and negative rail lead <NUM> via second power stage <NUM>. Second leg <NUM> electrically connects second power stage <NUM> trunk <NUM> and first left <NUM>, and therethrough to neutral rail <NUM> and positive rail lead <NUM> via first power stage <NUM>.

First power stage <NUM> is configured as a buck regulator and includes a power filter <NUM>, a DC/DC converter <NUM> and an energy storage module (ESM) <NUM>. ESM <NUM> can include one or more of a battery, a capacitor (e.g., a supercapacitor or an ultracapacitor), or a fuel cell, and is configured an adapted to source/absorb transient current pulses P flowing through positive rail <NUM> through positive rail lead <NUM> via DC/DC converter <NUM> and power filter <NUM>. DC/DC converter <NUM> electrically connects ESM <NUM> power filter <NUM>. Power filter <NUM> electrically connects DC/DC converter <NUM> to neutral rail lead <NUM> and positive rail lead <NUM>.

With reference to <FIG>, power filter <NUM> is shown. Power filter <NUM> includes an ESM positive lead <NUM>, an ESM negative lead <NUM>, a switch circuit <NUM> and a filter circuit <NUM>. Filter circuit <NUM> connects positive rail lead <NUM> and neutral rail lead <NUM> to ESM <NUM> (shown in <FIG>) and has a first capacitor <NUM>, a differential filter inductor <NUM> and a second capacitor <NUM>. First capacitor <NUM> is connected between positive rail lead <NUM> and neutral rail lead <NUM>. Second capacitor <NUM> is connected between ESM positive lead <NUM> and ESM negative lead <NUM>.

Differential filter inductor <NUM> includes a positive winding <NUM>, a neutral winding <NUM> and a ferrite core <NUM>. Positive winding <NUM> is connected in series between ESM positive lead <NUM> and positive rail lead <NUM> and is electromagnetically coupled to neutral winding <NUM> via ferrite core <NUM>. Neutral winding <NUM> is connected in series between ESM negative lead <NUM> and neutral rail lead <NUM> and is electromagnetically coupled to positive winding <NUM> via ferrite core <NUM>. As will be appreciated by those of skill in the art in view of the present disclosure, differential filter inductor <NUM> operates like a transformer with the difference that power does not flow through differential filter inductor <NUM>.

Switch circuit <NUM> includes a diode <NUM>, an inductor <NUM>, and a short circuit switch <NUM>. Inductor <NUM> and short circuit switch <NUM> are connected in series between positive winding <NUM> and ESM positive <NUM>, and therethrough to a positive terminal (shown in <FIG>) of ESM <NUM> (shown in <FIG>). Diode <NUM> is connected between ESM negative lead <NUM> and ESM positive lead <NUM>, connects to ESM negative lead <NUM> through short circuit switch <NUM>, connects to positive winding <NUM> through inductor <NUM>, and is arranged to oppose current flow through diode <NUM> to ESM negative lead <NUM>. Controller <NUM> (shown in <FIG>) is operably connected to short circuit switch <NUM> through a control lead <NUM> for controlling connectivity of ESM <NUM> to power bus <NUM> during operation.

As will be appreciated by those of skill in the art in view of the present disclosure, when a short circuit occurs on the power bus to which first power stage <NUM> is connected, the bus voltage drops to the voltage of the ESM <NUM>, and as such ESM <NUM> will source maximum current to the power bus. Short circuit switch <NUM> limits current flow from ESM <NUM> in the event of a short circuit on the power bus by acting as a buck regulator. This causes power stage <NUM> to source a predetermined amount of current for a predetermined period of time to attempt clearing the short circuit fault from the power bus.

With continuing reference to <FIG>, second power stage <NUM> is similar to first power stage <NUM> and additionally includes a power filter <NUM>, a DC/DC converter <NUM> and an ESM <NUM>. ESM <NUM> can include one or more of a battery and a capacitor, such as a supercapacitor or ultracapacitor, and is configured an adapted to source/absorb transient current pulses flowing through negative rail <NUM> through negative rail lead <NUM> via DC/DC converter <NUM> and power filter <NUM>. DC/DC converter <NUM> electrically connects ESM <NUM> to power filter <NUM>. Power filter <NUM> electrically connects DC/DC converter <NUM> to neutral rail lead <NUM> through second leg <NUM> and negative rail lead <NUM>.

In certain embodiments ESM <NUM> and ESM <NUM> (shown in <FIG>) are of a common type. In this respect ESM <NUM> and ESM <NUM> can both be, for example, ultracapacitors to handle repetitive loads or batteries for energy dense loads, for example. In accordance with certain embodiments the HESM can include two or more sets of power stages paralleled together to connect with the power bus, the first set or power stages having ESM's of a first type stacked in series, the second set of power stages having ESM's of a second type stacked in series.

With reference to <FIG>, power filter <NUM> is shown. Power filter <NUM> is similar to power filter <NUM> (shown in <FIG>) and is additionally arranged for sourcing/absorbing current pulses (shown in <FIG>) to negative rail <NUM> and neutral rail <NUM>. In this respect power filter <NUM> includes an ESM positive lead <NUM>, an ESM negative lead <NUM>, a switch circuit <NUM>, and a filter circuit <NUM>. Filter circuit <NUM> connects negative rail lead <NUM> and second leg <NUM> of neutral rail lead <NUM> to ESM <NUM> (shown in <FIG>) and has a first capacitor <NUM>, a differential filter inductor <NUM> and a second capacitor <NUM>. First capacitor <NUM> is connected between second leg <NUM> of neutral rail lead <NUM> and negative rail lead <NUM>. Second capacitor <NUM> is connected between ESM positive lead <NUM> and ESM negative lead <NUM>.

Differential filter inductor <NUM> has a neutral winding <NUM>, a negative winding <NUM> and a ferrite core <NUM>. Neutral winding <NUM> is connected in series between ESM positive lead <NUM> and second leg <NUM> of neutral rail lead <NUM> and is electromagnetically coupled to negative winding <NUM> via ferrite core <NUM>. Negative winding <NUM> is connected in series between negative rail lead <NUM> and neutral rail lead <NUM> and is electromagnetically coupled to neutral winding <NUM> via ferrite core <NUM>.

Switch circuit <NUM> includes a diode <NUM>, an inductor <NUM>, and a short circuit switch <NUM>. Inductor <NUM> and short circuit switch <NUM> are connected in series between neutral winding <NUM> and ESM positive lead <NUM>, and therethrough to a positive terminal (shown in <FIG>) of ESM <NUM> (shown in <FIG>). Diode <NUM> is connected between ESM positive lead <NUM> and ESM negative lead <NUM>, connects to ESM positive lead <NUM> through short circuit switch <NUM>, connects to neutral winding <NUM> through inductor <NUM>, and is arranged to oppose current flow through diode <NUM> to ESM negative lead <NUM>.

As will be appreciated by those of skill in the art in view of the present disclosure, because first power stage <NUM> (shown in <FIG>) and second power stage <NUM> are stacked in series, mismatch in efficiency of first power stage <NUM> and second power stage <NUM> can cause the state of charge of one of first power stage <NUM> and second power stage <NUM> to diverge from the stage of charge of the other of first power stage <NUM> and second power stage <NUM> during continuous operation, i.e., when absorbing/sourcing pulses on power bus <NUM> (shown in <FIG>). The divergence can accumulate to point where the divergent power stage exceeds predetermined overvoltage limit - at which point the power stage is disconnected from power bus <NUM> for service. To avoid such overvoltage fault conditions HESM system <NUM> includes controller <NUM>. Controller (shown in <FIG>) has a balancing mode wherein one of first power stage <NUM> and second power stage <NUM> are connected to power bus <NUM> through the other of first power stage <NUM> and second power stage <NUM> to dissipate charge to maintain state of charge of first power stage <NUM> and second power stage <NUM> below the overcharge limit.

With continuing reference to <FIG>, controller <NUM> includes an interface <NUM>, a processor <NUM>, and a memory <NUM>. Memory <NUM> includes a non-transitory machine readable medium and includes a plurality of program modules <NUM> having instructions recorded thereon that, when read by processor <NUM>, cause processor to undertake certain actions, e.g., steps of a method <NUM> (shown in <FIG>) for controlling connectivity of HESM system <NUM> to power bus <NUM>. More particularly, controller <NUM> is configured to open and close short circuit switch <NUM> and short circuit switch <NUM> according to switch states <NUM>-<NUM> (shown in <FIG>). More particularly, based on whether an input <NUM>/<NUM>/<NUM> is received at interface <NUM>, controller configures HESM system <NUM> in (a) absorb/source mode <NUM>, (b) a short detected mode <NUM>, (c) a discharge mode <NUM>, or a (d) balance mode <NUM>.

With reference to <FIG>, switch states of HESM system <NUM> are shown for (a) absorb/source mode <NUM>, (b) a short detected mode <NUM>, (c) a discharge mode <NUM>, or a (d) balance mode <NUM>. Absent receipt of input <NUM>/<NUM>/<NUM> at interface <NUM> controller configures HESM system <NUM> in absorb/source mode <NUM>. In the absorb/source mode <NUM> short circuit switch <NUM> and short circuit switch <NUM> in an electrically closed state. Closure of short circuit switch <NUM> places ESM <NUM> in electrical communication with positive rail <NUM> and neutral rail <NUM> to absorb/source current pulses flowing through positive rail <NUM>, thereby regulating voltage across positive rail <NUM> and neutral rail <NUM>, HESM system <NUM> thereby actively filtering of split HVDC power bus <NUM> by sourcing and absorbing current pulses from HVDC power bus <NUM> as required during operation.

When a shorted detected input <NUM> is received at interface <NUM> controller <NUM> configures HESM system <NUM> in short detected mode <NUM>. More particularly, in short detected mode <NUM>, controller <NUM> electrically opens both short circuit switch <NUM> and short circuit switch <NUM>. Opening short circuit switch <NUM> and short circuit switch <NUM> disconnects both ESM <NUM> and ESM <NUM> of first power stage <NUM> and second power stage <NUM>, limiting (or eliminating entirely) current flow from HESM system <NUM> when a short circuit is detected on power bus <NUM>. This can be done, for example, by applying a pulse-width modulated signal from controller <NUM> such that controller <NUM> operates as a hysteresis current regulator. For example, controller <NUM> can regulate current flow of a predetermined amount for a predetermined duration prior to opening the short circuit switches following an unrecoverable fault, e.g., <NUM> amps for <NUM> seconds).

When a discharge input <NUM> is received at interface <NUM> controller <NUM> configures HESM system <NUM> in discharge mode <NUM>. More particularly, controller <NUM> discharges one of first power stage <NUM> and second power stage <NUM> through the other of first power stage <NUM> and second power stage <NUM> by operating of short circuit switch <NUM> (shown in <FIG>) and short circuit switch <NUM> (shown in <FIG>). For example, when discharge input <NUM> indicates that ESM <NUM> of first power stage <NUM> is to be discharged controller <NUM> electrical opens short circuit switch <NUM>, disconnecting ESM <NUM> from power bus <NUM>, and electrically closes short circuit switch <NUM>, connecting ESM <NUM> to power bus <NUM>. As will be appreciated by those of skill in the art in view of the present disclosure, opening short circuit switch <NUM> while short circuit switch <NUM> is closed causes the discharge of ESM <NUM> of first power stage <NUM> through power filter <NUM> of second power stage <NUM>.

Similarly, when discharge input <NUM> indicates that ESM <NUM> of second power stage is to be discharged, controller <NUM> electrically closes short circuit switch <NUM> and electrically opens short circuit switch <NUM> to discharge ESM <NUM> through power filter <NUM> of first power stage <NUM>. Under either condition it is contemplated that controller <NUM> cause the ESM associated with discharge input <NUM> to discharge substantially in full through power bus <NUM>, such as for maintenance by way of non-limiting example.

When an overcharge detected input <NUM> is received at interface <NUM> controller <NUM> configures HESM system <NUM> in balance mode <NUM>. More particularly, controller <NUM> discharges one of first power stage <NUM> and second power stage <NUM> through the other of first power stage <NUM> and second power stage <NUM> by operating of short circuit switch <NUM> (shown in <FIG>) and short circuit switch <NUM> (shown in <FIG>). For example, when overcharge detected input <NUM> indicates that ESM <NUM> of first power stage <NUM> is overcharged, controller <NUM> electrical opens short circuit switch <NUM>, disconnecting ESM <NUM> from power bus <NUM> and electrically closes short circuit switch <NUM> to connect ESM <NUM> to power bus <NUM> to connect to ESM <NUM> of first power stage <NUM> through power filter <NUM> of second power stage <NUM>. It is contemplated that, responsive to receipt of overcharge detected input <NUM>, HESM system <NUM> retaining the switch settings for a relatively limited time interval, i.e., a time interval sufficient to return the charge of ESM <NUM> to within a predetermined charge limit and to not fully discharge ESM <NUM>. Overcharge detected input <NUM> can be provided, for example, via a sensor <NUM> (shown in <FIG>), coupled to ESM <NUM> and disposed in communication with controller <NUM>, and/or a sensor <NUM> (shown in <FIG>) coupled to ESM <NUM> (shown in <FIG>) and disposed in communication with controller <NUM>.

Similarly, when overcharge detected input <NUM> indicates that ESM <NUM> of second power stage is overcharged, controller <NUM> electrically closes short circuit switch <NUM> and electrically opens short circuit switch <NUM> to discharge ESM <NUM> through power filter <NUM> of first power stage <NUM>, as shown in (d) balance mode <NUM> in table <NUM> (shown in <FIG>). As above, it is contemplated that, responsive to receipt of overcharge detected input <NUM>, HESM system <NUM> retain the switch settings for a relatively limited time interval, i.e., a time interval sufficient to return state of charge of the respective ESM to within a predetermined limit and to not fully discharge ESM <NUM>.

With reference to <FIG>, a method <NUM> of controlling connectivity of an HESM system, e.g., HESM system <NUM> (shown in <FIG>), to a power bus, e.g., power bus <NUM> (shown in <FIG>), is shown. Method <NUM> includes absorbing/sourcing current pluses to the power bus with a first power stage and a second power stage, e.g., first power stage <NUM> and second power stage <NUM>, to regulated the power bus, as shown by box <NUM>. Absorbing/sourcing pulses to the power bus can include closing both a first power stage short circuit switch and a second power stage short circuit switch, e.g., first power stage short circuit switch <NUM> and second power stage short circuit switch <NUM>, as shown in (a) absorb/source mode <NUM> in table <NUM> (shown in <FIG>). Absorbing/sourcing can occur while the HESM system actively regulates current flow through the power bus, the HESM system regulating current flow through the power bus as a hysteresis current flow regulator.

Method <NUM> also includes disconnecting the HESM system from the power bus upon detecting a short circuit on the power bus, as shown by box <NUM>. Disconnecting can include disconnecting the first power stage and the second power stage upon detection of a short circuit on the power bus, as shown in (b) short detected mode <NUM> in table <NUM> (shown in <FIG>). For example the first power stage short circuit switch and/or the second power stage short circuit switch can be opened as shown by box <NUM>.

Method <NUM> additionally includes balancing SOC within the HESM system by determining whether the first power stage is overcharged, as shown with box <NUM>. Balancing SOC within the HESM system includes connecting the first power stage to the power bus through the second power stage, as shown with box <NUM>. In this respect the second power stage short circuit switch is opened and the first power stage short circuit switch closed such that the first power stage is connected to a negative rail, e.g., negative rail <NUM> (shown in <FIG>), of the power bus through the second power stage, as reflected in the second row of (d) balance mode <NUM> in table <NUM> (shown in <FIG>).

Balancing SOC the HESM system can include connecting the second power stage to the power bus through the first power stage, as shown with box <NUM>. In this respect the first power stage short circuit switch can be opened and the second power stage short circuit switch closed such that the second power stage is connected to a positive rail, e.g., positive rail <NUM> (shown in <FIG>), of the power bus through the second power stage, as shown with box <NUM>, as reflected in the first row of (d) balance mode <NUM> in table <NUM> (shown in <FIG>). It is contemplated that the balancing SOC include partially discharging one of the first and second power stage to within a predetermined limit to prevent overcharging of an ESM to accumulate such that the ESM need be taken off-line for maintenance.

Method <NUM> further includes discharging either (or both) the first power stage and second power stage, in full, as shown with box <NUM>. Discharging either (or both) the first power stage and the second power stage includes connecting the first power stage to the power bus through the second power stage, as shown with box <NUM>. In this respect the second power stage short circuit switch is opened and the first power stage short circuit switch closed such that the first power stage is connected to the negative rail of the power bus through the second power stage, as indicated in the second row of (c) discharge mode <NUM> in table <NUM> (shown in <FIG>).

Discharging either (or both) the first power stage and the second power stage can also include connecting the second power stage to the power bus through the first power stage, as shown with box <NUM>. In this respect the first power stage short circuit switch can be opened an the second power stage short circuit switch closed such that the second power stage is connected to the positive rail of the power bus through the second power stage, as shown with box <NUM>, as indicated in the first row of (c) discharge mode <NUM> in table <NUM> (shown in <FIG>). It is contemplated that the balancing state of charge include fully discharging at least one of the first and second power stage.

Claim 1:
A hybrid energy storage module, HESM, system, comprising:
a first power stage (<NUM>) with a first short circuit switch (<NUM>) to connect the first power stage to a DC power bus, the first power stage connected between a positive rail (<NUM>) and a neutral rail (<NUM>), the first power stage comprising:
a first energy storage module, ESM (<NUM>);
a first direct current/direct current, DC/DC, converter (<NUM>) connected in series to the first ESM;
a first power filter (<NUM>) connected in series to the first DC/DC converter; and
a first neutral lead;
a second power stage (<NUM>) stacked in series with the first power stage and with a second short circuit switch (<NUM>) to connect the second power stage to the power bus, the second power stage connected between a negative rail (<NUM>) and the neutral rail (<NUM>), the second power stage comprising:
a second ESM;
a second DC/DC converter connected in series to the second ESM;
a second power filter connected in series to the second DC/DC converter; and
a second neutral lead, the first neutral lead connecting the first power filter to the second power stage, the second neutral lead connecting the second power filter to the first power stage, wherein the first power filter includes a switch circuit having the first short circuit switch (<NUM>), a positive ESM lead (<NUM>), a negative ESM lead (<NUM>), and a diode arranged to oppose current flow from the positive ESM lead to the negative ESM lead; and
a controller (<NUM>) operably connected to the first short circuit switch (<NUM>) and the second short circuit switch (<NUM>), characterized in that:
the controller has a state of charge, SOC, balancing mode to discharge one of the first power stage and the second power stage by connecting one of the first and second power stages through the other of the first and second power stages with the other of the first and second power stages disconnected from the power bus.