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PowerBrick

Purpose

The purpose of the PowerBrick is providing power for portable devices; these days most of them are charged from USB, or at least from 5 volts (same voltage, different connector). And in some cases it is needed to power several power-hungry devices, typically smartphones and cameras, for prolonged time. For example when recording some outdoor events.

Construction

The PowerBrick is built around a 12V/5Ah lithium-ion polymer battery obtained from HobbyKing. This amount of power should be sufficient for most common needs; after converting it down to 5 volts it equals roughly 10 amp-hours of a 5-volt battery.

For maintaining high efficiency, the power circuit is realized as a buck converter, using a LM2576 chip, a 3-ampere step-down regulator, the adjustable version. (Handy chips; can be used as both voltage and current controlled power supplies, and are inexpensive when bought in semi-bulk over eBay. Worth having a stock.)

As most of current gadgets are charged or chargeable from USB, the USB-A connectors were chosen as the interface. Total of six is available. The connectors are set in the box with hot-melt adhesive. Each connector has its own LED indicating output voltage presence. The connectors are marked with red and blue to indicate the positive and negative side.

Two of three of the output pairs are fused with a 1.6-amp Polyswitch resettable fuse. However short-circuit tests suggest it is optional, as the chip shuts down on short circuit immediately, and on mere overload (tested with a 1-ohm resistor) the fuse reacts fairly late.

As the LM2576 can heat up when under longer-term higher-current load, it is mounted on a heatsink, realized as an aluminium strip along one of the device's long sides.

To prevent unpleasant surprises, as the battery is capable of giving over 100 amps of current and a short circuit could lead to fire and molten copper, the battery lead is fused with a 5-amp resettable fuse.

The output voltage for the ADJ version of the chip is set by a resistor divider. For fine tuning a trimpot is used to adjust the output between 4.3-5.8 volts. The location of the trimpot - a mechanical part subject to malfunction - was chosen in such way that increase of its resistance or total loss of electrical contact would lead to undivided output voltage appearing on the feedback pin, therefore dropping the output voltage down to about 1.2 volts, which should be safe for the attached devices.

The output voltage is set to 5.25 volts, in order to be within the USB charging specs, in the zone that indicates to the charged device that high current is available. The D+ and D- lines of each USB connector are shorted, according to the same spec.

As some of the possible failure modes involve letting the full input voltage through, a 5.8V transil is added to the output. In case of such failure it should protect the attached devices from damage. (There is a suspicion that the transil is underpowered for the application in question, and that it may fail if left tripped for longer time. An SCR-based crowbar may be more apt here.)

To charge the battery, a 7-pin DIN connector is used. The edgemost pins are connected to pairs to handle higher current. The three remaining pins are used for battery balancing; the two inner-cell interconnections, labeled white and yellow, are made accessible. The third pin is unused and reserved for devices with 4-cell batteries.

The charging connector can be also used for monitoring the battery against overdischarge (a serious issue with this design). Another possible use is directly tapping the 12V output on the battery, for powering 12V-requiring devices.

Mk1

An older model of PowerBrick was built earlier. As the time was the main limiting factor, shortcuts were taken. A LM2575-5 was used, limiting the output power dramatically to only 1 ampere (mistake done by attempting to avoid having to tune the resistor divider and using a fixed-voltage variant and forgetting that LM2575 is limited to only 1 amp of output). The heatsink was also dramatically underdimensioned and located inside of the case, leading to localized deformation of the plastic case during field operation.

The connectors were all located on the front panel, which proven to be unsuitable for carrying in pocket, as the attached cables would take too much space. For Mk2 model the charger and one of the USB connectors were moved to the opposite side of the box, and only two USB connectors were left on the top side.

Images


Mk1, inside view

Mk1, detail of converter

Mk1, detail of converter

Mk1, detail of charging connector

Mk1, top view

Mk1, top view, with charging monitor

Mk1, schematics

Heat damage of the case (heatsink from the other side)

Mk1-to-Mk2 conversion

After problems were encountered during field use, the unit was redesigned and upgraded.

The LM2575-5 chip was exchanged for a LM2576-ADJ one, boosting the output power to 3 amps. The heatsink was significantly enlarged to avoid overheating issues.

One of the USB connectors was moved to the side of the device, to allow its easier accessibility when transporting the brick in e.g. a pocket. The charging connector was also moved to the device side, opposite to the USB one, in order to facilitate easier use of the box as a 12V power supply.

The remaining holes in the case were backed with a piece of cardboard and filled with hot-melt adhesive.

Overdischarge protection

The battery used is "naked", without an undervoltage cutoff. As such, overdischarge, with a subsequent irrecoverable damage, is a serious risk. For field use an undervoltage cutoff was designed.

A full-scale battery protection circuit was sadly unavailable. As both time and money were in shortage, a field-expedient solution with one FET was chosen. The undervoltage cutoff is usually about 2.5 volts for the common boards; in multi-cell batteries the lowest-voltage cell triggers the cutoff. The battery must not drop below 2 volts, and below about 3.3-3.5 volts it does not hold significant amount of energy anymore. This is a fairly wide range. Many FET types have the Vgs threshold-saturation range conveniently in this range. An IRF630 N-FET was chosen for its suitable voltage, relatively low cost, and immediate availability.

The FET is connected in the negative line. Its gate is connected via a 10 kΩ resistor to the output of the first battery cell; the role of the resistor is to protect the battery in case of gate oxide breakdown, limiting the worst-case current to below half-milliamp - that may still discharge and destroy the one cell but it will at least not be a smoke-fire surprise. The other role of the resistor, in combination with the gate capacitance, is to smooth the transitions and prevent oscillations.

Use of the FET is not optimal; instead of a sharp cutoff the FET acts as a variable resistor, changing from 0.4 Ω (fully open, sense-cell voltage over 3.2 volts) to infinity (cell voltage below 2.8 volts) over a range of resistances, potentially wasting energy and generating heat while gradually throttling down the available current as the cell voltage falls.

The location of the threshold voltage fairly high above the minimum, and even above the usually employed cutoff voltage, trades a relatively negligible amount of battery capacity for wider safety margin, as the cells aren't usually created equal and the second and third cell's voltage could be lower than the sense-cell's one.

The overdischarge protection does not work for loads connected directly to the charging connector. (TODO: rectify this.)


Overdischarge FET characteristics, measurement circuit

TODO

Images


Inside, electronics

Detail of electronics

Detail of charging connector

Inside view

Detail of electronics

Battery

Battery

Inside, fully assembled

Inside, fully assembled, battery connected

Inside, fully assembled, battery connected

Outside view, unpainted

Outside view, unpainted

Outside view, charging connector detail

Outside view, charging connector detail

Outside view, side USB port detail

Outside view, bottom side

Outside view

Outside view

Outside view

Outside view

Overdischarge transistor heatsink access

Overdischarge transistor heatsink hole

Overdischarge parts

Overdischarge FET in place

Overdischarge FET in place

Overdischarge FET in place

Overdischarge FET in place

Overdischarge FET in place

Reassembled unit

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