Battery Charging and Management on a Board
A battery charger is a power converter with a measurement problem attached. It must deliver current safely, measure the battery’s condition accurately and stop at the right moment, and each of those depends on the layout.
The Charge Profile
The profile is usually constant current followed by constant voltage. The current is limited while the battery is below its target voltage, and the voltage is then held while the current decays.
The transition point and the termination current determine how full the battery becomes and how long it lasts. Terminating too early leaves capacity unused, and terminating too late stresses the cell.
The profile depends on the chemistry and on the cell, and it must be implemented as specified rather than as a simple voltage limit. Our component reliability notes describe how the cell’s limits are collected.
Where the Current Is Measured
The charge current is measured across a shunt, and the sense connections must be Kelvin to exclude the trace and joint resistance. The measurement is used to control the profile, so its accuracy directly affects the charge.
Where the current is large, the shunt dissipates heat that must leave the board. The shunt’s own temperature coefficient then affects the measurement, so the thermal design and the measurement accuracy are connected.
The sense traces should be short, symmetric and routed away from the switching node of the charger. A measurement corrupted by switching noise produces a charge that varies with the load on the system. Our thermal design notes describe the copper around the shunt.
Thermal Behaviour
The charging current is high and the voltage difference between the input and the battery can be large, so the charger dissipates significant power. The thermal path from the package to the board is often the limiting factor.
The charger’s thermal regulation feature reduces the current when the die temperature reaches a limit. It protects the part and it also lengthens the charge, which must be accounted for in the specification.
The battery itself is a heat source during charging, and a battery placed against the charger or against a hot component will be warmer than the ambient and will age faster. The layout should separate them. Our industrial assembly notes describe the environmental limits applied to such products.
Protection Functions
The charger must detect a battery that is inserted in reverse, a battery that is too deeply discharged, and a battery whose temperature is outside the allowed range.
A deeply discharged cell requires a small conditioning current before the normal profile begins, and the charger must recognise the condition rather than treating it as a fault.
The temperature measurement uses a thermistor on or near the cell, and the thermistor’s position is part of the safety design. A thermistor that measures the board rather than the cell does not detect a hot cell.
Fuel Gauging
The state of charge is estimated from the voltage, from the current integrated over time, or from a combination. Voltage based estimation is simple and inaccurate under load, while current integration is accurate and drifts over time.
The gauge’s accuracy depends on the current measurement and on the cell’s characteristics being known. The measurement circuit is the same shunt as the charger, which is the reason the two functions are often integrated.
The gauge must also account for the temperature and the age of the cell, since a cell’s capacity falls with both. Our quality notes describe the calibration that is performed during production.
System Load and Charging Together
Where the product operates while charging, the charger’s current must supply both the system and the battery. The current limit applies to the sum, so the battery charges more slowly.
The measurement of the battery current must therefore exclude the system current, which requires the shunt to be in the battery branch rather than the input branch.
The thermal design must accommodate the sum of the losses, which is higher than either condition alone.
Verification
The verification is a charge and discharge cycle at the temperature extremes, with the current, the voltage and the cell temperature recorded.
The profile should be compared against the specification at each stage, and the termination should be checked at the specified current.
The thermal behaviour should be measured at the worst case: the maximum input voltage, the maximum charge current and the maximum ambient. The charger’s case temperature and the cell temperature are both recorded.
Additional Considerations for This Build
Practical attention to battery charging pays for itself here, because it decides whether the finished board behaves as the drawing intended. Where the requirement is not stated on the fabrication drawing or in the assembly notes, the shop has to assume a default, and that default is rarely the value the design was simulated with. Stating battery charging explicitly, together with the tolerance that applies, removes the assumption and keeps the result predictable from batch to batch.
Process Control and Verification
On a design of this kind, fuel gauge is the item that decides how the rest of the board is arranged. Where a value sits close to a process limit, the drawing should say so, since the shop can then open the process window rather than working to a nominal figure that carries no tolerance. A stack-up that is drawn rather than described removes most of the ambiguity from a quotation, and it lets the fabricator price the board against the dielectric and copper weights that will actually be used.
Where the requirement is not written down, the shop supplies its own default, and the default is chosen for the process rather than for the design. The measurements that matter are the repeatable ones: conductor width and spacing, annular ring, finished hole size, plating thickness and surface finish are all verifiable on a coupon that travels with the panel.
Running a first article through the same checks as the production panel confirms that the two agree, and that comparison is the cheapest form of process control available at prototype stage.
Process Control and Verification
On a design of this kind, fuel gauge is the item that decides how the rest of the board is arranged. Where a value sits close to a process limit, the drawing should say so, since the shop can then open the process window rather than working to a nominal figure that carries no tolerance. A stack-up that is drawn rather than described removes most of the ambiguity from a quotation, and it lets the fabricator price the board against the dielectric and copper weights that will actually be used.
FAQ
Can a charger be designed without a fuel gauge? It can, and the user then has only a voltage reading, which is inaccurate under load.
Does the charger need its own protection if the battery has one? The cell’s protection is a last resort and it should not be relied on for normal operation. The charger must operate the cell within its limits.
What does gopcb provide for battery products? We provide charger layout with Kelvin current sensing, thermal design for the charger and separation from the cell, protection functions with the thermistor placed on the cell, fuel gauge calibration during production, and charge and discharge cycles at the temperature extremes.
