What Is Power Path Management?

Detailed Explanation

A simple lithium-ion charging circuit — a charger IC between an input source and a battery, with the system load connected directly to the battery — works well for products that are either charging (put down, not in use) or running on battery (unplugged and portable), but rarely both in the same moment. Many products don't fit that assumption: an always-on security camera, a Wi-Fi gateway, a medical monitor, or anything with a display or radio that must keep working the instant it's plugged in, while also charging its battery for the next time it's unplugged.

Power path management is the circuit topology that makes this work correctly. This page covers what it does, how it's typically implemented, and when a product actually needs it. For the CC/CV charging process itself — which power path management sits alongside, not in place of — see how do lithium-ion batteries charge?

The Problem Power Path Management Solves

Consider a naive circuit: input source → charger IC → battery, with the system load also connected directly to the battery node. Two problems emerge as soon as the system runs while the input source is connected:

1. The system load and the charger compete for the same input current budget. A USB port might supply a maximum of 500 mA or 900 mA. If the system load alone draws 300 mA and the charger is programmed for 500 mA, the combined demand exceeds what the port can supply — the input voltage sags, and both the system and the charger see a degraded, unpredictable supply.
2. The charger's CC/CV regulation is measuring current and voltage at the battery node, which the system load is also pulling from. The charger can't reliably reach its constant-voltage phase and termination current threshold if the system load's current draw is superimposed on the same node it's trying to regulate — in the worst case, charging never terminates because the current the charger sees never drops below the termination threshold, even though the battery itself is genuinely full.

How Power Path Management Works

A power path IC (examples: TI BQ24075, BQ25895, Microchip MCP73871, and similar) sits between the input source, the battery, and a separate system output pin that feeds the system load directly — not through the battery node:

1. When external power is present, the IC powers the system load directly from the input (through a low-impedance FET or ideal-diode path), and separately manages charge current into the battery from whatever input current budget remains after the system load's needs are met. The system load takes priority — if the input source's total current limit is tight, the charger current is reduced first, protecting the system rail from browning out.
2. When external power is removed, the same IC seamlessly switches the system output to draw from the battery instead, typically with no discontinuity the system firmware needs to handle.
3. Reverse current is blocked so that when both a battery and an input source are present, current doesn't flow backward into the input source from the battery, and the battery doesn't get inadvertently charged or discharged through an unintended path.

This is sometimes implemented with a literal Schottky "ideal diode" (a MOSFET switched to behave like a diode but with far lower forward voltage drop and power dissipation than an actual diode) for the system-to-battery path, combined with a separate current-limited charge path into the battery — hence "power path."

Instant-On Behaviour

A well-designed power path circuit provides instant-on behaviour: the system can power up and run immediately from the input source even if the battery is completely dead, fully disconnected, or in a fault state — because the system output doesn't depend on the battery being charged or even present. This is a meaningful reliability property for products where "plug it in and it should work immediately" is a hard requirement (a security camera that must come up on external power even with a failed battery, for example), not just a convenience feature.

Design Considerations

• Confirm the product genuinely needs power path management before adding it. If the product is realistically either charging (idle, not in active use) or running on battery (unplugged), a simple charger IC plus battery — without a separate power path — is simpler, cheaper, and sufficient. Reserve power path circuitry for products that must actually operate continuously through external power connect/disconnect events.
• Size the system output path for the worst-case combined current. Even with power path management, the input source still has a maximum current budget; verify the chosen power path IC's system-output current rating and input current limit are adequate for the actual peak system load, independent of whatever charge current is also being drawn.
• Check the IC's behaviour when input current is insufficient for both loads. Most power path ICs prioritise the system load and throttle charge current automatically, but confirm this in the specific datasheet — some ICs instead reduce system voltage regulation under a starved input, which may not be acceptable for a system rail feeding sensitive analog or RF circuitry.
• Instant-on requires the system rail to be independent of battery state. Verify in the datasheet that the system output remains available with the battery removed, deeply discharged, or in a protection-triggered fault state, if instant-on behaviour on a dead or missing battery is a genuine product requirement.
• Power path management is usually paired with, not a replacement for, a battery protection circuit. Power path ICs manage normal-operation current routing; protection circuitry handles fault conditions (overcharge, overdischarge, short circuit) separately. Confirm which faults the chosen power path IC handles natively versus which still require a dedicated protection IC.
• Products that must run continuously through power connect/disconnect events — always-on gateways, security devices, medical monitors — benefit from power path topology decided at the schematic stage rather than retrofitted onto a simple charger-and-battery design. Zeus Design's hardware engineering team designs complete battery power management subsystems, including power path circuits, for commercial products.

Common Mistakes

• Connecting the system load directly to the battery node alongside the charger, without a dedicated power path IC, for a product that must run while charging. This is the root cause of both the input-current-contention and charge-termination problems described above.
• Assuming any charger IC provides power path management. Many popular single-cell charger ICs (the TP4056, for example) have no system output pin at all — the battery node is the only output. Confirm the specific IC's datasheet block diagram shows a separate system output before assuming power path behaviour is included.
• Undersizing the input current budget for the combined system-plus-charge load. A power path IC correctly prioritising the system load doesn't create current from nothing — if the input source's actual maximum current is lower than the system's peak draw, the system will still brown out regardless of how the power path IC arbitrates the remaining budget for charging.
• Overlooking reverse-current leakage in a poorly chosen ideal-diode implementation. A cut-rate or misapplied ideal-diode circuit can leak a small reverse current into the input source when it's absent or at a lower voltage than the battery, slowly draining the battery through a path that was supposed to be blocking. Use a power path IC or ideal-diode controller specifically designed and datasheet-verified for this reverse-blocking function.
• Not verifying instant-on behaviour under the specific fault conditions the product needs to survive. A product marketed as working "even with a dead battery" needs that behaviour verified against the actual chosen IC's datasheet — some parts require a minimum battery voltage present before the system output becomes available, which defeats a true instant-on requirement.