The engineering decision
Provide a defined path from source to load and battery, with charge control, reverse-current behavior and source-loss response specified.
Have these inputs ready
- Source and charger ratings
- Common-port or separate-port BMS schematic
- Load profile during charging
- Required switchover and battery acceptance limits
The battery carries the net current
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Balance current at one node, power across converters
At a common DC node, current entering from the source splits between the load and battery. If the source cannot meet the load demand, the battery supplies the difference when the architecture permits it. Current direction is defined at the battery terminals; charging and discharging are not two independent simultaneous terminal currents.
Across a converter, use power and conversion efficiency. A 10 A input rating at one voltage cannot be subtracted directly from a 10 A load at another voltage. Source current limiting and thermal derating can further reduce the power available to replenish the battery.
In the power expression, η applies only to the source converter, and Pload is measured at its output node. A separate battery converter needs its own efficiency and direction-dependent power balance.
A running load changes recharge time
Consider an instantaneous shared-node example at 48 V: 20 A entering the node and 12 A drawn by the load leave 8 A toward the battery branch. A 25 A load instead requires 5 A from the battery. The actual charge controller must enforce the selected battery voltage and current profile; this node balance is not a fixed 48 V charging specification.
For 20 Ah of replenishment at a steady net 8 A, the constant-current arithmetic gives 2.5 hours. Current taper during constant-voltage charging, battery temperature limits, balancing and changing load extend or alter the actual recharge time. Check whether the available docking window reaches the required next-mission SOC.
| Source current | Load current | Battery current | State |
|---|---|---|---|
| 20 A | 12 A | +8 A | Net charging |
| 20 A | 20 A | 0 A | No replenishment |
| 20 A | 25 A | −5 A | Battery supplements source |
| 0 A | 12 A | −12 A | Battery supplies load |
Give every operating state a defined path
| Function | Question to resolve | Failure if omitted |
|---|---|---|
| Charge termination | Does the charger measure battery current separately? | Load current can mask taper |
| Reverse blocking | Can battery energy enter an inactive source? | Unexpected backfeed |
| BMS disconnect | Which port opens and what remains powered? | Loss of load or loss of charge control |
| Source removal | Is the transition within equipment hold-up time? | Controller reset |
| Battery unavailable | Can the source carry startup and peak load? | Bus collapse |
Exercise transitions, not only steady charging
Power-path charger documentation demonstrates separate system supply and battery supplement behavior, but those functions are device-specific. A common-port BMS does not by itself implement load sharing or uninterrupted transfer. Trace charge and discharge switching states through the complete schematic.
Test source connection and removal at low battery SOC and maximum load. Also test a full battery, a battery that blocks charging because of temperature, and loss of the battery path. Observe minimum and maximum bus voltage and check charger recovery.
If operation must continue without interruption, specify an allowable voltage-time excursion and an explicit energy reserve. The equipment hold-up requirement determines whether capacitors, battery supplement or another conversion stage is necessary.
Measurement & validation procedure
| Test / review | Record | Acceptance condition |
|---|---|---|
| Verify current accounting | Source, load and battery current with sign convention | Measured balance matches each state |
| Check source transitions | Bus minimum/maximum and interruption duration | Equipment remains inside its specified input envelope |
| Check charge completion | Battery current and termination status under load | Termination and restart follow the approved charging profile |
Continue this engineering task
Related engineering guidance
Prepare the evidence for review
- Power-path schematic
- Charger model and configuration
- Docked load and recharge window
- Allowed bus-voltage interruption
Technical sources & claim support
References support the stated engineering principles. Worked examples use their explicitly stated assumptions.
- Texas Instruments BQ25798 data sheet ↗
Power-path management; battery supplement mode; charge termination
Low-voltage power-path example covering load sharing, charge termination and battery supplement mode; apply the device voltage and current limits.
- Texas Instruments BQ76952 data sheet, SLUSE13B ↗
Sections 7 and 10–12: electrical characteristics, measurement, protection and FET control
Example of a monitor/protector architecture; capabilities depend on the selected IC and complete circuit.
Key claims and evidence
| Claim | Basis | Reference |
|---|---|---|
| Power-path chargers can supply a system and allow battery supplement | documentation | Texas Instruments BQ25798 data sheet |
| Net current follows current conservation at a shared node | calculation | calculation |
| A BMS port arrangement alone does not establish uninterrupted transfer | inference | Texas Instruments BQ76952 data sheet, SLUSE13B; Texas Instruments BQ25798 data sheet |