Recently, on many forums, Facebook groups, and the #WattCycle, many users have reported that the WattCycle LiFePO4 batteries with Bluetooth version behaving as if they “take turns” supplying a parallel bus: one battery supplies the load while another appears offline (BMS shows pack voltage lower than bus voltage and 0.0 A discharge). To respond promptly to customer concerns, our engineering team has executed repeatability tests, root-cause analysis and risk assessments, and has prepared interim user guidance and a roadmap for corrective measures. This document is the official technical statement: it clarifies the facts, explains the root cause, provides our safety conclusions, and gives practical operational recommendations. We conclude the behaviour is the expected result of deliberate BMS protection logic (designed to protect individual cells) interacting with parallel system electrical conditions. The behaviour is not a BMS hardware failure and our tests did not identify sustained, dangerous inter-battery charging currents or evidence of thermal/runaway risk.
We will release a firmware/app option (a “parallel high-capacity mode”) to mitigate low-load staggered-wake behaviour by end of September 2025.
Key Takeaway
WattCycle’s BMS intentionally isolates the pack from the external terminals when internal cell state or cell-level protection thresholds indicate charge termination or imbalance. The BMS uses a recovery/hysteresis margin (engineered at ~0.3 V between charge cutoff and re-connection) to prevent rapid repetitive switching near full charge. In parallel systems this protection logic can cause one pack to remain offline until the active pack’s terminal voltage drops enough to meet the protected pack’s re-connection conditions. This is a design trade-off: it prioritizes cell health and safety over enforcing simultaneous terminal participation at all times.
User-reported Key Phenomena
During charging when the pack approaches full state of charge, a WattCycle battery may first enter charge-cutoff (single-cell or pack-level protection triggered). This can result in the other paralleled batteries continuing to be charged to a higher bus/terminal voltage.
After the system switches to discharge, some users observed that the WattCycle battery does not participate in discharge for a short period (app shows “Discharge: Enabled” but measured current = 0.0 A). The pack only “wakes up” and begins discharging after the paralleled pack’s voltage falls by a certain differential.
The phenomenon is most readily observed under the combination: multiple parallel units + low system load (≤ 50 A) + packs at or near full charge. At higher discharge currents (e.g., 100 A, 200 A, 400 A) the system reaches a state in which all packs discharge concurrently more quickly.
Some users are concerned about mismatches between the APP indicators and actual terminal currents, and about the inability to adjust certain protection thresholds.
Technical Cause Explanation
The following explanation is based on WattCycle BMS design principles and engineering test data; it aims to describe, in technical terms, why the above phenomena occur.
1. BMS protection logic and disconnection mechanism
To protect cell longevity and individual cell safety, the WattCycle BMS will disconnect the cell stack from the external terminals via internal power switches (MOSFETs) when it detects single-cell over-voltage risk or when the pack is full while significant per-cell imbalance remains. This action blocks further charge/discharge current paths and prevents rapid repetitive charge/discharge or single-cell over-voltage stress.
When the charge-protection MOSFET is in OFF state, the terminal/bus voltage may be maintained by the external bus or other paralleled batteries while the BMS internal pack voltage (actual cell voltages) is isolated. This leads to the observable discrepancy between the terminal voltage and the voltage reported in the APP.
2. Recovery (re-connect) threshold and hysteresis setting
To prevent frequent on/off switching near full-charge (which would stress cells), the BMS enforces a recovery margin (hysteresis) between the charge over-voltage cutoff and the re-connection threshold of approximately 0.3 V at the cell level. This hysteresis prevents rapid repeated conduction toggles when pack voltages hover near the cutoff point, therefore protecting cell health.
Consequently, even if the external bus voltage is higher than the BMS’s internal pack voltage, the BMS will remain disconnected until the pack voltage falls (through discharge or internal balancing) into the re-connection window. This re-connection process takes longer under light loads.
3. MOSFET behaviour, body diode, and isolation topology
The MOSFET(s) in the BMS are the controlled switches that connect/disconnect the cell stack to the pack terminals. Typical implementations use MOSFETs in back-to-back arrangement to block the intrinsic body-diode conduction and achieve robust bidirectional isolation.
When MOSFETs are fully OFF and back-to-back topology is correctly implemented, conduction from the bus into the pack is blocked except via any small bleed paths or monitoring circuits. If a BMS used a single MOSFET or different topology, a body diode could permit one-way current during OFF, this would present different behaviour. WattCycle packs use MOSFET arrangements and logic tuned to prevent unsafe passive charging while still allowing controlled re-connection.
4. Circuit reasons for the “first-on then wake up” behaviour in parallel systems
When multiple battery packs are paralleled and most are in “charge-OFF” status, the system bus voltage is maintained by the bus or the first battery pack that becomes conductive. Once the first pack turns on and supplies the bus, the other packs that remain in “charge protection not yet recovered” state still have their MOSFETs OFF. For those MOSFETs to Turn ON, a triggering condition must be met — the voltage of the active battery pack must drop to a level close to the MOSFET body-diode forward voltage of the not-yet-awake pack, or the not-awake pack’s internal voltage must fall to its BMS re-connection threshold. This requires time, particularly for large-capacity packs under light load where voltage decays slowly.
Over short intervals, any voltage difference may cause brief equalization currents (tens of amps). In our engineering tests we did not observe sustained, high-magnitude battery pack charging currents(i.e., continuous current from one battery into another that would cause protective trips).
Below is a simplified schematic illustration of a typical battery protection board:
The circuit primarily controls charging protection and discharging protection through Q1 and Q2 connected in series.
When charging paralleled battery packs, as each pack reaches full, the charging MOSFET (Q1) will be turned OFF for that pack — visible in the APP as the “Charging” indicator turning Grey (i.e., charging circuit disconnected). At that moment the charging current path is interrupted.
Charging current path diagram: (Discharging ON, Charging OFF)
Discharging current path diagram:
Please note:
V P+P- = VB+B- - Vab
Normally VQ1 can range approximately 0.7 V to 1.5 V depending on the conduction path (body diode vs. Rds(on) conduction). This is one reason why, when the pack reaches protection, the measured terminal voltage can be lower than the internal battery pack voltage reported by the BMS.
During parallel discharge, one pack’s Q1 MOSFET may open first;
The current path for the pack that first turns Q1 ON is as shown in the diagram:
In that state VB+ to VB- ≈ VP+ to VP- and Q1 conducts with almost no voltage difference.
When a paralleled pack is in Charging OFF state while another is Charging ON, you can observe that the Charging OFF battery pack’s terminal voltage (due to the body diode voltage) will be lower by roughly 0.7V – 1.5V compared with the Charging ON pack. Following the principle that the higher-voltage source supplies first, the Charging OFF pack will not discharge until the Charging ON pack’s terminal voltage drops by at least ≈0.7V, which at point it becomes possible for the Charging OFF pack to begin discharging.
WattCycle Battery Test Data
WattCycle Technical Support Department performed repeatability discharge tests with three WattCycle packs in parallel. Key results are:
The connection method for parallel operation during the test is as follows:
Please special attention to the connection method of the parallel circuits. Connect the system positive (main +) to Battery A positive (+), and connect the system negative (main −) to Battery C negative (−). Do not take both the system positive and system negative from the same battery.
50 A discharge: first pack SOC ≈ 94% → all three packs begin discharging.
| 50A Discharging | |||
|---|---|---|---|
| Discharging current | 1#capacity(%) | 2#capacity(%) | 3#capacity(%) |
| First battery beginning discharging | 100 | 100 | 100 |
| Second battery beginning discharging | 98 | 100 | 100 |
| Third battery beginning discharging | 94 | 97 | 100 |
100 A discharge: first pack SOC ≈ 93% → all three packs begin discharging.
| 100A Discharging | |||
|---|---|---|---|
| Discharging current | 1#capacity(%) | 2#capacity(%) | 3#capacity(%) |
| First battery beginning discharging | 100 | 100 | 100 |
| Second battery beginning discharging | 96 | 100 | 100 |
| Third battery beginning discharging | 93 | 94 | 100 |
200 A discharge: first pack SOC ≈ 94% → all three packs begin discharging.
| 200A Discharging | |||
|---|---|---|---|
| Discharging current | 1#capacity(%) | 2#capacity(%) | 3#capacity(%) |
| First battery beginning discharging | 100 | 100 | 100 |
| Second battery beginning discharging | 97 | 100 | 100 |
| Third battery beginning discharging | 94 | 94 | 100 |
400 A discharge: first pack SOC ≈ 95% → all three packs begin discharging.
| 400A Discharging | |||
|---|---|---|---|
| Discharging current | 1#capacity(%) | 2#capacity(%) | 3#capacity(%) |
| First battery beginning discharging | 100 | 100 | 100 |
| Second battery beginning discharging | 99 | 100 | 100 |
| Third battery beginning discharging | 95 | 96 | 100 |
Test conclusion (high level): transient non-participation by some packs during parallel discharge does occur, but the state does not persist. When the active pack’s terminal voltage and the non-participating pack’s internal pack voltage differ by approximately 0.5 V, the non-participating pack will begin discharging. Higher discharge currents accelerate the transition to all packs discharging concurrently. No sustained inter-pack charging or protective tripping was observed in these tests.
WattCycle's Official Commitment
User safety first: Based on repeated tests and monitored data, the phenomenon is a repeatable, controlled behaviour produced by the BMS protection logic. In our current engineering evaluation we have found no evidence of sustained inter-pack charging currents, protective trips, or direct safety hazards such as thermal runaway.
Impact on cycle life: The BMS recovery margin and disconnect logic are intentionally designed to avoid frequent charge/discharge cycling near full charge; this approach helps extend cell life. However, in heterogeneous parallel systems (different brands, ages or SoC) it may produce short-term user experience differences.
Compatibility recommendation: To reduce the probability of experiencing this behaviour, we strongly recommend against paralleling WattCycle packs with batteries of different manufacturers, different capacities, or visibly degraded/aged units. Prior to paralleling, ensure all units have similar SoC and similar health status.
Suggestions for Users
Do not mix manufacturers/old & new batteries in permanent parallel unless you ensure that the batteries or cells SoC and health conditions of the batteries are compatible.
Commissioning best practice: Before paralleling, charge each pack individually to the same SoC and allow equalization time; confirm cell voltages within acceptable tolerance.
Cable and wiring: Use short, equal-length parallel bus connections and appropriately sized cable and fusing; high series resistance can increase imbalance and complicate equalization.
Charger configuration: Use specifically designed for LiFePO4 charger. Avoid charging voltages far above recommended targets unless specifically required—excessively high charger voltage increases chance of early single-cell cutoff.
If a pack shows 0.0 A while parallel: Temporarily isolate the external pack (open breaker) to let the isolated pack wake on its own, or follow the app sequence below to force participation. Reconnect and observe; expect a short equalization current spike in re-connection.
APP / Engineering Workaround
For three batteries parallel setups our engineers recommend the following APP procedure to bring all batteries into participation more quickly:
- In APP, disable discharge on the currently active pack (battery A).
- Confirm battery B begins supplying the load; then re-enable battery C. Battery A & B should now both be enabled.
- Then disable battery A & B and enable battery C; finally re-enable battery A & B. This sequence tends to get all three batteries into the ON state.
Increasing sustained load helps wake packs: If the paralleled system shows non-simultaneous discharge under low load, temporarily increasing the load (e.g., starting a large load device within allowed limits) will accelerate the transition to multi-pack discharge.
If you suspect abnormalities or a safety issue: Immediately disconnect the parallel connection and contact WattCycle customer service and technical support, and provide APP logs and system monitoring data for engineering analysis.
Official Measures and Following Up Plan
Design intent: WattCycle’s BMS protection logic is designed to prioritise cell safety and service life. Under full-charge conditions the BMS will intentionally disconnect to avoid rapid charge/discharge cycling that can damage individual cells.
Feature improvement plan: We will release a firmware/APP update by the end of September 2025 that includes a “Large-Pack Parallel Mode (Parallel Optimization Switch)” option. This mode is intended to improve the responsiveness of multiple large-capacity packs to discharge simultaneously under low continuous loads. The new release will provide more flexible strategies for parallel operation while maintaining protective behaviours for cell safety.
Technical support and user education: In the short term we will distribute detailed parallel-installation and commissioning manuals to channels and distributors, and publish FAQs and instructional videos on our website and forums to guide correct parallel usage and troubleshooting.
Further verification: On request, the company can provide more detailed test data and engineering documentation; if required, remote or on-site technical assistance can be arranged.
Practical FAQ
Will the isolated pack drain into the active pack and cause overcharge/discharge trips?
Under normal wired conditions and with WattCycle topology, we did not observe sustained large-magnitude inter-pack charging currents that trigger protective trips. Short equalization currents during re-connection are normal; persistent destructive currents were not observed.
Can I switch off BMS protections to force passive behaviour?
No. Basic cell-level protections are safety critical and not user disableable. WattCycle will not provide an option to remove fundamental over-voltage or over-current protections.
Is this unique to WattCycle?
Different vendors use different BMS strategies. The observed behaviour stems from the combination of aggressive per-cell protection, hysteresis settings, and parallel system conditions. Other vendors may implement different thresholds or behavior.
I have mixed packs in my RV / boat—what should I do now?
If possible, avoid long-term mixed parallel operation. If immediate change is impractical, ensure all packs are at similar SoC, monitor system behaviour, and contact WattCycle support for guided troubleshooting.
Conclusion
WattCycle designs BMS logic with cell safety and cycle life preservation as the highest priorities. The parallel behaviour reported on forums and FB group is a reproducible interaction between that protection logic and parallel system electrical conditions. We recognize the operational inconvenience this can cause in certain low-load, mixed-pack deployments and are delivering both documentation and a firmware/APP mitigation to address it. We appreciate user reports and will continue to publish test data and firmware notes to maintain transparency.
