Battery Boot Failure: Root Cause & Firmware Fix
By Arjun K Nair
Introduction
Bloom Tulip and Bloom Lily bonding routers are engineered as robust, multi-WAN, battery-powered devices designed to ensure uninterrupted connectivity. These routers integrate multiple LTE modules and a powerful microprocessor, drawing power from either a 12V DC adaptor or an internal rechargeable battery.
This dual power source operation is essential for portability and backup scenarios, but it introduced unexpected complexities during real-world deployments.
The Problem — Invisible Failure
During rigorous field testing, a critical and perplexing issue emerged: the routers worked perfectly when powered by the 12V DC adaptor, but experienced consistent boot failures when operating solely on battery power. The behavior was consistent:
The router functioned flawlessly when plugged into the wall, indicating that the core hardware components were completely operational.
Why It Worked on Adaptor but Not Battery
12V DC Adaptor
- Provides a continuous, high-capacity power supply.
- Can easily handle sudden, massive spikes in current demand without triggering protection circuits.
Battery with BMS
- The Battery Management System (BMS) strictly regulates power output to prevent damage, overheating, or fire.
- It has a hard limit on the maximum allowable current draw. Exceeding this limit triggers an immediate safety shutdown.
Root Cause Analysis
Extensive debugging of the boot sequence revealed a critical flaw in how the system handled power-up events. It wasn't a hardware failure, but rather an issue of timing and peak power demand.
The microprocessor initializes normally.
The firmware sends a command to power on all 4 LTE modules simultaneously.
Each module draws a significant "inrush current" precisely when it powers up.
The combined spike equals 4× the normal inrush current happening in a fraction of a second.
The battery's BMS detects this massive spike as an overcurrent event (a potential short circuit).
The battery instantly shuts off to protect itself, causing the device to fail mid-boot.
The hardware was fine. The issue lay entirely in the firmware's timing configuration.
Bonding Router — Battery Boot Issue & Fix
The Fix
To solve this, engineering had to choose between redesigning the hardware or intelligently altering the firmware.
| Factor | Hardware Approach | Firmware Approach ✓ |
|---|---|---|
| Method | Increase battery capacity and redesign BMS to handle higher current spikes. | Delay the boot sequence of each module. |
| Complexity | PCB changes required | No hardware changes required |
| Regulations | Re-certification required | No re-certification required |
| Cost | Increased BOM cost per unit | Zero additional cost |
| Time to Market | High (Redesign & Re-validation) | Low (Immediate deployment) |
Chosen Solution: Firmware Fix
We rewrote the boot sequence to implement a staggered startup routine:
Result
Key Takeaway
Understanding the intricate interaction between hardware constraints (BMS current limits) and firmware execution (boot timing) is critical. What initially appeared to be a hardware failure was elegantly solved through precise root cause analysis and an optimized firmware timing adjustment, saving substantial costs and development time while delivering a perfectly reliable product.
Download the Full Case Study
Discover how we resolved a critical battery boot failure in our Bloom Tulip bonding routers. This case study explores the root cause analysis of inrush current spikes and the elegant, zero-cost firmware fix that ensured reliable portable operation without hardware changes.
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