⏸️ Building: paused battery-management-system bms electronics power radio

radio-powersupply

for AIAA-UH | Amateur Radio Club: designing a BMS for a portable Ham-Pack with more accessories.

selected battery size and BMS. just double checking that it satisfies criteria when i get the chance.

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GitHub README

Ham Radio Power Supply

A custom battery pack power supply project for portable ham radio operations, featuring a 3S3P lithium-ion configuration designed to power the Retevis RT95 dual-band mobile ham radio. Thank you AIAA@UH and Shane :D

What's Included

Core Power System

  • Battery Configuration: 3S3P lithium-ion pack (12.6V, 15Ah)
    • 9x BAK N21700CG-50 cells (5Ah per cell, 15A max continuous discharge)
    • Provides 10.35Ah minimum capacity
  • Battery Management System (BMS): 3S-20S 30A BMS with cell balancing
    • 30A continuous discharge, 60A burst capability
    • 50mA balancing current per cell
    • Overcharge protection (4.2V/cell), overdischarge protection (2.8V/cell)
  • Wiring: 16 AWG for safe current handling
  • Charging: 9A Li-ion 3S charger (12.6V CC/CV) for ~1.1 hour charge time (TBD)

Target Radio

  • Model: Retevis RT95
  • Power Requirements: 1.735A average, 6A peak (12A recommended capacity), 13.8V DC ±15% operating voltage
  • Weight: 0.64 kg
  • Dimensions: 124mm (W) × 163mm (D) × 39mm (H)

Optional Accessories

  • 2.4" Sunlight-Readable TFT Display (TBD): To decorate the hampack!
    • Integration via NHD development board with ribbon cable
    • Perfect for outdoor operations and battery status display

KiCad Design Plans

This project uses KiCad for circuit design, PCB layout, and simulation:

Circuit Simulation

  • Power Distribution Simulation: Just for fun, exploring circuit behavior and validation
    • Verifying voltage regulation under various load conditions
    • Modeling discharge characteristics and runtime estimates
    • Testing protection circuit responses (BMS, fuses, etc.)
    • Learning experience with KiCad's simulation capabilities

The hampack/ directory contains all KiCad project files including schematics, PCB layouts, and custom symbols.

Project Status

Currently in progress. Next steps include:

  1. Finalizing component sourcing
  2. Designing the TFT display interface Schmematic and PCB in KiCad
  3. Running circuit simulations for validation
  4. Assembly and testing

Safety Notes

  • BMS balances all cells for safe operation
  • System includes overcharge/overdischarge protection
  • Proper fusing and wire gauge for current handling
  • Operating within manufacturer-specified voltage range (13.8V DC ±15%)

For detailed calculations and component specifications, see DESIGN.md

Project Journal

Initial BMS Design

Dec 21, 2025

Initial BMS Design

open journal page open source file

Design/Documentation

Due Date: December 19, 2025 → December 20, 2025 Projects: Ham Radio Powersupply (https://www.notion.so/Ham-Radio-Powersupply-2ccc50d201c2803da44de30d1e2cf3a4?pvs=21) Status: Ready for review

About the Radio

ModelRT95
Dimensions (mm)124 (W) x 163(D) x 39 (H)
Weightabout 0.64 kg
Manualright here!

Requirements

Average Amperage1.735 A
Peak Amperage6 A
Recommended Current Capacity12 A
Minimum A/H Capacity10.35 Ah
A/H Capacity + Reserve20.7 Ah
Operating Voltage13.8V DC ±15%

BMS must balance all cells. Pick BMS rated at least 12 A,

Pick a charger that matches the pack voltage and the charge current

Assumptions

To check max continuous and/or peak discharge (greater than or equal to 12 A), check cell discharge rating x parallel count, BMS continuous current rating, wire/fuse ratings.

Calculation

Configurations

BMS$16.00
Overcharge voltage per cell4.2 V
Overdischarge voltage per cell2.8 V
Discharge Continuous Amps30 A
10 second Discharge Burst60 A
Charge Continuous Amps30 A
Balancing Current50 mA
Li-Ion Cell$2.50 per cell
Nominal Voltage3.6 V
Fully Charged Voltage4.2 V
Fully Discharged Voltage2.5 V
Capacity5 Ah
Max Continuous Charge1C / 5A
Max Continuous Discharge3C / 15A

So, I propose a 3S3P (12.6 V, 15 Ah) configuration (with no accessories)

16 AWG

Charger specs
Chem & Series Ct.Li-ion 3S, or 12.6 V CC/CV
Charge Current9 A (~1.3 hr charge time)

t_ideal = [Ah]/[A]

~1.1 h = [10 Ah]/[9 A] | | Power connectors | Mini-Tamiya 2-pin connector |

Fusei didnt realise there alr was a fuse
Current rating10 A
Voltage rating> 12.6 V

Fun Accessories!

TFT Screen (Sunlight Readable)

Links:PriceNotes
TFT Screen$16.64
IDC Cable$20.00
IDC Cable + 2x20 Pin Headers$12.00most likely covers both the idc cable listing above, and the pin headers below.
2x20 Pin Headers$5.00

Note: I need some time to decide what accessories I want to use.

NHD Github profile

Personal Notes

Current Capacity (A)Ability to deliver # A of current at a point in time.
Ah or WhLike volume of fuel tank.
C-Ratethis is trying to tell you the CURRENT.

C-rate is an expression for the allowable charge/discharge current relative to the battery’s capacity.

A_output= [C-Rate]*[Ah Rating]

This is a constraint for how fast the battery can be charged or discharged safely. |