24V 18650 LiIon Battery System

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24V Modular 18650 LiIon Battery System

Release status: beta [box doku]

24v liion battery system.jpg
Description Modular 24V lithium ion battery system based on 18650 cells
Author(s)  Uli (Uli)
Last Version  0.2.2 (01.05.2020)

Introduction

The battery system described here is a modular system designed as 24V total voltage. The modules are stacked on each other with brass standoffs and additionally interconnected with ribbon cables.

Modules

Each battery system consists of one or several battery cell holder modules containing each 14 18650 cells and at least one battery management system module to charge the cells with surplus energy and protect the cells from over- and under- voltage. Optionally a state of charge display module shows the voltage of each parallel cell string.

Battery Cell Holder Module

Designed as a 24V battery, 7 cells are wired in series (7s) and each of these consists of 2 cells in parallel (2p) which makes 14 cells per module (7s2p). As the nominal voltage of a typical 18650 lithium ion cell is around 3.6V, 7 of them in parallel results in a nominal voltage of 25,2V. A typical end-of-discharge voltage is 2.8-3.0V making 19.6-21.0V in a 7s system. A fully charged cell usually has no more than 4.2V, resulting in a maximum voltage of 29.4V for the whole 7s battery system.

Bill of materials (BOM) for one 7s2p battery module holding 14 18650 cells (total ~11.05$ excluding shipping):

- 2 x single 18650 battery holder 0.44$ (0.88$)
- 6 x dual 18650 battery holder 0.66$ (3.96$)
- 1 x XT60PW Plug 1.50$ (0.75$)
- 2 x 10PCS Gold 1808 125V 250V AC 0451 SMD Fast blow Fuse 15A 2410 ultra-rapid fuses 1.69$ (0.34$)
- 1 x 10PCS DC3 20P 2.54mm Socket Header Connector ISP Male Double-spaced Straight needle Curved needle 1.17$ (0.12$)
- 1 x PCB ~ 5$

Cells

Thanks to Paul, who gave me a hint on which cells to order, I am pretty happy with the Panasonic NCR18650B cells which I ordered in this store (unfortunately they don't ship to Germany anymore, but to Austria for example). The cells are rated with 3300-3400 mAh per cell and can be charged and discharged with 1C continuously, which is pretty awesome, I think. When I discharged them continuously with 500mA, I had no cell under 3300mAh and some with even more than 3600mAh so far.

My first setup consists of 3 battery holder modules (with each containing 14 18650 cells) and one BMS module and therefore contains 42 cells in total. This setup has a capacity of roughly 500Wh (3.6V * 3300mAh * 42 cells), can be discharged with 18A or 453 Watts continuously (3 Amps * 6 parallel cells * 25.2V nominal voltage). The BMS module is designed to take 60A (3 * 20A) which results in about 1,4 A maximum charging per cell. I am charging them with two solar panels (each 100W, so 200 Watt Peak in total), which results in max 8 Amps charge load for the whole 25V system, or 2,66A per battery holder module or 1,33A per cell. In practice it will usually be quite less.

The whole battery systems with 42 cells weighs 2.8kg (1 cell weighs 47 gramms).

Battery Management System Module

description yet to come...

BOM (total ~22.81$ excluding shipping):

- 3 x Battery Protection Board Module 24V 20A 7S Lithium Li-ion LiFePO4 Battery Battery BMS Protection Board with Balancing 5.27$ (15.81$)
- 2 x 20pcs Car Fuse Holder Mini For PCB Board 2.55$ (0.26$)
- 2 x 30pcs 30A Mini Blade Fuses Green 1.33$ (0.09$)
- 1 x 10pcs DC3 20P 2.54mm Socket Header Connector ISP Male Double-spaced Straight needle Curved needle 1.17$ (0.12$)
- 1 x AMASS XT90 Male Female Connector Plug Pigtails with 150mm 10AWG Silicone Wire RC Battery Cable Wire for RC Lipo Battery 1.74$
- 1 x PCB ~5.00$
- some thick wire

State of Charge Display Module

yet to come...

Addidtional Parts and Materials

- 10pcs Brass standoffs M4 hex 40mm 3.51$ (1.40$ per module) (additional shrink insulation recommended)
- 20pcs nylon spacer M4 6mm 1.48$ (0.30$) (for additional insulation of brass standoffs at very top and bottom)
- ribbon cable 16 WAY Flat Color Rainbow 
- 10Pcs Female 2.54mm Pitch FC IDC Connector For Flat Ribbon Cable 20P

Useful tools

- Liitokala Lii-500 battery charger for 4 18650 cells
- ISDT BattGo BG-8S Smart Battery Checker Balancer
- insulation tape (e.g. for DC3 cable)
- soldering iron
- solder wire
- wire cutters and pliers 

Assembly

1. Order PCBs, electronics parts, 18650 cells, cables, and screws
2. Assemble 7s battery cell holder module
3. Assemble battery management system module
4. Assemble current state of charge display module
5. Assemble and connect cables and wires
6. Test basic setup
7. Test battery under load
First batch of the Panasonic NCR18650B cells arrived.
Desk full of tools, electronics and cells while putting together all the parts for the 18650 battery system.
First working version of the 18650 battery system ready for testing.

Debugging

- !warning!: insulate ribbon cables and brass standoffs or you will risk a short!

Housing

There is no design for a housing or case yet, but it will be updated here, when there is one.

Open Issues

- waterproof housing for mobile and outside use