LiFePo4 Battery Pack – Build Phase
Based on the requirements determined earlier, and the related components having been obtained, the actual build is able to start. This page shows the various steps and a narrative showing the build.
An Assortment of Parts
I started collecting parts from a variety of sources. Here’a a list of bits, in no particular order:
LiFePo4 25Ah cells (4)
BMS with RS232 and Bluetooth
10A boost converter (charging side)
30A boost converter (output side)
Dual USB socket
Single Powerpole holder – 3D print
Dual Powerpole holder – 3D print
Powerpole connectors (3 sets)
Copper plumbing strap – 1/2″
Crimp connectors – various types
Mics hardware – screws, nuts, washers
Battery retaining clamps – 3D print
Wire – various gauges
The enclosure selected is a 6510 case made by UK Products Canada. It is similar to those cases offered by Pelican.
Working from some preliminary paper sketches, I did some component layouts to verify fit, alignment, and potential clashes.
I also had a wiring diagram drawn up.
The battery pack component layout will try to keep wire lengths as short as possible, where practical. For best fit in the case, the cells are laid on their narrow edge.
Once the layout made sense, it was time to mark the case and to start drilling holes. A step drill made easy work of the large switch and meter openings.
The large boost converter came with its own brass standoffs, so I marked and drilled small holes to screw it in place from the bottom of the case.
Assembling the Components
Next, I needed some sort of inter-cell wiring, or buss bars, to tie the cells together in series. The battery supplier was out of stock when we ordered the cells, so I had to come up with something else. I used flexible copper tubing normally used for refrigerator water lines and such, hammered it flat on an anvil, then cut short pieces to appropriate length for the cell spacing, and drilled holes for the cell terminals. Star washers and 6mm nuts provide solid connections between the cells.
Standard crimp terminals are used for the other wiring throughout. Except for the BMS sense wires and wiring for the USB plug and voltmeter, everything else is wired with 10 and 12 gauge wire, to allow for up to the maximum 30 Amp output of the boost converter. The fact that all the wires are very short means literally no loss or voltage drops in the wiring.
The User Interface
The ‘user interface’ end of the case has a dual port switchable USB jack, a battery voltage and ammeter on the load side, a switch to turn all the loads on and off, a single power pole jack for the charging input, along with an input voltmeter. And last, the dual power pole provides raw battery output on the top jack, and ‘boosted’ output on the bottom jack. The boosted output is set to 13.8 volts and is intended to power radio equipment at their full rated output power based on what will appear to be a regulated power supply input.
It’s been documented and demonstrated often that a 100 watt radio’s RF power output will decrease as battery voltage decreases. This boost converter presents a constant voltage to the radio, allowing it to produce full output regardless of the actual battery voltage.
The boost converter’s low voltage cutoff is set to protect the battery from discharging too low. That setting, in conjunction with the BMS, will prevent battery abuse.
Next, the wiring is coming together. Notice the wire gauge is appropriate for the 30 amp rating of most components.
Besides regular crimp connectors, a couple of Wago connectors are used for the multi-wire connections.
Smoke Test (not literally)
Initial testing is successful – charging works, direct battery output works, and the boost converter works as expected.
As seen in a picture above, the round dongle is a Bluetooth interface to the BMS. I can connect with my iPhone app and read a number of parameters, as well as set some important parameters.
In addition to the construction shown, Mark N6MTS made the recommendation that the battery terminals and buss bars should be protected from accidental contact. After all, there is enough amperage in this battery pack to probably do some welding. I 3D printed these covers and covers on the main + and – terminals.
The Battery Management System
The BMS needs to know the Amp/hour capacity of the assembled battery pack, and other values such as maximum voltages and minimum voltages.
Here is the main screen of the iOS app showing the state of charge and individual cell voltages. You can see how close the balance is between the cells (at the time of the screen grab).
There are several other screens in the BMS application for detailed configuration, but once configured, this main screen is all I look at from time to time.
In actual use, I want to monitor state of charge, along with voltage and current, cell voltage deltas. When charging, I watch voltage and current, and also the battery temperature.
One last important consideration is to secure the cells within the enclosure. They’ll get bumped and jostled around in everyday use, so they must be securely held in place. It is also critical that none of the mounting hardware can perforate the cells. My idea was to use narrow (1/2″) copper plumbing strap, but this would need some cushioning at the corners, and a way to ensure the cells are wedged in place. I designed and printed some corner brackets and strain relief ends for the straps. These are shown here as parts, and after installation.
The final weight of the entire pack is 12.6 Lbs or 5.7 Kg. With the folding handle, it is quite reasonable to carry without a handy pack mule.
Ready to go to work, and with appropriate warning labels:
Next step: let’s test this thing. Continue reading…