Introduction

With solar minimum band conditions, QRP operation in the field is not as fun as before. So, you want to take your QRO radio with you instead. Many of the mobile HF rigs are great to take out in the field.

Now with your mobile HF transceiver you can output 100 watts RF power and gain up to 2 S-units at the other end. However you need a DC power source that can provide you with approximately 20A DC current at a solid 12.8 volts.

In the past, to meet this power requirement, you either needed to stay in or near your vehicle, or carry a very large capacity lead acid battery.

Nowadays there is a battery chemistry that is (relatively) safe to use and allows for rapid draws of energy from very small packs. That chemistry is LiFePO4.

In this guide, we'll cover the basics of selecting appropriate LiFePO4 cells, how to build a pack and share some thoughts on how to get the most out of your pack.

 

Warning

Batteries can be dangerous. Please exercise caution when building your pack. The LiFePO4 cells referenced below can discharge very large amounts of energy quickly, almost like large capacitors. Use caution whenever you handle your pack, triple check your wiring, fuse where possible, yada, yada, yada. You are responsible for your own project. Cross check the information outlined below as you will own what you decide to implement.

 

The Cells

First, don't confuse LiFePO4 with LiPo or other "Lithium" chemistry cells. Dig deep into the vendor's specs for the cells until you find the true chemical make-up. This guide only applies to LiFePO4 cells with a nominal voltage of 3.2V DC per cell.

You will need 4 cells in series (4S) to create a nominal 12.8 volt pack. But how big a cell do you need? To determine that, we need to do a bit of math.

First, estimate how you will use the radio you plan to power with this pack. Will you be 25% transmit and 75% listen (a caller), or 5% transmit and 95% listen (a hunt and pouncer)?

Next, decide on how long you will want to be able to operate for without a recharge. We are assuming a solar panel and suitable charge controller are not yet in the picture.

And finally, determine how much energy your radio draws on both RX and TX (for the power level you plan to use). It might be a good idea to go with a worst case scenario for power draw and just assume the maximum your radio will consume on it's highest RF power setting. So, for the TX, use the manufacturer's specification for your radio. For the RX, (optionally hook up a dummy load to the antenna connector so you don't accidentally transmit with no antenna attached and) use a DC power meter to monitor how many amps your radio draws while listening.

Let's use my own rig and situation as an example:

Radio: Yaesu FT-891
RX amp draw: 1.1A (with button backlighting turned down as low as possible)
TX amp draw: 10.45A (at 40 watt SSB output - my usual TX power level - 1 S unit better than 10 watt phone QRP)
Use pattern: 25/75 (a caller for SOTA, POTA, CNPOTA and WWFF/VEFF activations)
Use duration: 2 hours

So, that's 1.1A x 0.75 (75% of the time) x 2h = 1.65Ah required for RX. And then 10.45A x 0.25 (25% of the time) x 2h = 5.225Ah required for TX.

Add up the RX and TX Ah requirements and we have 6.875Ah required.

Now, with the old lead acid cells, (if running at the stated 20h consumption rate or less) you could use approximately 50% of the battery's rated capacity. With LiFePO4, at any consumption rate not exceeding 2C (2 x the rated capacity of the pack), you can use approximately 70-80% of the rated capacity. For this example we'll go with 70%.

So, we take the required 6.875Ah and divide by the 0.7 usable capacity and determine that I need a 9.82 Ah LiFePO4 pack. We'll round up to 10Ah. So, 4 x 10A 3.2V nominal LiFePO4 cells need to be acquired.

There are many LiFePO4 cells on the market, but recently HAMs have been reporting good results with Headway brand cells. These cylindrical cells come in capacities such as 8Ah, 10Ah, 12Ah and 15Ah. I bought 4 of the Headway model 38120S 10A cells here.

 

The BMS

A BMS, or Battery Management System, is an electronic means of protecting the pack from overcharge, overcurrent and the dreaded over-discharge. Depending on the BMS model, it may also protect from charging your pack at less than 0 degrees Celsius. Also, depending on the model, it may perform top balancing of your cells when the cells are being charged (more on that later). Some basic BMSes are FET based and require very little power from the battery to operate. Some more expensive and elaborate BMSes require external contactors to ensure protection of the pack.

In my camper I have an Orion JR BMS running Gigavac contactors (utilizing economizer circuits) to protect the pack. This elaborate BMS also supports 3 battery pack temperature probes to ensure the pack temperature is accurately represented. This BMS is fully programmable for varying battery chemistries and use patterns... and it always faults to the battery being completely disconnected from all charge and discharge circuits. If there is an overdischarge event, zero load will be placed on the batteries - even the BMS will be cut off.

We will not be using a BMS as elaborate and expensive as the one in my camper for our little portable battery packs. And we also need a solution that draws less power from our battery to provide the basic protection we need.

We can simply eliminate the requirement for temperature protection by simply never charging the pack when it's cold. But we do ideally need safeties for overdischarge and overcurrent.

Now, let's back up a bit and take a moment to outline some key/core attributes of the LiFePO4 battery technology:

  1. Overcharge decreases the capacity of the cells.
  2. Overcurrent (charge or discharge) simply isn't safe.
  3. Charging at a temperature of less than 0 degrees Celsius will greatly impact the capacity of the cells. You never want to do this.
  4. Overdischarge just once KILLS your cells immediately. You NEVER want to do this.

Now let's look at the manufacturer's specs for the 10A Headway cells I've selected for my portable pack build:

1C maximum charge current, 0.3C - 0.5C suggested charge current
1C - 5C maximum constant discharge current, 10C maximum burst discharge current
3.65V maximum charge voltage, 3.8V maximum cell voltage
2.0V minimum cell voltage
0-45 degrees Celsius for charging
-20-65 degrees Celsius for discharging

Now let's look at the specs of some other LiFePO4 cells (in this case the much-more-expensive CALB CA180 180Ah cells I have in my camper):

1C maximum charge current
2C maximum constant discharge current
3.65V maximum cell voltage
2.5V minimum cell voltage
0-45 degrees Celsius for charging
-20-55 degrees Celsius for discharging

The figures are similar, with the ranges a little tighter on the CALB cells. However, these are the limits. You never want to hit these levels, and if you do by accident, you need your BMS to protect the cells from your oopsie.

So, for the portable pack build, I've selected this BMS.

It's critical stats are as follows:

allows up to 10A charge current
allows up to 120A discharge current
allows up to 3.75V maximum cell voltage
allows down to 2.1V minimum cell voltage
optional temperature-based control
self consumption less than 30uA normal, less than 20uA when over discharged

I don't care too much about discharge current protection as I will be fusing both the positive and negative leads with 25A fuses. I also don't care too much about the charge current limit as I'll be using a charger where I set the charge current limit to 0.2C. But otherwise, the protections fit the spec of the Headway cells. I will, however, do two things to prevent a pack-killing overdischarge event:

  1. When storing the pack, charge to 50% and then disconnect the BMS. The BMS always draws power, and even if it's protecting from over discharge at 2.1V per cell, eventually it's sub-20uA draw will drain the cells to 2.0V or lower and kill them.
  2. As CALB says, go no lower than 2.5V per cell, I don't want to get anywhere near 2.1V. In fact, I don't want to go much below 3.0V per cell, ever. So I will be adding a LVD (low voltage disconnect device) to catch an under-voltage situation very early.

 

The LVD

The low voltage disconnect device is my personal safety to ensure that if I'm operating at 100 watts RF output and my pack is low in remaining capacity, the pack will disconnect the DC load well before the cells are drawn down to dangerously low voltage levels.

LiFePO4 voltage over time goes something like this: each cell starts off fully charged with a resting voltage of around 3.4V per cell. Soon that drops to somewhere between 3.3V and 3.2V and that's where the cell voltage levels stay for the majority of the time. Then there's a lower knee where all of a sudden voltage drops down to 3.0V per cell and then you know you're pretty much empty. Further discharge of the pack will quickly pull the cells down into the 2.xV range where you can risk destroying the cells.

With all protection devices, they draw energy to keep you safe. So, you need a device that draws as little energy as possible so that you can "play radio" for as long as possible.

For my LVD, I've chosen Victron Energy's Smart Battery Protect 65. There's also a classic Battery Protect 65 without bluetooth, but the amp draw (when in use) of the new Smart model is less than that of the older classic (with the bluetooth turned off after you've set your protection parameters).

With the Smart version, I pay for the additional protection it provides with 1.2mA of additional current draw. I set the low voltage disconnect at 12V (3V per cell) and enjoy the disconnect delay of 90 seconds. I may in the future install a buzzer so that I can be alarmed when a disconnection is imminent.

Smart BatteryProtect

 

The DC Power Analyzer

Like every good HAM, I like to see stats for my DC power draws. I enjoy seeing how many amps I've drawn from the pack, my current amp draw, as well as my lowest pack voltage so far. This gives me some indication as to where I am in regards to pack capacity remaining.

There are many DC power anaylzers on the market. For more information on the one I use, please check it this news article.

Image result for DC power analyzer

At the end of the day, the DC power analyzer that I use requires 50mA to operate, so like all other safety and monitoring devices, consider the cost of it's service. I think, however, it is very worthwhile to use. If the analyzer indicates you've got lots of power left when you need one more contact to activate a summit, turn up your radio's power to maximum and reach for that distant station. If, on the other hand, if the analyzer indicates that you are near exhaustion of your pack's usable capacity, perhaps turn your radio's RF output power down, or disconnect other devices that you may be powering from the pack in order to hold out a little bit longer for that last contact or to finish up with a pile-up.

Of course, if you plan to implement a solar charge solution in the field, you will need two analyzers - one for the DC output and one for the DC input. Or, perhaps just save 100mA each hour and don't use any as the sun will sustain your fun.

 

Optional but Recommended: Cell Balancing

Cells, even from the same lot/batch, may have differing capacities. Without going into massive detail on the topic, I'll just state that when cells have different capacities, their voltages will drift away from each other either at top (charged) or bottom (discharged) charge levels.

I am a big fan of only charging my LiFePO4 packs to 90%. CALB suggests this practice for their batteries and if we recall key attribute 1 of LiFePO4: overcharge reduces capacity. Now, if I'm only going to 90% charge level, do I care if my cells have slightly different voltages at 90% full? No.

Now when I take the pack down to the bottom, if one of my cells is a little lower in capacity than the rest, it may show a significantly lower voltage than the other cells. If I take a cell below 2.5V I worry about it, not to mention the 2.0V minimum outlined by Headway. Ideally, when my LVD cuts me off at 12V, each of my cells will be at exactly 3.0V - all nicely out of the 2.xV range with zero risk to any cell.

Hitting that LVD voltage is going to be common for me... as I know I'll be trying to get just one more contact required to make the SOTA/POTA/CNPOTA activation valid and I will likely not be carrying a solar panel and charge controller with me to the summit (to limit weight).

So, I want to bottom balance my cells. To do this, I'm going to place a load on each cell individually until the cell reaches 2.8V. With these 10A cells, I'm going to start with a 2A load to bring the voltage down to 2.8V and then immediately remove the load. Then once the voltage bounces back up, I'll put a 0.5A load on the cell taking it down to 2.8V again and immediately remove the load. Finally, once it bounces up again I'll do one more draw at 0.5A to 2.8V and then remove the load to get a final resting voltage just under 3.0V for each cell. This is just below my LVD set point of 12V or 3.0V per cell. So, now, when my LVD kicks in, all cells will be balanced at 3.0V.

Once bottom balanced, I'll assemble the pack in 4S configuration and charge to 90%. I won't let my BMS do any top balancing, otherwise my bottom balance will be messed up. On the more elaborate BMSes, you can turn balancing off. On the basic BMS I've chosen for this build, I know balancing doesn't start until 3.6V per cell. But as I only plan to charge to 90% anyways, I will charge at a much lower voltage of 3.45V per cell. More on charging later.

So, what's an easy, no-risk way to discharge cells at exact amp rates with load disconnects immediately when the bottom voltage is reached? Use a battery workstation from the Remote Control world. I personally use a Revolelectrix Cellpro Powerlab 8 v2. This device is a bit of overkill if you don't normally build LiFePO4 packs, but it is flawless at disconnecting the load when the low voltage is reached. It also provides a metering of energy removed from the cell which allows for capacity testing on both the cell and pack levels.