In the first two articles, we covered the PowerFilm F15-1200, and the Genasun GV-5 charge controller for lithium iron phosphate batteries. Now we’re turning our attention to energy storage by introducing you to the Headway 38120 10A lithium iron phosphate cells.
Building my system
When building a man portable off-grid power solution for amateur radio communications, we should attempt to find the balance between portability and energy storage. If we get that balance wrong, we risk having a system which may be great for storage, but too cumbersome for portability. There is also the flip side. A system lightweight and portable, but can’t keep up with the needs of our comms gear, despite having an excellent solar panel and charge controller.
How do we define a system?
Our man-portable, off-grid power system, is made up of several components all integrated into a single system. The entire system is modular, meaning any one component can be exchanged for another, to meet the demands of our field Ops. Those components are:
- The individual cells making up the battery pack
- The battery management system protecting the individual cells
- The solar charge controller
- The solar panels
- The power distribution box (if one is used)
Here at 65° North, finding that balance is always a difficult endeavor. During summer months we have nearly 24 hours of daylight. This means we can get away with small storage capacity, provided we have enough collection from our solar panels. In winter we have the opposite challenge. Higher storage capacity, to offset the problem of having only a few hours of daylight. So we must consider increasing both storage capacity, and solar collection.
Although this article is from the perspective of a man portable operator at 65° North, the challenges are valid for an operator anywhere. So, I believe my ultimate goal is to build two independent battery packs. One for summer, the other for winter. I’ll explain, then move on.
- Summer system with modest energy storage capacity. In this option, the solar panel and charge controller combination replenishes energy storage as we use it. The system is much lighter because energy storage is kept to a minimum. (5Ah LiFePO4 18650 in 4S2P).
- Winter system with higher storage and solar collection capabilities. The solar panels job is more about decelerating discharge from our energy storage. Increased storage capacity also means increased weight. (10Ah LiFePO4 38120 in 4S1P)
When talking about the PowerFilm thin-film flexible solar panels vs energy storage. It’s almost always lighter and more efficient to carry more or larger thin-film solar panels, than more energy storage! To avoid risks, I test panels on cloudy or overcast days as well as sunny days.
With all that said, this article will focus on a winter system (in part). This battery pack is based on the Headway 38120S 10Ah LiFePO4 cells. The battery pack that we make from the cells is only part of the solution. The battery must also be integrated with “battery management system” to protect and balance the cells. The BMS is the only interface to the battery from the solar charge controller. I know this sounds complicated but this is how the Bioenno LiFePO4 packs, the QRP Ranger, and The Goal Zero Sherpa series work. It’s not complicated. It only looks and sounds complicated, because they are put inside pretty boxes!
HUOM if you haven’t read the first two parts of this series please go back and read those first
Headway 38120S LiFePO4 cells
I decided on these Headway 38120 10Ah lithium iron phosphate cells. As a mostly QRP operator, a 10Ah battery pack connected to 20/40 watt solar panel, would give the little ft-817 and an efficient laptop or tablet, an enormous amount of operating time. Even with the planned ft-857/897 at a maximum of 20 watts (emcomm), I could still anticipate a generous operating time, provided I double up on my solar panels. Don’t forget the lithium iron phosphate cells can be drawn down to 20% the total capacity. In contrast, the SLAB should only be drawn down 50% of its total capacity before permanent damage becomes apparent. So there are lots of benefits to the Headway 38120 lithium iron phosphate cells, such as:
- More capacity
- Less weight
- 100A peak current draw
- Initial cost
- Perceived complexity.
Battery specs: Headway 38120S 10Ah 10C LiFePO4 Cylindrical Battery Cells with screws.
- Normal Voltage:3.2Volts.
- Capacity: 10Ah.
- Energy density 105 (Wh/kg)
- Weight: 350Grams each.
- Dimension: Diameter(mm) X High(mm) = 38±1mm X 122±1mm(132±1mm).
- Maximal Discharge C-Rate: 15C (150Amps).
- Maximal Charge Current: 5C (50Amps).
- Maximal Continuous Discharge C-Rate: 10C(100Amps).
- Overvoltage Protection: 3.65±0.05Volts.
- Undervoltage Protection: 2.0Volts.
- Impedance: ≤4mΩ.
- Lifecycle: 2000Cycles.
I configured the cells in a 4S1P configuration. This is a simple series configuration, technically identical to the original qrp battery pack. I also included a battery management system, to protect the cells from:
- Over discharge
- Short Circuit
The battery management system has one output for charge and discharge. It has 5 outputs for battery pack plus and minus, and balance leads between cell 3 & 4, 2 & 3, 1 & 2. The charge and discharge port connects to the battery port, on the Genasun GV-5. The GV-5 will manage the constant current constant voltage required to maintain the pack. All we have to do is connect our load which could be a radio, a laptop or something like that, and of course connect our solar panels. So that’s two connections to the solar charge controller and 5 connections to the battery cells themselves.
Why not a commercial battery pack?
I know some of you are thinking about the Goal Zero Sherpa, the Bioenno batteries, the QRP Ranger from Hardened Power Systems, the Tracer battery packs and so on. Regarding the Goal Zero Sherpa series, it’s a one-size-fits-all solution which works but has no upgrade path or spare parts. The Hardened Power Systems QRP Ranger has a long way to go in regards to cost and reliability. I love the idea, but it’s design goal seems to have been “make it as cheap as possible, but look cool”. It is also very fragile! If someone is going to build and sale something like this, it should be based on quality parts and manufacturing. If that is not the case, I can build it myself, without using any cheap Chinese parts. Like the Goal Zero, there is also no upgrade path. So “no thanks” at the moment. The Bioenno batteries are absolutely brilliant, with excellent availability in North America. If I still lived in North America, I would love for them to get one here for testing. Unfortunately they weren’t easily available in Europe, so that was kind of a deal-breaker (if you’re outside of NA). Finally that leaves the Tracer lithium iron phosphate packs, which are the closest things to the Bioenno packs we have in Europe. Unfortunately they are cost-prohibitive outside the UK. Tracer should reevaluate its shipping policy.
Now in addition to those things I mentioned, there is the issue of not wanting a brick for a battery pack. Building a bespoke battery pack myself means I can shape it however I like. There’s no rule that says it has to be shaped like a motorcycle battery. With that in mind, I would rather have a small flat pack or something that fit in a small Pelican case. Whichever way the build goes, it’s definitely better to have the choice, than to be locked into a brick sized motorcycle battery looking battery pack.
Words of wisdom
There are lots of clever people on the internet! There are even more who only think they’re clever! There are also people like you and I, who do our due diligence in the form of research. No matter what you read on the Internet, or watch on YouTube, always always always use a battery management system when building your lithium iron phosphate battery packs, for use with the solar charge controller. My battery pack will never be connected to a balance charge controller like the HobbyKing packs for example. The combination of my solar charge controller, and my battery management system when combined, make up my balance charge controller! If your bespoke battery packs are never going to be simultaneously charged and discharged, then you can get away with not having the battery management system, since you are recharging them with your balance charge controller. Using the BMS means you don’t have to worry about low voltage.
I’ll start putting together the build video.