Individual LiFePO4 cells have a nominal voltage of about 3.2V or 3.3V. We use multiple cells in series (usually 4) to make up a lithium iron phosphate battery pack.
- Using four lithium iron phosphate cells in series, gives us roughly ~12.8-14.2 volts pack when full. This is the closest thing we’re going to find to a traditional lead-acid or AGM battery.
- Lithium iron phosphate cells have greater cell density than lead acid, at a fraction of the weight.
- Lithium iron phosphate cells have less cell density than lithium ion. This makes them less volatile, safer to use, an offers almost an one-to-one replacement for AGM packs.
- To reach the same density as lithium-ion cells, we need to stack lithium iron phosphate cells in parallel to increase their capacity. So lithium iron phosphate battery packs with the same capacity of a lithium ion cell, will be larger, as it requires more cells in parallel to achieve the same capacity.
- Lithium iron phosphate cells can be used in high- temperature environments, where lithium ion cells should never be used above +60 Celsius.
- The typical estimated life of a Lithium iron phosphate battery is 1500-2000 charge cycles for up to 10 years.
- Typically a lithium iron phosphate pack will hold its charge for 350 days.
- lithium iron phosphate cells have four times (4x) the capacity of lead acid batteries.
Individual Lithium-ion cells usually have a nominal voltage of 3.6V or 3.7 volts. We use multiple cells in series (usually 3) to make up a ~12 volt lithium ion battery pack.
- To use lithium-ion cells for a 12v power bank, we place them 3 in series to get a 12.6 volt pack. This is the closest we can get to the nominal voltage of a sealed lead acid battery, using lithium ion cells
- Lithium ion cells have a higher cell density than lithium iron phosphate we spoke about above. This means we use fewer of them for the desired capacity. Higher cell density comes at the expensive of greater volatility.
- As with lithium iron phosphate, we can also stack Lithium-ion cells in parallel to increase the capacity of our packs.
- The typical estimated life of a Lithium Ion battery is two to three years or 300 to 500 charge cycles.
- Typically a Lithium-Ion pack will hold its charge for 300 days.
I’ll add this section based on feedback from one of our Facebook followers.
The reason we use 3 cells in series for Lithium-ion battery packs is the voltage. A 4S lithium ion pack has too high voltage (~16.8v) when full. In contrast there are some radios which require more voltage than the low side of a 3s lithium-ion pack can provide at the end of its voltage curve. If we still want to use a 4S lithium ion pack, we need to integrate a DC DC regulator, to manage voltage output. Or, as I alluded to in the second paragraph, we can also use lithium iron phosphate cells, which have 14.2-14.4v fully charged. This is perfectly fine for most radios, but read the voltage requirements for your radio.
charging lithium iron phosphate + lithium ion cells is very similar. Both use constant-current and then constant voltage for charging. If we’re talking about one of the DIY battery packs from the channel, solar or desktop charging is usually done by two pieces of gear.
- First we have the voltage and current source. This can be an adjustable buck, or a solar panel for example.
- Next we have the charge controller. This regulates the voltage and current coming out of our voltage/current source, feeding the BMS.
- Finally, the BMS sends the regulated voltage to the pack. It also bleeds off voltage from cells which have a higher voltage than the others. This gives the others a chance to catch up. Despite what Bioenno says, never directly connect an unregulated source to your battery (BMS or not!).
As with all batteries, the cold affects the ability for lithium ion or lithium iron phosphate cells to be charged. So we need to do something to ensure the battery doesn’t drop below freezing. Battery charging is one of the reasons I deploy a shelter during cold weather. It’s relatively easy to keep the temperature inside the shelter above freezing, while your solar power or generator remains outside the tent. One trick utilized to keep these cells above freezing, is keeping them and the radio equipment, inside an enclosure. All radios make heat, so restricting (to some degree) ventilation, heat from the radio will significantly warm the space around the battery. Another trick is to use chemical hand warmers near or inside the battery compartment. The point is to use common sense. Since we know we shouldn’t charge batteries up below freezing, a simple change of operating practices can easily Rectify this.
If you’re building a pack with more than one cell in series, you’ll need to balance the cells in the pack or in the charger.
It’s important to point out just because someone can make a YouTube video or blog showing you how to build a pack, doesn’t necessarily mean they know exactly what they’re doing.
The bottom line, you either need to manually balance your cells, or actively balance your cells. if you’re building one of my battery pack projects, AND you’re going to use that pack while simultaneously charging and discharging it, active balancing is the way to go. On the other hand, if you’re using that pack for discharging only, taking them out to the field for discharge, then charging once you’re back home, technically you don’t need any balancing while discharging the pack. If you’re going to charge the cells as a complete 4s or 3s pack, you’ll need a balance charge, or charge them individually. Of course if you’re using 18650 batteries, and your charger accommodates charging more than one cell at a time, you’re all good!
Choosing a BMS
The following paragraph relates only to those of you who would like to build a complete battery pack. Now that you’ve read the paragraphs above, you understand the voltages between Lithium ion and lithium iron phosphate are unique. This also means the BMS you use for your battery Pack s are specific to lithium ion or lithium iron phosphate. You can find a variety of different balancing boards in the projects on the channel. We choose balancing boards by the capabilities we require from them. Before choosing a board we need to know:
- How many amps we want to pull through the board
- How many cells are in series
- Whether lithium ion or lithium iron phosphate cells will be used
- Does the board offer cell balancing ( if you’re using a BMS always get one with cell balancing)
When you have these numbers, you can use them to choose the right BMS from your supplier. You shouldn’t even be looking at the price until you understand your requirements. You should also take care about eBay and Alibaba sellers. They often incorrectly label BMS boards with much greater capabilities than they actually provide. So use your common sense. If I know I’m going to be pulling 15 amps out of a BMS, I usually purchase one from eBay which is rated for 30 amps.
Why else might you want to integrate a BMS into your project? A good BMS also offers these features:
- Over-voltage protection
- Under-voltage protection
- Short circuit protection
When people are telling you not to use a BMS or balancing isn’t required , they do so without understanding the additional protection a BMS provides. Food for thought!
Lithium vs SLA Discharge graph
Sometimes no matter how hard I try, operators still hold on to the illusion that a sealed lead acid battery of the same capacity is no different or even better than a lithium ion or lithium iron phosphate pack. This is usually based on the price. That’s utter nonsense!
Here are a few facts.
- The number one reason for not using a lead acid battery is weight. Lithium and lithium iron phosphate packs are a fraction of the weight while offering greater cell density. This translates into greater operating time, or the ability to power our gear for much longer in the field, without an increase in size/weight.
- Small sealed lead acid batteries have an extreme voltage drop under heavy load. They were never designed for high amperage applications. In fact small sealed lead acid batteries were designed to have a small load on them over a long period of time. Applying the typical 15 to 20 amps from a modern 100 watt radio, we experience a significant voltage drop. A properly built lithium ion or lithium iron phosphate pack does not show the same voltage drop as a lead acid battery. In fact under load, the voltage is relatively flat while discharging lithium ion and lithium iron phosphate packs.
- One of the Illusions about lithium-ion or the lithium iron phosphate battery packs, is “they are difficult to charge”. In fact lithium ion and lithium iron phosphate packs are easier to charge than a sealed lead acid battery, if we just open our minds to it. All we need to know is how many cells we have in series, and the voltage of the individual cells in the pack. Then use that number to apply constant voltage constant-current to the pack. This is basic math! There is no float voltage or any stages when charging lithium or lithium iron phosphate packs. Just constant voltage constant-current. When the battery reaches the top of its voltage curve, it’s full. No floating, or absorbtion, .. it’s just full when it reaches the top of its voltage curve.
So there’s a lot of misinformation on the internet. There’s even more on YouTube, driven by YouTubers who either don’t know or haven’t done the research. Not slamming them, but it’s important for each of us to do our own research. I would agree that on the surface it seems a lead acid battery would be cheaper to buy, than the lithium-ion or the lithium iron phosphate pack. There are so many other things to look at beyond price, which give us the real answer to that question. I don’t even consider using lead acid batteries in any of my projects anymore. So that leaves lithium ion and lithium iron phosphate. Which one should you use in a project? Well here is how I choose.
- If I’m trying to go ultralight traveling quite a distance on foot, lithium ion is probably the better way to go. Greater cell density gives a longer run time in the smaller package than lithium iron phosphate,
- If I’m looking for something easy to work with, a greater amount of watt hours over the 3S Li-Ion, where I had traditionally used in SLA battery, LiFePO4 is the better choice.
- If I’m looking for best investment for storage batteries in an off grid solar generator, 1500-2000 cycles, zero maintenance, and 10 or more years sounds pretty amazing.
Like anything in the world, the results of our projects are based on the research we do. I often get criticism about not publishing so many videos, but when you do the research and the background work, it’s impossible to throw out any old crummy video everyday. So do the research guys. In the end it’ll be very rewarding.
Travelling with Lithium batteries
Rules change from one jurisdiction to another as easily as day turns to night. At the moment it seems the heaviest restrictions on lithium batteries are found flying into or out of North America. According to both the FAA and TSA websites, Lithium batteries with more than 100 watt hours may be allowed in carry-on bags with airline approval, but are limited to two spare batteries per passenger. Loose lithium batteries are prohibited in checked bags. Neither the FAA or TSA make any difference between lithium ion or lithium iron phosphate.