Have you heard about this (relatively) new dating app Tinder? I’m still single and ready to mingle so I decided I’d give it a shot. The premise is that pictures of random people come up with a bio and you swipe right if you like them, and left if you dislike them. If you swipe right and they swipe right too, you can chat with each other. It turned out I should have spent a little more time reading the bios instead of looking at the pictures. Chatting with my “matches” smothered whatever spark the pictures had lit. Embedded systems can sometimes leave you feeling the same way, especially when they have battery issues. You spend tons of time designing a great PCB, only to have the battery degrade too quickly or fail from temperature issues. If you’re really unlucky your battery may even start a spark in your circuit. I may not be able to match you with the perfect girl, but I can help you choose the right battery for your board. Two of the most popular choices for embedded systems are lithium-ion batteries (Li-Ion) and lithium iron phosphate batteries (Li-phosphate or LiFePO4). These two types have very different charging and discharging characteristics. Sometimes you could use either or, but usually one is better than another. Read on to see which kind will benefit your application the most.
Love has a variety of different meanings in English. I say I love my iPhone, but I also say I love my girlfriend. Hopefully, these loves differ in a few important ways. In the same vein, when people say “Li-Ion” they could be talking about several different varieties of Li-Ion batteries. The ones we’re discussing here are the most common, lithium cobalt oxide (LiCoO2). These Li-Ion batteries use graphite for their anode. Let’s look a look at Li-Ion’s bio.
Voltages: 3.6 V nominal, ranging from 3.0 V- 4.2 V.
Specific Energy: 150-200 Wh/kg
Charge Rate: 0.7 C - 1 C, charging above 1 C will damage battery
Discharge Rate: 1 C. You may be unfamiliar with the “C” rate. It means that if a battery is rated for 2400 mAh it can discharge with a maximum current of 2.4 A without being damaged.
Life Cycle: 500-1,000 cycles. The life cycle depends heavily on operating temperature and depth of discharge (DoD).
Thermal Runaway: 150°C, this is what causes Li-Ion batteries to explode.
Charging Temperature Range: 0-40°C
Discharging Temperature Range: -25-60°C
Let’s see if you match with Li-Ion or Li-phosphate. Editorial credit: Happy Zoe / Shutterstock.com
Maybe you’ve already decided to swipe right on Li-Ion, but let’s give Li-phosphate a chance as well. Its anode is also made of Carbon, but that’s one of the few characteristics it shares with Li-Ion.
Voltages: 3.2 or 3.3 V nominal, ranging from 2.5-3.65 V.
Specific Energy: 90-120 Wh/kg
Charge Rate: 1 C
Discharge Rate: 1-25 C, possibly with 40 A pulses.
Life Cycle: 1,000-10,000 cycles, highly temperature dependent.
Thermal Runaway: 270°C
Charging Temperature Range: 0-45°C
Discharging Temperature Range: -20-60°C
Once you’ve done all your swiping on Tinder, you get the chance to message your matches. Then you can find out a little more about them, and possibly even compare them. While I don’t recommend objective comparison for dating, I do think it’s a good idea for battery cells. Let’s look at how Li-Ion and Li-phosphate compare on: safety, discharge, and charge capacity.
As you saw earlier, Li-Ion batteries have a very high energy density. This makes them attractive for power hungry portable devices like mobile phones and computers. Sometimes extremely attractive people can be a bit unstable, and apparently that applies to batteries as well. Li-Ion’s high energy content makes it vulnerable to things like explosions. Multiple failure modes make it difficult to cover all your bases on Li-Ion batteries. One of the failure modes is rapid discharging and recharging, leading to thermal runaway. In things like handheld electronics, cycling depends on how often the user drains and recharges their device. This leaves the battery’s fate in the hands of an end user who has no knowledge of battery failure modes. Granted, that kind of failure is not likely, but better to be safe than sorry. Li-phosphate is a little bit calmer than Li-Ion. It has a lower energy density and more stable chemistry. These features mean it won’t burn, even if it does fail. All in all, Li-phosphate batteries are much safer than Li-Ion batteries.
It’s also important to look at the discharge characteristics of these batteries. Li-Ion’s voltage discharge is good, but not great. A good voltage discharge curve is flat, meaning the voltage doesn’t degrade as battery charge capacity reduces. Li-phosphate, on the other hand, has an excellent voltage discharge curve at the right temperatures. Discharge current comes into play as well for electric motor applications. Li-Ion’s low discharge current rate of 1 C pales in comparison to Li-phosphates which can be up to 25 C. Another win for Li-phosphate.
Some people like to chat with multiple matches at once. I just don’t have the mental or emotional capacity for that. It’s always important to consider cell capacity in love and in batteries. Despite Li-phosphate’s wins in safety and discharge, Li-Ion can simply store more power. When you’re designing portable electronics, size often matters. Li-Ion batteries can store up to twice as much energy per kg over Li-phosphate. That makes a big difference when you’re trying to hit strict space targets. So there’s finally a match for Li-Ion.
Catastrophic failure in a Li-Ion battery.
There’s a lot of information to take in on Tinder, so sometimes I get my friends to help me swipe. I’ve got a little advice to help you choose the best battery for your application.
If you’re a more stable kind of person who looks for safety and reliability, I would go with Li-phosphate. It’s best for applications like electric vehicles (EV) or medical instruments where catastrophic failure is not an option. EV motors will also benefit from the high discharge rate of Li-phosphate batteries.
Maybe you’re a bit more on the edge and want to pack as much fun into life as possible. If your devices mimic you, they’ll need a Li-Ion battery. Things like mobile phones, computers, and cameras need as much juice as they can get. They also usually have a lifetime of around 2 years, which Li-Ion can match or exceed.
Now that you know more than you ever wanted to about batteries, it’s time to design. I can give you design advice in blogs, but I can’t be there when you’re working. Luckily, programs like CircuitStudio can. CircuitStudio has a great array of advanced tools that can help you design the perfect PCB.
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