Autonomie project

Transcription

In this video we're going to talk about rechargeable battery chemistries. Lithium, Nickel Metal Hydride, Lead- acid, there are so many options, what's the right battery for your project? And how do you choose a battery? I'll show you an example later in the video. Here's a graph I made comparing the mass of a 1 Watt-hour battery using different chemistries. As you can see, Lithium polymer batteries are the lightest, so they may seem like the best. So why don't we use them for everything? Well it's not that simple. Let's talk about the advantages and disadvantages of each type of battery, starting with Lithium polymer. Lithium polymer, or "LiPo", batteries are the highest energy density batteries available, meaning they store the most amount of energy for a given size. That's why your phone, your laptop, and other lightweight devices use lithium polymer batteries. But this high energy density comes at a cost - literally, they are expensive, and it's not just the batteries themselves. When working with lithium polymer batteries you have to spend money on special circuitry to keep them safe. For example, if you are charging a LiPo battery you have to make sure that you never exceed 4.2 volts per cell. LiPo batteries will EXPLODE if you overcharge them. And it's not just charging them that's tricky. You have to be careful when discharging them too. If you discharge a LiPo below 3 volts per cell, you might permanently lose some capacity, and sometimes you get this dangerous puffy battery situation. Finally, you have to keep a close eye on temperature at all times. If the battery gets too hot, bad things can happen. Despite all of these complications, lithium polymer batteries are the best way to power a small, lightweight project. Don't be afraid of using them. You just need to make sure that you have a good charging circuit, a good low voltage discharge cutoff circuit and a good temperature monitoring circuit. I'll put links to some suitable hardware that you can use in the video description section. Another option you should look at is LiFePO4. These are kind of like lithium polymer batteries but without the Michael Bay explosions. You still have to charge and discharge them carefully using appropriate circuitry, but if you make a mistake you'll just permanently lose some battery capacity. The downside is that they're a little heavier than lithium polymer and that's why you'll never see LiFePO4 being used in a phone. They are typically used in lightweight robots, power tools, and radio control toys because they are really good at delivering high amounts of power in a small, lightweight form factor. Okay, what if you don't have a lot of money to spend, and the thought of complex charging circuits makes you uncomfortable? Well then you probably want a sealed lead-acid or SLA battery. These are a lot more durable than lithium batteries and are better at coping with accidental overcharges or overdischarges. They are excellent value for money and they are also the best type of battery out there for handling temperature extremes. This is why lead acid batteries are the standard choice for cars, motorcycles, solar energy storage and emergency power supplies. The disadvantage of lead-acid batteries is that they are big and heavy. Nobody uses lead acid for portable applications. Speaking of things that nobody uses, let's talk about Nickel Cadmium batteries. If you ever had a portable device in the 80s it probably had a nickel cadmium battery. Ni-Cds are cheap, and in theory last longer than lead acid batteries, but they are really annoying because they suffer from the "memory effect". This means you have to regularly fully discharge then fully recharge them in order for them to maintain a high capacity. And since nobody has the time to do that, you end up losing battery life. There are very few situations where you would want to use a Nickel Cadmium battery. Instead, you should look at Nickel Metal hydride batteries. These have much higher energy density than NiCd, they are only a little more expensive, and have no memory effect. They are bigger and heavier than Lithium batteries, but they're very safe to work with, and that's why NiMH has become the king of consumer rechargeable batteries. You can get them in any supermarket and charging them is cheap and easy. You should be aware that ordinary NiMH batteries have an annoyingly high self discharge rate. This means that even if you aren't draining the battery, they'll discharge themselves over a couple of months. This makes them a terrible choice for TV remotes and video game controllers. If you are going to buy NiMH batteries, I strongly recommend buying low self discharge types, like Panasonic Eneloops, which have an exceptionally long shelf life. I bought a dozen of these 6 years ago and have been putting them in my TV remotes and robots and they are still going strong. It's a lot cheaper in the long run than buying alkalines. So there you have it. There is no such thing as a perfect battery chemistry, only the best battery chemistry for your particular project. It's up to you to weigh the pros and cons and make a decision. (Intermission) Next, I'd like to give you an example of how to choose a specific battery. Let's say you wanted to build a portable audio system using the class D amplifier circuit I showed you in a previous video. You want it to be lightweight, but you want it to also be safe to use in a wide variety of weather conditions since you will be outdoors. The logical choice of battery chemistry here would be LiFePO4. And let's say we want it to have a battery life of at least 8 hours, so it'll last all day. The first thing we need to talk about is the voltage your device needs. According to the datasheet the TPA3122 expects 10 to 30 volts on the input. So whatever battery we choose should have a voltage range that works with this. Let's start exploring our options at batteryspace.com. I've been buying from these guys for 9 years and I think they have the best selection on the internet. As you can see we have a lot of choices for batteries with different voltages. These 19.2 volt packs would be right in the middle of the 10 to 30 volt requirement we have, so let's check them out. Here's a random battery I clicked on. According to the specifications, it's going to give us a minimum of 12 volts, a maximum of 21.9 volts, and an average of 19.2 volts. This is perfectly within our 10 to 30 volt requirement, so it looks like something similar to this will work. Now let's try to figure out the peak current requirements of the battery. Let's say we have built our amplifier, and using our bench power supply set to 21.9 volts we have found that the peak current draw for the system is 1.1 amps. So whatever battery we choose will have to handle peaks of at least 1.1 amps. It turns out that even the cheapest option we have can deliver 7 amps, so it looks like peak current capability is not going to be an issue for this project. If your project draws more power, pay very close attention to how much current your battery can safely deliver. Okay, that's the peak current taken care of. Now in order to figure out battery life, and make a final choice of battery, we want to figure out the average current draw in our project. Let's say we power the amplifier with 19.2 volts from our bench power supply, and at normal listening volumes we find that it draws an average of 300mA. 19.2 volts multiplied by 0.3 amps gives us an average power consumption of 5.76 watts. Earlier in the video we decided that we want the amplifier to run for 8 hours. So 8 hours multiplied by 5.76 watts gives us 46.08 Watt hours. So we need a battery with a capacity of at least 46 Watt hours. If you need more information about battery capacity calculations, check out my battery capacity tutorial video. Alright, let's go back to batteryspace, sort by price, and look for battery packs with at least a 46 watt-hour capacity. This one looks like it will work! Let's see if it meets our project requirements. Alright, this thing has a capacity of 63 watt hours, so it's going to give us significantly more battery life than what we need, but when you take into account the fact that all batteries lose capacity over time, in 2 years this battery pack might just barely satisfy the 46 watt hour requirement. So for now it's overkill, but it's a good long term investment. Next let's double check the voltage is okay. It looks like this battery pack has a different cutoff than the first pack we randomly looked at, but 15 to 21.9 volts is still exactly right for the project. We've got a max discharge rate of 7 amps, which is a lot more than the 1.1 amps we need, so no problems there. It has a protection circuit for overcharging, overdischarging, and overcurrent, so it should be pretty safe to use. I strongly recommend buying or building protection circuits whenever you work with lithium batteries. Finally, it would be a good idea to pick up a suitable charger for the battery pack. Alright, now you know the basics of different battery types, and know how to pick a battery for your project. If you want more detailed information about batteries, I recommend browsing batteryuniversity.com. Thank you for watching, and if you would like me to keep making videos like these, please check out my support page in the video description. Make sure you check out Patreon, which is kind of like an ongoing crowdfunding campaign that will help fund more videos. If you become my Patron, you get to see videos two weeks before everyone else!