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Comment by skohan | Hacker News Reader

How efficient is wireless charging?

As a pandemic hobby, I've gotten into building batteries to power light vehicles, and it's an interesting problem to try to fit the maximum capacity you can into as small a package as possible. Efficiency is a huge concern here, as the more energy you lose to, for instance, a voltage conversion, the bigger your battery has to be to meet the required use-case.

So this kind of wireless charging is neat, but if there's a significant efficiency difference to just plugging in a power bank, it would be hard to feel like it's worth it.

This might be cool for something like a camera, where hot-swapping batteries faster has a major benefit to the use-case.

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Not especially efficient in practice. In general you can expect in the ballpark of a 50% loss in efficiency with consumer electronics. This is mostly due to design/usability compromises though. In theory you can get 90+% efficiency with ideal conditions.

It's just a transformer right so under ideal-ideal circumstances you could get 99% or more. Large power transformers are ~99.7% IIRC.

There's no possibility of a core here though, which makes it inductive coupling, not a transformer, and reduces the coupling efficiency.

Indeed, thank you for the clarification! Electrical engineering is more of a hobby for me, so I'm always eager to learn more.

Can you tell us more about what kind of vehicles you are building? Are they big enough for people? I'd love to hear more about your experiences around this!

I'm actually not building the vehicles, just the batteries for aquatic applications. I've got a small zodiac, and I've been experimenting with powering a small electric trolling motor.

The thing is, most trolling motors for a boat this size run on 12V, because lead-acid batteries are the standard for marine batteries. The energy density of lead-acid batteries is terrible, so you need a pretty big battery for a little tiny boat which doesn't make a lot of sense. You can do a little bit better with a lithium-iron-phosphate battery, but since these are the "new" tech in marine batteries, you pay a really big premium for these which is absurd when you look at the price of the cells themself. So you are paying like 3-5x the price of materials just to get it in a waterproof case.

Also LiFePO4 batteries have a pretty poor energy density when compared to lithion-ion batteries, which are the current standard bearer. The problem with li-ion batteries is that there's not a great way to get them to 12V. Depending on the configuration, you can get a voltage nominal of 10.8V, or 14.4V, both of which put you a little outside the range of what's optimal to power most 12V motors when you're close to min or max voltage. Also there's some advantages to building a 24V battery instead of 12V. Also you have to be really careful about waterproofing, since li-ion batteries are extremely volatile.

So those problems are solvable, and I managed to make a battery which is about the size of a box of tissues, and can power my boat for up to a couple of hours, depending on how many people I bring with me. It's pretty fun to do, and I have all the stuff now, so I'm thinking about doing an e-bike or electric scooter next.

Also I'm thinking about trying to drive the boat with a brushless aquatic thruster, since the trolling motor itself is not that efficient by current standards. But there's a lot of fabrication involved there which I would still have to teach myself.

That sounds like an incredible project, if you have a write-up I'd love to read it.

Thanks! I haven't documented it yet, but I will have to get around to it.

The energy density of LiFePO4 shouldn't be an issue in that application, unless you're having to carry a lot of them on foot or scratch on weight limits.

Regarding the motor, I'd suggest to look at enthusiast RC hardware, which tends to be efficient by necessity: great power density at these sizes is a question of cooling, which is easier if the components are highly-efficient.

Yeah where I am boating, I have to go a couple hundred meters by foot to get to the water, so optimizing for weight was one of my primary design constraints. Also it was partially just out of interest to see how small I could go.

And I have been looking at RC motors! The main challenge there would be figuring out how to mount it - those are the skills I think I need to level up a little bit.

For such a footpath, I'd suggest to get a vest-style carrying harness for them. May work to just have that come with a plug and throw it in a storage case/bag on the boat.

It's surprisingly comfortable to carry a lot of weight if you're just heavier than normal.

I really love hearing about your personal experiences with this, and I think a lot of other people do too.

if it were low power electronics you were powering, I would suggest taking the battery to 14.4v and using some diodes to drop the voltage down a little bit. There would be some power dissipation in the diodes and that would increase with current, so it's probably not a great solution.

It might not be that bad to run the motor out of spec a bit. It sounds like you have a good solution. Did you consider putting the lead acid battery in a towable floatation device of some sort?

> Also LiFePO4 batteries have a pretty poor energy density when compared to lithion-ion batteries, which are the current standard bearer.

In comparison to generic, consumer LiCo, they are better, much better.

How do you even get started building a battery? You should blog about it, I'd read it :)

I've done it too, honestly there's not that much to write about it I thin.

Heat management during fast charging cycles is the reason why you end up with a less dense structure than the theoretical maximum.

Yeah, tbh, the hardest thing for building a custom E-Bike battery pack is the ridiculous prices and downright unavailability of cells!

Best I can get are 32700 LiFePO4 6000mAh cells for 5 Euros each... A prebuilt Li-Ion Chinese pack with an extra 2Ah is just 30 Euros more than the cells I would need alone (housing+BMS cost not included). Just sad.

Idk, cells are commodity priced, so currently I think you just have to bite the bullet and shell out a couple hundred bucks if you want to build a battery.

Used cells are also an option, and there's some really good ones out there.

Anyway Li-ion and LiFePO4 cells are going to seem cheap once solid-state cells hit the market.

Nah, cells aren't commodity prices, not even 18650 and certainly not anything LiFePO4. If you establish a relationship with the factory or a supplier you can send prices crashing down.

Maybe because typical Li-Ion 18650 cells are significantly cheaper per Wh than LiFePO4?

Sadly, no, 18650's cost the same for half the capacity. Likely because they're in high demand right now and car manufacturers have hogged up the best of them.

Hence, LiFePO4, heavier, lower voltage, less capacity, but also safer tbh.

> How efficient is wireless charging?

Not efficient

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