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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!

skohan 5y ago
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.
- seryoiupfurds 5y ago
  That sounds like an incredible project, if you have a write-up I'd love to read it.
  - skohan 5y ago
    Thanks! I haven't documented it yet, but I will have to get around to it.
- namibj 5y ago
  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.
  - skohan 5y ago
    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.
    - namibj 5y ago
      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.
- a_conservative 5y ago
  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?
- baybal2 5y ago
  > 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.

skohan 5y ago

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.

seryoiupfurds 5y ago
That sounds like an incredible project, if you have a write-up I'd love to read it.
- skohan 5y ago
  Thanks! I haven't documented it yet, but I will have to get around to it.
namibj 5y ago
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.
- skohan 5y ago
  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.
  - namibj 5y ago
    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.
a_conservative 5y ago
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?
baybal2 5y ago
> 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.