Comment by ben_w | Hacker News Reader

Comment by ben_w | Hacker News Reader

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The Unitree links claim a 9000mAh battery will last it 2 hours; I don't know the voltage, but even at 5.5 V cells, that's only about 25 W average power consumption.

I find this difficult to believe, both because that's a very low power draw and because that would be penny-pinching on battery capacity.

My expectation on timescale is that genuinely general-purpose robots will need at least as much compute as a self-driving car (possibly more, that's a minimum), and have at most 1/10th the available power to do that (because they're physically smaller).

Between algorithmic improvements and Koomey's law, I think this will take at least 5 years between any given category of customer being able to afford no-steering-wheel-needed self driving cars, and the equivalent for androids.

Given the Waymos were doing geo-fenced cars with no safety drivers in the vehicles in 2017 (but still had an employee in the back with access to an emergency stop button)*, this gap is compatible with the recent press releases and youtube videos going around about robotics — but as nobody has yet started actually shipping this level of self-driving cars directly to end users**, my only guess for personal general-purpose domestic androids is: some time after 2030 as a minimum, but probably later than that.

* https://phys.org/news/2017-11-waymo-autonomous-vans-human-dr...

** No, Tesla's current one doesn't count; it will only count when it actually ships without a steering wheel or when people are actually allowed to use it while sleeping.

You misunderstand battery specs. It's a 13 string (more commonly, 13s) battery- a string is a number of cells wired in series. By convention that's a 48 volt battery, which matches with the 54 volt charger. In total that's a 421 watt-hour battery, for an average power of ~210 watts over 2 hours.

When you connect 2 9000 mAh cells in series, the resulting battery has 2x the voltage but the same mAh capacity. In parallel, the battery has the same voltage but 2x the mAh.

> My expectation on timescale is that genuinely general-purpose robots will need at least as much compute as a self-driving car (possibly more, that's a minimum), and have at most 1/10th the available power to do that (because they're physically smaller).

This seems logically flawed:

1. A car travels hundreds of miles from home. Why would a robot walk more than a while from a transport or home base? At short distances, if you truly need that power, it probably makes more sense to stream data/video over a direct low-latency connection. Long distance networking has a latency comparable to camera frame time, but even a normal wifi router can keep a lower latency than human nerve delay.

2. Humanoid robots will never be running at 85 mph past a bunch of people. They probably don't need to have the same compute throughput as a car, and definitely don't need to have the same reaction time.

Thanks for the clarification, that makes a lot more sense — 210 W / 421 Wh suddenly makes it seem totally reasonable on both counts.

Correcting the correction:

> When you connect 2 9000 mAh cells in series, the resulting battery has 2x the voltage but the same mAh capacity. In parallel, the battery has the same voltage but 2x the mAh.

The relevant units are:

* Capacity (Q, in mAh, Ah, kWh, etc)

* Power (P, in Watts)

* Voltage (U, in Volts)

* Current (I, in Amperes)

* Duration (t, in mostly measured in hours)

And the relevant formulas are:

* P = U x I or Power equals Voltage(difference) times Current

* Q = P x t or Capacity equals Power times duration

From this we can establish that connecting batteries in series or in parallel will not change their Capacity. When having 13 batteries of 29000mAh, or 29Ah, you have 13 x 29 = 377Ah or 377000mAh. Connecting batteries in series or parallel does make a difference in voltage and current: a string in series will increase the voltage while keeping the current the same (theoretically, in practice you get less than the current of the weakest cell); a parallel setup will increase the maximum current while keeping the voltage the same (again, in theory).

If you have two 1000 mAh cells connected in series, they will provide 1 amp for 1 hour. The battery is still 1000 mAh even though it is made of two cells with 2000 mAh total.

You are equating amp-hours with watt-hours, which is not reasonable. Q is charge, and is only proportional to a number of electrons. E is the energy, or a number of electrons at a voltage.

> When having 13 batteries of 29000mAh, or 29Ah, you have 13 x 29 = 377Ah or 377000mAh

By this logic, with those cells in series the whole battery would be at 48 volts and 337 amp-hours, giving it a storage of 18.1 kWh. That's despite being made of 13 cells with only 107 watt-hours each.

The only thing I don't like about car comparisons here is that, unlike cars, a household robot could have a plugged in offloaded computer. The robot could be stripped to sensors and motors, and whatever is necessary for failsafe operation, such as remaining balanced. Most other things a household robot will do could handle latency limiting it to a few hundred updates per second, and you could generally engineer around that latency... So the only power you need on the robot is enough to control the motors and sensors, and the limited onboard compute.

Edit: to clarify, my expectation is that the compute is on-location, so the latency is in the scale of <1ms more so than 10-100ms from cloud offloading

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