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I must admit, I'm an electronics noob. I work in software, and I'm starting to realize that the world of electronics is more complicated than I had anticipated.

In a previous project, my brother built an electric truck using 24 deep-cycle lead-acid batteries. The batteries were similar to this battery, which has the following specs:

  • 6V high capacity Deep Cycle power
  • 230AH at 20 hour rate
  • Weight of 24 batteries: 696kg

As some of you may have heard, Tesla just announced the Powerwall 2, which has the following specs:

  • 14kWh
  • Cost per kWh: $393
  • Comes with an inverter (I have no idea what this is or if it's necessary)
  • Weight: 110kg

The electric motor for this car conversion has a controller that maxes out at 3000 amps. Normal cruising amps is around 1000.

Including a Powerwall unit might not be practical for use in an automobile, but I'd like to get a better apples-to-apples comparison of one over the other, as it's possible that a similar type of battery might become available for use in cars.

Obviously the Powerwall unit weighs a lot less, but does it provide the same amount of power? And how does it compare in terms of energy storage capacity?

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  • \$\begingroup\$ 6V * 230 Ah == 1380 Wh == 1.38 kWh. *24 == 33 kWh. Anything more you need? \$\endgroup\$
    – Janka
    Commented Nov 1, 2016 at 20:49
  • \$\begingroup\$ The inverter converts battery DC to house AC. You'd need a different system - a speed controller. \$\endgroup\$
    – pjc50
    Commented Nov 1, 2016 at 20:51
  • \$\begingroup\$ as it's possible that a similar type of battery might become available for use in cars Uhm, I'm quite sure the Teslas I see driving around here and there do not use Lead-Acid batteries, they use Lithium based batteries. \$\endgroup\$
    – Bimpelrekkie
    Commented Nov 1, 2016 at 20:52
  • \$\begingroup\$ @Janka Thank you! That's the conversion I was missing. Curious, I had assumed that the Powerwall would have more storage capacity than 24 oldschool lead-acid batteries \$\endgroup\$
    – samcorcos
    Commented Nov 3, 2016 at 2:20
  • \$\begingroup\$ @pjc50 Interesting, that's good to know. It looks like these inverters cost ~$1000. Is that normal? Is that what a speed controller would cost? \$\endgroup\$
    – samcorcos
    Commented Nov 3, 2016 at 2:21

2 Answers 2

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You need a 1-3kW ac inverter and it will last 20 hrs at a 3kW flat load.

230Ah*6 = 1380 Wh/bat *24 = 33 kWh

or 33 kWh / 696 kg = 48 Wh/kg vs 127 Wh/kg for Powerwall

  • $110 * 24 = $2640 .../33kWh = $80/kWh DC only for your Lead Acid

  • vs $393/kWh for Powerwall2 which includes DC-DC and DC-AC

  • a 3kW inverter might be around $800 + extra features

    • ** The inverter gives AC power out so you can use during AC power rates during the day.**

  • thus your cost /kWh increases ~40%

So yours appears cheaper but more bulky and thus longer more expensive cables and consumes more storage space and weight > 700 kg

  • meanwhile nuclear product costs are around $0.03/ kWh and hydro power is the cheapest
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  • \$\begingroup\$ Just what I was looking for! Thank you. The weight is the biggest problem. If the batteries weren't so heavy, the electric truck could likely go twice as fast and three times as long. Not sure if there are any other reasonable alternatives... Sounds like lithium ion is quite dangerous to drive around with without encasing it in titanium or some such metal. \$\endgroup\$
    – samcorcos
    Commented Nov 3, 2016 at 2:27
  • \$\begingroup\$ Tesla modules are pretty rigid. I've seen the load tests. \$\endgroup\$
    – D.A.S.
    Commented Nov 3, 2016 at 2:44
  • \$\begingroup\$ The danger of lithium ion has little to do with the the enclosure (aside from crashes I suppose) -- the problem in general is energy density. More energy density is more volatile and more dangerous. The hard thing is how to charge them. You have to manage a chemical reaction that changes over time and can't let it get out of hand. Also, I had to refill the lead acid batteries every few months with deionized water which was a pain. \$\endgroup\$
    – Chet
    Commented Nov 3, 2016 at 3:15
  • \$\begingroup\$ @Chet but isn't energy density the whole point? Gasoline is more energy dense and more unstable, but we still use that as a fuel. \$\endgroup\$
    – samcorcos
    Commented Nov 4, 2016 at 1:54
  • \$\begingroup\$ the critical temp Tc is the key turning point for thermal runaway. somewhere after 85'C internal \$\endgroup\$
    – D.A.S.
    Commented Nov 4, 2016 at 2:31
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The powerwall is an adaptation of Tesla's car battery systems for domestic use. The car power pack can be seen here: https://electrek.co/2016/02/03/tesla-battery-tear-down-85-kwh/

544kg; 85kWh; maximum power output 350kW. Obviously beats the lead-acid batteries by a long way, but requires more careful handling. Has to be kept cool and could potentially cause a nasty, unextinguishable fire if punctured. That's why it's kept in a titanium box. The use of lots of individual cells rather than one huge cell helps with cooling and safety.

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  • \$\begingroup\$ Yikes, that's good to know! Maybe I shouldn't mess around with lithium ion then! \$\endgroup\$
    – samcorcos
    Commented Nov 3, 2016 at 2:23

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