2

According to the Tesla website the new Powerwall operates at 350 to 450 volts? Why such a high voltage when most solar battery banks operate at 48V?

3

The Powerwall is/will be based on the lithium ion battery technology used in the Tesla cars, so instead of a few large lead acid batteries (like in PV battery banks) it will contain a zillion tiny 18650 cells, which work better if you put them in series and let the voltage soar. This keeps the current low while still allowing the same amount of power output (good old ohms law). The hardware to turn the current into a sine wave for consumption (called an inverter) also becomes simpler to design because it has high voltage to start with, so it doesn't need a boost circuit or a transformer to generate 120/208/240v RMS.

  • That's great. Thanks. Any idea why traditional solar battery banks don't do this too? (given the ease of inverting later) – Not a chance May 18 '15 at 2:16
  • Another question arises (should it be a new topic?) how do you charge at this voltage? Seems you'd need all your solar panels in series (good for cheaper wiring) which may require a minimum number of panels and may make charging difficult when panels degrade. – Not a chance May 18 '15 at 2:25
  • Lower cost of 12v lead acid batteries (the current cheapest bulk storage medium) and the relative danger of having a >48v bus that is not 100% self contained. With batteries, once you get past 200V in a string you are in the instant-death territory if you come between a terminal and the ground, and since PV systems are custom built and sit in the open most home users don't want this risk. You would need to build a very sturdy enclosure (i.e. a locked room with a steel door) to do it safely. Enter the Tesla solution: wall mounted, completely sealed off from the user. – Jeff Meden May 21 '15 at 13:10
  • Sorry to bug you. Wouldn't stepping up to charge the batteries (in the powerwall case) cause as much difficulty as stepping up to invert (in a low voltage bank case)? – Not a chance May 22 '15 at 3:26
  • No because Lithium cells are often charged in smaller sections for balancing purposes. I dont know the exact details of the internals of the Tesla Co. batteries (few do) but in most Lithium applications you want to charge in smaller chunks so that you can more evenly charge the string, since Lithium cells are very sensitive to over/under charge. So, they could very well be charging in 30-40v sections and only using the full 350-450v string when discharging. – Jeff Meden May 22 '15 at 12:10

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.