Robot batteries and the hurricane

[Cuog]

That's a pretty large efficiency gap. I did this calculation using the efficiency of the inverter I have(83% at full load) And still come up with a bit over 2 hours.

When calculating anything which can be highly variable you will of course need to build in a fudge factor. For example, we don't really know how close a battery after being used for FRC is to its original 17.2AH rating.

There is a reason that battery meters on laptops can be so inaccurate a lot of the time. There's a lot of variables to account for and trying to pin down an exact length of time something will last is only going to a better chance of being wrong.

Do your math for the best case scenario and figure out what worst case scenario is acceptable, and see if that leaves a realistic fudge factor.

[Tristan Lall]

Keep your screen brightness to a minimum, turn off the wireless radios, and set your computer to aggressively conserve power. It will draw far less than 85 W.

As an anecdote, I used to use a 5-cell, 8 A·h lithium polymer battery (5 × 3.7 V = 18.5 V) to drive a Pentium 4 Mobile laptop (which normally used a 95 W AC adapter with 19 V DC output). I rigged a barrel plug and an extension lead to the battery, and was able to get about an extra hour, or maybe an hour and a half out of it. (The computer thought it was running from AC, so it didn't take advantage of some of the meagre power-saving features in Windows XP, plus it probably tried to charge the computer's battery from the external pack.)

Any chance you can rig a system like this, that avoids the DC-AC-DC conversion?

[Mike AA]

Originally Posted by Tristan Lall

Keep your screen brightness to a minimum, turn off the wireless radios, and set your computer to aggressively conserve power. It will draw far less than 85 W.

As an anecdote, I used to use a 5-cell, 8 A·h lithium polymer battery (5 × 3.7 V = 18.5 V) to drive a Pentium 4 Mobile laptop (which normally used a 95 W AC adapter with 19 V DC output). I rigged a barrel plug and an extension lead to the battery, and was able to get about an extra hour, or maybe an hour and a half out of it. (The computer thought it was running from AC, so it didn't take advantage of some of the meagre power-saving features in Windows XP, plus it probably tried to charge the computer's battery from the external pack.)

Any chance you can rig a system like this, that avoids the DC-AC-DC conversion?

There are adaptors that are made for plugging into a car cigarett lighter or for airplanes. They're more expensive but a lot more efficient.

[Al Skierkiewicz]

Time to add a few things to the discussion.
Some inverters reach maximum efficiency only at full output.
Even at peak, there is a lot of loss, 83% at full output is likely less. Use 75% for your calculations.
The 17.2 AH rating is speced at a 1.8 amp load. If you draw more the rating goes down. You will get 17 hours of battery (until the terminal voltage falls to 8 volts) at a 1 amp draw. You will not get 1 hour of battery with a 17 amp draw. (curves say about 35 minutes) At 7 amps figure between 14 and 15 AH.
Batteries have a defined life of about 400 charge/discharge cycles. Less if the batteries are used hard in competition.