Robot batteries and the hurricane

Hi all,
I was wondering what you think of the following idea (and whether or not anyone knows the math to calculate how long the batteries will last).

Due to hurricane Irene my house will probably lose power. In order to deal with that and keep cell phones / laptops charged I had an idea. I have a couple batteries from the KOP at my house and I am planning on getting a 12V socket and hooking it up to the battery. I have a power inverter that plugs into a 12V socket and outputs two AC sockets and a USB socket. Does this seem like a feasible plan? If I am running an 85Watt power adapter off it my calculations say I should get ~24 hours but I didn’t know how to factor in the AC/DC conversion so I’m hoping you guys might be able to help me with that. Thanks!!
Oliver

It should work, but keep in mind each battery will only last for a limited number of charge/discharge cycles. Don’t do anything you will regret next year, but if it is necessary, I wouldn’t hesitate to do it. Just prioritize electricity usage, don’t leave the inverters plugged in when not in use, and don’t over-discharge the batteries.

Also, the number of amp-hours the battery will put out depends on battery condition, state of charge, discharge rate , and temperature. On top of this, efficiency is lost in the conversion from DC to AC and then for AC to DC, so it would be even more inefficient than a single conversion would be.

The KOP batteries are 17.2AH batteries. Your adapter can convert up to 85W so we’ll use that as the max operating condition since such things are usually rated by the amount they draw from the source, and not the output(so that they can use higher more impressive sounding numbers).

85W drawn on 12V taken from P=IV(Power = Volts times Current) 85=12(I)

Current drawn will be about 7 amps. Therefore at full draw you can run for ~2.5 hours. Depending on how much current draw your devices take you can run a lot longer since even a laptop charger doesn’t draw 85W(typically).

If you have a phone charger that only draws 10W then you can last for about 20 hours.

I wouldn’t worry about discharge/charge cycles, my team has abused batteries we’ve owned since 2001 and they’re still serviceable.

A quick way to figure out the draw of your inverter off the battery take the wattage of the draw from the inverter and divide by 10. This number will be the current draw in amps pulled from the battery. I.e. a 80watt load (laptop adaptor) will draw 8 amp at 12volt.

Dare I suggest that you try measuring the current draws as mentioned above? You could then get some closer estimates for your batteries’ life.

Thanks guys! I went ahead and gave it a shot just to test and it worked!! I decided to use the PD board (since the robot is at my house) because I figured it was safer that way. The system basically goes Battery -> PD Board -> 12V Socket -> DC to AC inverter -> AC Adapter for whatever device I want to charge.

We purchased an inverter from AndyMark that comes with an Anderson plug already on it so it can plug right onto a battery. it works great for powering the Classmate on the robot cart or at demos.

This is assuming 100% efficiency of the inverter. Assuming anywhere from 50%-90% efficiency and 85W output, the current draw on the battery side will actually be higher than 7A per whatever your efficiency is. May as well go with 14A, so cut all your times in half.

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.

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.

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.