Commuting - help, calculate how long a battery should last

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tibikefor2
08-22-05, 09:09 AM
All:

I was wondering if someone out there knows how to convert volts and milliamps to determine how long a bettery should run on a charge.

I will be getting a new custom battery pack this week. The old battery pack had 10 AA NimH batteries, each with 500 milliamps. The baterries were in packs of 5 and run parallel. Therefore each pack had 6 volts and the pack has a total of 1,000 milliamps.

My new pack will also have 10 AA Nimh batteries, but the difference is that each battery will have 4,000 milliamps instead of 500. I believe that the two parallel sets of 5 batteries will yield a total of 8,000 milliamps compared to the 1,000 milliamps of the old pack.

Does anyone know the equation to calculate how long a 10, 20 halogen and a combination of the light bulbs should burn with the new batter pack?

Thanks.

Ed Holland
08-22-05, 09:29 AM
The standard measure for battery capacity is not milliamperes but milliampere-hours (mAH). That is, a 1000 mAH battery will supply 500mA for 2 hours, 1000mA for 1 hour, 2000mA for 1/2 hour etcetera. This is the amount of charge that the cell can hold.

Next you need to know the current taken by a bulb, or combination of bulbs.

Power = volts x amps.

By rearranging this equation, a 10W, 6V halogen will draw 10/6 = 1.66A. Compared to the battery charge capacity, you might expect a 4000mA hour pack to run for a time defined by the amp hour capacity (4amp-hours) devided by the current draw (1.66 amps):

4/1.66 = 2.4 hours.

Two bulbs wired in parallel would draw twice the current of one, so the run time is halved and so on.

In practice, the full capacity of the batteries is rearely realised, so run times deviate (below) the simple theoretical calculation. This is due to factors such as the self-discharge that occurs even when the battery is not used, temperature of the cells during use etc.

Even so, in going from 500mAH to 4000mAH batteries, you should get close to 8 times the run time, for the same bulb setup.

Hope that helps,

Ed

So your new battery pack, with 4000maH cells should last 8 times longer in ideal conditions than the 500mAH cells.

tibikefor2
08-22-05, 09:41 AM
Ed:

Thanks. The added capacity should allow me to run 20 watts of light on one charge for both my morning and evening commute. Yeah!!!!!

Ed Holland
08-22-05, 11:19 AM
Well, that is the theory anyway. As I said, in practice you may not realise the full battery capacity suggested by the specifications - it would be advisable to test the system to determine the actual running time.

Good luck,

Ed

tibikefor2
08-22-05, 12:49 PM
Ed:

The pack will actually have 8,000 milliamps, as it will have two 4,000 packs run in parallel, or 8,000 milliamps. It is interesting as I was getting two hours of run time from the 2, 500 milliamp packs on the 10w bulb. The 2 hours was a little short for my commute round trip. If I keep the lights at 10w, I should have (in theory) no problems doing a round trip on one charge.

Thanks again for my help.

slvoid
08-22-05, 03:09 PM
Just remember too, the potential across the battery will drop, not very dramatic, but it will drop, as it discharges. So if you have a 4hr battery life, expect around 3.5 at best. That's also assuming it's fully charged every time.
That's also assuming it's warm. At 32 degrees F, or freezing, expect your battery to drop 30%.
Nimh's also provide only 1.2 volts give or take 1, rather than 1.5.

John Ridley
08-22-05, 04:17 PM
I'm a bit suspicious of your numbers. If you're really talking about milliamp-HOURS, which is the way that capacity of batteries is measured, then your numbers are way off. I don't think there ever was such a thing as a AA NiMH that only delivers 500 mAH. Current models are 2500 mAH, even the oldest ones I have were rated 1600, I've never seen anything below 1400.

500 mA might be the recommended max drain rate for those cells. If you were drawing close to that or exceeding that, then you would NOT be getting full capacity from the cells.

That's because batteries need to be derated the faster you discharge them. A 4000 mAH battery will only really deliver 4000 mAH if you drain it slowly, like 50 mA rate, which should keep going for 4000/50 = 80 hours. If you start draining it at an amp, you'd think you should get 4 hours, but really you'll probably get 3 to 3.5. If you drain it at 2 amps (2000 mA), you'll be lucky to get an hour and a half.

Also, keep in mind that incandescents are VERY inefficient when attached to a battery, because as the battery discharges, the voltage drops, the filament cools a bit, it puts out less and more yellow light, and it starts drawing MORE current. That runs the battery down even faster.

So at the tail end of the battery life, your light will get dimmer fast and the battery will tank.

Try not to EVER let the battery totally tank, it's bad for them. NiMH does not have a significant memory effect so there's no need to cycle them. CERTAINLY don't ever leave the light running unattended.

After all that, the real answer is, you're going to have to try it to know the real runtime, though it sounds like you'll have plenty. Heck, you can probably do with just one of those packs. Also the runtime will be less if you ride it in the cold, and it'll drop as the pack ages.

slvoid
08-22-05, 04:47 PM
Maybe he means AAA?

tibikefor2
08-22-05, 05:07 PM
John:

The batteries said 500 ma on them. If I can get 3 hours, I would be quite happy. My commute is on average 1:05 to 1:15. If I get 3 hours, then I have 30-50 minutes of a fudge factor. I will have a backup LED in case I ever run out of juice and then there is always the cell phone where Mrs TiBike will be on the other end. I just know that the 2 hour runtime that I was getting is insuffcient.

The return of the battery pack will be Thursday and I will run a test on Friday.

Thanks for your input, I appreciate getting information from people who know way more than I do.