I’m a runner and have one of these GPS gizmos that straps on my arm, and transmits data to a wristwatch that keeps track of time and distance. The thing requires three AAA batteries. Depending on (well, that’s my question), I get anywhere from 6 to maybe 12 hours of use from a set of batteries. Since the thing needs three, and batteries are sold in even numbers, I will occasionally have to mix batteries of different brands, and quality. By “quality” I just mean the capacity of the battery, or, how long it lasts. Since I sometimes have to buy these batteries “on the run” so to speak, I wouldn’t expect consistency in what’s available.
So my GQ is, if I mix a high-end battery with one or two cheapies, what determines the duration of use for the set? I can think of two possibilites:
The set will last as long as the lowest capacity battery in the set. In other words, when the lowest capacity battery is drained, the device goes dead, and whatever energy was left in the high-end battery is wasted.
The duration of the set is governed by the sum of the capacities of all three batteries. In this case, there’s no waste, but you get less time by mixing in a cheap battery than if you used three good ones.
So, I appeal to the experts of the SDMB. Which option is true, or closest to the truth? Or is truth somewhere in between? Are there other factors to consider?
Don’t ever mix battery types or old with new batteries (really cells, but everyone calls them batteries). Always replace them as a fresh set of the same type. When you mix batteries, one cell will go dead before the others, and the remaining batteries will try to push a current through it, causing overheating and leakage. No, it probably won’t explode, but battery electrolyte kills electronics by promoting rapid corrosion.
I’ll second that rechargeable suggestion. They’re expensive initially, but will save you a fortune in the long run. Go with the NiMH ones, keep 'em cool, and don’t fast-charge them.
The weaker battery will be the determining factor. Lets say that your device has a shut off voltage of 3V. Normally a fresh set of Alks will start at 1.7V each for a total of 5.1V. The voltage will fall till each has about 1V left, then your GPSR will shut off.
If you have a week battery or slightly used as they deplete you may get into a situation where one is at 0.6V and the other 2 are at 1.3V. The 0.6V is getting likely to leak at this point, the other 2 have some life left.
If you take a totally drained battery 1V and combine it with 2 new ones 1.7V, you could possibly drive that one to 0V and still have enough to continuine using the GPSR then you could reverse charge it.
If you only replace 1 battery you would be in better shape as the device should turn off before the other 2 start leaking.
Any device that takes 3 batteries is going to be a problem for frequent use just due to the way batteries are packaged, try to buy 12/24 at a time from the same lot.
As stated above rechargables are great for such usage. In my GPSR I get 12-14 hrs from Nimh and 18-20 from alks so about 2/3rds the runtime. I find the real benifit is not having to worry about wasting the batteries, I can use the backlight on full, leave the thing on in the car, that is unless I think I’m not going to get to replace them soon.
Okay, I’m learning something.
How about this - I have an old Radio Shack VOM somewhere around the house. I could check the voltage of each cell. Is it fair to say that I should try to match cells with the same voltage in order to get the best result? That way, I could weed out the weak ones.
My main concern is when I go out on a 2-3 hour run. I can (and usually do) carry spares, but I hate stopping to change batteries. But I also hate tossing out a potentially useable one.
(yeah, I know what your’re thinking - this guy bought a fancy $150 wristwatch gizmo to use in the most low-tech sport in history, and he’s worried about a lousy buck or two worth of batteries?)
No, since cell open-circuit voltage isn’t indicative of cell capacity, even among cells of similiar type. Howver, youc an generally consider a cell to be “new” if it’s been unused and kept in a temperate environment for quite some time after it was purchased. This is especially true for alkaline types, which generally have a long shelf life and low internal current leakage.
With rechargeables you can start every day with a fully charged set. Another way to save big on batteries is to rig an external pack of D size batteries which you can carry in a belt pouch or other convenient place. I sometimes do that with my digital camera. The cost per watt*hour will be somewhere between 1/5 and 1/10th of using AA. You will be amazed how long they’ll last you.
you could use a voltmeter Race Bannon but it’s just going to give you a rough idea. A battery tester would be a better choice as they will give a load to the battery being tested.
In my experence alks having 1.2v or less when not just used (lets say sitting in the battery draw) are pretty much shot.
It seems like the idea of rechargables are not reaching your eyes, are they totally out of the picture?
Kanicbird, No, rechargables are not out of the question for me, but they do seem to be more trouble than they’re worth. I may change my mind. I know I have a Ni-MH cell in a mini-disc player that I also run with (but the cell is not something you see in your average retail store) and it seems to last forever. If I could get that kind of duration for my GPS receiver, I’d be very happy.
My questions are more along the lines of understanding the variation and limitations of different retail batteries. I’m not interested in mixing “old” batteries with "fresh"ones. But, due to the fact that I sometimes have to buy what I can find while I’m out in the middle of a run, I’d like to know why I should or shouldn’t mix my leftover (but still relatively “fresh”) high priced cells with some cheap ones I grabbed at the 7-11.
When I’m out running, I’m more interested in my own energy/ hydration than the condition of my gizmos.
What QED said. Even if all batteries are fresh, you don’t want to mix different types because some will run down faster than others. You risk damaging the device.
Rechargeables are OK, but the problem is that it’s difficult to find a charger that handles 3 batteries. It’s annoying to have to use up two sets of 3 and charge them 4 or 2 at a time. I think the best solution is to stick to one brand of battery, so you can always find 2 matching batteries for that leftover battery. Or buy batteries in 12 packs.
One bicycle light manufacturer (CatEye) solved this 3-battery problem by designing their LED lights to accept 4 batteries. It only takes about 4 volts to run an LED so 3 batteries are enough, but they take 4 anyway and use a resistor to burn off the excess energy. (Yes I checked with a voltmeter; there’s almost 2 volts across the resistor.)
If you’re using 3 cells, then a lithium-ion polymer cell should work fine, as well as having much more capacity. Li-poly cells are 3.6V. There is a very popular company called Kokam making them…you can get them at www.b-p-p.com. These cells are taking the radio-contolled airplane world to the next level, making it possible to get the same duration as gas-fueled models. You’ll have to get a special charger, but in the long run they’ll pay from themselves, and one cell (perhaps a 1020mah) should last you long enough…there are many larger sizes as well.
To expand on this, a typical red LED has a forward voltage drop of approximately 2 volts, and a maximum forward current on the order of 20 mA. Typically, they are not run anywhere near max current, so a resistor is chosen to limit the current through the LED to a design value of around 10 mA. If the resistor were not present, the current would rise dramatically after the forward turn-on violtage was reached, quickly approaching infinity (in practice this is limited by the internal resistance of the battery, which is generally fairly low.) The resistor value is calculated bye the formula (V[sub]bat[/sub] - V [sub]F[/sub]) / I[sub]limit[/sub]. Using the above values, and assuming 4 x 1.5 V cells, 6 V - 2 V / 10 mA = 400 Ohms. 402 Ohms is a standard value, which is close enough for government work.
Then why does every other white LED flashlight on the market use 3 cells? I’ll grant you that the extra voltage allows for a more flat discharge curve, but I’d hardly call that necessary.
Well, it depends on the design, the number and type of LEDs and how they are connected. LEDs do not have a liner resistance with voltage and the current has to be limited somehow and most often this is done with resistors which can be external or embedded in the LED case.
White LEDs have a typical forward voltage drop on the order of 5 volts. 4 cells would be necessary to turn these on. Some of the newer white LEDs have forward voltages less than 4.5 volts and so can use 3 cells.
Here’s a suggestion for you. Check out ebay, search for AAA batteries. Just looking very quickly I found this
You might be able to find a better deal if you look more. When I got my digital camera, I went on eBay and found a box of 50 batteries (Duracell) for like $9.00 + shipping. I figure, even if 20 of them die before I can use them it’s still worth it.
I don’t mean to make a big deal out of this, but which LED has a 5 volt voltage drop? The white Nichia LEDs have a “typical” drop of 3.6 V and a maximum allowable votlage of 4.0 v. I just checked the Luxeon 1W LEDs and they are 3.42/3.99 V (typ/max).
I’m well aware that a current limiting resistor is necessary, and that the larger the voltage across the resistor, the more constant the current. However, most manufacturers have chosen to use 3 cells and a current limiting resistor that creates a 0.9-volt drop. CatEye alone uses 4 cells and a 2.2-volt drop (or something like that, I forget the actual measurement) across the resistor.