Replacing batteries
October 9, 2006 7:42 AM Subscribe
On a device with 2 batteries, say for an example, a remote control with 2 AA batteries, the instructions for the device say, when the time comes, to replace both batteries, not just one. As if replacing one battery, while it will supply enough energy to run the device, will somehow damage the device. Is this just a ruse to sell more batteries?
Well, IANAEE, but I was under the impression that if you put in one dead battery and one full battery in, it would be attempting to draw the full voltage (2x1.5, in the case of AA?) from the full battery (since if I recall, they're always connected in series?), which either just wouldn't work, or would overload the full battery? Or something along those lines? But I could just be talking out of me arse.
Aside from that, I've found that if one battery is dead, the other one is going to be pretty much dead as well, and it's not like they sell batteries in singles, so it just makes sense to chuck both the dead ones, and replace them with two fresh ones so I don't have to replace them again in the future (and at the same time, I don't have to remember which one was the fresh one and which the dead, the next time I replace my batteries)
posted by antifuse at 8:15 AM on October 9, 2006
Aside from that, I've found that if one battery is dead, the other one is going to be pretty much dead as well, and it's not like they sell batteries in singles, so it just makes sense to chuck both the dead ones, and replace them with two fresh ones so I don't have to replace them again in the future (and at the same time, I don't have to remember which one was the fresh one and which the dead, the next time I replace my batteries)
posted by antifuse at 8:15 AM on October 9, 2006
They are in series, and their voltages add. By the time the need replacing, they will have declines from about 1.5 V to about 1.0 V or so.
Their internal resistance also increases as they discharge. If you replace one, it will boost the combined voltage and they will probably power the remote for a while, but eventually, the weaker one will limit the available voltage and current to the circuit, and you'll have to replace it, too.
These things usually power an infrared LED that you can't see. It is brightest when the batteries can provide adequate current. Two fresh batteries will do a better job than one weak and one fresh.
If you want to see the LED, you can shine it into your camcorder or digital camera with the viewfinder on. Then you can see the brightness difference between your "replace one battery" strategy and the recommended one.
IAAEE, and I recommend changing them both. Use lithium for maximum life... Alkaline if you're cheap. Carbon zinc if you are really cheap or get free batteries somewhere.
posted by FauxScot at 8:41 AM on October 9, 2006
Their internal resistance also increases as they discharge. If you replace one, it will boost the combined voltage and they will probably power the remote for a while, but eventually, the weaker one will limit the available voltage and current to the circuit, and you'll have to replace it, too.
These things usually power an infrared LED that you can't see. It is brightest when the batteries can provide adequate current. Two fresh batteries will do a better job than one weak and one fresh.
If you want to see the LED, you can shine it into your camcorder or digital camera with the viewfinder on. Then you can see the brightness difference between your "replace one battery" strategy and the recommended one.
IAAEE, and I recommend changing them both. Use lithium for maximum life... Alkaline if you're cheap. Carbon zinc if you are really cheap or get free batteries somewhere.
posted by FauxScot at 8:41 AM on October 9, 2006
These days, batteries are available with different internal chemistries, for different expected uses, and so what I say here will differ in some details, according to the physics of different chemistry packages.
But generally, things like TV remotes which use a couple of AA batteries in series to get a 3V power source for their internal electronics, work best with either carbon or alkaline batteries. These cells generally deliver pretty close to 1.5 volts DC, until they are more than 90% discharged, but as they discharge, their internal resistance goes up, and they're capable of delivering less and less current to the circuit while maintaining rated voltage. Eventually, when nearly fully discharged, they can still produce 1.15 to 1.25 volts into a nearly infinite impedance (open circuit), but drop voltage immediately when asked to supply any current. Shortly after that, the chemical cell can fail mechanically, and then they can produce no voltage, period. And leaking batteries, particularly carbon or alkaline, do nasty things to the inside of TV remotes. So, you want to see the death of batteries in devices they power as an early leakage warning, and get them out of the device, so they don't corrode it when their chemical cells burst and leak.
So if you replace only one of 2 cells in a series set up, they can perhaps, slowly charge the small anti-noise capacitors on the TV remote's internal circuit board through the high internal resistance of the weak cell, enough that you may actually even get the remote to work for one or two channel changes at a time. But the internal resistance of the weak cell is enough to prevent full current being delivered through the cell pair, and it continues to rise, until the unit doesn't work at all. In the meantime, if there are some low voltage gallium arsenide LEDs used in your TV remote, they may be driven by a 1.5 volt power point, tapped off the middle of the battery bridge formed by the 2 series battery cells. So, in those cases, you can have a power source for the LED's that is good, but an unreliable power source for the nominally 3 VDC control electronics, with some possibility to prematurely discharge the "good" battery through the output LED's (and maybe damage the LED's through continuous duty operation). This latter possibility is pretty low probability these days, but sometimes happened with older controls using simpler control logic designs.
So, the practical advice is that batteries are cheap, so why not replace in pairs? You save nothing really by only replacing one, and you have the aggravation of an unreliable or damaged TV remote, until you have 2 good cells working together.
posted by paulsc at 8:47 AM on October 9, 2006
But generally, things like TV remotes which use a couple of AA batteries in series to get a 3V power source for their internal electronics, work best with either carbon or alkaline batteries. These cells generally deliver pretty close to 1.5 volts DC, until they are more than 90% discharged, but as they discharge, their internal resistance goes up, and they're capable of delivering less and less current to the circuit while maintaining rated voltage. Eventually, when nearly fully discharged, they can still produce 1.15 to 1.25 volts into a nearly infinite impedance (open circuit), but drop voltage immediately when asked to supply any current. Shortly after that, the chemical cell can fail mechanically, and then they can produce no voltage, period. And leaking batteries, particularly carbon or alkaline, do nasty things to the inside of TV remotes. So, you want to see the death of batteries in devices they power as an early leakage warning, and get them out of the device, so they don't corrode it when their chemical cells burst and leak.
So if you replace only one of 2 cells in a series set up, they can perhaps, slowly charge the small anti-noise capacitors on the TV remote's internal circuit board through the high internal resistance of the weak cell, enough that you may actually even get the remote to work for one or two channel changes at a time. But the internal resistance of the weak cell is enough to prevent full current being delivered through the cell pair, and it continues to rise, until the unit doesn't work at all. In the meantime, if there are some low voltage gallium arsenide LEDs used in your TV remote, they may be driven by a 1.5 volt power point, tapped off the middle of the battery bridge formed by the 2 series battery cells. So, in those cases, you can have a power source for the LED's that is good, but an unreliable power source for the nominally 3 VDC control electronics, with some possibility to prematurely discharge the "good" battery through the output LED's (and maybe damage the LED's through continuous duty operation). This latter possibility is pretty low probability these days, but sometimes happened with older controls using simpler control logic designs.
So, the practical advice is that batteries are cheap, so why not replace in pairs? You save nothing really by only replacing one, and you have the aggravation of an unreliable or damaged TV remote, until you have 2 good cells working together.
posted by paulsc at 8:47 AM on October 9, 2006
eventually, the weaker one will limit the available voltage and current to the circuit, and you'll have to replace it, too.
Yes, and so doubling the number of times you have figure out which one is the culprit for little if any benefit. Some devices have 3 or 4 batteries - depending on how many you have, your change-only-the-most-depleted strategy would have you changing batteries constantly. I guess it would help to label each battery with an installation date so you know which is the most likely weakest link but still...
posted by scheptech at 8:56 AM on October 9, 2006
Yes, and so doubling the number of times you have figure out which one is the culprit for little if any benefit. Some devices have 3 or 4 batteries - depending on how many you have, your change-only-the-most-depleted strategy would have you changing batteries constantly. I guess it would help to label each battery with an installation date so you know which is the most likely weakest link but still...
posted by scheptech at 8:56 AM on October 9, 2006
There are two reasons to replace both batteries. The first has to do with voltage. Each new battery produces 1.5 volts which add together to make 3.0 volts. As the batteries are discharged, their voltage goes down to, let's say, 1.1 volts (2.2 volts total) and the device stops working. If you replace with just one battery, you would think that you have fixed things so that they will run half as long. But you really have a 1.1 volt battery and a 1.5 volt battery for a total of 2.6 volts instead of the 3.0 volts the device expects so it may not work as well.
The second reason is internal resistance. All batteries have internal resistance that uses up some of the energy in the battery and wastes it as heat. Internal resistance goes up as the battery is discharged. Since the batteries are connected in series, all of the current has to pass through both batteries. If you replace only one battery, the second still has a high resistance and wastes more energy from the good battery. So you get even less useful energy from the good battery. The internal resistance also lowers the voltage output from the battery combination, so in the example above you will get even less than the 2.6 volts you might expect.
In high current applications, it is possible that the higher resistance in the bad battery could cause it to get warm enough to leak, although battery leakage is rare in today's batteries.
posted by JackFlash at 8:59 AM on October 9, 2006
The second reason is internal resistance. All batteries have internal resistance that uses up some of the energy in the battery and wastes it as heat. Internal resistance goes up as the battery is discharged. Since the batteries are connected in series, all of the current has to pass through both batteries. If you replace only one battery, the second still has a high resistance and wastes more energy from the good battery. So you get even less useful energy from the good battery. The internal resistance also lowers the voltage output from the battery combination, so in the example above you will get even less than the 2.6 volts you might expect.
In high current applications, it is possible that the higher resistance in the bad battery could cause it to get warm enough to leak, although battery leakage is rare in today's batteries.
posted by JackFlash at 8:59 AM on October 9, 2006
If you don't replace them together, the strong battery will attempt to recharge the weaker battery. Since alkaline batteries are not rechargable, this will only result in the weak battery overheating, and perhaps leaking as well.
posted by gmarceau at 9:01 AM on October 9, 2006
posted by gmarceau at 9:01 AM on October 9, 2006
Just as a counterpoint as to why not just replace both -- if, as happened to me this summer, you have only two fresh batteries and your headlight takes three then you have to make the choice whether to try to get by without the headlight or, as I did, power it on two fresh and one tired battery and find out first hand if the warnings were real. So there are plenty of cases where one might want to do this and so its a reasonable question.
The headlight worked fine and when I could get more batteries (I was working in a pretty remote location) I popped in a third fresh. I've had to use the same process with GPS batteries as well and it can get you through in a pinch and doesnt seem to hurt the device. (speaking of, you can stick exhausted GPS batteries into LED flashlights and they are almost as good as new...)
posted by Rumple at 11:01 AM on October 9, 2006
The headlight worked fine and when I could get more batteries (I was working in a pretty remote location) I popped in a third fresh. I've had to use the same process with GPS batteries as well and it can get you through in a pinch and doesnt seem to hurt the device. (speaking of, you can stick exhausted GPS batteries into LED flashlights and they are almost as good as new...)
posted by Rumple at 11:01 AM on October 9, 2006
The instructions are designed so that any moron can successfully get optimal and satisfactory performance from the device as intended. The issues and caveats raised by replacing just one battery require a fairly lengthy explanation (as seen by the answers above), and product designers know that Joe Moron will blame the product, not his own actions, if the remote signal isn't strong enough to work without being precisely aimed at the TV, or if it seems requires a new battery constantly.
posted by -harlequin- at 1:11 PM on October 9, 2006
posted by -harlequin- at 1:11 PM on October 9, 2006
This thread is closed to new comments.
posted by dsword at 8:12 AM on October 9, 2006