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Battery Tips!

Last update: February 2013


Which kind and brand of battery is best?

See the home page for which kind of battery is best, and which brand of battery is best.  For NiMH specifically, see my comparison of NiMH brands.

You can mix brands together

The manufacturers tell you that you should use only the same brand batteries together, but they don't provide any good justification for that.  Whether alkalines or rechargeable, voltage is voltage, and your device neither knows nor cares who made the batteries.  Your device is certainly not going to tattle on you to the manufacturers.  Now, if your alkalines had very different capacities then that could promote leaking, but the difference between most alkalines isn't that great, and you shouldn't really be using alkalines for most purposes anyway.  For rechargeable batteries, again, just try to match the capacities as close as possible, not to prevent leaking (since rechargeables almost never leak), but to avoid reducing the cycle life of the lower-capacity battery.


How much money do I save with rechargeable batteries instead of alkaline?

For a device that goes through a set of four AA's every 15 days (e.g., use the device 1 hour a day and a set gives 15 hours of runtime), switching to rechargeables saves about $100 a year.  And yes, that includes the cost of recharging the batteries, which is negligible.  The alkalines cost around $99 a year, vs. only 31¢/year in electricity to charge the NiMH batteries, after a first-year cost of $25 for a charger and a set of batteries.   (Showing the ret of my work:  365 days a year ÷ 15 days/set = 24.3 sets/year,  x $3.78(+8% tax) = $99 for the alkalines.  Charging cost is figured as: 20 wH to charge a single AA, x4 in a set, x 24.3 sets a year, x 16¢ per kWh.)

How much does it cost to charge a NiMH AA battery?

In electrical costs, less than a penny.  Charging it three times would cost about a penny.  This assumes an electrical rate of 16¢/kWh.


Possible voltage problem with NiMH batteries

NiMH batteries put out less voltage than their alkaline counterparts (1.2V vs. 1.5V).  Usually the difference doesn't matter, but sometimes it can.  If a device really wants a lot of voltage then supplying it with less means you'll get much less runtime.  Many devices, like digital cameras, are actually designed for the lower voltage of NiMH's so those devices are never a problem.  But for other devices, the more batteries your device uses, the greater the risk that the voltage will be too low.  For example, with devices that take...
  • 1 battery.  The 0.3V difference between 1.5V and 1.2V is rarely a problem.
  • 2 batteries.  Now the device expects 3.0V from alkalines but you give it only 2.4V from NiMH.  The 0.6V is more likely to be a problem than it would be in a 1-battery device, but you're still probably fine.
  • 3 batteries.  Now the differences are much more likely to manifest.  I have lots of headband flashlights, and they're noticably dimmer and give less runtime with the 3.6V provided by the NiMH vs. the 4.5V that they're expecting from alkalines.  Unfortunately there are few headlamps designed with NiMHs in mind, but both the Zebralight and the Fenix HL21 take a single AA and are super-bright.  I have one of each, and I love them.  (Here's a comparison of the various Zebralight models.)
  • 4 batteries.  Now we're looking at a 6.0 - 4.8V = 1.2V difference.  That's pretty sizable, and I'm not surprised by poorer performance at this point.
  • 6-8 batteries.  You will almost certainly have problems here.  Giving 9.6V when the device is expecting 12V is just asking for poor performance.
So how do you deal with this?  One solution is to use NiZN batteries instead, since they're rechargeable like NiMH, but they have plenty of voltage, 1.65V.  Then of course you run into the opposite problem:  NiZn are so powerful that they can fry sensitive devices, and that problem increases the more batteries the device takes.  My own solution is to mix NiMH and NiZn in devices that take 3+ batteries.  For example, 2 NiZn's @ 1.65V and 1 NiMH @ 1.2V = 4.5, exactly what I'd get from alkalines.  However...

If you're not careful about mixing NiZn and NiMH together, you can easily kill your NiZn's.  Your NiZn's will usually run out faster when you mix them with NiMH's, and if you don't stop and charge the NiZn's soon enough, the NiZn's will be damaged or dead.  The solution here is to either make sure that all the mixed batteries have a similar mAh capacity (not mWh), or else be really diligent about monitoring the voltage of your NiZn's, charging them well before they get down to 0.5V (preferably charging when they drop to around 1.0-1.1V).  As to the former (matching mAh capacity), that means you'll have to find and use lower-capacity NiMH's, because most NiMH's have a larger mAh capacity than NiZn's.

You might think you have to match only mWh and not mAh, because mWh is the total energy.  That would be convenient, because mWh for common NiZn's and NiMH's is similar.  But you really do have to match mAh, not mWh.  The reason is that in devices which don't limit the voltage to about 1.2V, they'll make use of the NiZns' extra voltage.  For example, the light will burn brighter, or the toothbrush will spin faster.  So even if the total energy expended between both kinds of batteries is the same, the NiZn expends it faster, because it's running at a higher rate.  Another way to look at it is that, as NLee points out, the same amount of current (amps) flows through both batteries, regardless of what the voltage is.  So if the device is drawing 350 mA (or whatever) from each battery, then the device with the fewer mAh is going to run out faster.  That's usually going to be the NiZn.

Another solution to the problem of insufficient voltage from NiMH batteries is to make a power pack.  If your device has an input for an AC/DC adapter, you can get battery holders from Radio Shack for just a few bucks, and wire them together (red wire to black wire), then attach a power plug so you can plug it into your device.  If that doesn't make sense, just go into Radio Shack with your device, tell them you want to make a battery pack with battery holders for it, and ask them what you need.  Be sure to tell them you'll be using 1.2V NiMH batteries and that you want to use an extra battery or two to get the right voltage.  For example, if your device expects 6V and takes 4 batteries, then you'd actually use five batteries, because 5 x 1.2V = 6.0V.

Milking every last drop out of your alkalines

For most purposes you should be using NiMH instead of alkalines, but if you have some alkalines for whatever reason, these tips will help you get all the energy available from them.

Different devices stop working at different low voltage levels
(the "cutoff voltage").  It could be 1.3V in a halogen flashlight, but only 1.0V in a clock or 0.8V in a radio or remote control.  The tip here is that if your alkaline has "died" in a high-demand device like a flashlight, it could have a second life in a lower-demand device like a remote control or a clock.  (Popular Mechanics has a table of cutoff voltages.)

Also, you can make a battery pack with a battery holder from Radio Shack or Amazon to milk your alkalines completely.  For example, I have some battery-powered Christmas lights that expect 4.5V (three 1.5V AA's).  I have some alkaline AA's that are around 1.15V.  (I don't buy alkalines, I salvaged these from elsewhere.)  Three of them would be only 3 x 1.15V = 3.45V, which would be kind of weak for the lights.  But putting four of them in a battery pack gives me 4 x 1.15V = 4.6V, which is just about perfect.

Alkaline voltage drops sharply after hitting 0.9V, so consider a 0.9V alkaline as completely dead and useless.

Battery Leaks

Since it's really only alkaline batteries that leak, see my section on alkaline batteries for info about leaking.

Batteries in sets don't all die at the same time

When the batteries in your device go dead, there's a good chance that only one is really dead.  The rest likely still have some juice in them.  How can you use this info?  Depends on the battery type:

For alkalines, measure the voltage of the batteries with a battery tester or volt meter.  If you find that any of the batteries still have a good charge (>~1.35V), you can toss just the bad ones and keep using the good ones.  Once I was in Radio Shack and a customer came in to buy replacements for four "dead" batteries.  I asked if I could have the old ones and he gave them to me.  I tested them at home and only one was dead; the other four were close to 1.5V, and had lots of life left in them.
        If you use this tip then try to use only old batteries in the same condition together.  You don't want to mix fresh and old batteries in the same device, because that promotes leaking.  In any event, if there's one thing you take from this site, it's that you should probably be using NiMH or NiZn batteries instead of alkaline anyway.

For rechargeable batteries (e.g., NiMH or NiZn):  If one battery discharged early, it's limiting your total runtime.  The bad battery is the weak link in the chain.  Test the voltage of all the batteries and if one or more is above ~1.17V while others are closer to 1.10V, refresh the dead cell(s) in a smart charger.

Reviving dead rechargeable batteries

I cover how to bring dead rechargeables back to life on the Charging Tips page.

What is a "dead" battery?

Unfortunately, it's common to use the single word "dead" to refer to two different conditions:  One, a battery that has been discharged (used all its capacity), and two, a rechargeable battery that can't be recharged any more.  "Dead" is a poor word for a NiMH that's simply empty, because if you can bring it back to life, then it's not really dead.  A better term for a discharged NiMH might be "sleepy".  Though on this site, I try to use "discharged" to refer to a battery whose capacity has been drained, and "dead" to refer to a battery which can't be resurrected.

When is a battery dead / discharged?

NiMH, NiZn, NiCd.  Charge them before they hit 1.0V.  You could wait until they hit 0.9V, but you'll get more cycle life if you recharge them sooner.

Alkaline.  When alkalines hit 1.1V they might no longer power energy-hungry items, but they'll often have a second life in clocks (down to 1.0V) or remote controls (down to 0.8V).  Alkaline voltage plummets after 0.9V-1.0V (depending on the brand and rate of discharge). (Powerstream)

9V.  Depends on the device you're powering, since different devices require different minimum voltage.  I imagine that no device will run on less than 6.0V.  Smoke alarms typically start chirping around 7.6V. (source, PDF)
See below for how to test battery voltage.

How to tell how much energy is left in a battery

Alkalines.  Alkalines start at 1.5V and lose voltage at a pretty steady rate until about 1.0V, at which point the voltage plummets.  So if you tested an alkaline and it was halfway between 1.0V and 1.5V (i.e., 1.25V), it would be about half drained.

NiMH/NiCd.  These spend almost all their runtime between about 1.18V and 1.16V, at which point they suddenly plummet down to 1.1V or less.  So you can't reliably gauge how much capacity is left in the battery by testing the voltage.  That's why devices that try to tell you how much battery life you have left are notoriously inaccurate when powered with user-supplied NiMH batteries. (more)

StefanV has a good chart showing the voltage curve of alkalines vs. NiMH.

See below for how to test battery voltage.

How to test battery voltage

Battery testers are fine.  But multimeters connected directly to an unloaded battery often give inaccurate readings.  "Unloaded" means not actually powering something, and unloaded voltage is often higher than the loaded voltage.  Battery testers supply a small load.  Multitesters do not.  To test voltage with a multitester, stick the battery in the device, turn it on, and then test the voltage.

An unloaded 9V might test as 10V unloaded but only 5.6V with a small load applied. (sounce, PDF, p. 9)  That doesn't mean that every battery that tests as 10V unloaded will test as 5.6V under load; most batteries will usually test just a little lower under load, but some will test a lot lower.  Every battery is different, so that's why you can't trust the reading unless you test under load.

Understanding electrical terms

Volts (V).  Voltage is a measure of how hard the electricity comes out.  It's electrical pressure.  So in devices with motors, applying a higher voltage will make the motor spin faster.  A device doesn't choose the voltage it wants, it just receives whatever voltage is supplied by the battery.  That is, it's the battery that decides the voltage.  See "Voltage" above for how this relates to AA and AAA batteries.

Amps (A).  Amps are a measure of current, which means how many electrons are flowing.  A device will try to draw as many amps as it needs from the battery.  That is, it's the device that decides the amp rate, not the battery.  (Note that this is the opposite of voltage.)  A high-drain device is one that needs lots of amps quickly.  Regular alkaline batteries are terrible for those devices because they can't pump out the amps fast enough, but NiMH are great.

Amp-hours (Ah).  This is a measure of how many amps are stored in a battery.  That is, it's a measure of the capacity of the battery.  A battery with more amp-hours has a higher capacity and will give more runtime before being depleted.  A battery with more amp-hours does not make a motor spin any faster.  Because batteries are small, we measure their capacity in milliamp-hours (mAh).

Watts.  Watts is a measure of power, and is a combination of volts and amps.  Actually its volts times amps.  When you take how hard the electricity is pumping (volts) combined with how many electrons are flowing (amps), then you've got your total power.  Think about it:  If you hit me with a paper plane going 30mph it's not going to hurt me very much.  That's like high pressure (the 30 mph speed) but low flow (since you're not throwing much matter), so the pressure x flow isn't very much.   Similarly, if you slide a refrigerator into me at 0.1 mph it's not going to hurt me very much.  That's low pressure (the 0.1 mph) with high flow (a huge refrigerator, lots of matter), so again the total power isn't very much.  But if you hit me with a refrigerator going 30 mph, then suddenly you can see how that's a lot of power.

Watt-hours.  We just saw that watts is a measure of the rate of power at any given instant.  The amount of energy consumed is measured in watt-hours.  (Or in the case of batteries, milliwatt-hours, since batteries are small.)  Say you've got a 1.2V battery and a device that draws 150 mA.  The rate of power is 1.2V x 150mA = 180 mW (milliwatts).  Now let's look at the battery.  Your battery is rated as 2000 mA.  So your battery has 1.2V x 2000mAh = 2400 mWh.  Your 2400 mWh battery divided by the 180 mW draw from your device means you could run your device for about 2400 mWh ÷ 180 mW = 13.3 hours.

What do I do if I swallow a battery?

Call the National Battery Ingestion Hotline immediately at 1-202-625-3333 (open 24 hours, and can call collect if necessary).  You can also call your local poison center at 1-800-222-1222.  (Calls are automatically routed to the poison center for your area.)

Here's information about swallowed batteries.

NiCd or NiMH for solar lawn lights?

While NiCad cells have been replaced by NiMH almost everywhere, you still see them in solar-powered landscape lights.  This made me wonder if NiCds actually perform better in that application for some reason?  When I get a chance I'll test them head to head, but in the meantime...
  1. Westinghouse (a manufacturer) says that either Ni-Cd or Ni-MH are fine.
  2. A member at Candlepower Forums says that NiMh underperform NiCd in solar lights because NiMH don't perform well when being charged by very low currents, never getting fully charged, and sitting for long periods without being charged.
  3. Others say that NiMH don't tolerate the higher trickle charge that the light is set up to give the NiCds, but I doubt that's an issue since the NiMH's have higher capacity and won't get full anyway.
  4. Detect Energy says that the batteries should be charged in a regular battery charger every three months to keep them healthy, but that even if you do so they'll last only 1-2 years.

Why you don't get shocked

As you may have guessed, there's just not enough power in household batteries to do you any harm. It's the exact same kind of electricity that can kill you, but there's just not nearly enough of it.
Batteries put out juice when something comes between the positive and negative ends of the battery. When you come between the two ends by touching them, your body provides resistance to the tiny 1.5 V in a household battery, so the current can't flow from one end of the battery to the other, and no real circuit is formed.  When that battery is in your appliance, however, the metal parts readily accept the current, and the juice flows out one end of the battery, through the device where most of it's used up, and then the remaining current goes back into the other end of the battery.
Once I had a 9V battery and a wire dish scrubber in my pocket. (Don't ask me why.) After a few hours the items shifted and the scrubber was touching both terminals of the battery. I noticed something hot in my pocket, getting hotter, almost painfully hot, before I realized what it was.  If I didn't take them out of my pocket and separate them then I tend to think I would have burned a hole in my pants.  So don't put batteries in the same pocket as keys or coins—especially 9V batteries.

AA, AAA, C, and D are really "cells", not "batteries"

In technical terms, a cell is a single energy-producing unit, while a battery is multiple cells strung together.  So AA's aren't really batteries, they're actually cells.  A 9V is truly a battery, because it contains 6, 7, or 8 individual cells.  (That's what you'd see if you unwrapped it.)
Is this important?  Of course not.  The general public uses the word "battery" instead of "cell", there's no way to stop them, and even if we could there would be absolutely no benefit in doing so.  So I use the familiar term "battery" instead of "cell" throughout this site, because that's what people are familiar with.  I put this section here to let the nitpickers know that I do understand the difference, and to point them to it when they email me to complain.  (Yes, some people really don't have anything better to do.)

You can also see our charging tips.

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