Battery Charging FAQ

What kind of charger to use

• My recommendations.  If you don't already own a charger, see my battery charger recommendations for what charger to get.
  
• Use a smart charger!  Cheaper chargers can't tell when your battery is full, and either undercharge it (robbing you of runtime) or overcharge it (shortening the capacity and/or cycle life of the battery).  Make sure your charger is a smart charger.  Every charger I recommend is a smart charger.
  
• NiMH chargers handle NiCds too.  Most chargers made since about 2005 charge both NiMH and NiCd batteries.  But some newer chargers charge only NiMH, not NiCd.
• Old NiCd chargers aren't good for NiMH batteries.  Old NiCd chargers can't tell when to stop charging a NiMH. Depending on the charger, it'll either undercharge your batteries (so you get short runtime), or overcharge them (shortening the life of the battery).
  
• Any company's charger will charge any company's battery.  For example, a Panasonic charger will charge a Sony battery and vice-versa.  You have to match only the type of battery (e.g., a NiMH battery with a NiMH charger), but the brand doesn't matter.  I do recommend using a smart charger, though.
  
• Low Self-Discharge NiMH's don't need a special charger.  Any NiMH charger will charge both regular NiMH batteries and LSD NiMH's.
  
• NiZn batteries require a special charger.  NiMH and NiCd chargers will not work.  First off, a NiMH/NiCd charger charges at around 1.3-1.6V, while a NiZn charger uses around 1.9V.  Next, the capacity of NiZn's is a lot smaller, and I'm thinking it could be possible for a NiMH charger to try to put too much energy into a NiZn battery, even at a lower charging voltage.  Finally, a NiMH charger might just refuse to charge the NiZn anyway, if it checks for impedence.  (The resistance in NiZn cells is much higher than in NiMh.)
  
• Alkaline battery chargers are a joke.  You can charge the batteries only a few times, they lose voltage and capacity each cycle, and charged alkalines are prone to leaking.  See my article on problems with charging alkaline batteries.
  

Beware of "dumb" chargers!

Lots of chargers, maybe most of them, don't stop charging when the battery is full.  They either stop too early, too late, or not at all—continuing to charge until you remove the battery.  This is bad, because undercharging means you don't get as much runtime as you should from your device, and overcharging reduces cycle life (the number of times you can charge the battery before its capacity is significantly reduced).

By contrast, smart chargers give the batteries exactly as much juice as they need.  This is the kind you should be using.  Fortunately, these days, there are lots of affordable smart chargers.

"Smart" has another important meaning:  True smart chargers charge each battery separately, giving each one exactly as much energy as it needs.  It has to be that way, because how could a charger be "smart" about stopping the charge if it didn't have the ability to stop charging different batteries at different times?  Careful though: Some manufacturers call their chargers "smart" even if they require charging in pairs (at least two batteries at a time), e.g. EBL's model 802.  It's unlikely that such chargers manage each battery separately (and if so, the manufacturer doesn't bother to say, and if not, then it's not a true smart charger).

You usually can't tell what kind of charger you have by just looking at it.  A dumb charger is never labeled "dumb", a timer charger is never labeled "timer" (and there's no visible timer, it's internal), and a smart charger is rarely labeled "smart" on the product itself, and even if it is, some manufacturers use the word "smart" even if the charger requires charging in pairs.  You have to check the product description "smart" or something like "manages each battery separately", and that it either manages each battery separately or doesn't require charging in pairs.  Or you could just go with one of my recommended smart chargers.  Here's Bluejay's Law of Battery Chargers:

Any charger that's not described as being a smart charger, is definitely a dumb charger.
Conversely, any charger that's indeed a smart charger, will almost always be described as such (even though some chargers listed as "smart" still require charging in pairs; must check the details).

Here are the details on the different kinds of chargers.

1. Dumb Chargers (aka Manual Chargers).  These chargers never stop.  The user manual will estimate how long you should leave various size batteries in for, and you're supposed to take them out exactly after that much time has passed.  Since every battery is different, then no matter how diligent you are, you'll either undercharge or overcharge your batteries this way.  Undercharging means you get less run time, and overcharging means you reduce the batteries' cycle life.  Dumb chargers are also slow, because pumping the juice in fast when there's no automatic shutoff could overheat the battery, cause it to pop its seals and start a fire.
   
   
2. Timer Chargers.  Chargers with an (invisible, internal) timer will shut off after a certain amount of time, which means that you'll undercharge or overcharge your batteries.  Some timers are set to a really long cutoff time, which means that batteries get consistently overcharged.  And if the timer is set to fill up the average battery and you buy the highest-capacity batteries, the charger will never fill them up completely  Finally, some timer-based chargers require that you unplug and replug them in between each use, to reset the timer. (example, PDF)
   
   
3. Smart Chargers.  These chargers figure out when each battery is full and then automatically stop charging that channel.  The typical way is to check for the voltage reversing.  ("Negative delta-V".)  If that fails, backup methods could include a timer, a maximum voltage, and/or a maximum temperature.  Note, negative delta-V is for NiMH batteries; lithium chargers look for a decrease in charging current, since lithium batteries absorb only as much as they need. (source)  See my recommended smart chargers.  (more on termination from Battery University, MPower, and CandlePower Forums, and NLee the Engineer)
   

Mixing battery types, sizes, and charged levels at the same time

If your NiMH charger is smart, then you can mix NiMh and NiCD, different sizes, and semi-fresh and nearly-dead batteries at the same time.  If it doesn't then you can't; if you insist on mixing them anyway, some of your batteries will be undercharged and some will be overcharged...more so than usual.

Is it better to charge fast or slow?

First of all, you almost never have a choice, so for practical purposes this rarely matters.  Most chargers don't let you set the charge speed.

But let's say you have a fancy charger that does let you set the charge rate.  Even then, the manufacturer default is likely what's best for your battery. 

But let's say you're able to set the charge rate and you really want to know which is better.  So here you go:  In general, slower is better, because it preserves the cycle life of the battery, and but for AA there's such a thing as too slow.  So here's how to do it:

• For AAA, go with 200 mA.  500 mA is acceptable but could reduce cycle life.  Never use 700 mA.
• For AA go with 500 mA.  200 mA is so slow that it could make the charger miss the cutoff signal and overcharge the battery. (NLee the Engineer)  700 mA is acceptable.

La Crosse suggests that the optimal charge rate is 200mA, and that's what the default is for its BC series, though again, for AA, you should use 500 mA instead.

At the recommended levels, a battery will take about 4-5 hours to charge.  Faster than that potentially reduces cycle life, and too much slower than that could possibly cause the charger to miss the cutoff, overcharging the battery, and reducing cycle life.

Reviving batteries that won't charge

If a battery is really dead, most chargers will reject it.  But never fear, I'm going to show you how to revive dead batteries so the chargers will recognize them. Basically, you just force some voltage into the battery to get it up to a minimum voltage (e.g., ≥0.5V for La Crosse BC series chargers).  Note that revived batteries might suffer from reduced capacity or reduced cycle life, but sometimes they bounce back very well:  I just revived a battery which had gone down to 0.03V, and after charging it tested at 820mAh, better than its label rating of only 800mAh.  Anyway, here are the ways to resurrect dead rechargeable batteries:

1. Dumb Charger method.  If you have another, cheaper charger that doesn't test the battery voltage, just stick in the dead battery for a while until it's charged enough to be recognized in your good charger.
   
   

Jump-start methods.  To give you battery a jump-start, you connect the terminals of a fresh battery to those of the dead battery (positive to positive, negative to negative). You keep them connected for a few seconds, or for just a fraction of a second if you're using a power tool battery as the good battery.  After jump-starting, try to charge the battery again.  If the charger still doesn't recognize the battery, then try to jump-start the battery again, this time for a longer length of time (or more swipes in the case of the power tool battery).  While I've never had any problem using these methods, they're definitely use at your own risk.  Anyway, here are various ways to do a jump-start:

2. "Any metal" method.  Stand the two batteries up on a metal surface like a stainless steel countertop or cooking pan(not non-stick).  If you can't find a metal surface then lay a paperclip down on the table and then presto, you have a metal surface.  Then lay something like a screwdriver or knife across the positive terminals.  Alternatively, you could lay the batteries on their sides and touch one screwdriver or knife across the positive terminals, and a screwdriver or paper clip across the negative terminals.
   
3. Paper Clip method.  UNPLUG your charger!  Then insert a freshly-charged battery and your dead battery, side by side.  Unbend a paper clip and touch the ends to the positive end of each battery for a few seconds.  If you notice the paper clip getting hot, then the dead battery has a short and you should stop trying to charge it and just recycle it.  Thanks to NLee for this idea.
   
4. Battery Holder method.  Get a 2-cell battery holder.  Insert a freshly-charged battery and the dead battery in the same direction.  Since you're inserting one of the batteries opposite of the normal way, make sure the spring is contacting the positive head.  Touch the black and red wires together for a few seconds.
5. Power Tool Battery method.  This method involves sparking and seems dangerous, so this is definitely use at your own risk.  Anyway, remove the battery from a power tool such as a cordless drill.  Put the dead battery in a battery holder, hold the red wire to the positive terminal of the power tool battery, and then briefly touch the black wire to the negative terminal of the power tool battery (less than one second).  When you brush the negative wire to the power tool battery you'll likely see a spark.  One swipe is likely enough, but I often do 2-3 for the heck of it.
   

How far to drain batteries before charging

NiMH:  You can charge at any point (you don't have to wait until the battery is mostly discharged), but the sooner you recharge the better, because you'll get more cycle life out of the battery.  So charging NiMH's when they deplete to 1.1V is better than waiting until they get to 1.0V.

NiCads: Charge when most (but not all) power is gone.  Performance will have decreased at this point (i.e., a light will dim or a tape player will slow down), and the voltage will be around1.0-1.1V.  More on charging NiCads.

Lithium:  You can charge at any point, but if you're not going to use the battery for a while, leave it only halfway charged.  A lithium battery stored at room temperature for a year permanently loses 4% of its capacity if stored at 40% charge, versus a 20% loss if stored at a 100% charge.  Also, heat above room temperature kills capacity permanently.  So, for example, keep eBike batteries indoors in the summer.  Finally, the slower the charge, the longer the battery will last; don't charge a depleted battery in faster than two hours. (somEV)

Alkalines (inc. RA's):  Remember you need a special charger for alkalines, and that I don't recommend charging alkalines anyway, even "rechargeable alkalines", because you get very few recharge cycles, capacity drops precipitously on each cycle, and because all flavors of alkalines are prone to leaking.  That said, if you insist on charging alkalines, then the sooner you charge them the better, as you'll get more cycle life that way.  That's actually true of most kinds of battery, but it's extra true for alkalines.

Problems with charging NiMH and High Capacity NiCd C- and D-cells

Many (probably most) chargers that handle C- and D- cells won't fully charge them.  Solution? Use a better battery charger.

When I was researching for the debut of this page in 1999, I was amazed to discover that NONE of the Radio Shack chargers in their 2000 catalog would fully charge the High Capacity NiCad batteries that Radio Shack sold! (This was according to a rep I spoke with a t1-800-THE-SHACK.)  Likewise, the $50 Battery Manager available from Real Goods sold would fully charge the High Cap D's that Real Goods sold (even though the picture of the charger in their catalog showed a High Cap D inside the charger!).  My test with the Battery Manager showed that it charged a High Cap NiCad D less than half full.

I don't keep track of which chargers don't fill a C- or D-, properly, but I do recommend a charger which does.

Remove the batteries when they're done

Most chargers give a "trickle" charge after charging is finished.  Some chargers give too much of a trickle, and even those which give a reasonable trickle can still overcharge if the batteries are left in the charger too long.  A good compromise between convenience and not overcharging is leave the batteries in the charger for no more than a day after charging.

How much electricity does it take to recharge a battery?

It takes less than a penny's worth of electricity to charge a battery!  Batteries just don't store that much energy.  Charging 4 AA batteries in one of my chargers uses 34 watt-hours, or 8.5 watt-hours to charge one battery.  At a sample electricity rate of 16¢ per kWh, that would be 0.0085 x 16¢ = 0.14¢.  That's not fourteen cents, that's fourteen hundredths of one cent.  So, charging seven batteries would cost you about a penny.  For more information about electricity costs and household electricity usage, visit my Saving Electricity site.

References:

• EasyBattery.com FAQ
• Dans Data
• GreenBatteries FAQ