LarsBlog

I seem to use quite a few AA and AAA rechargeable batteries.  For convenience, I couldn't go past buying a couple of USB Battery chargers - one for home and one for work!  They are available for just a few bucks on eBay, but unfortunately these cheap chargers don't stop charging when the battery is full.  Overcharging a battery can result in the battery heating up and ultimately exploding.

So... It's very important to know how long to charge your batteries for!  Knowing nothing about electricity except some dim memories from high school science class, I did some googling and came up with the below.

1. Charging time = Battery Capacity / Charger Power Output

On the bottom of my USB Battery charger, it states it has a power output of 150mA.  On my NiMH AAA battery, it states a capacity of 1000mAh.  So in simple terms: 1000mAh / 150mA = 6.66 hours.  Typically, you're also going to get around 30% efficiency loss during the charging process.  This brings our charge total to around 8.6 hours when fully discharged.

There are various calculators around on the web to help determine charge time based on the output of your charger and battery capacity.  I wasn't sure whether or not to trust them, then I found the manual for my Uniross USB charger online, and thankfully it agreed with the calculators, quoting 8 hours to charge a 1000mAh battery from full discharge.

Remember, if in doubt, always under-estimate!  It's apparently better to undercharge NiMH batteries than overcharge them!

2. Prolonging Battery Life

Wikpedia had some interesting tips on this topic.  According to them, NiMH and NiCd batteries like my rechargable AA's and AAA's should be fully discharged once a month, or once every 30 charges, in order to maximise their life.  Otherwise the "memory effect" causes the battery to lose it's capacity over time.

More surprising to me was that the lithium-ion batteries used in laptops and ipods will actually degrade in capacity if fully charged, due to cell oxidation.  High temperatures and full discharges are also very bad, which is why running a laptop off AC can be bad for it's battery as it results in full charge and high temperatures.  A 40% charge level and refridgeration is recommended for storage of unused lithium-ion batteries!  Apparently, full charge cycles can also eventually decrease the capacity of NiMH/NiCd batteries too.

3. Battery Recycling

Batteries leech all sorts of toxic crap into ground water and other undesirable places when in land fill.  I believe ultimately this stuff will wind up as contaminants in our food.  When you're finished with your batteries (rechargable or otherwise) it's important to dispose of them responsibly!  Check out http://recyclingnearyou.com.au/ to find out where.

4. Further Reading

Powerelectronics.com has a good intro to USB charging, technical yet I could more or less understand it!
BatteryUniversity seems to have reams of information about batteries in general -in English and German!