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Page history last edited by PBworks 14 years, 7 months ago

The Lithium Report   by Noah Vawter and Annina RĂ¼st

 

Presenting a completed design along with all of the useful information we have researched about using rechargeable lithium batteries in homebrew electronics for beginners and seasoned veterans.

 

 

F.A.Q.

How to Recharge Lithium Batteries

Our Design

Links

 

 

F.A.Q.

 

Why use rechargeable lithium batteries?

 

Lithium batteries are the most dense source of electrical power available to hobbyists.  This means you can use them to power things like motors, robots, bright lights, loud speakers, etc., yet still have a lightweight design.  Disadvantages of lithium batteries include their cost, danger and complexity of recharging.  The outcome of our research is a simple recharger.  We can't do anything to alleviate cost.

 

 

Primary lithium vs. secondary Lithium

 

Primary lithium means non-rechargeable lithium batteries.  Secondary Lithium means rechargeable ones.  This wiki and project are only going to be concerned with rechargeable lithium batteries.

 

 

Lithium-Poly vs. Lithium-Ion (a.k.a.  Lithium +)

 

In doing our project, we ran into many sources of information warning us against using lithium-poly batteries in lithium-ion chargers and vice versa. 

 

 

 

How to Recharge Lithium Batteries

 

Lithium batteries are best charged with a two-part cycle:

  1. Constant current
  2. Constant voltage

 

Constant Current

Your charger should measure the voltage on the battery to determine which cycle to be in.  When the battery is low, the charging begins in constant current mode.  As the battery fills with charge, its voltage will increase.  Keep filling it with a steady current, until it reaches the threshold voltage.  The optimal threshold varies with the battery, but is nominally 4.1V.  That means it will work for most batteries.  More powerful batteries can go to 4.2 to 4.5v, but don't try overcharging unless a spec sheet says it.  You might create fire and explosion if you do. 

 

The amount of current used during the constant current cycle should be 1C.  What does 1C mean?  That refers to the charge rate of your battery, which is related to its capacity.  For example, if the capacity of your battery is 500 milliAmp Hours (mAh), then you can safely charge it at 500mA.  Some batteries can be charged faster, at a higher current, but don't do it unless you are sure your battery's spec sheet says it can handle it.  You might create fire and explosion if you do.

 

 

Constant Voltage

During the Constant voltage portion of the cycle, the charger maintains a voltage of either 4.1V or 4.2V across the battery,

allowing as much current to flow as the battery wants to accept.  As time passes and the battery reaches its capacity, the current flow will get slower and slower.  Eventually, it will reach next to nothing.  When that happens,

 

Caveats

Some lithium chargers, such as the one offered on the Sparkfun website that is based on the MAX1555 chip, only perform the constant current portion of the cycle.  This is acceptable, but it will not fully charge the battery.  We've read various estimates stating that the battery will only charge from 50% to 80% of its capacity. 

 

Trickling

Unlike Ni-Cad and NiMH chargers, lithium chargers should not apply "trickle" charges to keep the battery topped off.  If you do this, you may cause explosion and fire.  If you must keep the battery near its maxium charge, wait until it loses a good chunk of its charge, the restart the constant voltage cycle.

 

 

Our Simple Charger Design - Noah and Annina's Charger

 

This charger is based on the typical circuit of the LM3622.  It can be used to charge lithium-ion or lithium-poly

batteries.  It can charge one lithium cell or two cells in series.  It can supply current at 10 different rates, from 100mA (the very smallest from sparkfun)

up to 1200mA (pretty hefty). 

 

chips

  1. Parts for lithium charger:

    IC1 = LM3622 (datasheet and free samples)

    Q2 = STS4DPF30L

    C1, C2 = 10 uF

     

    D1 = DIODE SCHOTTKY 60V 3A SMC    B360-FDICT-ND   

    rotary switch for current control = grayhill 94HAB10WT = digikey  GH7253-ND

    resistors for current control:

    R1 = 0.1 Ohm (1/2 Watt) (good tolerance)

    R2 = 0.2 Ohm

    R3 = 0.5 Ohm

    R4 = 1.0 Ohm

     

    3.3V regulator.  must be very low dropout (169-287mV) regulator from david b:

    Supply must be 3.587 or more:   TPS77633  (datasheet)

    The schematics are here:

    http://thelithiumreport.pbwiki.com/f/lm3622.zip

     

Links

 

Where to Buy Lithium Batteries

    Sparkfun.com 

    Batteryspace.com

 

Where to Learn More About Lithium Batteries

    Batteryuniversity.com

    Wikipedia - Lithium-Ion Batteries

 

 

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