#3660 HIGH MP Star: battery not safe for submersion

[Zarro Boogs per Child]

#3660: battery not safe for submersion
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  Reporter:  AlbertCahalan  |       Owner:  rsmith  
      Type:  defect         |      Status:  closed  
  Priority:  high           |   Milestone:  MP Start
 Component:  hardware       |     Version:          
Resolution:  invalid        |    Keywords:          
  Verified:  0              |  
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Changes (by rsmith):

  * status:  new => closed
  * resolution:  => invalid


Comment:

 Replying to [ticket:3660 AlbertCahalan]:

 > At least according to c_scott on #olpc, the battery is not
 > safe when submersed. Despite any warnings given, batteries
 > are sure to get submersed.

 I don't know what the context of this discussion was but since the max
 voltage on the batteries is 7.5V we are pretty safe even while dunking it
 in the ocean.

 Google tells me:  http://www.biophysica.com/conductivity.htm

 =======================
 The unit of measure commonly used in Siemens per centimeter (S/cm). The
 conductivity of water is usually expressed as microSiemens/cm (µS/cm)
 which is 10-6 S/CM. The relation between conductivity and dissolved solids
 is approximately:

     2 µS/cm = 1 ppm (which is the same as 1 mg/l)

 The conductivity of water from various sources is

     Absolute pure water - 0.055 µS/cm
     Distilled water - 0.5 µS/cm
     Mountain water - 1.0 µS/cm
     Most drinking water sources - 500 to 800 µS/cm
     Sea water - 56 mS/cm
     Max for potable water - 1055 µS/cm

 Sea water   = 1.7 ohms/mm
 max potable = 95 ohms/mm

 The min spacing between the positive and negative terminals on the batter
 is about 2.3 mm:

 So in the ocean you will draw 2.1A and max potable 34mA.

 Long term submersion will drain a battery but it does not pose much of a
 threat involving high currents as both of these numbers are inside the
 normal operating range of the batteries.

 > Current needs to be limited as necessary.
 > A fuze would solve this. Fuzes are cheap.

 The batteries are already internally fused.

-- 
Ticket URL: <https://dev.laptop.org/ticket/3660#comment:2>
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