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I want to create a circuit that will power my Raspberry Pi using Lithium Polymer. My knowledge of LiPo is rather low and I believe they can get dangerous if used incorrectly so I turn here.

THe intention is to create a LiPo battery (of two cells) that I can charge with a LiPo charge circuit chip of suitable type, that part I'm ok with.

Then I hope to use something like the LM2676 3A switching regulator to get the 5V out that I require.

My main question is, do I need any sort of circuitry when running the LiPo cells for protection, for example to ensure they don't get discharged too much?

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    \$\begingroup\$ LI-PO batteries are prone to explosion if just about anything goes wrong. You overcharge them--They explode, you discharge them too rapidly--They explode, You drop them--They explode, you get them too hot--They explode, you look at them the wrong way--They explode \$\endgroup\$ – Reid Aug 27 '12 at 20:20
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Protection circuitry is a highly critical element in a safely usable Lithium-polymer or Lithium-ion cell. Raw (unprotected) cells are less commonly sold, i.e. many times, the manufacturer of the battery includes protection circuitry as part of the battery; however, for safety, do not assume this is the case and check the datasheet of the battery.

Purpose of Li-ion/LiPo protection circuitry

The protection circuitry serves to protect against short-circuit, over-discharge, and over-charge conditions. Loosely speaking, the circuitry relies on an over-current cutoff, an under-voltage cutoff and an over-voltage cutoff respectively for ensuring against those three conditions. Sometimes, protection circuitry also includes thermistor-based overtemperature protection.

Unprotected versus Protected batteries

You MIGHT conceivably get by with an unprotected battery, for simplicity, if you have certain components on your PCB, e.g., a fuse (prevents overcurrent/short-circuit), an undervoltage-supervisor IC (prevents overdischarge), and a smart battery-charging IC (prevents overcharge).

But my personal assessment is: Unless you are able to constantly monitor the battery, do NOT use an unprotected battery; protection circuitries are fairly simple to implement (e.g. using pre-packaged protection ICs), or alternatively, a protection-added battery is easy to purchase.

How to implement Protection circuitry (for a Two-cell pack)

If you choose to implement it yourself (versus buying a protected pack), then here are two protection ICs you can consider for your scenario (i.e. two-cell Li-ion or LiPo):

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  • \$\begingroup\$ @Hamid: Why are you using a two-cell configuration? Why not use a stepup regulator/boost-converter to 5V? \$\endgroup\$ – boardbite Aug 28 '12 at 7:54
  • \$\begingroup\$ Very good question, why aren't I? Do you recommend doing so, considering I need at least 1A typical out of it? I didn't think that kind of power would be feasable with a single cell. \$\endgroup\$ – Hamid Aug 28 '12 at 9:17
  • \$\begingroup\$ Actually, Li-ion/poly are usually rated for 2C discharge to my knowledge (thus 1000 mAh battery can handle 2 amps), however, depending on the integrated protection circuitry on board the battery, they may discharge only maximum of 1 amp discharge; even then you should be fine for your case. For example, this one seems to have a limit of 1 amp discharge: Sparkfun LiPo 1000mAh. As a bonus, it has protection on-board as the description states, so you should be good to go! \$\endgroup\$ – boardbite Aug 28 '12 at 9:28
  • \$\begingroup\$ Thanks, that makes sense, however, I was hoping to run it for a little longer than an hour :( lets say I did go for this, which has the protection circuitry on board, what circuitry can I therefore exclude from the electronics I build? I'll obviously need a charger IC (MAX1555 equiv)... \$\endgroup\$ – Hamid Aug 28 '12 at 9:30
  • \$\begingroup\$ Well, please read the Sparkfun description of that battery, and the datasheet, and you'll understand what's already on-board the battery, which is sufficient for most apps. But you might want to add a 2nd protection on PCB for undervoltage cutoff and a fuse. Charger IC: I recommend MCP73831T, which is used for example in this board). Boost converter that can handle 1A discharge -- play with the "Output current" filter on this search engine at Mouser. \$\endgroup\$ – boardbite Aug 28 '12 at 9:38
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TE Connectivity Circuit Protection provides a host of information as it pertains to Lithium Polymer cells and how to provide the proper level of protection to help prevent against catastrophic events from occuring. I host a blog "Around the Circuit" and we just posted a short video from one of our design engineers where he talks about pack protection. I invite you to check this out at http://blog.circuitprotection.com/blognews/battery-cell-and-pack-protection-%e2%80%93-te-talks/[Battery Cell and Pack Protection][1]

It may be of some use to you as you try and resolve your protection problem.

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