# Building Battery Packs



## noela (May 22, 2008)

I am going to be building some battery packs, and want to make sure I have all my ducks in a row.

My first ones will be LiFePo4 units, 6-cell, 3300mah, 19.2v. I am trying to find out how I can install a polyswitch within the pack for resettable protection. Do I use a strap type polyswitch and put it between two of the cells, or do I use a circular type polyswitch and put it between the + terminal of the pack and the connector. 
Also, based on the above ratings, how do I determine which polyswitch is rated properly? My normal run current will be 1.2a.-2.4a., and max draw would be 7a. 
Thank you.


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## TonyWalsham (Jan 2, 2008)

Polyswitches are very good for overall system protection. 
I use the RUE series rectangular ones. 
Given that your maximum current draw is going to be 7 amps then logic should tell you that a 7 amp Polyswitch is what is needed. 
The problem will be that Polyswitches are guaranteed to hold their rated trip current indefinitely. As such they will often pass double that current for a little while depending on what that overload actually is. That lag in trip time could be disastrous. 
To help assure quicker tripping at maximum load I often use two 3 amp Polyswitches in parallel for a total of 6 amps. Even though the load is shared between two of them, no two Polyswitches are exactly the same, and one will trip before the other. Once the first one trips the other will trip almost instantly. 

I am afraid I cannot advise where they should be in a battery pack.


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