Polyswitch fuses.

[vereybowring]

I have a question for the more technically minded amongst our group.
I am planning to build a mod using 18650 protected batteries in parallel with a charging circuit.

I have already thought I might want a little extra protection for the batteries so would like to fuse them individually to protect them from each other and the charge circuit.
Better safe than sorry is what I think.

So I don't have to mess around I came across polyswitches that surface mount in various amp ratings.
10Pcs Polyswitch SMT SMD Resettable Fuses 1812 1.5A on eBay (end time 13-Apr-11 11:19:02 BST)
Has anyone got any experience with them ?
Will putting one in front of each battery be useful ?
At 3.7-4.2V with 3Ohm atomizer will 1.5A fuses be enough or do I need Higher ?

I really need to go and read up on all my electrical stuff as I'm at least 15 years out of date with stuff and have forgotten too much.

 

[RatInDaHat]

I= E/R or I=V/R depending if you were military or not. so Your total current = 4.2/3 = 1.4 Amps. 1.5 may cut it assuming your resistance doesn't go below 3 ohms.

 

[vereybowring]

Excellent point which I should have seen, but this is why I'm posting everything as I go so people can point out my mistakes.
Haven't done much detailed electrical stuff for going on 20 years.
Will probably go with 3A then since so many atomizers are 1.7 Ohm to 3 Ohm and I just happen to be using the higher end at the moment.

 

[Java_Az]

Poly switch might be considered overkill. The protection circuit is supposed to stop the battery from over discharging. Some pack makers do add them in the battery packs they sell as a second failsafe(cut down on potential law suits = priceless) Since your running two of these one on each battery it would let about 3 amps threw to the load side 1.5 amps x2 = 3amps. But if you look at the input side these poly switches start to open at 1.5 amps they in a way regulate the output or load side on the switch to 1.5 amps But it takes 3 amps for the switch to fully open and not let current threw. So on the battery side two of these would need 6 amps to open or turn off the polyswitches. Since your doing a parallel battery configuration i think the 1.5 amps x2 would be about right.

EDIT: here is a link to a data sheet on a 1.5 amp poly switch Notice there is a hold current and a trip current for them. http://www.tycoelectronics.com/commerce/DocumentDelivery/DDEController?Action=showdoc&DocId=Specification+Or+Standard%7FSCD25260%7FI%7Fpdf%7FEnglish%7FENG_SS_SCD25260_I.pdf%7FRF1358-000

 

[vereybowring]

Thanks Java_Az,
Interesting, I am learning how little I know every day at the moment.
So 1.5A at each battery should be fine then ?
Wish I knew so much more about circuitry, but I'm trying to play safe.
Do I need the fuses at all then ?
I would really like to get the build right first time since I don't have money to squander on mistakes (even the small costs of a strip of fuses).

 

[Java_Az]

Do I need the fuses at all then ?

I would say no as long as your using quality made protected cells.

 

[RatInDaHat]

Why would you run two fuses? are you looking for protection for each cell individually?