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I'm designing a device that has 6 18650 elements (all parallel) in holders, made so that the end user will be able to pop out the batteries when they're discharged and plug in a set of new batteries. It's possible to put in 4 charged batteries when 2 discharged remain inside (the device is like 2-layer box with lids at top and bottom, having 4 cells in the top compartment and 2 cells + electronics in the bottom one, so the user has to open both to change the batteries), therefore allowing the charged batteries to charge the discharged ones, which AFAIK is a disaster because of high currents.

To protect the device (and the user) against this, can I just put a resettable PTC fuse between the 4-cell compartment negative terminal wire and 2-cell compartment negative wire? I'm not thinking about the scenario where the user mixes charged and non-charged batteries in the same compartment. Also, if the fuse approach is conceptually right, will there be any drawback to using the resettable fuse, t.i. would a regular non-resettable fuse perform substantially better in terms of safety?

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I think that the resettable ptc fuse is not the best way to engineer your solution. I agree that you need some sort of protection, but ptc resettable fuses work in an interesting way that isn't ideal for you.

When the current is too high for the ptc fuse, it blocks the current until you remove that current - correct. But, how does it do this? As the current reaches the specific 'trip' current of the ptc fuse, the fuse heats up. As the fuse heats up, the particular material that it is made up of starts to increase in resistance. When the material of the ptc fuse hits a certain temperature - the temp at the trip current, the resistance becomes extremely high and blocks most or all of the current and continues to until the current is removed and the ptc cools down.

I would be concerned about some one placing a charged and uncharged cell together, the ptc fuse trips, but the cells are left in the unit and the fuse remains hot and if near the battery thats not good and the fuse could eventually fail.

Could you have your unit measure the voltage of the cell inserted, and if the same as the other cells then it's allowed to connect to the other cells?

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I would strongly recommend against such a scheme where you connect, disconnect and then reconnect cells in parallel again ! You can only connect Lithium based calls in parallel after properly balancing them. This means that they must be allowed to reach the same voltage and stay connected in parallel

Sure you can use a fuse or PTC or whatever to protect the cells charging each other but it is still not an elegant solution. I predict that the situation will occur where the user will be unable to find cells that can be inserted without the fuse activating.

A much better solution is to make packs of balanced batteries and connect/disconnect them as a whole.

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  • \$\begingroup\$ Hi! Could you elaborate on your position? What are the factors that you are considering when stating this? \$\endgroup\$ – Арсений Пичугин Feb 5 '16 at 8:35
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    \$\begingroup\$ The factors I consider are: power lost because cells charging each other (when there is a small voltage difference not enough to trigger the protection). Users can mix up partly charged/fully charged cells, trigger the protection, get frustrated. Cells wearing out more quickly because they have to re-balance every time you insert them. How will you guarantee all cells are charged to the same level outside your device ? Cells that are just charged will charge the cells that were charged longer ago. \$\endgroup\$ – Bimpelrekkie Feb 5 '16 at 8:52

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