18650pack cell protection idea. PTC + fuse & diode).

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Redeque   1 µW

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18650pack cell protection idea. PTC + fuse & diode).

Post by Redeque » Sep 16 2017 8:12pm

After seeing different types of 18650 pack builds I have been thinking about protection.

I am going to build a 14S6P pack for my E-bike using Panasonic NCR18650PF 2900mAh - 10A.
This will be used with a 60amp BMS and 30amp controller.

After looking at the current/temperature curve for this cell I sure don't want to ever pull more then 10amps per cell.
To avoid this happening (in some case I can't think of yet) using a PTC fuse would be great.
Problem with a PTC fuse is that, if a problem does accure, you can't find it!
So the Tesla style fusing comes to mind but that makes a blown fuse very permanent.
That is when I thought of combining the two options.
The PTC fuse as cell overcurrent protection and a 1A tesla style fuse with diode as inrush protection, in paralel.
With this setup you can never pull more then 10amps per cell due to the PTC and if a cell dies the fuse will blow at 11Amps.

Price wise I am not bothered since it is around 10,- extra for the entire pack.
(100x PTC = 9,- / 100x Fuse = 1,50,- /100x Diode = 1,50)

I this "overkill crazy", "does't work crazy" or just "brilliantly crazy"....?!
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flippy   10 kW

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Re: 18650pack cell protection idea. PTC + fuse & diode).

Post by flippy » Sep 17 2017 3:45am

Read into how a PTC works. More power means more heat wich means more resistance. It is not a "switch". You will physically burn out the pack due to heat generation.

Buy a proper bms, its cheaper once you add up the cost of building such a rube goldberg sysyem.
Lithium beats liquid dinosaurs.

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DanGT86   10 kW

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Re: 18650pack cell protection idea. PTC + fuse & diode).

Post by DanGT86 » Oct 12 2018 9:45am

What about using PTC fuses on the signal wires/balance leads to the bms?

I would like to implement this on a custom pack build for safety. The small wires that go to each cell seem easy to damage and could be a direct short if the battery got hit in a crash. I know several companies use them on all the BMS wires. I built one pack with 18gauge signal/balance leads because it was all I had on hand. I sometimes worry about that one because I feel like as a fuseable link the 18ga. wire could definitely start a fire in the battery bag before shorting itself open.

Do PTC fuses alter the voltage under normal use temperatures such that the BMS cutoff points need to compensate for them? Id like to be able to still use a balance charger with 100-200mA of balancing current. Would that get 22-24 gauge wires with a ptc fuse warm enough to throw off the reading drastically?

Is it as easy as buying a few $ worth of a common size of PTC fuse and just adding them in or is it more complicated than that?

What would be the thought process when selecting the propper PTC fuse?

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flippy   10 kW

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Re: 18650pack cell protection idea. PTC + fuse & diode).

Post by flippy » Oct 12 2018 11:43am

Please be more accurate, a PTC inside a cell is called Pressiure Temperature Current. A regular PTC in electrical terms a polymeric positive temperature coefficient, aka: a self resetting temp fuse. They both work as a fuse but one is permanent and the other is just a fancy resistor
Lithium beats liquid dinosaurs.

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DanGT86   10 kW

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Re: 18650pack cell protection idea. PTC + fuse & diode).

Post by DanGT86 » Oct 12 2018 11:53am

I am talking about the auto resetting fuse. Im guessing based on your description that the resistance goes up with temp until its basically "tripped" to stop the current.

I guess I'm wondering if there is such a ptc that would have little perceptable effect on the voltage signal under most normal temps/use of the pack so I dont need to recalibrate the BMS Voltage limits.

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flippy   10 kW

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Re: 18650pack cell protection idea. PTC + fuse & diode).

Post by flippy » Oct 12 2018 12:00pm

If you want to go that route then you need a regular fuse, never ever put passive or active parts between the bms and the cells.

Please note that it is still academic, the risk of a cell popping does not cause a immediate problem, you just lose capacity and the bms will keep trying to balanc out the voltage difference created by the popped cell.
Lithium beats liquid dinosaurs.

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