18650 protection IC's... - Page 1
Forum section: electronics / beginners

author: alank2
author_rank: Super Contributor
timestamp: March 08, 2016, 03:38:25 am
content: I'm thinking of using an 18650 in a project, but after trying a protected cell in a socket, it is really tight and a PITA. I guess the protection circuit adds too much length.So I've got a couple of questions--Is it an actual IC? Why not just a simple PTC?Also, one concern I have with this type of battery is that I don't want it shorting to anything and creating a high current situation. I was planning on using a PTC on the positive side along with a schottky diode to prevent reverse current flow, but then it occurred to me, what if the exposed part of the battery holder touched anything conductive against another ground on the board. Then I thought, I could put two PTC"s in. One on the negative side and one on the positive side - what about that idea?

author: Audioguru
author_rank: Super Contributor
timestamp: March 08, 2016, 03:47:17 am
content: A PTC will heat up and limit the charging or discharging current.But it will not limit the charging voltage. It will not detect a full charge and shut off the charging. It also will not detect a low voltage and shut off the load and it will not refuse to charge a battery with a voltage that is too low.Then you will probably have a nice hot fire or explosion without a protection circuit.

author: MK14
author_rank: Super Contributor
timestamp: March 08, 2016, 03:50:25 am
content: Quote from: alank2 on March 08, 2016, 03:38:25 amOff-hand I DON'T know the full functionality of the protection chip, but being an IC (and in all likelihood, very well designed as a result), I would imagine it does a lot of protection features, some of which might be completely non-obvious, such as cell temperature management.How did you envisage the PTC preventing over-charging (as in sensible charge rates, but for TOO LONG), or attempts at too deep a discharge, or enforced shutdown if things go bad and/or the cell has expired.Also what about overvoltage ?I'd suggest sticking to keeping the safety circuit, because lithium cell faults can set buildings on fire and stuff, when they go wrong otherwise.

author: amyk
author_rank: Super Contributor
timestamp: March 08, 2016, 03:55:32 am
content: A PTC is only overcurrent protection, you also need undervoltage and overvoltage.The DW-01 and its companion dual MOSFET is probably the cheapest solution that has all the protections you need.

author: alank2
author_rank: Super Contributor
timestamp: March 08, 2016, 04:03:24 am
content: I plan on still using a protected cell - I want to add some protection on top on that. Does the idea of a PTC on the positive and negative side have any merit?

author: MK14
author_rank: Super Contributor
timestamp: March 08, 2016, 04:09:03 am
content: Quote from: alank2 on March 08, 2016, 04:03:24 amIf YOUR circuit (completely external to the battery and its built in protection circuits) can benefit from the PTC, then great.Why are you talking about PTCs, on BOTH the positive and negative sides ?(Assuming the cell remains insulated, inaccessible, to the rest of the circuit, except its terminals, obviously).

author: sleemanj
author_rank: Super Contributor
timestamp: March 08, 2016, 04:42:06 am
content: Quote from: alank2 on March 08, 2016, 04:03:24 amWhat extra protection - that is to say, what are the additional issues that you are trying to prevent that the protection circuit doesn't already handle?As above the most common single cell protection circuit has a DW01 protection IC and 8205A dual mosfet, at least for external protection boards, have a read of the DW01 datasheet and see what protections it implements. There is no sense in adding protection against failures that are already protected against.PS: The 18650 holders with a proper coiled spring for negative will fit protected cells better than the ones that just have a bit of bent metal.PS2: You can also get 18650 holders with a protection circuit built into the holder itself, for use with unprotected cells.

author: alank2
author_rank: Super Contributor
timestamp: March 08, 2016, 01:02:20 pm
content: Quote from: MK14 on March 08, 2016, 04:09:03 amThis assumption can't be assumed. The battery will be on the underside of something and some parts of the battery terminals will be accessible.

author: MK14
author_rank: Super Contributor
timestamp: March 08, 2016, 02:45:23 pm
content: Quote from: alank2 on March 08, 2016, 01:02:20 pmIt makes more sense now.In general, it is usually best that the battery(s), its holder and any wires connecting it to the main PCB(s). Should be insulated, physically protected, encased etc etc. Enough so that the batteries, are safely away from any kind of accidental short circuits or something.I'm not exactly sure what your project would look like, or do. But it does seem to be beginning to sound like you want to fix one or two PTCs, to make up for the fact that the battery installation is rather/potentially weak and sustainable to shorting out.The thing is, occasionally/rarely (in theory), the safety circuit can fail. So if that happened in your setup, not much else would need to go wrong, for there to potentially be a fire hazard, created.Really everything should be sorted out, the final step being the last compile and download.

author: alank2
author_rank: Super Contributor
timestamp: March 08, 2016, 02:47:33 pm
content: Quote from: MK14 on March 08, 2016, 02:45:23 pmI guess that is what I'm thinking. With the protection IC, and a pretty low current PTC (0.5A) on each terminal, that that might be a pretty solid plan for all situations.

author: MK14
author_rank: Super Contributor
timestamp: March 08, 2016, 02:54:04 pm
content: Quote from: alank2 on March 08, 2016, 02:47:33 pmI think I'm beginning to get it now.Are you saying that if you ONLY had one PTC, say on the positive terminal. The danger could be that the somewhat weak installation of your battery, *could* connect the positive terminal of the battery, WITHOUT going through the PTC. So if it also makes contact with something which goes to the negative terminal. You could have a bad short circuit.But if you have the PTCs on BOTH battery terminals. Then if one fails, and shorts to something. The other battery terminal, still has its PTC, so the current will (hopefully) be limited to a safe 0.5A (500mA).

author: alank2
author_rank: Super Contributor
timestamp: March 08, 2016, 03:16:38 pm
content: Quote from: MK14 on March 08, 2016, 02:54:04 pmExactly. With both PTC's if the the positive terminal shorts to any local ground, it would still have that PTC on the negative terminal to go through...