How hard can I push a protected 18650 battery? - Page 1
Forum section: electronics / general

author: smashedProton
author_rank: Frequent Contributor
timestamp: November 30, 2013, 03:18:51 am
content: I'm doing a project where I need to heat up nichrome wire with an 18650 battery. Since the battery is protected, can I just use a simple switch in between? I want the project to be as small as possible too. What is the correct way to do this? thanks in advance

author: Psi
author_rank: Super Contributor
timestamp: November 30, 2013, 03:56:47 am
content: Some 18650 say they have protection but do not.Also the low voltage cutout protection usually kicks in at a much lower voltage than you would want for long battery life.(You really want to stop discharging a lipo before 3.3V but often the cutout doesn't kick in until 2.8V)

author: smashedProton
author_rank: Frequent Contributor
timestamp: November 30, 2013, 04:16:18 am
content: I'm mainly talking about overcurrent protection here

author: peter.mitchell
author_rank: Super Contributor
timestamp: November 30, 2013, 04:25:18 am
content: the cut out is up to manufacturer of the protection boards discretion, however, if you look at the cell datasheets you should be able to get a good idea of what is safe.

author: chibiace
author_rank: Regular Contributor
timestamp: November 30, 2013, 04:59:08 am
content: Whats the project your working on. Sounds interesting

author: nuhamind2
author_rank: Regular Contributor
timestamp: November 30, 2013, 05:15:14 am
content: something that involve heating the nicrhome wire to cut something, stryofoam maybe ?

author: amyk
author_rank: Super Contributor
timestamp: November 30, 2013, 10:27:14 am
content: What's the specifications on the 18650? Some are designed for much higher currents than others (but have correspondingly lower capacity).

author: M. András
author_rank: Super Contributor
timestamp: November 30, 2013, 12:50:40 pm
content: i bought some aw branded cells for my flashlight it cuts out around 3.2volts and rated for approx 5A discharge its a 2.6Ah cell

author: Legit-Design
author_rank: Frequent Contributor
timestamp: November 30, 2013, 01:02:48 pm
content: Quote from: Psi on November 30, 2013, 03:56:47 amSome claim to have cut off at 2.5V, that seriously affects life of the cells, not much addition to capacity under 3.3V either.

author: Mike S
author_rank: Contributor
timestamp: November 30, 2013, 02:42:15 pm
content: The way I understood it is that Lithium Cobalt is the chemistry that typically has a protection circuit added onto the cell and their max discharge is something like 2 to 3C.Lithium Manganese (sometimes labeled as IMR) is the safer chemistry with lower capacity, but can discharge at 10 to 15C. These are far less likely to "vent with flame", and I don't think I've ever seen a built in protection circuit for this type of cell.If you can take the hit in capacity, it might be worth looking into LiMn or LiNiMnCo.

author: Neilm
author_rank: Super Contributor
timestamp: November 30, 2013, 06:17:52 pm
content: Quote from: Psi on November 30, 2013, 03:56:47 amA properly built 18650 cell from a good manufacturer has a small level of internal protection. This protection consists of an over current / over temperature device. This is a single shot device that when heated up breaks the circuit. It does not do anything for over discharge or over charging except if the cell gets hot enough to activate the cut out.

author: Psi
author_rank: Super Contributor
timestamp: December 01, 2013, 12:01:29 pm
content: That's not a protection circuit, thats a fuseIt needs a mosfet and control IC to be done properly.

author: justanothercanuck
author_rank: Frequent Contributor
timestamp: December 02, 2013, 08:52:38 am
content: 18650, nichrome... this almost sounds like an electronic cigarette.

author: Neilm
author_rank: Super Contributor
timestamp: December 02, 2013, 07:02:01 pm
content: Quote from: smashedProton on December 02, 2013, 06:21:36 amNo I don't - I work in safety and compliance testing.

author: Psi
author_rank: Super Contributor
timestamp: December 02, 2013, 11:36:44 pm
content: This is what we're talking about, a protected 18650 battery has some smarts inside.They have a PCB with control IC and mosfets to disconnect the load under low discharge and overcharge cutoff points as well as over current conditions.

author: thm_w
author_rank: Super Contributor
timestamp: December 03, 2013, 01:28:16 am
content: If you look at some protection circuits available from DX, reported range seems to be 2-5A:http://dx.com/p/charge-discharge-protective-circuit-board-for-rechargeable-li-ion-batteries-17-4mm-1-9mm-26112http://dx.com/p/charge-discharge-protective-circuit-board-for-rechargeable-li-ion-batteries-13-4mm-1-7mm-26114http://dx.com/p/charge-discharge-protective-circuit-board-for-rechargeable-li-ion-batteries-15-8mm-2-3mm-26113However, this would depend on mosfets used/controller IC. I'm sure it varies widely.The IC is VM7068:https://docs.google.com/file/d/0Bwe-Ui1_paOQYlJnS2FVU3l6SE0/edit?pli=1Default for this chip is limited to 0.1V across the FET (S8205A). Which has ~0.03 ohm rds on, so 3.3A overcurrent (or 1.7A since there are two fets in series?).

author: reagle
author_rank: Unknown
timestamp: December 03, 2013, 01:29:06 am
content: I'd almost say you need a current regulator there. Otherwise you'll just keep tripping SC protection on that board. Of course it all depends on the wire resistance and the resulting current. if its safely below protection limit, it may work. Keep in mind protection circuits have a rather wide tolerance on current. (these chips are typically optimized for voltage accuracy)

author: smashedProton
author_rank: Frequent Contributor
timestamp: December 03, 2013, 04:07:53 am
content: What is the best current limit circuit for a low resistance load? I cant just use a resistor.

author: peter.mitchell
author_rank: Super Contributor
timestamp: December 03, 2013, 08:16:27 am
content: if you want to limit the current of nichrome wire, just use more of it

author: amyk
author_rank: Super Contributor
timestamp: December 03, 2013, 12:02:09 pm
content: Quote from: justanothercanuck on December 02, 2013, 08:52:38 amAn 18650? That's more like an electroniccigar...Don't rely on the protection PCB for current limiting or voltage limiting, it's designed as a safety feature and might just fail if you keep hitting its limits.