Increasing portable emergency light battery capacity - is it safe?   - Page 1
Forum section: electronics / beginners

author: BlackBeret
author_rank: Newbie
timestamp: January 18, 2023, 10:36:53 am
content: Hello. I have a portable emergency light, that operates on a single 18560 battery:https://www.arteleta.it/media/catalogo/prodotti/0/1467033918364dl1077.jpgI thought of increasing its runtime by either:1. Replacing the single, low-capacity, 18560 with multiple high-capacity 18560s, connected in parallel.2. Replacing the single 18560 with a single high-capacity 26650.Is it safe to do so?(I would rather go with option #1, if that's safe enough).

author: tunk
author_rank: Super Contributor
timestamp: January 18, 2023, 11:10:49 am
content: Option #2 has fewer failure points.

author: BillyO
author_rank: Super Contributor
timestamp: January 18, 2023, 12:35:29 pm
content: Provided the charging circuit is up to the task it should be fine. It will take longer to fully charge though.

author: Infraviolet
author_rank: Super Contributor
timestamp: January 20, 2023, 02:57:58 am
content: Option #1 is a recipe for disaster, paralleled lithium batteries can charge in to each other and discharge each other, cells can easily get below their minimum or above their maximum safe voltages. Flames result, unless you are very lucky and the cells have built-in protective circuits, in which case dead cells result nonetheless.Either use a higher capacity single cell, or replace the cell entirely with a regulated power supply set for the sort of voltage the current cell has when mostly charged, and have that regulated power supply powered from a series stack of cells with a higher overall voltage.

author: CatalinaWOW
author_rank: Super Contributor
timestamp: January 20, 2023, 05:18:29 am
content: Quote from: Infraviolet on January 20, 2023, 02:57:58 amI don't understand the hazard in parallel lithium batteries, except at the time of initial connection. At initial connection if they don't have the same state of charge the higher one will charge the lower one at a rate limited by the impedance of the batteries, which could indeed be a problem. This could be addressed by connecting them through an appropriate resistor which after current in the resistor dropped to zero would be shorted and removed.Once shorted together they will both be forced to the same voltage and as long as the charger maintains them within safe limits there should be no serious problem. They might not accept or deliver current equally, but they will accurately follow each other in voltage. The only problem would be if the user was trying to draw substantially more than the maximum current acceptable for a single cell, and any other paralleled cells had degraded so much that they were contributing little to the packs total output. In the OPs case this is an unlikely fault since the single cell is already handling the current and it appears that OP wants to add Amp-Hours, not Amps.Series connections do have the potential for charge becoming seriously unbalanced and do require some sort of monitoring/balancing to avoid problems.

author: Mr.B
author_rank: Unknown
timestamp: January 20, 2023, 05:26:16 am
content: Quote from: CatalinaWOW on January 20, 2023, 05:18:29 amThis^I have used this method numerous times, with identical spec cells, and never had an issue.

author: Ian.M
author_rank: Super Contributor
timestamp: January 20, 2023, 06:17:46 am
content: There is a potential issue with large numbers of unprotected cells in parallel - if one cell develops an internal fault that causes excessive self-discharge, the others will dump current into it, increasing the risk of a runaway exothermal event. However unless you are building EV packs or similar you are unlikely to ever parallel enough cells for this to be an issue.

author: coppercone2
author_rank: Super Contributor
timestamp: January 20, 2023, 06:49:11 am
content: and you can fuse them

author: james_s
author_rank: Super Contributor
timestamp: January 20, 2023, 07:10:36 am
content: Quote from: Infraviolet on January 20, 2023, 02:57:58 amHuh? Paralleled lithium ion batteries is extremely common, most laptops have them, I have model airplanes that are set up for multiple packs in parallel. EVs have large numbers of cells in parallel, I've built numerous packs myself that have parallel cells. It's no issue at all.

author: BeBuLamar
author_rank: Super Contributor
timestamp: January 20, 2023, 01:45:32 pm
content: Quote from: Infraviolet on January 20, 2023, 02:57:58 amMost power tool power packs that are capable of more than 3.0 Ah have parallel Li-Ion cells. They are generally parallel in group of 2 for 12V or 5 for 20V in series and have 2 or 3 of these groups. Yes there is the possibility of the problem you said but it's a common practice. There is also a 20/60V pack that switch between 20 to 60V by changing the configuration.

author: BeBuLamar
author_rank: Super Contributor
timestamp: January 20, 2023, 01:47:34 pm
content: Quote from: Mr.B on January 20, 2023, 05:26:16 amAnd in fact EV packs are series / parallel to get both voltage and capacity needed for the EV. They don't make one cell with a lot of Ah capacity.

author: Infraviolet
author_rank: Super Contributor
timestamp: January 21, 2023, 01:36:08 am
content: The advice I'd always seen is that cell balancing is trickier for paralleled cells than series ones, Id never been sure why, but in most of the projects I've seen people only have parallel cells when those cells were paralleled at the factory (with extremely precise matching of characteristics), but then seriesing them up only needed a balancing circuit the like of which is commonly available as a module.My comments about seriesing cells for higher voltages were with the idea that this would then feed to a step down regulator, when drawing the normally needed current from this regulator the current drawn from the batteries will be lower by a factor close to the ratio of the old cell's voltage vs the new series' voltage, and therefore the series stack will be able to supply the current for longer.The OP's option 2 sounds pretty good, assuming the higher capacity cell is still the same voltage (same numebr of in series cells insie it, probably just a single cell given that an 18650 is that) as the old one. The higher capacity cell will be able to supply any given current for longer than the old cell before its voltage gets below the device's cut-off threshold.

author: james_s
author_rank: Super Contributor
timestamp: January 21, 2023, 01:39:29 am
content: Either option is perfectly fine. Putting cells in parallel is totally ok and very common. Even if they are not quite at the exact same state of charge it is rarely an issue, consider that the difference between fully charged and 50% charged is only a few hundred mV for Li-ion, you've got to have an extremely low internal resistance in order for a dangerous amount of current to flow with for example a 200mV potential difference and it's easy to get them more closely matched than that.