BMS discharge cut-off voltage - Page 1
Forum section: electronics / beginners

author: carpetpaul2129
author_rank: Contributor
timestamp: May 28, 2022, 09:07:42 am
content: Hi guys, quick question if I may. I'm in the process of working on additional auxiliary battery packs for an Ebike. I'm using ex-Dyson vacuum 18650's. I've made up a test pack of 10S and used a balancing BMS. it works fine, except the range is 5 miles as compared to a new 10S pack that has almost double the range, part of this is understandable based on the fact they're used cells, however, when the pack is cut-off by the BMS, the resting voltage is then about 3.52v and fairly even across all 10 cells (I've not tried to monitor the cells under load, that's the next plan of attack). note : in use, the cells don't even get warm.1, I wonder if the BMS is just cutting out early ?2, I wonder if I have one spectacularly duff cell that's being noticed by the BMS and that's causing it to cut out early ?3, Perhaps under load, it's dipping under the cut-off voltage (all it says in specs is 'over discharge protection 2.6v)4, why do the cells instantly show 3.52v when supposedly depleted ? or is it normal for them to spring back from under 3v when the load is removed ?5, would watching the voltage during use be a good idea ?as ever, many thanks, comments appreciatedPaul

author: tooki
author_rank: Super Contributor
timestamp: May 29, 2022, 11:44:52 am
content: #4 is key here, I suspect. The no-load voltage of a cell (of any battery type, not just LiIon) tells you very little about the true state of charge and of the state of chargefor your load. (By that I mean that a battery that is dead enough to not power a heavy load could still be fine for a light load, so “state of charge” actually needs to be calculated for the load in question.)And yes, it’s very normal for voltage to recover quickly when the load is removed.

author: rsjsouza
author_rank: Super Contributor
timestamp: May 29, 2022, 12:00:46 pm
content: Indeed. The internal resistance* of a battery tends to increase as it depletes or wears out. That is why most battery testers have a load to verify its status.If you charge these 18650s, they might be able to power a flashlight or a radio for very long periods of time. This is one of the reasons why I have various half-used AA/AAA/9V batteries in my battery box, as the kids' toys tend to be high draw and I use the half depleted ones in remotes, wall clocks and other gizmos.*an analog representation of the internal state of a battery, not a real resistor.

author: Ian.M
author_rank: Super Contributor
timestamp: May 29, 2022, 12:23:09 pm
content: I wouldn't want to keep used 18650 cells that have been abused by discharging them at a current high enough that their terminal voltage dropped below 2.6V/cell under load, as they are an incendiary bomb waiting to happen.e.g. see Louis Rossmann's eBike fire - new cells in the battery, but 'B' grade or possible fakes with lower capacity than claimed:https://youtu.be/JCaDz_2YcGQ?t=65If you do keep them, keep them outside in a steel ammo can or similar with the gasket removed so it cant build up pressure and explode, and an inch of dry sand in the bottom, on a concrete slab away from flammable objects. Seal the pack in a Ziploc bag with a fresh silica gel desiccant sachet, to prevent dust from the sand blocking the cell vents or condensation buildup causing corrosion. One wrap of PVC insulating tape round the lid/can joint would be acceptable to keep rain out, as it would blow off if they start venting.

author: tooki
author_rank: Super Contributor
timestamp: May 29, 2022, 12:56:30 pm
content: Agreed. The entire concept of using salvaged lithium rechargeables gives me the heebie-jeebies.