Led charging indicator circuit - Page 1
Forum section: electronics / projects

author: drummerdimitri
author_rank: Frequent Contributor
timestamp: May 27, 2019, 08:08:26 pm
content: I am converting my walkie talkie from NiMH to Li-Ion and had to re-wire the charging dock's circuit as it used to convert an AC voltage and rectify it into DC to charge the batteries but now I would like to use fully integrated DC charger and would like to retain the original led charging indicator LEDs for each unit but since I've wired them to be charged in parallel, what circuit would I need to build in order to have the light come on for each unit separately?

author: mikerj
author_rank: Super Contributor
timestamp: May 27, 2019, 08:56:58 pm
content: Lithium ion charge termination is determined by voltage, so if you charge packs in parallel then by definition they will complete charging at the same time.This isn't a good idea however, if you plug a fully drained pack into one slot with a full pack in the other, you could get damaging levels of current flowing from the full pack to the discharged one. Better to have have fully independent charging circuits.

author: drummerdimitri
author_rank: Frequent Contributor
timestamp: May 27, 2019, 09:20:48 pm
content: Quote from: mikerj on May 27, 2019, 08:56:58 pmI see what you mean, however i meant I would like for each led to light up when the corresponding unit is placed in the docking station for charging.As with regards to the over current flowing I am not concerned about this since they will be used together at all times so their state of charge will be more or less the same.

author: mikerj
author_rank: Super Contributor
timestamp: May 30, 2019, 11:20:46 am
content: Are there any other pins on the battery packs, such as a thermistor? If so you may be able to utilise these these to sense when a battery is plugged into one of the charging ports.

author: Siwastaja
author_rank: Super Contributor
timestamp: May 31, 2019, 09:11:08 am
content: You can't connect li-ion packs in parallel just like that, they will be destroyed, possibly catastrophically. Having a feeling they are "more or less" at the same SoC when connecting together is not enough.You can connect the li-ion cells parallel permanently, by carefully making sure they are at the same voltage within about 50 mV, but any chance of getting them disconnected and put together is a massive risk of forgetting to measure before connecting them together again.Example calculation:Typical 18650 power tool cell, DC resistance at room temp 20 mOhm, max specified charge current 2.5A, contact/wire resistance total 10 mOhm.Cell 1, 80%, 4.00VCell 2, 60%, 3.80VCurrent when paralleled: 0.2V/(20+20+10mOhm) = 4A, exceeds max charge current, damage very likely.Example 2:Cell 1, 100%, 4.20VCell 2, 0%, 3.40VCells are warm, DC resistance is only 10 mOhm, connection is solid with only 5mOhm resistance,Current when paralleled: 0.8V/(10+10+5mOhm) = 32A, instant destruction of the cell, cell safety systems are likely to react and prevent a fire, but the cell is volatile and dangerous after the fact.Use separate chargers and separate loads so they are never connected in parallel directly, or decide to build a big enough pack with permanently paralleled cells.

author: drummerdimitri
author_rank: Frequent Contributor
timestamp: May 31, 2019, 09:41:52 am
content: Thanks for this info.I will not be using the charging station in that case and will instead charge them individually with the mini USB port on each unit.

author: angelicajames
author_rank: Newbie
timestamp: May 31, 2019, 10:02:55 am
content: That's not a great approach if you charge packs in parallel at the same time and then by meaning, they will achieve charging. Because Lithium-ion charge termination is circumscribed by the voltageThe better way is to have enough detached charging circuits.