18650 charging, Philosophically speaking  - Page 1
Forum section: electronics / general

author: jwhitmore
author_rank: Regular Contributor
timestamp: October 09, 2018, 08:08:41 pm
content: This is probably going to get me shot down but I'm hoping to learn something.So if you were to charge a single 18650 it's relatively easy, you can get a ready made TP4056 PCB which contains charging and protection of the shelf. Plug in any single 18650 and it just does it's thing.The next evolutionary step is when you get multiple 18650s and combine them into a bigger battery pack, 7S1P seems to be quite a common configuration. Now you're going to charge seven 18650s in series. This complicates things as you have to now monitor the individual 18650s so that an individual 18650 doesn't get over charged. So you need a Battery Management System, which as I understand it, will burn off excess energy to stop any individual battery getting over charged. To minimise the risk of this happening people seem to spend hours pre-testing 18650s to match them against each other, before combining suitable, well matched batteries into a pack.I might have that wrong, but if I have it right my problem is with burning off energy to stop over charging. That seems to contradict what it is you're actually trying to do, harvest energy.So if cost was not an issue, or practical considerations would it make a degree of sense to charge 18650 batteries individually and then discharge them in series? You would still need the BMS when you're discharging the 7S1P because you'd have to monitor that a battery's voltage wasn't dropping like a stone compared to the others in the pack, but at least you're not potentially burning off energy during the charging process.I'm sure somebody more knowledgeable then me will come straight back and say that you'd never be burning of anything other then negligible energy so it's not worth the effort. Then again it's not just the energy but the whole matching process. And given the individual charge and BMS managed discharge you could, I image, have SW highlight a battery which is worth dumping out of a pack.

author: ConKbot
author_rank: Super Contributor
timestamp: October 10, 2018, 02:14:09 am
content: QuoteYou got it! When you charge a pack to being balanced, and all the batteries are at a 100% state of charge with a nice nominal 4.200 V when resting at no load. You take 2.5AH out, and put 2.5AH+ a tad extra for coulomb inefficiency back in, and you should be back at 4.200V/cell, even if your weak cells reached 5% SOC, and the good ones were only down to 20% SOC. Even if the cells are slightly mismatched capacity and impedance wise, the current flowing though them all has to be equal, so everything stays nice and balanced.What makes for the need for balancing is differences in leakage current, difference in coulomb effiency (i.e. 1 battery may give 99mAh out for every 100mAh put in, while another gives 98.7. Not a big deal over one cycle, but it adds up) and other effects that may start showing up under more extreme use (high discharge causing heating, uneven temps in the pack leading to differences in leakage or efficiency, etc)There are methods of balancing that can reduce losses by using charge pumps or transformers to move current to where it needs to go, but they really arent considered until you get into the tens or hundreds of AH range. Or for applications where you need every joule you can out of the battery, even if its in a degraded condition, and the balancer would be trying to charge the weak cell while moving energy out of other cells to keep it in a safe range. Something like a satellite where you want to get as many cycles as possible from a pack.www.cobham.com/media/1961450/Cobham_2016_NASA_Battery_Workshop.pdfLaptop batteries used to not even come with balancing, just monitoring and protection, relying on good matching from the factory to give a battery that stayed in balance for the service life of the battery, so one cell of two welded in parallel would go weak and start limiting pack capacity, get further and further out of balance till you have ~5 minutes of runtime, tear the pack open and find 4 good perfectly serviceable cells, 1 dead, and 1 discharged flat by the dead cell.

author: jwhitmore
author_rank: Regular Contributor
timestamp: October 10, 2018, 08:26:32 am
content: Thank you. Pragmatism beats philosophy

author: jwhitmore
author_rank: Regular Contributor
timestamp: October 10, 2018, 05:36:07 pm
content: So a bit more thought and whilst the burning off of energy whilst charging is not a big swing. The matching of batteries into packs seems to be a lot of work that his to be undertaken.And on that subject if I create a 20P pack with 18650s, it will have a capacity rating. If I at a later date want to create a new pack to add to the system do I have to find batteries which are doing to match the previous pack in the system? Battery management is subject in itself. I'm sure this balancing of packs is an issue both when discharging and when charging. Have to dig out stuff to read up on this BMS and balancing.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 10, 2018, 11:06:46 pm
content: You could make a circuit that dumps the excess charge into neighboring cell(s), and so on and so forth until the last cell is fully charged. I'm not real sure what happens if you try overcharging such a circuit, though: each clamp dumps charge into its neighbor, and you're applying power but it has nowhere to go. You'd have to have a battery manager on the outside that notes when all cells are fully charged, and either disconnects (terminal voltage goes up) or dummy loads (terminal current stays high) the charge.Maybe a voltage balancing chain would be better than a clamping chain...Anyway, for the lossy version, it's only supposed to be a small loss in charge efficiency, overall the same loss as the maximum difference in matching. If one cell is 90% and the rest are 100%, then that cell has to burn the remaining 10% on every discharge-charge cycle, and the full pack is limited to that 90% charge figure. Well, 90% discharge means the 100%-ers aren't fully discharged, so they only need to go 90%, so it's more like a 10% of 10% difference, and it's not so bad at all, a few percent of charge efficiency, not the same as the full difference. In the average case that is. The worst case (all start at "0%" charge) would still be the 10% loss case. But again only 10% for that cell, so for a 4S pack, say, it's still 2.5% max overall.So yeah, a few percent is down in the noise of normal variation and charge efficiency to begin with, so it's really not a bad method in general.Tim

author: IanMacdonald
author_rank: Frequent Contributor
timestamp: October 15, 2018, 04:12:37 pm
content: Basically, any setup using cells in series is more problematic than cells in parallel. The problem with cells in parallel, though, is the low voltage and high current, which calls for a well-designed inverter if losses are to be kept under control. Which is why it's rarely used.Even primary cells in series can suffer from one of a set being over-discharged and suffering reverse polarity if it is weaker than the rest. For example, alkaline 1.5v cells will very rarely leak and damage equipment if used in equal sets, but mixing old and new cells in series can still result in leakage and equipment damage.A truck driver I knew ruined two monster lead-acid batteries by connecting a 12v heater fan to the centre tap. The drain on one battery, though only a few amps, unbalanced the charging sufficiently that damage was done over time.