2 Lipo cells in series: can they savely be charged in series? - Page 1
Forum section: electronics / projects

author: cdwijs
author_rank: Regular Contributor
timestamp: May 29, 2015, 10:39:14 am
content: Hi All,I have a project that's to be powered by 2 LiPo cells in series. I have to design the charger to the project.As far as my understanding of LiPo cells goes, they have to be charged current-limited, and voltage limited to 4.2V. If I now charge 2 of them in series, one of them will be fully charged, while the other is not. As the total voltage is then not yet 8.4V, the charger will continue, and I think the first cell will be damaged by overcharging.I have found a charger IC that can charge 2 LiPo's in series, but it only measures the total voltage, not the individual cells.http://nl.farnell.com/texas-instruments/bq2057wsn/ic-charge-manager-li-ion-soic/dp/1212263?ost=1212263This IC has one NTC input, can I put an NTC on each cell, and then connect the 2 NTC's in series? Will the charger IC then see that one cell is getting overcharged?Is this a real problem? This is the first time I ever make a charger for LiPo, what are your experiences?Is it worth it to use 2 1-cell charger IC's in series?Kind regards,Cedric

author: hayatepilot
author_rank: Frequent Contributor
timestamp: May 29, 2015, 12:24:03 pm
content: Yes, for a long lifetime of the cells it is essential to balance the battery pack.As you said the cells are gonna drift apart over time/cycles until one of the cells gets damaged when fully charged. This can go as far as battery explosions or fire.I would recommend using a cell balancing chip. For example:http://www.farnell.com/datasheets/1887557.pdfWhen LiPos where new in RC-planes, there where no balancing chargers available. The cells inevitabli started to drift apart and had a very short life. If you where lucky you could get maybe 50 charge cycles out of the battery pack.Today all of the chargers have balancing capabilities so the battery life is much better and charging is safer.Greetings

author: cdwijs
author_rank: Regular Contributor
timestamp: May 29, 2015, 01:04:45 pm
content: Thanks for pointing out balancing is a real issue.I think I'll use this limiter IC, all the others are either not in stock, or are hard to solder by handhttp://nl.farnell.com/texas-instruments/bq29419pw/volt-protection-2-4-cell-8tssop/dp/1647907I have checked all 2-cell charger IC's from farnell, not a single one has an integrated balancer, or limiter. I don't understand this, as they are marketed to be able to correctly charge the cells.Kind regards,Cedric

author: amyk
author_rank: Super Contributor
timestamp: May 29, 2015, 01:49:25 pm
content: Why not put them in parallel instead? Then they can be treated as one big cell and won't need any balancing...

author: hayatepilot
author_rank: Frequent Contributor
timestamp: May 29, 2015, 01:56:42 pm
content: This chip would not work as a balancer, the output pin just goes HIGH if one of the cells has overvoltage.You could use this signal to stop charging. So either stop charging when 8.4V is reached or if the protection chip says stop.This way the cells will not get damaged, but you loose a capacity if the cells drift since one cell will never reach full charge.A better way is to use a protector chip with balancing function like the one i mentioned above. Maybe there are other more DIY friendly chips with the same functionality...Why does it need to be easily soldered by hand? Just for the prototype, or is it a one-off project?You can solder QFN packages (for prototyping) without solder paste and stencils, you just need a hot air gun:Greetings

author: cdwijs
author_rank: Regular Contributor
timestamp: May 29, 2015, 02:06:41 pm
content: I can indeed use one cell (or multiple in parallel), but then I need a boost converter, and power the arduino due via the USB port. Maybe this is also a viable option, but I have not yet explored it.This would require an IC that has these properties:-Charge one Lipocell,-Create 5V,-Stops before the cell is drained too far-Available at farnell,-can be hand-soldered (it's a one-off product)Do you have a suggestion for such an IC?Kind regards,Cedric

author: mark03
author_rank: Frequent Contributor
timestamp: May 29, 2015, 06:40:28 pm
content: Olympus PEN cameras are one example of a mass-market consumer product using two Li-ion cells in series with no tap point for balancing. So, clearly it is acceptable in some situations. It may help if Olympus uses matched cells at the factory. Also, there may be some form of lossy balancing circuitry inside the battery. I haven't cracked one open to find out.But anyway, I just built a micro-USB charger for these batteries, based around an MCP73842 which is designed for a 2S cell configuration (without balancing). Works fine.

author: cdwijs
author_rank: Regular Contributor
timestamp: May 29, 2015, 08:41:14 pm
content: Hi all,I think 2 cells in series is a bad idea. I'll use a 1 cell charger with a boost converter. I don't have to create this myself, I can just buy this circuit:https://www.sparkfun.com/products/11231I will alter the circuit to have a cutoff voltage of 3.5V, to prevent damage from deep discharge, if the clients wants longer runtime, I can just add more cells in parallel. sparkfun has 3x2000mAh, that should be enough :-)Kind regards,Cedric

author: nukie
author_rank: Frequent Contributor
timestamp: June 03, 2015, 02:12:44 am
content: Bosch Power tools 18v 4.0Ah battery is a 5S2P connected 10x 18650 cells. They charge them without individual monitoring. I guess as long as you have quality cells that is factory matched it should be alright??!!

author: cdwijs
author_rank: Regular Contributor
timestamp: June 03, 2015, 05:08:34 am
content: Quote from: nukie on June 03, 2015, 02:12:44 amI cannot get a hold of factory matched cells. Also, I think it's a feed forward loop. If one cell is just a little bit less capacity than the other, it will be overcharged a little, and then it looses a bit of capacity, so the next cycle is worse. So I don't think it even matters if the cells are mached in the beginning. I don't know however what happens exactly when a cell is being overcharged. I think it looses capacity immediately, it will heat up, and so on, but I don't know for sure.Does anybody know about a study on overcharging li-ion and li-po cells? I don't need to know anymore for my project, but I'm starting to get facinatedKind regards,Cedric

author: NiHaoMike
author_rank: Super Contributor
timestamp: June 03, 2015, 05:48:01 am
content: All of the lithium ion multicell packs I have seen that don't appear to have balancing connections (practically all consumer packs) have a balancing circuit built into the protection board. LiFePO4 can get away without balancing, but it's still a good idea.

author: Siwastaja
author_rank: Super Contributor
timestamp: June 03, 2015, 09:24:52 pm
content: Quote from: cdwijs on June 03, 2015, 05:08:34 amNot immediately - the interesting thing in LCO/NCA/NMC/LMO cells is that they are not "fully" charged at 100%! (That is, the chemical reaction is not completed.) Thus, the 100%, 4.2V point, is kind of arbitrarily chosen value (not exactly, but you can think it like this). So, you can overcharge these cells, they take up charge, the voltage goes up quite linearly like it did before 4.2V, and you get more Ah in, more Ah out, a higher capacity cell. The problem is, both lifetime and safety goes down. But it's not a sudden thing. (This works the other way, too - using a 4.2V cell at 4.1V will increase the lifetime and safety considerably, at the expense of capacity.)What you need when doing series packs without cell level monitoring is to leave a bit more extra leeway on both ends.The voltage curve of these cells isn't flat at all, so you need considerable amount of charge imbalance before you get considerable voltage imbalance, so it doesn't happen just like that -- this is good, the cells are kept in balance easily.4.25V is often considered the safe maximum with no catastrophic effect on lifetime. So if you charge to 4.15V/cell by design, you have a whole 0.1V of leeway, which corresponds to about 8% state-of-charge imbalance, before anything bad starts to happen. It would take a huge amount of unwanted reactions in the cell, i.e., a very bad cell, to develop this much imbalance.The cell that sits at a higher voltage, will calendar fade more quickly. While it loses capacity, at the same time, it loses charge (self-discharges). This tends to balance (not unbalance) the cells at the top -- on the other hand, it causes an imbalance at the bottom.So when your pack is balanced at top, you need to watch out for the LOW limit by adding even more leeway there. With only 2s, it's not too bad. But even when you manage to get the top balance right, if the capacities differ, you can have a big difference at the low end; 2.5V may be a valid cut-off under load at 0% SoC, but another cell might be at 3.2V at 5% SoC! So, for a 2s pack, I'd use something like 3.1-3.2V/cell as a lower cutoff. An application that uses low discharge currents (like <0.2C) can use even higher low limits (say, 3.3V) because there is little voltage sag.It's of course recommended to separately monitor the cell voltages, but you can fail at it too, and it happens all the time; BMS is easy to fail at miserably. On the other hand, consumer electronics have always been full of 2s, non-tapped, non-BMS solutions with good track record. BOSCH extends it to 4s and apparently even further, but I would stop at 2s or maybe 3s; after that, use a cell-level BMS.TLDR; charge separately to 4.15V, make sure they really are at the same voltage, then connect to series, don't connect anything to the center tap, and keep tighter limits (4.15V/3.10V per cell for example, instead of 4.20V/2.50V for a single cell)