Using multiple LiPo chargers in parallel to increase charge current - Page 1
Forum section: electronics / beginners

author: sky99
author_rank: Newbie
timestamp: February 08, 2016, 05:25:57 am
content: Hi, all!I am doing embedded projects, and for that purpose, i bought 18650 LiPo cells (3400mAh ones, this vastlybeats the price/wh of the pouch cells i could find) with protection circuitery. To put it behind : these are notsomething_fire fake/crap cells, but samsung ones; i also checked the capacity and it was accurate. So i have good quality cells, i have a balancing charger to balance them properly before doing packs, and i'm super cautious.So here is the problem. I have those 3.4Ah cells, and i want to use multiple cells in parallel for increased capacity. I keep them in parallel to avoid the need of a BMS. Now, in my application, i don't want to extract the batteries to charge them. So i use a lipo charging circuit. At the moment an adafruit one that is great, but a bit expensive. The problem is that it can only provide up to 1 amp of charging current to the batteries. So with 4 batteries, i'm in for more than 13 hours.Firthermore, i'd like my device to be able to work like a laptop : if i plug it to charge it, i can keep using it. So i need more current.So what i want to do is to use multiple charging circuits in parallel to do that. The reasoning is that current will increase, and since the boards will all read the same tension from the batteries, they will have similar behaviours.However, i am concerned by the fact that if a charger sends current while another reads, the reader will get a 4.2V tension and stop chargin. I don't think that it would matter at end of charge, if one charger stops slightly before another, but during the charge i fear that i may be problematic.Do you think that i can use multiple lipo charge circuits in parallel to charge the batteries (each time, bellow 1C, even 1/3C) and thus increase the current or not?Otherwise, i considered using a charger per battery/couple of batteries, then connect the positives poles of the batteries to a diode, then merging all + together after the diodes, so that the current couldn't flow from one charger to another. Is it a valid aproach?So, if one would tell me if any of those two aproaches is valid or not, and if not why? By the way, if you want to bring precisions even if it is valid, feel free, i like reading and learning stuffI tried searching on the web, but all i find is how to connect two lipo in parallel or in series.Many thanks in advance!

author: Psi
author_rank: Super Contributor
timestamp: February 08, 2016, 05:28:19 am
content: nope, you cant put multiple chargers on the same battery.As you say, they will get horribly confused because lipo charger periodically stop charging to sense the voltage, and this may not happen at the same time.Have a look for a simple RC lipo charger that's suitable and can charge at higher current.This might be suitablehttp://www.hobbyking.com/hobbyking/store/__73942__Turnigy_B4_Compact_35W_4A_Automatic_Balance_Charger_2_4S_Lipoly.htmlEDIT: Actually i think that needs 2 cells min

author: sky99
author_rank: Newbie
timestamp: February 08, 2016, 02:10:41 pm
content: Hi,thank you for your answer!So no multiple chargers for me then! Thanks for your answerAbout the hobbyking charger, sadly, it won't do, because i need to integrate it to the device, hence the need for circuits.I'm starting to think that there must be a reason if i don't find anything with 1 cell charge circuit with large current...Perhaps circuits tracks thickness, that limits max current? However, even 2A, that's not unheard of, on a circuit i think?Well, anyway, what do you think about the second solution :one charge circuit per battery, and each battery gets connected to the load via a diode, so that the current from one charger can only go through it's battery, but never through the others?(diode)+ ----|charger1|-------|+ battery1 +|--------|>-------- B1L- ----| |-------|- -|--- ...+ ----|charger2|-------|+ battery2 +|--------|>--------B2L- ----| |-------|- -|--- ...So, on the scheme up here, each battery positive lead gets connected to the load AFTER the diode (of course, all grounds are tied), on the points B1L and B2L, ..., BNL.What do you think?Thanks in advance

author: tszaboo
author_rank: Super Contributor
timestamp: February 08, 2016, 03:00:52 pm
content: I think you are not looking hard enough. There are up to 10A charger ICs on ti.com, up to 20A on linear.com.

author: amyk
author_rank: Super Contributor
timestamp: February 08, 2016, 03:24:13 pm
content: Quote from: Psi on February 08, 2016, 05:28:19 amDepends what type. The simple ones that are just a voltage+current regulator will work fine in parallel, doing exactly what you'd expect.

author: Morgoroth
author_rank: Regular Contributor
timestamp: February 08, 2016, 04:33:42 pm
content: seems what are you looking for is power path management, and that means feed the battery and the load at same time in a smart way.take a look at this:http://www.ti.com/lit/ds/symlink/bq24031-q1.pdfmicrochip have another line of this battery chargers to look in for:http://www.microchip.com/Developmenttools/ProductDetails.aspx?PartNO=MCP7383XRD-PPMput one for each battery and they would do the job, the problem could be (in free thinking mode) that the batteries must see a 'slow' discharge or 'slow' changes on the load to not get problems with the control, probably a low pass filter after all the batteries and before the load, and another low pass filter for each battery could solve the problem, you need to try or simulate. Probably a good idea is to ask to forums manufacturers and see what they say about it.

author: Audioguru
author_rank: Super Contributor
timestamp: February 08, 2016, 05:01:19 pm
content: A Lithium battery charger does not stop charging to detect the voltage. Instead it charges and limits the voltage to 4.2V and keeps on charging until the charging current drops to 1/25th to 1/40th the mAh rating then it shuts off.I have the battery from a cell phone. It has two Li-Ion cells in parallel and has only one protection circuit.If your battery cells are all the same then you can connect them in parallel (use series resistors first to prevent an explosion when one tries to charge another without current limiting) and use a high currewnt charger to charge them all in parallel.I think the battery will become overcharged and unstable (it might explode or catch on fire) if the charger is also used to power whatever the battery normally powers. There are some charger ICs that have a separate output that can power the product while the separated battery is charging.

author: sky99
author_rank: Newbie
timestamp: February 08, 2016, 08:15:20 pm
content: Quote from: NANDBlog on February 08, 2016, 03:00:52 pmHi,I'm sure that there are chips that does that, but it would require a support board, and i have no circuit etching capacity at the moment. Furthermore, i don't know how to properly use those circuits, unless they are as simple as an ATMEGA(i generally use mine without an external crystal to keep it even more simple). Most chips are also surface mount, something that i can't solder. Hence my search for complete and ready to use boards. And thereby, i'm looking for a way to use multiple ones to increase charge current

author: sky99
author_rank: Newbie
timestamp: February 08, 2016, 08:39:35 pm
content: Quote from: Morgoroth on February 08, 2016, 04:33:42 pmThis chip sounds great, but i only see it in surface mount packages. This is why i look for charge circuits already made!Quote from: Audioguru on February 08, 2016, 05:01:19 pmInteresting to know, so it is only dependant on the charge current for charge end detection.About the batteries in parallel, befor assembling them, i do a charge to balance those out. Indeed, once they are at exactly the same tension (+/- 1/100th of a volt) i connect them in parallel. That is the method used by people making 18650 battery packs for their electric scooters and such things, and sounds reliable without the resistor in series. The batteries are bought together, from the same manufacturer, and likely the same batch.My current battery charger board (the one from adafruit) is meant to be used to charge AND use the device, but provides too litle current (1A).Thanks all for the insights, and do you think i that the second assembly i mentioned early is valid?By the way, the load won't be heavy (2 raspberry pi 2 + an SSD, one raspi with wifi equipement to serve as an hotspot). I want large batteries not for high currents but high runtimes. One of the uses of the device will be off grid, with a solar panel charging it. So i need to be able to spend the night on battery only, and even more a security factor. I prefer to oversize the batteries, and have them discharge less often, thus increasing the life expectancy of the batteries.However, be it with solar o DC plug, i need to increase the current to charge faster. As of now, i plan to use 4 3.4Ah lipo, but i have 10 of this model. At 1A it would thus take 34h to charge the pack... (i know that i'm always taking the worst case, but i prefer to always be super conservative, and obtain real life performance equal or superior to my calculations made for the worst scenari).

author: Audioguru
author_rank: Super Contributor
timestamp: February 08, 2016, 09:12:33 pm
content: The Adafruit 259 chargershould notpower a product while it is charging a Lithium battery.Did you know that your system MUST detect a low battery voltage of about 3.2V then completely disconnect the battery or the Lithium battery is ruined and might explode or catch on fire if it is recharged.Read all about Lithium batteries atwww.batteryuniversity.com/learn.

author: sky99
author_rank: Newbie
timestamp: February 09, 2016, 12:29:17 am
content: Quote from: Audioguru on February 08, 2016, 09:12:33 pmHi, the battery charger that i'm using for this project is this one :https://www.adafruit.com/products/390However your warning is of interrest, since i have been using the other charger in another project (a raspi A+ and a camera module).I'm familiar with the website you recommend, but i newer saw anything about batteries exploding if charging while on. The only thing i found there was this : "Some portable devices sit in a charge cradle in the on position. The current drawn through the device is called the parasitic load and can distort the charge cycle. Battery manufacturers advise against parasitic loads while charging because it induces mini-cycles, but this cannot always be avoided; a laptop connected to the AC main is such a case. The battery is being charged to 4.20V/cell and then discharged by the device. The stress level on the battery is high because the cycles occur at the voltage threshold, often also at elevated temperature."In my case, the current drawn by the device is really small compared to the rating of the battery, and the charge current is low compared to the C rating, as i charge between C/3 and C/6. Thusly, the battery is never even warm.So, is your warning a general warning, or specific to this product? If it's this product, that probably means that i should stop using this charger, since it's pointless if i can't use it that way.If general, this is problematic : how to explain that all cell phones function properly this way, all computers, etc?And in this case, what is the solution to this problem? If i have a device that must NEVER be turned off, but still has to be recharged, how to do? I even designed my device so that i could change the batteries while the system runs (one half, then the second half for instance).And how is it possible to make a robot that charges itself up, if it has to power down? moreover, how is it possible to make a battery powered system with solar charging?

author: Audioguru
author_rank: Super Contributor
timestamp: February 09, 2016, 01:57:59 am
content: A Lithium battery must NEVER be trickle charged. When it is detected to be fully charged, the charging must be turned off.Adafruit did it by making the battery output and the load output on their charger completely separate but most products have them connected together with the on-off switch which is wrong for charging.

author: sky99
author_rank: Newbie
timestamp: February 09, 2016, 04:37:53 am
content: Quote from: Audioguru on February 09, 2016, 01:57:59 amOk, i'm lost now.I don't understand why you talk about trickle charge?I'm not using it since i'm using lipo specific chargers! I just mentioned that i was using much lower charge current than i could. Generally C/3 to C/6. My batteries ranges from 3 to 12Ah, i charge them with 1A, so it effectively does 1/3*C and 1/12*C (in this case, it's too low, hence why i'm looking for a way to charge with multiple chargers).Or maybe i misunderstand trickle charge, from what i read it is a very small charge current applied after the charge of NiMH to keep it topped (generally C/100).Here i'm not charging at low current after a charge, i have a MAX charge current equal to C/3 to C/6.I want the system to be able to be fully charged in 3 to 6 hours, but i need to be able to have it continuing it's work while beeing charged.After your previous post, i understood that the adafruit charger 259 can't do that. However now you seem to say that it is possible?

author: amyk
author_rank: Super Contributor
timestamp: February 09, 2016, 09:00:30 am
content: Quote from: Audioguru on February 08, 2016, 09:12:33 pmRead all about the misinformation of "Battery University" athttps://www.eevblog.com/forum/projects/diagnosing-lithium-cells/(in particular, seethis post, andthis post specifically about charging)

author: Psi
author_rank: Super Contributor
timestamp: February 09, 2016, 09:15:26 am
content: Quote from: Audioguru on February 08, 2016, 05:01:19 pmI dunno about all lithium chargers but the RC ones stop charging to check the voltage periodically.The cell voltage under charge current doesn't reflect the true cell voltage, so it stops charging to get an accurate reading and resumes charging if the cell was under 4.2V.Obviously the charge current is reduced as needed to keep the voltage under 4.2v at all times.The cell is not considered charged until it stays at 4.2V when charging stops.Some of the reasons for this maybe that hobby lipo cells can be charged at higher C ratings. 5-10C charge etc.

author: sky99
author_rank: Newbie
timestamp: February 09, 2016, 04:28:58 pm
content: Hello,thanks for the reading, i'm getting plenty of interesting informations about batteries.I find it, indeed, difficult to find much informations about lithium batteries online,often, i can find basic informations, or unbacked up claims, but not precise, technical data,about when it does X or Y, when the effect Z occurs, etc...This is also why i registered here, i figured this would be a good place to find interesting stuff

author: sky99
author_rank: Newbie
timestamp: February 10, 2016, 07:47:49 am
content: Hi, i have gotten confirmation from Adafruit that their charger can be used with the load connected (https://forums.adafruit.com/viewtopic.php?f=19&t=89585&p=451614#p451614). So unless someone else has evidence that this is dangerous, i'll consider it safe.Now back on the matter : do you guys think that one charger per battery, then connecting the batteries to the load via a diode to prevent current flowing back to other batteries/chargers is a valid solution, or do you forsee any problem i have not thought of?

author: KL27x
author_rank: Super Contributor
timestamp: February 10, 2016, 09:24:47 am
content: That would work fine, afaic.

author: sky99
author_rank: Newbie
timestamp: February 13, 2016, 01:53:51 am
content: I'll order a bunch of chargers and test it out!Thanks, all!

author: DivineChaos
author_rank: Regular Contributor
timestamp: February 13, 2016, 08:49:22 pm
content: Quote from: sky99 on February 08, 2016, 05:25:57 amAre you SURE you have genuineSamsung18650's? I looked and they do not make anything over 2500mAh. The ones I specifically saw with the highest capacity fromSamsungwere the INR18650-25RIf however you meantPanasonic, the genuine NCR18650B goes up to 3400mAh.I only care because 18650's from non reputable sources can literally be death devices and I never want to see anyone hurt by dodgy adverts.

author: jesse.c.millwood
author_rank: Newbie
timestamp: May 27, 2016, 11:07:53 pm
content: @sky99 did you ever try this out?Sent from my Nexus 5 using Tapatalk

author: ebclr
author_rank: Super Contributor
timestamp: May 27, 2016, 11:38:17 pm
content: A bench power supply with a current and voltage limit , will solve your problemI charged 10 18650 in parallel on a daily basys with a 15V 10A power supply who can limit current and voltage.