Parallel charging of Li-Poly cells - Page 1
Forum section: electronics / general

author: Elios000
author_rank: Newbie
timestamp: January 31, 2015, 03:40:31 pm
content: ok so i need some help in explaining how and more so WHY this works at allthere are still a lot people in the RC world that seem to think that parallel charging of Li-poly packs is not safe or that packs cells in the packs cant ever balance rightwhat i know form doing this on my setup is the packs come out right at 4.2v per cell on the dotbut i my self dont really get whats going on over allso setup that i have used is as followsPowerLab8 40amp charger > 'paraboard' > X6 6S 3200mAh 30C Li-poly packsnow here is the thing the main power leads are all in parallel AS WELL as the balance tapsthis means wile each 6S packs is parallel all packs Cell 1...6 are in parallel with all other packs Cell 1...6and i have pushed some pretty crazy amps in this setup like all 40amps the charger can out putso thats 40amps at an avg of 24v or so some where from 900W to 1000W and yes wires get a bit warmbut whats cool is all six packs come out at 25.2v bang on same with each cell in all the packs bang on 4.2veven crazier is this works with packs of different mAh ratings and every thing works outi can toss on say three 3200mAh packs and three 3000mAh packs and nothing gets over or under charged every thing comes out exactly as it shouldnow i do know that internal resistance has some thing to do with this and that with Li-po cells it changes as function of the cell voltage but thats about itthanks for the help in advancecharger that i am usinghttp://www.revolectrix.com/pl8_specs_tab.htmparaboard if you have not seen one

author: electr_peter
author_rank: Unknown
timestamp: January 31, 2015, 03:59:14 pm
content: The problem with parallel charging is that some cells can get too much current (which increase heat and may pose damage to battery or initiate fire). Inevitably, some batteries will have lower internal resistance and will consume non-proportional current in respect to other batteries. Best way to charge is to have separate charging circuit with balancing for each battery.Simple example: 6 discharged LiPo batteries in parallel, 5 old (high resistance) and 1 new (low resistance). Say, all have same nominal capacity and require 2A each of charge current. Charger provides 6*2A = 12A current, but battery with lowest internal resistance consumes more than 2A in some stages of charging.Severity of over current depends on exact setup, type and state of batteries. Situation is worse when different capacity rated batteries are connected together. Charging almost identical batteries in parallel is allowable if charging setup is monitored all the time (especially for temperature).

author: Elios000
author_rank: Newbie
timestamp: January 31, 2015, 04:10:13 pm
content: the charger monitors each set of cells ie Cell 1 from all packs as one cell in this case and can ether discharge or add current to bring that cell set up or down to 4.2 as neededeven at 40amps in my case which comes out to about 2C charge rate in my case(6x 3.2amps) the packs dont get any warmer then if i did each on there ownand at 20amps effectively 1C they dont even get over ambient and i have even had them cool off wile chargingin this case it was two packs that i had just flown and put on at 1C the packs where pretty warm and ended the charge cycle cooler then they startedi put them on at 10amps for two 3200mAh packsheating has never been an issue these packs get pretty abused in flightthe heli i run them in uses 12S two 6S 3200mAh packs in series and i get ~7min of flight time with out hitting the LVC which is set at 3v per cell loadedpacks come down at about 3.3 per cell restingmy charger shows im pulling 3Ah out of the packs in 5 to 7 min flight at avg 42v

author: electr_peter
author_rank: Unknown
timestamp: January 31, 2015, 04:26:33 pm
content: When you connect LiPo cells in parallel, charger sees it as a one big LiPo cell. As voltage is monitored all the time, overvoltage is not a problem - problem could be over current.From description it seems that you charge six 6S cells of ~3100mAh each with ~40A current. That is over 2C charge rate. Charging rate is aggressive, but LiPo batteries can accept 2C charge with almost no complaints. Also, 3200mAh and 3000mAh is almost identical capacity.Over current should not be a problem in described setup (considering that temperature is low as well).

author: Elios000
author_rank: Newbie
timestamp: January 31, 2015, 05:00:39 pm
content: so the question is why do cells come out to 4.2v ?make it easier lets just talk about a two 2S packspack 1 cell 1 is in parallel with pack 2 cell 1 via the balance leads as are Cell 2now both packs main leads are also in parallelthe case of most chargers they can discharge each cell of the over all pack the power lab can also shunt current to each cell well the bring up weak cells

author: Marco
author_rank: Super Contributor
timestamp: January 31, 2015, 05:31:41 pm
content: What would happen if you try to do two 5C capable packs at 5C (so 5*3*2=30A for 3000 mAh packs) with one pack discharged and one pack charged? Normally when the cell voltages a relatively close together and the charge rate per node is not too close to the edge ESR can handle the balancing in parallel charging ... but when you start pushing the limits I expect failure.When an individual cell in one of the packs is failed open a parallel charge near the C limit will result in cascade failure as well.

author: IanB
author_rank: Super Contributor
timestamp: January 31, 2015, 05:34:17 pm
content: Quote from: Elios000 on January 31, 2015, 03:40:31 pmQuestion: why can you successfully charge Li-poly in parallel?To answer this, we can think about what is inside a Li-poly cell and how it is constructed. We look at it as "one cell", but if we looked inside it, we would see that it could also be considered as 10 little cells in parallel, or 100 even littler cells in parallel, or perhaps even thousands of really tiny cells in parallel. This is how a Li-poly cell gets made with higher capacity: the designers just add more and more parallel "stuff" to the cell. They make the electrodes bigger (more area) and provide more polymer electrolyte between the electrodes as required.So in reality, two Li-poly cells in parallel is much the same as one big Li-poly cell with twice the capacity. In each case you have double the electrode area and enough extra electrolyte to make it all work.This is why you can (usually, with some limitations) charge them in parallel.Things to be careful about:1. If you are going to charge batteries in parallel, make sure they are both at the same state of charge before connecting them. Otherwise large currents could flow that could be damaging.2. Try to charge only batteries of the same type and capacity and age. This helps to ensure that currents are balanced evenly during the charging.3. Do not charge too many batteries in parallel at once. Two or three is about the safe limit. More than that and a current imbalance could occur that makes one battery exceed its safe operating region.

author: Elios000
author_rank: Newbie
timestamp: January 31, 2015, 05:39:45 pm
content: if one cell is out of range (high or low) the charger will terminate the charge cycle

author: electr_peter
author_rank: Unknown
timestamp: January 31, 2015, 05:39:48 pm
content: Quote from: Elios000 on January 31, 2015, 05:00:39 pmFinal charge voltage will always be 4.2V (or what charger thinks is OK for LiPo) for all cells connected in parallel - because of parallel connection (=equal voltages) and charging to 4.2V voltage limit.You should understand that charging batteries in parallel can be dangerous. By your posts it seems that youthinkparallel charging is easy and very safebecausedescribed setup worked few times. It takes one failure in battery to start cascade of failures as Marco mentioned.

author: IanB
author_rank: Super Contributor
timestamp: January 31, 2015, 05:52:48 pm
content: Quote from: Elios000 on January 31, 2015, 05:39:45 pmThat's a bit late if it happens after the damage is done...

author: Elios000
author_rank: Newbie
timestamp: January 31, 2015, 05:53:57 pm
content: its worked more then a few times more like a few 100 times for me and othersits pretty common

author: Elios000
author_rank: Newbie
timestamp: January 31, 2015, 05:57:24 pm
content: Quote from: IanB on January 31, 2015, 05:52:48 pmthe charger will also not start unless the cells are in the right range as well

author: IanB
author_rank: Super Contributor
timestamp: January 31, 2015, 06:01:46 pm
content: Quote from: Elios000 on January 31, 2015, 05:57:24 pmBut you didn't read and make note of what I said. If you connect packs together that are in a different state of charge (extreme case, you connect a charged pack to an uncharged pack), then very high currents will flow between the packs that can melt wires and in extreme cases might cause a fire.The charger cannot stop this happening since it would be you that did the damage, not the charger.Quote from: Elios000 on January 31, 2015, 05:53:57 pmA thought experiment:"I have run across the street without looking 100 times so far and I'm still alive. So it must be safe to run across the street without looking."True or false?

author: Elios000
author_rank: Newbie
timestamp: January 31, 2015, 06:07:02 pm
content: as to different charge levels well you dontyou would level out the packs first to be with 0.5v per cell of each otherafter bit of time on the paraboard the packs will level out with in 0.01v or so pre cell

author: rexxar
author_rank: Frequent Contributor
timestamp: January 31, 2015, 08:39:18 pm
content: What if you use a board like Elios posted, but had a 10R or so resistor between each connector and the input? That way the resistance between two cells is 20R, and the current should be limited to something like 2A if you connect a flat cell to a fully charged one.