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I have more batteries from the same manufacturer and wanted to make higher capacity packs by putting two cells in parallel. The two cells come with their own PCB, but I only kept one of them, as I soldered their leads together.
It seems to work and they are charged and discharged just like regular batteries 3. However, not all the packs I made work smoothly and I get a failure rate of 2 in every 10 packs.
They all start out normally, but occasionally when the batteries are not charged for some time, or are used up, they tend to not work anymore. So far, in only 1 pack I found the voltage to be different both now below 2V, aka dead. All the controllers I looked into was designed for multiple cells in series, not parallel. Am I fundamentally missing something here?
Connecting cells in parallel is common practise with professional battery pack manufacturers, so there is nothing wrong with it. What pros do is they assemble packs from cells out of the same box same shipment, same lot , having very tight tolerances.
This way, you even get packs with serially connected cells eg.
This is quite a considerable achievement, you can calculate yourself how much self discharge difference the cells can have in order not to have significant unbalancing. Your case is much simpler 1s2p so I guess you are using low quality cells. The above is only valid for top quality cells Panasonic, Samsung etc , a lot of "noname" cells like used in aftermarket camera batteries are just crap. Which cells are you using?
The essential bit [not explicitly mentioned in the other answers] is [by all accounts] matching the internal resistances of the cells. Which according to sellers can only be done reliably at factory. The MIT boffins have studied this too and quantified the impact of mismatched resistance:. And since you'll probably be interested in measuring this internal resistance yourself, this is how they've done it for 2.
While there is no commonly accepted standard for measuring the internal resistance of lithium-ion batteries, we chose this current and time profile because it is relevant to the duty cycle seen by these cells in hybrid vehicles and power tools. A comparison of several methods for the internal resistance of lithium-ion cells is provided by Schweiger et al.
The 15 s current pulse allows the effects of the mass-transfer limited reaction to show. Longer delay times can lead to significant self-heating of the cell which affects the measured internal resistance.
This 17C discharge rate is within the specified rating for this high-power cell, of 32C continuous discharge and 55C for s peaks. The characterization tests were done on bare cells in a background room temperature of 25 C. The resistance difference between the most and least resistive cells was The maximum difference in capacity in this same batch of cells one full discharge cycle at 17C continuous discharge current was 3.
For the purposes of this experiment, the differences in initial capacity were considered to be negligible compared to the differences in internal resistance.
Schweiger, O. Obeidi, O. Komesker, A. Raschke, M. Schiemann, C. Zehner, M. Gehnen, M. Keller, P. Li-Ion cells in parallel work very well as long as they are in exactly the same condition. If one cell is degraded, it will cause the other s to slowly discharge in order to compensate for the lower voltage the degraded one will have. Then measure the voltage of them all and make a pack out of the ones that have the nearest voltage compared to each other.
For example, if you have 5 cells and you need to make a 3-pack and they show 3,8, 3.
It means the first is in bad shape, the last is in excellent shape and the other 3 are good enough to be used. I think if the diode would work only for 1, one battery will charge the other, and the other cannot circulate its energy back into the first?
Cells are very low quality very likely with noname cells and have high and different self-discharge. It is a nightmare to keep such cells in balance and generally from dying in several months.
The cells were mechanically or thermally damaged during assembly or soldering. They must be welded, not soldered. I know RC packs are soldered, but it does not mean they do it right. The cells were overdischarged. When overdischarged below specified levels, metallic dendrites start to grow between electrodes and cell develops rapid self-discharge and in some cases even internal short-circuit, which can be vary dangerous and burn your house to the ground.
Especially if there are more cells connected in parallel. To avoid such chain reaction, most reputable battery manufacturers add fuses on each cell, especially if more than two are in parallel. The most famous for this solution is Tesla, also you might get some inspiration from small company called Energus Power Solutions.
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How to Parallel Charge
Asked 5 years, 4 months ago. Active 1 year, 9 months ago. Viewed 39k times. By design, they work together just fine. Tauseef Latif.
Tauseef Latif Tauseef Latif 1 1 gold badge 2 2 silver badges 11 11 bronze badges. In the off chance the battery is not charged or is exhausted, the battery refuses to charge.
One more thing I just noticed now: I took apart one battery and took the 2 cells apart. The voltage reading was 3. After a few hours the 2v one went back up to 3.
Is this an anomaly or a human error on my part? Martin Martin 66 1 1 bronze badge. I got it in bulk, and I got them in the same shipment, etc. LiPo cells are packaged just in a plastic bag You could keep the batteries as they are including the protection circuit and connect them in parallel using the provided leads.
Are you sure they come with a schottky diode? If my method is fundamentally sound, I could try and solve this from the vendor side. If you refer to self discharge, the combination will just have the sum of both cell's self discharge current.
No problem as well. I'd try to parallel the packs including protection PCB and see how things go then. Maybe you damage them by removing the protection PCB. Fizz Fizz Overmind Overmind 1 1 silver badge 6 6 bronze badges. I personally use a 7-LED Cree flashlight for all battery testing. Just using it 1 minute can give me pretty good readings on how good or bad cells are.
Just gotta put the lab coat on for a minute and do some science! I like it. Carl Carl 1. Null 5, 10 10 gold badges 23 23 silver badges 34 34 bronze badges. Sign up or log in Sign up using Google. Sign up using Facebook. Sign up using Email and Password. Post as a guest Name.
Paralleling li ion batteries pdf
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