Charging Li-ion batteries in parallel

Question

I wanted some opinion on my set up here, just to make sure I got the basics right.

I have a Li-ion battery charging circuit based on the MCP73113. This is designed to be a single-cell battery charger.

The battery itself (3.7V, 650mAh) comes with its own PCB with Schottky diode and current regulators as protection. EDIT: Not a Schottky diode. Current limiter and a Protection IC.

By design, they work together just fine.

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 (3.7V, now 1300 mAh).

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).

I assume my method is not the right approach to ensure 100% success rate, so I was wondering how can this be solved. All the controllers I looked into was designed for multiple cells in series, not parallel. Am I fundamentally missing something here?

Answer 1

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. 3s1p) where the cell voltages stay closely matched over the lifetime of the pack (several years) - without balancing of course. 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?

Answer 2

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:

a 20% difference in cell internal resistance between two cells cycled in parallel can lead to approximately 40% reduction in cycle life when compared to two cells parallel-connected with very similar internal resistance

And since you'll probably be interested in measuring this internal resistance yourself, this is how they've done it for 2.2 Ah batteries:

Internal resistance was measured at 50% state of charge (SOC) with a 15 s DC pulse of 40 A (17C). 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. [3]. 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 10-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 24.7%. The maximum difference in capacity in this same batch of cells (one full discharge cycle at 17C continuous discharge current) was 3.6%. For the purposes of this experiment, the differences in initial capacity were considered to be negligible compared to the differences in internal resistance.

[3] H.-G. Schweiger, O. Obeidi, O. Komesker, A. Raschke, M. Schiemann, C. Zehner, M. Gehnen, M. Keller, P. Birke, Sensors 10 (6) (2010) 5604e5625, http://dx.doi.org/10.3390/s100605604

Answer 3

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. Your pack will inevitable have lower capacity and will present problems when charging (bad cell may overheat so much that it even starts a fire while the other(s) will be unable to reach 100% charge).

As a good practice, charge individual cells to full, then discharge them to around 80% (use same load and same time interval for each) and then let them stabilize for a while (few hours is very safe, but minimum 15 minutes will do). 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, 3.92, 3.93, 3.95, 4.01, exclude the 3.8 and 4.01 ones from the pack. It means the first is in bad shape, the last is in excellent shape and the other 3 are good enough to be used.

Answer 4

it may just be that on deeply discharged packs that the initial current draw looks like a short circuit or faulty cell to the charger, try putting a low value resistor in series with the battery when charging the cell to start with and allow the cell voltage to rise a bit before removing the resistor and allowing normal charging to resume, many second hand laptops i have purchased refuse to charge their batteries for this reason (deep discharge from being sat for months unused) but after connecting directly to each cell and putting a little charge in almost all batteries will then charge normally and capacity is generally restored to a good level with a few charge discharge cycles (be careful not to discharge too low the first few times)

Answer 5

Lose the diode or keep it parallel.

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?

Answer 6

If your cells self-discharge after some time, it indicates any combination of these problems:

1. 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.

2. 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.

3. 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. There is an image of the fuse on their Technology page: https://www.energusps.com/page/technology