Samsung has conceded that its attempts to push the Note 7’s performance to its limit was a factor in a string of overheating problems that led to the device being recalled. But it says both sets of batteries used in the device were fundamentally flawed.

The company has also detailed the expanded testing program it has instituted for batteries in future handsets.

Whether it was the phone itself or the battery that was the main problem has been a hot topic in the mobile industry. After the initial limited recall it seemed the problem was just a bad batch of batteries, but when the problem still occurred in some reissued handsets, the phone itself came under closer scrutiny.

Independently commissioned studies suggested the root cause may have been the attempts to make the phone as thin as possible, which meant the battery housing wound up putting too much pressure on the battery, leading to an internal short circuit. Samsung has now published the findings of its own investigations and says there was a specific problem with each of the two battery types.

In both cases the problem was with the way the battery is made up of flat layers (including the negative and positive electrodes and a separator) that are then curved as in a jelly roll.

With the original battery, the negative electrode was “deflected” – in other words, bent – in one corner. The effect of that was worsened by the tip of the electrode being placed on the curve of the “jelly roll” rather than on a flat section. That made it much more likely the negative electrode would end up pushed through the separator, contacting the positive electrode, and causing a short circuit.

The replacement battery’s problem was with the positive electrode itself having higher-then-usual welding burrs. These pushed up the layer above it (namely a conductive tab) such that in some cases it pushed through both a layer of insulating tape and the separator and was directly touching the negative electrode. Again there was a secondary worsening effect, namely that some of the batteries didn’t have the insulating tape at all.

On the face of it, Samsung was the victim of a bad set of coincidences: two different batteries each had two separate faults that combined to cause problems. However, Samsung said it had been “aggressive” in trying to maximize battery capacity and that it needs to step up the testing process to make sure that any resulting faults are spotted.

It’s taking three main steps to do this. The first is setting up a group of independent, external advisors to help set guidelines for how it can safely try to increase battery performance. The second is boosting its testing process to cover eight different tests [pictured]. The third is to add a new bracket to the battery housing that gives more protection against dropped phones putting stress on the battery.