Dear forum members
I have had a fascinating experience dealing with china these last two weeks have passed along rather quickly as I awaited rather impatiently to receive my 7s3p a123 modules. I am happy to state that my experience with xin wang's a123 cells proved to be immaculate. She was helpful till the last, helping with the bank transfer issue (where there was an issue with the bank company name), specifying cell types, sending me proper cells instead of the worse cells mentioned by some users.
On the whole, the result was good. I received two 7s3p modules, both measuring a voltage of 23volts upon arrival, both required 25AH of bulk charge, and then a further 3-7AH balance charge to have the two packs fully balance charged to
50.4volts 60AH.
COSMETIC CONDITION OF THE UNITS
The units arrived to me with minimal markings. Upon closer inspection there were no hairline cracks visible in the plastics, the unit had some indentations due to the battery's handling process of removal, where I assume upon inspection that the battery modules were housed in a thick plastic that required some crowbar work for complete removal. There are some minor indentations on the aluminium fins on the left hand side,some sticky (yellow) tape on the side covers which are made of plastic with six hex nuts holding them in place. These plastic side covers, cover the top of the laser-welded interconnections of the 3parallel assembly.
The unit further has some scratch markings to the rear of the cover assembly, there are some minor scratches on the sides of the unit and these require some straightening, but should not affect the cell's performance as they appear to be merely cosmetic.
INSPECTION OF INDIVIDUAL INTERNAL RESISTANCE OF THE CELLS
The cells hold an internal resistance of 0.03ohms. The resistance of the modules suggests that they may be cycled, as some of the parallel cells showed significant charge difference, so judging by the state of charge formula for the estimation of cell charge a relationship exists between the open circuit voltage measured and the SOC. I further moved to measure the SOC under load conditions and the state variable approach with the help of an ABC-150 power processing system used at university. According to the thevenin power model, I used the figures from terminal voltage of the battery, internal resistance (Rb) + VOC + RC/RD) and the polarization capacitance(C).
I did this to minimize the errors between measured voltage and current data from the battery, and the estimated terminal voltage as obtained from the linear terminal voltage equation of the battery.
The overall chemical reaction taking place is measured by using the formula PBO2 + PB + 2H2SO4 = 2PBSO4 + 2H2O
It is so, that the electrode reactions show that maximum open circuit voltage developed by a123 prismatic pouch cells of the model AMP20H is 3.32V as it is a normal practice to connect these cells in series or parallel to acquire the required battery voltage.
Instead of the traditional spiral wound positive and negative electrode separators made of pure lead grids mixed with lead oxide, these nanoparticle a123 pouch cells were designed with phosphorous compounds to permeate the proper active materials when the cell receives and emits it's first high charge and discharge.
The plastic internal container acts as an isolator from the nanoconductors and fibrous separators, instead of glass a carbon mixture is used as a semiconductor separator.
The cells have the attributes of the high quality properties of the more expensive A123 pouch cells of which I have purchased one or two of. The electrolytic resistance, plate resistance, and fluid resistance vary similarly on an oscilloscope upon over-discharge resistance.
POLARIZATION CAPACITANCE
During the engineering test, the battery's temperature, electrical capacitance, were consistent with the load which was continuous drop in battery capacity, based on peukert's formula of T = C / In (or C = T * In)
I cannot comment on the internal assembly of the batteries, however externally, the units seem to react positively to overload discharges, and only time can tell whether there will appear any shortening in battery cycle life from a possible defect in one of the cells, however so far, it is mentioned that there may have been a mechanical failure with the module assembly itself rather than the cell build quality. I have released the aluminium ties around the top and bottom of the cells leaving more room for the cells from compression, I will be adding some electrical separators between the short tabs.
IMAGES
I attach some photographs of the modules below, both of which, as it stands, are being bulk-charged with 25 large (6A+) laptop chargers.
