When looking to make the switch to Lithium there are many benefits, however not all Lithium Batteries are made the same.
There’s Prismatic and there is Prismatic…
Prismatic Lithium Cells
Prismatic Cells are the superior type of Lithium cell for uses in any battery that is in a non-stationary environment.
However, there’s more to the construction of a Lithium Battery, including cell type, assembly, and materials used.
Cylindrical or Prismatic
Cylindrical cells are typically made quicker and cheaper in comparison to Prismatic Lithium Cells, but at what cost?
Cylindrical 28650 arrays require dozens of spot weld connections. Spot welds are by nature a brittle fixing and the fact you are relying on so many connections make the likelihood of failure greater. Specialised equipment is also required to make the spot welds and they are difficult to fix if there are any issues with an individual weld.
When comparing Prismatic cells there are also multiple tiers of quality in the construction and the connection options.
Cost vs Quality
Some prismatic cells are constructed with plastic outer cases. The type of plastic varies, and the cheaper cells use lightweight plastics of the cheapest composition available. The casing is an important point if there are problems with the Lithium cell. The melting point of cheaper plastics can be as low as 150C. Even hard case prismatic cases using better ABS plastic have a melting point of 200C.
Why is the melting point of plastic cases important?
If a Lithium cell is damaged or has a manufacturing fault, then it can go into error. Cheaper cells manufactured under less strict conditions may get something as simple as a single grain of dirt encapsulated in the cell which becomes a high resistant point and will heat up and can cause a cell to fail. Therefore, it is very important that the cells are constructed, and systems audited under strict international ISO standards to ensure these manufacturing issues don’t occur.
The cell fails by creating a short between the anode and cathode layer. In the unlikely event, the temperature can rise quickly. When the melting point of the case is low then very soon the case will melt around that hot point. With an ingress of oxygen, the reaction is fuelled and becomes very difficult to stop. There are three elements to combustion. Heat, fuel, and oxygen. Cheaper prismatic cells can heat with manufacturing intolerances being the cause, the plastic itself becomes the fuel and when it melts it allows the third ingredient, oxygen.
The safest alternative is utilising hard case metal cell enclosures. Aluminium has a melting point of 660C. A substantially higher temperature than the 200C of the plastic cell cases and the battery outer cases. Aluminium enclosures are harder to penetrate and do not provide fuel like plastic does. The outer battery case acts as the second layer of protection and when made of hard metal like Aluminium, it will be less likely to fail and allow the rush of oxygen in.
Within the hard case metal, there are also different fixing options. The cheapest option is a metal tab that presents the positive and negative link. These tabs also require a spot weld to make a connection. The best option is a threaded bolt connection. This enables the most robust link and is easily serviced if required.
How the cells are linked together into a battery array is also a critical point. To make a 12V battery you need either 4 or 8 cells put into a serial array. The cheapest way is with individual links. The more robust connection is a customised Printed Circuit Board (PCB) which has embedded copper connections and turns the whole array into a mechanically rigid block.
What to look out for?
In summary, there are many considerations when manufacturing a battery and simple claims of having prismatic cells is a small part of the battery.
When looking at purchasing your next Lithium Battery, the below checklist is something you should take into consideration:
- Does the battery use prismatic cells
- Is the prismatic cell enclosed in an Aluminium outer case
- Are the connections made using bolts and not welded tabs
- Is the array held into a block with a rigid PCB cell link board
- Are the cells made by a company with international ISO accreditation
- Are the cells compliant with 62619 international standards
- Is the final assembly gone through charge and discharge tests during an auditable QA system with a QMS system in place
- Triple Guard Protection (Hard case metal cell protection, Electrical BMS protection, and hard case metal outer battery enclosure)
Baintech Lithium Batteries are manufactured with all the above elements and QA processes.
The 62619 tests have been done by German-based TUV SUD Labs and are contained in report number 64.280.19.00140.01
Don’t be fooled by cheaper batteries fully imported from China claiming to have prismatic cells.
There is always more to the story. Read more here