Specification v. Data Sheet
Identify What Is Needed!
We all know that a good product is underpinned by being well specified, even if that specification is adapted during the development phase. Having that baseline to work from is essential. For a development programme to succeed the baseline specification describing the overall system needs is a vital document. Unfortunately it is often the case that the sub-systems the development team are not directly responsible for designing then rely on the interpretation of commercial data sheets to predict performance.
Don’t Fall Into The Trap
Battery powered equipment, especially if it is portable, often gets treated this way. It is difficult to nail down power and energy requirements at the start of a project as there will inevitably be some evolution of the needs during development and testing. But, just to treat the battery as a simple add on as the project matures is fraught with danger. To keep the power requirement ‘risk’ under perceived control, designers will turn to manufacturers data sheets and base requirements on the information they contain. The trouble is that the data on these sheets, especially at cell level, is only accurate under the specific conditions stated which are rarely encountered in the users world. Allowance needs to account for the data being generated for a cell being tested in ‘perfect’ conditions and fresh from the production line. The chances are that even if the requirement is for a single cell, in use it will not be in a well ventilated, temperature controlled environment. The main equipment may be, but the cell will be tucked away in its own little compartment insulated from the world around it. The cell/battery data sheet will probably show profiles of various discharge rates, but these will be steady load continuous discharges, fine if that’s the way the kit operates, but unlikely for most applications. What happens as a load varies, or is interrupted? Then, looking at multi cell batteries, it cannot just be assumed that the sum of a multi series/parallel layout is just taking the cell numbers and calculating accordingly. How the battery is constructed, additional necessary safety devices, and even cell orientation can have a significant effect on final performance. It is no wonder that cell and battery data sheets come with a warning about there being no contractual obligation on the cell manufacturers part! Discovering this when batteries are reaching the end of their life after three years use instead of five is not the time to be finding this out.
We Don’t Know Everything
Cell and battery manufacturers do have a large amount of data and experience that they can call on when needed. The problem is that the battery requirement from the equipment developer is not sufficiently detailed, often reduced to a voltage / capacity / temperature range statement based on the broad envelope of the data sheet numbers. This is not enough to then produce a life and performance prediction where so many different factors need to be considered. The battery supplier can only base life predictions on the information supplied. Gaps in the information about how the battery will be used will result in a less accurate response from the cell/battery supplier. There is also the risk that one battery supplier will take an assumption on a gap in the requirement, whilst another will not, or assume something completely different. Either way, the result is a technology or financial risk depending on the supplier the equipment developer chooses. If the battery designer was not informed about a particular aspect of the requirement, they will not be obliged to assist when the kit does not work as intended.
A far better approach is to produce a proper specification for the battery, even if it is just a single cell. At the early stage of a project the reasons for not doing this seem many, but the effort in producing a comprehensive specification will pay off in the end. Being able to give competing suppliers a clear and complete requirement will reap benefits in the long term. By being in control of the battery specification, unfortunate problems like ‘sole supplier’ can be overcome and ownership is kept of the design – vital if the supplier originally selected is no longer able to produce the battery. Having a battery supplier provide a compliance statement against the specification gives better recourse in the event of performance shortfalls.
An independent battery consultant will be able to guide, and produce, a sufficiently detailed specification. This can then be updated as requirements mature in the main equipment development so the specification is a comprehensive description of all the parameters that need to be addressed by a potential battery supplier. This then gives the equipment developer the ability to run a fair competition between potential battery suppliers, with the obvious benefits!