Design Requirements

Following on from the previous post, here I set out the design requirements. Initially, I'm going to use a technique out of the Agile software development process of defining functionality through a set of User Stories; A user story is the smallest unit of work in an agile framework. It's an end goal, not a feature, expressed from the (software) user's perspective. The purpose of a user story is to articulate how a piece of work will deliver a particular value back to the customer.

Note that the user story is, by definition, a small piece of work. What is meant by small is open to interpretation, but it is meant to be something that's relatively quick to implement and has an identifiable end-to-end value to the user. What I will do here is use the technique to slice & dice the problem, creating an overall view of the system and identifying the individual strands of functionality that need implementing to create the whole, whether they be hardware or software.

Now, some will say, "why bother, its obvious whats needed", and that may be true for this is a simple system, but the technique works for much more complex problems. So, lets get started...

Our first attempt at a user story might be:

As a user I want to view the charging voltage and current on a graph by time

As stories go this is rather ambitious, in fact in Agile terms its probably an Epic, a story too big to be assessed, but lets break it down. Firstly, in bold, I've identified who needs or owns this story - 'user'. That user has a specific action in mind to 'view' some values 'voltage', 'current'&'time' through some medium 'graph'. Note we've said nothing about how this is done, nor anything about the physical processes, storage, timeliness, etc. These will be fleshed out later should it be necessary to do so.

Let's start breaking this epic down into smaller stories. Here are a few obvious ones (as I said above, this isn't a hugely complex problem, but it has its nuances):

  1. As a user I want to measure the battery terminal voltage at 1 sec intervals.
  2. As a user I want to measure the charge current from the charger at 1 sec intervals.
  3. As a user I want to record the time each measurement is taken.
  4. As a user I want the tuple {time, voltage, current} to be stored for later processing.

What I've done here is broken the bigger requirement into much smaller items and added some restrictive parameters to control the scope of the story, the idea being to minimise the complexity and maximise delivery of a functional element. It can be argued that stories 3 and 4 as written pose problems; in Agile terms they don't in themselves deliver business value as they are dependencies of stories 1 and 2 which themselves could be argued as not having any business value without stories 3 and 4.

In the end we would probably end up with:

  1. As a user I want to measure the battery terminal voltage at 1 sec intervals and store the tuple (time, voltage) on a PC for later processing.
  2. As a user I want to measure the charge current from the charger at 1 sec intervals and store the tuple (time, current) on a PC for later processing.

Why do these make better stories? Well, apart from the business value, another Agile tenet is that you start with the simplest, dirtiest, quickest solution and then write stories to extend, rationalise & refactor the solution, adding testable value at each step. It's fairly clear that adding the current measurement in story 2 is an extension of story 1 with some added front end hardware and some added software.

So lets start looking at story 1. What is the simplest solution we can come up with? The next article will do just that.


1. On Monday, April 13 2020, 05:21 by ex-gooserider

Nothing 'wrong' w/ 1 second intervals, but it seems excessively fine-grained. I would think the purpose would be served by 1 or even 10 minute intervals... One second intervals over a 12 hour cycle is a LOT of data - not easy to 'eyeball' it w/o graphing, and would fill the data recorder quickly.

Perhaps the ideal might be to say sample the data at 1 second intervals, but log only if the either the voltage or current value has changed by some amount. This would capture rapid changes, but not overload the user w/ data from the 'dull and boring' times (i.e. much of the CV stage)
It would still allow graphing w/o much hassle based on the time stamps.

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