I submitted the latest board that includes an external ADC. As I mentioned in my last post, this should improve resolution a Lot!
Someone asked about the fact that I have only one switch to control everything. How does it work? Like this:
Just want a charge? Flip the switch so it is not in the cycle position (LED by the word cycle is off) and plug it in.
Want to cycle your batteries to break them in or give them a refresh? Flip the switch to cycle and plug it in.
What if you want to drain your batteries fully before charging them? Flip the switch to cycle, plug it in, and once it starts draining them, Flip the switch again (the other way so the cycle light turns off) and walk away.
What if you want to check the full capacity of your batteries but you don't want to wait for the normal cycle setting? Power it up with the switch Not in the cycle position and then once it starts charging the batteries, Flip to the cycle setting. The charger will fully charge your batteries, then discharge them once to check their capacity, then charge them fully again.
A lot of people (including in some of the comments) have been asking "So what are you going to do with this?" "Are you planning on selling these?" "Are you going to make it Open Source and/or Open Hardware?" Hopefully this post will help address some of these questions.
Originally I started out with the plan to make a 4 battery charger that could handle both AA and AAA NiMh batteries. It would include an LCD that displayed everything real time and it would all be packaged in a sweet metal case. My first prototype board design included an LCD and handled 4 batteries. However there were some mistakes (as there almost always are the first time you design something). So I ended up with only a partially functioning prototype that cost me over $100 (with no sweet metal case). I realized I could not keep this up for very long without making some changes. This is where the design you see pictured in all the post comes in. This two battery version was created as a way for me to test ideas, prove design concepts and perfect the software, at a lower cost each iteration than the larger 4 battery version. I never meant to sell it, just use it for development. However I have had many requests to make it available as well.
Here is my plan:
In two weeks I am going to post this two battery version on Kickstarter. I say two weeks because after getting input from several people I have decided to make one last design change. I am going to add an external ADC with a built in voltage reference in place of the 1.8V ref (and a few other parts) that I added previously. What does this mean? It means after I finish this last modification to my design I should get resolutions down to much less than a millivolt and my temperature plots will be nice and smooth. In general this will be a great performance increase but only a $2 increase to the parts cost (the ADC cost more than that but I am saving cost by dropping the 1.8V ref and a few other parts).
If all goes well on Kickstarter then I will make this design all open source and use any money raised for the development of my more feature rich 4 cell version.
I used one of my old boards and hacked on an LCD. This is a very basic test, just to make sure things are working (if you can see, I didn't even include decimal places in the numbers). My goal is to have a similar LCD on my 4 battery version that would show real-time plots of everything for each battery.
Sorry for the low image quality, I just snapped it with my phone.
I got back the PCB for my latest board revision of my prototype NiMh battery charger. When I say prototype, I don't mean form, but function. By keeping it small I save on board cost during development and by only working with 2 batteries instead of 4, it helps with savings on parts. Even so, each board at such small quantities cost me around $45 (not including time).
On this go around I have added a 1.8V Precision Voltage Reference to increase the resolution of analog readings. I have also changed the Electrolytic capacitors to Polymer capacitors (often referred to as "Solid State Caps"). The biggest upgrades though were in the software. I added countless safety checks, refined everything and added some extra features that help it all work a little more polished and smooth.
I will post data from this charger in a few days.