Selecting a Solar Charge Controller
Intro to Charge Controllers
The first thing you need to know about solar charge controllers is that they're needed to regulate the power coming in from the solar panels in a way that will charge your battery--but not fry it. When your battery is fully charged, they will disconnect your panels from your battery.
The second thing you need to know about solar charge controllers is that they come in two types: Pulse Width Modulation (PWM) and Maximum PowerPoint Tracking (MPPT). The full PWM vs MPPT debate isn't one I'm going to rehash here: I'll just give you the basics. MPPT is more efficient at cooler temperatures and can convert surplus voltage into extra charging current. In a PWM system, this extra energy would instead be lost. However, MPPT is much more expensive, and a lot of the cheap charge controllers that claim to have it actually don't if you open them up and poke around inside. The benefits of PWM is that it's cheaper, and a cheap PWM module is much less likely to do something retarded than a cheap MPPT device--it's less complicated, so there's fewer things to go wrong. The major drawback is that your panel voltage needs to match your battery voltage. For large solar installations, wiring panels in series is a great way to get a higher voltage, which means smaller cables, which means less voltage drop and/or money spent on high gauge copper. I have only two panels though, so wiring in parallel isn't exactly a great hardship. I didn't really need MPPT.
About those LiFePO4s...
Finding a LiFePO4 compatible charge controller proved to be harder than expected. Virtually every charge controller on the market is programmed with a charging algorithm designed for some variant of lead acid batteries. That's great if you have lead acid batteries, but not such a good idea if you're using an alternative chemistry. Your lead acid batteries will be pleased when you treat them to a desulfating. Your LiFePO4s will cry and commit suicide.
As I researched what LiFePO4 batteries liked and disliked, I quickly came to realize that I would need a fully programmable charge controller in order to be able to turn all these annoying and unwanted lead acid battery "features" off. I started my search on Amazon, but found myself quickly deluged by cheap chinese charge controllers that claimed to be programmable. Very few had any documentation, so I'd read through dozens of amazon reviews just to inevitably find the one guy complaining that "programmable" meant you could choose between presets for Lead Acid (LA), Sealed Lead Acid (SLA), and Absorbent Gel Mat (AGM).
Clearly, more research was required. So I went off to google and read through many forums topics dedicated to solar LiFePO4 usage. I found that:
1. Genasun was the only company I could find that specialized in solar charge controllers specifically for use with LiFePO4 chemistry. However, my solar panels could be configured in either 6A @ 24V or 12A @ 12V, and at the time I was doing my build, Genasun didn't offer a charge controller suitable for use with either option. They also require you to send them in to Genasun to have their programming reconfigured.
2. Biennopower also offered some, but I couldn't find many reviews or details, so opted to skip them. There were scant few other options on the web, most of which were either sketchy, mysteriously vague, or very, very pricey ($300-600).
3. Solar charge controllers that allow you to customize every single thing do exist, but are expensive. They're meant for lead acid, but since they're (actually) programmable, you can change the parameters to something that will keep your LiFePO4s happy.
Enter the Morningstar
The cheapest option proved to be the Morningstar Tristar. It's a PWM solar charge controller with EXCELLENT documentation. You can make it do whatever you want, and it will also log charge data for you. (If you're reading this, you probably feel the same way I do about having all the performance details possible for a technical project...)
The are two steps to setting up the Morningstar Tristar. The first is to configure the DIP switches, which are insanely small. I used this to set the charge profile to "custom."
The second step is to actually design the custom charging profile. This is accomplished with a computer and a TS-232 to USB connector cable. Morningstar offers a free program called MSView which you can use to tweak everything just to your liking. It's a very basic GUI interface--it looks like something straight out of the 90s, and wouldn't look particularly out of place on a MS-DOS platform. But it gets the job done, and that's what counts.