This tractor originally had 6 8V golf-car-type flooded lead acid batteries, but I replaced them with 4 12V AGM batteries. (I'm using Universal Battery UB12350.) AGM batteries require more careful charging than flooded batteries. After trying various things, I decided to charge the 4 batteries independently. A set of batteries costs around $1300, so I wanted to maximize their lifespan by not undercharging or overcharging, and by having chargers that would monitor the battery temperature and adjust the charging voltage accordingly.
In the end, this was quite an adventure, taking more than a year. Here I review the three chargers that I tried: the Minn Kota MK460PC, the Dual Pro PS4, and the CTEK Multi-US 25000.
My first thought was to get a a multi-bank charger that would be a single unit with the capacity to charge the 4 batteries. There are several of these chargers, usually intended for boats with electric trolling motors.
The first charger I tried was the Minn Kota MK460PC. It claimed to have all the features I wanted. But when I got it and connected it up, it did not behave at all as I had expected. It started charging very slowly and took more than 10 minutes to be running at full power. Then it seemed to be charging the batteries for too long in the finishing cycle. So I called Minn Kota customer support to ask for the details of the charging algorithm and they refused to tell me! It's a secret! According to their customer service agent, "We don't make all the info on our items available to any and everyone so they don't try to steal them."
I didn't like the implication that I was a thief, but apart from that, someone who wanted to reverse engineer the charger could simply measure how it operates to learn this algorithm. So there's no reason for them to insist that the charging profiles used by their charger are proprietary information.
This made me unhappy, but I already had the charger, so I decided to use it anyway. In order to mount it, I had to cut one of the cables to feed it through a hole. That's when I discovered that there was no temperature sensor for the battery. There were only the two wires carrying the charging current. That's not what it said in the manual! In fact, the manual said very clearly that there was a sensor in the housing where the cable splits into the wires to the two terminals. I guess older models did have sensors there, but not mine. Since I wanted sensors on the individual batteries, I gave up on this charger.
So I tried again with the 4-bank charger from Dual Pro. Burned by my previous experience, I tried to be very careful this time. Before buying the charger, I called the company and spoke to an engineer about the algorithm. They were happy to discuss it. It seemed fine. So I bought it. I connected it up, I tried to verify that the temperature sensors worked. I heated and cooled the junction boxes where the sensors were supposed to be, and there was no effect. I called Dual Pro again. They confirmed that the temperature sensors were where I thought and claimed that the complexities of their algorithm meant that the simple test I was doing was not reliable.
So I took the outer insulation off one of the cables. Look! Only two wires. No temperature sensor! I called again, talked to their customer service representative. He claimed he hadn't known until the day before I called that the newest models of their charger had only ambient temperature sensing -- no sensors in the cables.
I demanded to return the charger, since they lied to me about the design. "Fine," they said, but since I had bought it from a dealer I had to return it to the dealer. So I sent it back to Hodges Marine, which was kind enough to refund my money, even though I'd taken the insulation off of one of the cables and the problem was none of their making. Unfortunately, I had to pay shipping both ways.
Twice burned now, I emailed a detailed list of questions to customer support. Remarkably, they emailed me back. After some further discussion, I was satisfied that this was the right charger. I bought four units from l2r2 on eBay. I paid $224 each, a big discount from the $325 list price. Still pretty expensive, but these chargers actually work. They do sensible things, and you can see the adjustment of the charging voltage by heating and cooling the temperature sensor.
Unfortunately, these 4 chargers were too large to mount anywhere
inside my tractor. So I mounted them in my garage.
I needed a connector to allow allow me to easily connect and
disconnect the power and temperature sensor wires. Since the
batteries are in series, I connected the charger outputs in series to
each other, so I only need 5 and not 8 power wires.
It's great. The CTEK chargers work just as they should. They come up to full power in a few seconds and charge the batteries, limiting the voltage according to the temperature of each individual battery. They shut off when they are done, so it is "plug and forget". I'm completely satisfied.
Was it worth it? Probably not. The system cost around $1000, and was a lot of work to build. While it's cheaper than a set of new batteries, I doubt I'll get double the battery life that I would from a somewhat inferior system that only monitored the ambient temperature. On the other hand, I have the best 4x12V charging system ever, and that makes me happy.
Feel free to write me at kdo@cosmos.phy.tufts.edu if you want to know more.