A friend of mine purchased a battery-powered Lobster Elite 3 tennis ball machine. Within a month or two, the stock charger had crapped out. The original charger was a cheap-o transformer-based trickle style that is normally found on low-cost equipment and I was surprised to see it on a high-dollar ball machine. The company does sell two other types of chargers, a 1 amp "Fast" charger for $100 and a 3 amp "Premium" charger for $150! Outrageous!
Since the battery is a very common Sealed Lead Acid (SLA) style with an 18 Amp-hour capacity, I thought to myself, "Self, why not help this nice lady out and build her a charger?"
Step 1: Machine Wiring
The first thing was to determine how this machine was wired. As part of figuring out that the stock battery charger was the problem, I ended up pulling off the control panel. All the connections looked good and I noted that the external battery charging port was connected directly to the battery.
It took some research, but I determined that the connector was a fairly common XLR type (3-pin, obviously).
3-Pin XLR Female Connector
Pin 1 = Red Wire, Battery Positive, +12 VDC
Pin 2 = No Connection
Pin 3 = Black Wire, Battery Negative, Common
Step 2: Buy Some Parts!
In terms of small battery chargers, there are a bunch of them out there. My requirements for buying one for this project were:
- Not too big (current-wise) - A 15 amp charger with a starting circuit intended for a car battery isn't what I was looking for as it would probably fry this little battery over time.
- Relatively inexpensive - After all, what's the point if this costs as much as the manufacturer's charger?
- Multi-stage with maintainer circuitry - One drawback of the stock charger is that if you leave it on the battery too long after it is charged, it will eventually kill the battery. Consequently, the owner has to disconnect it after the ball machine is charged and then connect it from time to time during storage to keep self-discharge from killing the battery.
I settled on a 4 Amp Suaoki Part #DDDPUT25-MLY for $36 from Amazon. It met all the requirements and was even color coordinated with the ball machine!
Next up, the connector. As mentioned in the last step, the machine has a 3-pin female XLR connector. I took a look at a few connector options and then decided to buy a patch cable and cut it up. I picked one up from, you guessed it, Amazon, for $5.60:
Once the parts came in (2 day prime shipping!), I was pleasantly surprised to find that the charger came with a nice little carry-case, detachable clips, and a detachable mounting set with a fuse and ring connectors.
Step 3: Putting It Together
The first things was to whack the female connector off the cord and ring out the conductors. Using my trusty meter, I quickly determined that pin 1 was the cable shield and pin 2 was the red wire. Although not ideal, at these low currents and voltages, no problem. I trimmed off the white wire and plugged in my soldering iron.
Cable Pin Out
Pin 1 - Cable Shield (bare)
Pin 2 - Red Wire
Pin 3 - White Wire (unused)
Note: Your cable may not have the same pin out. If you don't have a meter handy, you can use a battery and a small light bulb or rig up some other method of ringing out the correct wire.
I decided to keep the battery clips and cannibalize the ring connector accessory. The insulation was exposed on this guy so no meter needed here. Some solder and a bit of heat shrink over the connections took care of this one. While a more cautious and/or less confident man would have tested the whole rig prior to sealing it all up in heat shrink, I had no such qualms. Not shown in any of the pictures is the larger sleeve of heat shrink I put over both connections to make it nice and pretty.
Pro Tip - Make sure you have the heat shrink on the wire prior to soldering. That's experience talking...
Step 4: The Big Test!
So did it work?!?
Of course it did. I'm an electrical engineer and it's just two wires and a battery.
Summary: For less than $50 and an hour or so of fiddling around (FYI: that includes taking the pictures, but not writing the instructable), you too can build a setup that is better than what the manufacturer of these machines is charging $150 for. If you mess it up and get the connection backwards, the charger is smart enough not to explode or fry the battery. It will just pop up a wrong polarity error message and stand by without doing any damage.
PS. As a shout out to Lobster, they did the right thing and quickly replaced my friend's dead battery charger with the next model up. Now she is sitting pretty with a "fast" charger and a better than "premium" charger.
PPS. Even though they have really expensive accessories, these are really nice ball machines. If you are considering buying one, Treat Yourself!