Looking for Wall Replacement for Interstate Battery

Hi everyone,

I’m looking for a wired version of the battery (a transformer) that steps down 120V wall current to 12V DC with a similar load capacity as the interstate batteries found in FRC. I know that our batteries are 18Ah but I don’t know the max load. Based on the wire rating for 6AWG wires, it’s around 37amps. I need to order this transformer from McMaster because my project is on a tight deadline, but all I can find is transformers like these: https://www.mcmaster.com/7010k38

What are my options to achieve the same rating as the Interstate batteries with a wired transformer?

What electrical load do you intend to power? And what AC source do you intend to draw power from?

What you are asking about is really a DC power supply. It will convert 120V AC to 12V DC, at some power limit.

The batteries we use in FRC are able to supply momentary loads up to 400A; at that extreme the terminal voltage will drop significantly, likely below 8V depending on state of charge. Does your project need such an extreme momentary load?

2 Likes

Keep in mind that some teams do trip the main breaker, which is rated for 120A (the datasheet for it will tell you that you can go 400% of that for ~5 seconds before tripping it!). A CIM motor maxes out at 133A at a full powered stall. Batteries also have different behavior than you would probably see from a power supply - the power supply is going to hold voltage steady for you, while a battery under the load of a few hundred amps is going to drop voltage significantly. This may impact the behavior or performance of your design in ways you don’t want.

What are you trying to power?

This power supply from Automation Direct will give you 20 amps. Bigger ones get expensive fast. You need to size you power supply to the load, not the wire size.

Good questions, guys!

And it’s impossible to answer his question without knowing this.

1 Like

As a ball park some computer power supplies can handle 30-40 amps on the 12V rail, and some have 2 12 volt rails that can each handle that. I just looked at one with those specs that cost around $70.

If you’re looking to actually power a robot, and need 200 to 300 amps, you’re starting to creep up in price. The two AC to DC power supplies that I found were in the $400-$500 range.

We use the 100 amp version of this: https://powermaxconverters.com/product/pm3-12v-series/

Although we don’t use it for driver practice due to the >100A spikes that would occur during normal operation, we often use it for:

  • testing code that requires the robot to be on for a long time (vision calibration, etc.)
  • prototyping mechanisms (hand drill method connected to PSU for powering brushed motors)
  • testing and tuning mechanisms – tuning our arm and elevator took several hours and a power supply was really useful

As noted above you need to know your power requirements, which have a significant impact on cost and size as they go higher.

For electrical parts I would try Automation Direct, Allied Electronics, or Newark over a more mechanically focused company like McMaster. Alternatively talk to a local electrical distributor who might have parts on the shelf in your town and can help more directly meet your requirements (there are quite a few around, Automation Direct is even headquartered in Atlanta). You are looking for a large DC power supply or a power converter.

This is a 24V power supply. The OP needs a 12V power supply. Another thing to note is that cheap power supplies can be quite noisy on their outputs, which may not be a nice thing for some of the electronics they’re powering. A good quality high amperage 12V supply can get into the several hundreds of dollars.

You are right off course. Industrial low voltage DC (at least in my field) is mostly 24. I just had it on my brain.

Sorry for the late reply. Been busy at work. I am building an application that uses the same motors and hardware as an FRC robot. I am running 2 775pro motors and 3 miniCIM motors along with some other hardware (fans, lights, buttons, roboRIO power, and 2 linear actuators that have a max load of 2A. So I would say that the max load is going to be similar to that of a typical FRC robot, although I’m not sure exactly what that is. I’m not over-torquing motors because nothing is going to be hitting walls (like FRC robots). I don’t think I need it to reach a momentary load of 400A, maybe 120A at most.

What would be the difficulty of fabricating at 12 step-down circuit with perf-board ?

120 amps at 12vdc is a hefty power supply! Batteries are really neat, they can charge slowly at low current, then deliver a whole bunch of current very quickly.

I don’t have any good ideas of how to do what you want to do. It’s a tough requirement.

“Typical” is not enough information. Our “typical” robot is tuned to pull ~180A consistently because of our drivetrain sizing choices.

You can use a tool like the jvn calculator to ballpark the electrical loads your mechanism will generate. Plug your mechanical loads, motors, gearboxes in and out pops a ballpark electrical load.

You can use a tool like the Driver station data logger to find the average load in your system if you’re using a modern PDP. Run your mechanism on a battery for ten-fifteen minutes and then pull the “match log” from the driver station, and use the traces to find your max & average load.

You could consider keeping a battery in the circuit as normal and then bolting a 6A-10A fast charger to it directly. (A lot of off-grid PV systems will do something like that, where the PV DC is fed into a mppt battery charger, and then an inverter pulls power off the battery.)

If you have more than 120W (12v*10a) average power consumption, I’d set up a second (and third…) battery and fast charger in parallel rather than attach a second charger to a single battery, for heat management reasons. Lead-acid batteries are pretty stable and happy to get banked together, not like fussy lithium chemistries.

Alternatively you can go to a single much larger car battery and get closer to 100-150Ah, if you have something like a “daytime hours only” runtime requirement and can just let the battery run down all day and then charge overnight.

If you’re going to bank batteries, buy them at the same time, preferably from the same batch and keep the bank together. Batteries of different provenance, age, and/or charging histories do not always play nicely together.

1 Like

Good to know, thank you

I think the solution here could be to mount 5 or 6 30A AC to DC power supplies in parallel on a DIN rail from a 120VAC main line then have that combined 150-180A converge into a fuse box which distributes power to all electronics in the system.

2x https://www.grainger.com/product/SIEMENS-DC-Power-Supply-12A049 for 120A ($2k + tax)

or

4x https://www.grainger.com/product/SIEMENS-DC-Power-Supply-20AZ64 to get a little under 80A ($800)

The point though is that we can pull momentary spikes of 600A through the robot batteries when starting multiple motors at once. Our batteries can handle this through some complicated chemical processes by dropping their output voltage. AC-DC power supplies will not handle this 5x current spike well, and you’ll likely blow fuses.

1 Like

Okay, so what you’re saying is, for the sake of simplicity and also saving money, to stick with an FRC battery or something like that to power an FRC-like system since it can handle the high momentary load requirements, while a wall-mounted system would have to have several DIN-mounted power supplies all hooked up to a large fuse box which would require electrical permits to install and dozens of man hours to build such a system.

1 Like

Basically, yeah. If you keep a rotation of 2-3 batteries you should be able to always have one ready for bench testing while the others are charging.