Powering RAMPS from Battery. What to do with VCC?

Hi all. I am currently using ramps 1.4 with a PC PSU. Starting up with the M80 command and shutting down with M81 via the PS-ON connection, this has been working great.

I would now like to power my bot from a charging battery to handle power flicks and small power outages. Kind of like a 12 volt UPS. I understand I won't be able to use the M80/M81 command anymore, no need as there is no PSU to shutdown anymore. I will use gcode commands to shutdown motors and heater instead.

My question is, can I safely put 12V on to the VCC from my battery? Currently I use the 5Vsb of my PSU. Will it effect anything like LCD brightness or motor voltages? May be I should insert D1 and power the Arduino board from that? Or should i just regulate 5V off the battery and apply that to VCC? I have read the RAMPS 1.4 wiki but still feel unclear about what is best/or what not to do. Don't want to blow my board.
PS: I would also like to remove the USB cable once print is up and running so relying on USB to power the Arduino is out of the question.

Thanks in advance
Brad Cole

Charging while printing is a bit hard to figure out. You need to look at the voltage your charger puts out (absolute max) without the battery connected (or when it's just been connected to a fully charged battery), and also the max voltage the battery itself can hold when fully charged. The charger needs to use a slightly higher voltage than the battery gets to allow current to flow into the battery to charge it, so you need to take this voltage into account when connecting it to your RAMPS board.

eg: A lot of RC battery chargers use 15V to charge 13.8V batteries. There is circuitry in the middle that takes "some" of this voltage, but you might see spikes of say 14.5V on the battery, which will pass on to your circuitry.

If it's within the limits of what your Arduino Mega can handle, then put D1 back onto your RAMPS board and you should be right to go. A general rule of thumb is that standard Arduino Mega boards will work with a max of about 14V in a RAMPS setup reliably. Beyond that, you're working on tolerances. If you have a clone that supports higher voltages (eg: Taurino Power) then you might be fine. The issue here is that if the power from the battery/charger combo is noisy (eg: when it's charging) you may find that the Arduino does weird things, like resets. You could probably put a big cap between VIN and Ground (this is after D1) but I don't know how well that will work (having not tried it).

Otherwise, you need to find a way to power the Arduino Mega with clean voltage. You could use a 5V USB car charger (or some other way to regulate the 12V down to 5V) and run the Arduino Mega (and all the other 5V stuff like LCD, etc) off that. Advantage of a standard USB charger is that you can test it can provide the current with standard cables (no soldering) just by plugging it into your Arduino Mega, then once you're happy you can hard-wire it. Beware with some USB chargers and 5V regulator boards, as they may not have a common Ground between input and output, so you will want to test. 12V Gnd needs to be the same as the 5V Gnd, as it's this way on RAMPS.

Whatever you do, DO NOT connect the 12V directly to the VCC. You will fry the Arduino Mega, your LCD and all the motor driver boards.

Also: Make sure you put a real FUSE (fast blow) between the battery and the printer, as otherwise a short could lead to high current burning out tracks, wiring, etc. A battery can supply as hell of a lot more instantaneous current that most PSU's, and the PTC fuses will definitely not be fast enough to protect your setup.

Hope this helps.

Glad I asked about the 12 volt on VCC! That was the first thing I was going to try. Thanks for the advice Cefiar, you have given me some things to consider. I was going to use a 18 Amp Battery with a Ctek 5 Amp Smart Charger. But I am willing to change that config if neccassary. The printer is for PLA only so no heatbed is needed. The charger has a max voltage output of 14.8 V so that might cause some problems connecting in D1. Plus it would be noisy so I am leaning towards regulating off 5V from the battery (maby big cap on the 5V side to suppress any noise) to feed the Arduino. Still a little confused, would I put that 5V on VCC of RAMPS or VIN of Arduino or are they the same thing? Also would the noise from the Smart charger cause problems with RAMPS or just Arduino?

Another problem that I may have to face is I am not sure if the smart charger can charge the battery while it has a load connected. If it shuts down I may have to consider a dumb charger, like a 13.8V 5 amp power adapter. With the lower voltage, this will open up the possability of using D1 again. Wow decisions. I will put some more thought into it for a while.

Sorry for all the questions but let me simplify my questions and simply ask, If you wanted to connect your bot (non-heated bed) to a battery, how would you go about it?

VCC on RAMPS is the 5V line that feeds the Arduino as well.

The VIN is for unregulated input from ~7V up to 12V, and feeds the voltage "pretty much" into the same input as the barrel jack.

I say "pretty much", as the barrel jack has a diode in-line to protect from reverse voltages, whereas VIN has no such protection (D1 tries to provide this protection on RAMPS).

The problem with the Regulator on the Arduino is that if you feed it noisy voltages, you will most likely get noisy voltages out. You can put a cap on the 5V line (VCC) to help smooth it out, but this isn't always a perfect solution.

Also, it's worth noting that noisy voltages could make your motors behave erratically, so this is also something you may wish to consider.

That said, you've definitely got less to worry about if you're not using a heated bed.

Edited 1 time(s). Last edit at 11/03/2013 12:25AM by Cefiar.

Thanks Cefiar, you have given me a much clearer picture of how the RAMPS Arduino power system works. Will consider my options now.