Battery backed power supply...

CNCModeller

Hi all,
I've been looking at 24v battery backed power supplies for fire alarms, however apparently these supplies actually output 27.6 volts.

These are good as they output a fault signal if the supply fails.

Would 27.6v be an issue with a duet 3 mini?

Any constructive thoughts would be much appreciated.

All the best
Barry M

jay_s_uk

@CNCModeller the duet 3 mini can go up to 32V

dc42

@CNCModeller @jay_s_uk actually the published maximum is 25V. The reason is that the low-current mosfets are rated at 30V, and we wanted to allow some headroom to allow for the supply voltage rising when you turn off all the stepper motors and they feed power back into the supply rail. If there is a battery across the supply rail then it should absorb this power, so 27.6V would be OK.

deckingman

@CNCModeller For additional info, I've been running a DC UPS (two 12v batteries in series to give me 24V) for 2 or 3 years. The reason the PSU has to be set to 27.6 V is so that the output will charge the batteries when power is available. I haven't had any issues running that voltage but that's with a 6HC and three 3HC expansion boards rather than your mini.

For yet more additional information, I run a separate 12v PSU for non-essential things like lights, and the bed is heated by 240v. When mains power is lost, the DC UPS switches instantly to battery power which keeps the Duet boards and anything connected to them running without any interruptions. Anything non-essential which might be connected to the separate 12V supply does turn off. This is deliberate and designed to maximise the run time on batteries. For the same reason, the bed heater goes off because it's 800W and would require a huge amount of battery storage and a mains inverter to keep it going for any length of time. But because it's 12mm thick and insulated, it takes a very long time to cool.

It's saved a couple of multi-hour prints two or three times so probably worth doing. Although now that I only have my pension income to live on, I probably couldn't afford to do it nowadays.

CNCModeller

@deckingman, @dc42 thanks folks, great information.

I'm thinking similar, some of my prints are up to 5 days, using multiple kilos of filament. Allowing the printer to park up and the nozzle to cool before a controlled shutdown is where my mind is at.

So all heaters and ancillary functions stop the head retracts and parks and most importantly the extruder is brought back to temperatures where thermal creep won't cause a jam.

I suppose I could continue printing on battery until the voltage drops below say 20v to allow for short power interruptions.

I also had a power supply failure recently so battery backed supply after the PSU feels like the way to go, and would pay for itself with just one print saved.

Anyway again thanks for the info.

All the best
Barry M

deckingman

@CNCModeller If you cut out the bed heater and any ancillaries like lights and so forth, it's quite surprising how little power is used to continue printing - in my case with two XY gantries throwing 5Kgs around and hefty 6 input hot end with an 80Watt heater. I aimed for 2 or 3 hours on battery power - any longer than that, and the bed will cool to the point where the part falls off in any case.

Going down to 20V will likely kill the batteries. Around 11.5 V for a 12v Li Ion is reckoned to be as low as you can go - so 23V for two in series. Sealed lead acid will likely expire if you go below 12V each so 24V min.

samlogan87

As @deckingman said, the dc-dc ups is definitely the way to go. Phoenix Contact do quite a nice solution that I think you can connect up to 7 battery packs to so plenty of capacity. We used them a lot on PLC’s and telemetry units we needed to keep up for compliance purposes

Sam

deckingman

@samlogan87 It was your advice that pointed me in the right direction - thanks Sam. Although I ended up with a Meanwell device because it was about half the price of the Phoenix here in the UK.

For yet more information, there is a set of relay contacts which make and break depending on if the machine is running on mains power or batteries. I use these as an external trigger to change the fault tolerance settings of the bed heater so that I don't get an error when the bed temperature drops. They could be used for other purposes like turning off any "non essential" lights or fans that might be connected to the Duet board(s).

o_lampe

For 12 or 24V applications LiPo isn't the best match. Better look for LiFePO4 batteries.
The voltage level of a 4s block is 1:1 replacement for lead acid batteries.
A single cell can discharge to 2.5V under load, but the cell voltage drops very fast at the end, so there is not much time gained.

deckingman

@o_lampe .......if you can afford them. I'd love to go that route but here in the UK, LiFePo4 batteries are about 4 to 5 times the price of sealed lead acid. For example, I can pick up a modest 30Ah sealed lead acid for around £30 but an equivalent 30Ah LiFePo4 is £140. So two in series to give me 24V costs around £60 for sealed lead acid but a whopping £280 for LiFePo4. That's more than 1 weeks income for a pensioner like me. On a cost/benefit basis, I'd have to have an awful lot of lost prints due to power outages to justify that sort of money.

o_lampe

@deckingman Absolutely right! For stationary use, lead acid can't be beat. There are deep cycle versions that can stand such use case. Starter batteries not so much...
I was just comparing various Lithium battery chemistries.

samlogan87

@deckingman hey no worries. Glad it’s helped a few times. Why more people don’t use them I don’t know. Schenider have quite an old model but I have been told they are releasing a new model soon. They may have already actually. They other one to look at is ECB’s (Electronic Circuit Breakers) instead of fuses. You can have alarm outputs on them when they trip so you can go figure out why and reset them