UPS/Battery backup thoughts and questions



  • Background

    My machine has a mains powered bed heater. The bed itself is 10mm thick aluminium with 12mm of semi rigid insulation underneath, so once up to temperature it takes a long time to cool. Despite the fact that it's a large machine, without the bed heater it takes very little electrical power to run. Because it is a large machine, I print large objects which can take days rather than hours to complete. Power outages are rare where I live but when they do happen, it gets costly in both time and filament. So I am thinking of adding some sort of UPS.

    Solution 1

    It occurs to me that with Duet 3, I could utilise the RPi to interface with the UPS such that when power is interrupted and the UPS starts to run on battery power the RPi could turn off the bed heater and other non-essential items such as lights, and maybe even disable my third load balancing gantry to lengthen the run time. After a pre-determined period of time, the RPi could then instigate an orderly shutdown/resume.

    Alternative ideas

    I might still go down that route but another thought occurs to me. A UPS uses an inverter to convert DC voltage to AC voltage at an efficiency of say 85%. The 24V PSU then uses that AC voltage and converts it back to DC at a similar level of efficiency. Which all seems a bit daft and I could be losing 30% of the available run time simply through the process of converting DC to AC and then from AC back to DC.

    I suspect the answer to the following question is a big NO NO but what would happen if I simply connect a big 24V deep cycle leisure battery in parallel with the 24V PSU? This would automatically turn off the bed heater in the event of a power failure because that runs off the mains.

    If the answer to the above is big NO NO, could it be done with some simple circuitry? Would feeding 24 into the output terminals of at PSU cause damage to the PSU? If so could a diode mitigate that? What happens when the battery runs low and power is then restored. Will the 24 PSU charge the battery or will it "fry" either the battery of the PSU? If so, could something be done to limit the current going back into the battery? What else have I not thought about and how could it be mitigated?

    Over to you clever people.......



  • @deckingman Morning!

    Firstly you would need your PSU at around 13.5V (or I'm guessing about 27V for two in series?) as I think even the leisure batteries charge at that.

    Secondly fuses in the supply are essential. Lead acid and gel batteries have huge cranking amps, around 500A is not unusual, again this is the main rather than leisure batteries but I would have thought they are capable of the same.

    Finally, perhaps you could run a charger on the leisure battery a DC regulator to drop from the charging voltage to the machines 24V, not sure how the charger would react to the loading of the machine, but seeing as my charger has a jump start mode that's spun over my Rover V8 with 8 injectors with coils some of them must be reasonably robust!



  • @DocTrucker Morning Wes.

    PSU is 24V. I'm showing VIn as being 23.7 so a little low. Too low to recharge a battery I'd guess so for sure I'd need an external charger. Do you know if you can get 24V leisure batteries or are they all 12v so I'd need 2 in series?



  • We use Phoenix Contact Power supplies and 24vdc ups’s for our pump station controllers. They have to continue to power the modem and RTU for compliance reasons for up to 30 days. The batteries are 2x standard 12v lead acid batteries connected in series. They would be your best option. They are tidy wee din rail mount units. If you get the Quint version they have various comms expansion cards that you can get all sorts of information out. They do also have various io on them such as battery mode that you can set up as an input into the RPi. The trio is an integrated power supply and ups version that is slightly cheaper also by Phoenix contact.

    Regards
    Sam



  • 10A Quint (you can use your current power supply as they are 24in 24 out)

    https://www.phoenixcontact.com/online/portal/nz?uri=pxc-oc-itemdetail:pid=2320225&library=nzen&tab=1



  • @deckingman I've found 12v planet a gold mine of information for the truck.

    https://www.12voltplanet.co.uk/

    Winch and starter currents can creep and make you fastidious in lesser wiring! 😄



  • Ian, if you cool down the bed, your parts may warp and pop off... For PLA, I don't heat the bed anymore: I have a LockBuild surface, and it sticks very well with hair spray. I already printed 300x300 parts without issue.

    About 24V PSU, I have one of these Quint stuff (a bigger one), and plan to use it with two 12V batteries. We used them at work, and it works fine.



  • @fma As per my OP, it takes an awfully long time for the bed temp to drop to the point where parts might fall off - longer than a UPS could reasonably be expected to run the rest of the printer.



  • @deckingman said in UPS/Battery backup thoughts and questions:

    I suspect the answer to the following question is a big NO NO but what would happen if I simply connect a big 24V deep cycle leisure battery in parallel with the 24V PSU?

    as hinted to you'd only utilize part of the capacity of the batteries as the full charge voltage is a few volts over the nominal voltage. however you could run the duet at full charge voltage of up to 28.8v and have full use of the battery. but it would charge just fine back to 24v if used with 24v psu - the catch is a fraction of the capacity

    meanwell makes psu's with built in battery backup, but you'll still be using the cell voltage not regulated to nominal voltage. i use a few of them with 12v access controll setups, cheap and works well and even din rail mounting.



  • @deckingman You could always look into surplus Cell site backup systems where you have 24V Rectifiers that supply a float charge to the batteries and the batteries run the equipment we used to use 24V Systems and there are still a few in service even now.

    Suppliers I know have are POWEC, Inverses, Harmer and Simmonds and Erskine.

    Trouble is that they tend to be bulky.



  • @bearer Thanks and that makes sense about a fraction of the battery capacity. But doesn't a the inverter in a UPS suffer the same limitation?



  • @Dougal1957 Cheers Doug. I'm not too worried about bulk. The plan is that I'd instigate this after I've first moved the printer into my garage so I'll have more room and it won't matter what it looks like.



  • @deckingman Next time I see any on site I'll take a pic or 2 for you



  • Hi,

    The simplest approach would be a UPS of needed capacity to handle the outages.

    Is that just too expensive?

    Frederick



  • @fcwilt said in UPS/Battery backup thoughts and questions:

    Hi,

    The simplest approach would be a UPS of needed capacity to handle the outages.

    Is that just too expensive?

    Frederick

    As per my OP if you read it, you'll see that is the solution I put forward. But it just seems a bit daft to have an inverter converting from DC to 220V AC at around 85% efficiency, then a PSU converting that AC back to 24v DC with another loss due to efficiency. Why not cut out the "middle men" and just have DC backup?



  • Unless I missed it, we are missing a key fact from this discussion: What is the pattern of outages at your printing site?

    For example, at my house, I have a fair number of mains outages that last less than 3 minutes. Often just a few seconds. In the three years I've lived here, outages that last more than a few minutes have only happened twice. And both of those were 8+ hours. A UPS that could keep my 1300+ watt mains bed heater for 8+ hours is certainly physically possible, but it is more bulk and expense than I desire.

    Therefore...

    I've elected to put my smaller, 12 or 24 volt bed, printers on a small UPS. My printer that is big enough to have a mains bed, the logic (and motors and so forth) are on another small UPS, the bed is not UPS, it is mains only. Obviously, the printer has to have to power cords.

    This has worked very well. All short outages have been perfectly covered, and I've not had a long outage during a long print. Maybe someday I will; if it happens, Cest La Vie.

    So let's ask it as a question: What is the typical pattern that you are trying to mitigate? Have you considered covering only the "90th percentile" (using that term very loosely) outage occurrences ?



  • @deckingman said in UPS/Battery backup thoughts and questions:

    As per my OP if you read it, you'll see that is the solution I put forward. But it just seems a bit daft to have an inverter converting from DC to 220V AC at around 85% efficiency, then a PSU converting that AC back to 24v DC with another loss due to efficiency. Why not cut out the "middle men" and just have DC backup?

    There is engineering elegance to do it directly, bypassing the 220V stages. There is operational elegance, and robustness, in purchasing a totally "off the shelf" solution and just plugging it in and having it work.

    Me, personally, on a 'backup' system, I'm going to go for operational elegance & robustness every time.

    Having said that, Telcos have done this for ages. They run ALL THE TIME on 24V from batteries (or is it 48V? I don't remember) and their incoming utility and power distribution keeps those batteries charged.

    Assuming that Lead Acid batteries are used, "simple parallel" at 24V is NOT a good strategy. Lead Acid batteries will lose capacity rapidly if held at their nominal voltage for long periods of time. They will not "be there when you need them" in terms of ability to deep discharge.

    Instead, they need a battery charger designed for lead acid batteries. One that will 'charge' vs. 'float' at proper voltages. These voltages are all within the operating range of a Duet. So a three way connection of battery charger, batteries, Duet should work just fine.

    With one caveat: Some battery chargers are also "maintainers" or "de-sulfate" devices. These put out very short spikes of voltage WAY WAY above charge/float, to stop (or even remove) plate sulfation. Obviously, such spikes should not go into a duet.

    Summary: I, personally, would not worry about the losses and would just use off the shelf components. If you do choose to go 24V battery, it seems OK to connect charger/battery/duet all in parallel. Pick a charger that does NOT de-sulfate.



  • @Danal most telco stuff now is -48Volt supply but in the earlier days of Cell sites it was +24V and there are still some of the kit around the batteries tend to be deep discharge Gell type cells and the control systems are designed to disconnect the batteries from the output when they fall to a certain voltage to save the cells (They can be expensive to replace)

    Doug



  • @deckingman

    Hi,

    Seems to me I recall seeing UPS devices with DC output.

    Frederick



  • We run quite a few Harmer & Simmons modules in the field (for 48V telco stuff 🙂 ).
    These are quite expensive but last very long (we have some modules older than 20 years).
    I would personally go for "operational elegance" as Danal nicely describes because dealing with DC charging and batteries can be dangerous stuff (I've experienced a 24V battery explosion (due to bad maintenance), it was not pretty...)



  • @whosrdaddy said in UPS/Battery backup thoughts and questions:

    We run quite a few Harmer & Simmons modules in the field (for 48V telco stuff 🙂 ).
    These are quite expensive but last very long (we have some modules older than 20 years).
    I would personally go for "operational elegance" as Danal nicely describes because dealing with DC charging and batteries can be dangerous stuff (I've experienced a 24V battery explosion (due to bad maintenance), it was not pretty...)

    This of course is very true I to have seen this the Harmer and Simmonds I have used where all 24V Systems using SM24-50 or SM1800 rectifiers but they do tend to be huge.



  • @deckingman this Quint UPS’s are a 24vdc in 2vdc out. So you power supply is providing the power to the ups, which obviously charges the batteries and also runs your system. When power is lost, it switches over to batteries. There is no conversion between Ac and dc past the initial power supply. As far as I am aware with them, you can install as big of a battery system as you like. Having separate power supplies means you don’t have to replace yours



  • @samlogan87 said in UPS/Battery backup thoughts and questions:

    @deckingman this Quint UPS’s are a 24vdc in 2vdc out. So you power supply is providing the power to the ups, which obviously charges the batteries and also runs your system. When power is lost, it switches over to batteries. There is no conversion between Ac and dc past the initial power supply. As far as I am aware with them, you can install as big of a battery system as you like. Having separate power supplies means you don’t have to replace yours

    That almost sounds too good to be true. I'll have to give that some serious thought. Many thanks.

    BTW are you based in New Zealand?



  • So the one question nobody's asked is "How much do you want to spend?" 🙂
    Solar charge systems (without inverters) and 24v LiPo batteries are becoming cheaper and cheaper. There are even used LiPo batteries for sale on Ebay at a fraction of the cost of new ones and while it's really easy to kill lead acid batteries it's really hard to kill a LiPo so they usually have plenty of life to them.



  • @gtj0 I guess if I went the "normal" UPS route I'd be looking at £150 to £200 ish. So that sort of budget. I like the idea of using Li-ion batteries. All my power saws and drill drivers that I used to use for deck building are Li-ion (18 and 36v). They don't run down slowly like NiMh or NiCd but keep full power right up until the time when they need charging. I've seen a few 24v golf cart/electric bike batteries that might do the job.


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