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Forum Index : Microcontroller and PC projects : Super Capacitor Battery
| Author | Message | ||||
| PhenixRising Guru  Joined: 07/11/2023 Location: United KingdomPosts: 2069 |
What do we think? It sometimes happens that we need to cycle the main power of a machine (not because of the control freezing  ) and it's a pain in the buttski to have to re-home the machine.I intend to maintain power to the PicoMites and encoders for instant recovery. This only needs to be for a few minutes. I DO have some of those 5V UPS units with 18650s but maybe Super Capacitors might be a more robust solution(?) |
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phil99 Guru Joined: 11/02/2018 Location: AustraliaPosts: 3366 |
Found a datasheet Claimed nominal life 10 years or 1,000,000 cycles. High internal resistance which rises with temperature (and thus current), but much better than the first generation super caps. |
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| PhenixRising Guru Joined: 07/11/2023 Location: United KingdomPosts: 2069 |
Thanks Phil. Do you think that I could get away without the boost module? I'm only talking a few minutes. |
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Mixtel90 Guru Joined: 05/10/2019 Location: United KingdomPosts: 9088 |
2 capacitors in series give half the capacity of one of them so this is a 250F power bank, not 500F. The output voltage will fall due to the internal resistance, which is additive so you see twice the internal resistance of a single cap. Still very low, but it's there. You also need to keep supercaps as cool as possible as the capacity falls away with temperature rise, so keep them away from heat generating devices. Don't trust the 35mins operational time unless you've tested it! 20% tolerance is a lot on a cap this size. Mick Zilog Inside! nascom.info for Nascom & Gemini Preliminary MMBasic docs & my PCB designs |
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Volhout Guru Joined: 05/03/2018 Location: NetherlandsPosts: 6062 |
Phenix, If you use genuine pico's, you can simply power them from Vbus, and connect a 5V supercap to Vsys (add some charging current limit ,i.e. resistor and bypass shottky diode). You do not need to power the encoders and level shifters when you make a minor change to the pico program since the pico can check Vbus (GPIO24) and thus detect power drop and power return. On power down the program can stop the PIO's accordingly after reading fifo's until empty to get the last status of the encoders. That should be a milisecond action, and keeps the current location stored in the programs variables. After power returns, wait until encoders stabilize (1 second ?), then simply restart the PIO's. So your supercap only needs to power the pico itself. A voltage drop on the supercap will be compensated by the (genuine pico !!) buck-boost converter until the 4.5V supercap voltage drops below 1.8V. That is a delta voltage dV = 4.5 - 1.8 = 2.7V C * dV = I * dT With a estimated power consumption of 100mA that means that for every second operation you need 0.037 F (farad). For 1 minute operation that would be 2.2F. Using a 5F supercap (a 5V part, not a 2.7V part) would guarantee 1 minute, even after years of degradation of the capacitor. Regards, Volhout P.S. a 5V supercap is essentially 2 supercaps in series. Since the supercap technology is 2.7V. Kyocera /AVX have 2.5F 5.0V caps that are essentially a sleeve around 2 individual supercaps. They are available at farnell for around 3 euro each. P.P.S. supercaps only have their full capacity after they have been charged for long. A 10 minute charge (even if the voltage is achieved) will only give you 20% capacity, but after 48 hours, they show same voltage, but have full capacity. We learned this for an application that needed to run 2 week of of supercaps. This behaviour is different from regular E-caps. When these are charged to a certain voltage, charging them for 48 hours extra does not change anything in their capacity. . Edited 2026-08-27 20:51 by Volhout PicomiteVGA PETSCII ROBOTS |
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| PhenixRising Guru Joined: 07/11/2023 Location: United KingdomPosts: 2069 |
Hi Harm, I have to consider the fact that, during power-down, the axes could be disturbed. One of them is a linear slide with rack & pinion and is quite easily pushed. Stuff that shouldn't happen does happen and I need to maintain 0.01mm. |
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| Volhout Guru Joined: 05/03/2018 Location: NetherlandsPosts: 6062 |
OK, understood. Then you need a "UPS" alike function for both sensors, converters andd pico's. Using a rechargeable battery sounds like the way to go. If you need only 5V then a commercial power bank could do the job. Volhout PicomiteVGA PETSCII ROBOTS |
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| Mixtel90 Guru Joined: 05/10/2019 Location: United KingdomPosts: 9088 |
What powers the linear slide? If the actuator loses its supply, which I suppose it would do during a power down, you've lost any electrical braking so you may not be maintaining position anyway. The only way is to re-home it. Mick Zilog Inside! nascom.info for Nascom & Gemini Preliminary MMBasic docs & my PCB designs |
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| Volhout Guru Joined: 05/03/2018 Location: NetherlandsPosts: 6062 |
Hi Mick, As long as the encoders keep working, you know where you are. Homing is not needed. But when the drive gets out of control, you may damage the product you are making. Regardless if you know where you are. That may be acceptable if the product is cheap. But I have seen machines milling a solid block of aluminum the sized 2 meters by 1.5 meter by 1 meter (milling took 2 days) into a very accurate vacuum chamber (ASML). Don't want to damage the product then... Volhout PicomiteVGA PETSCII ROBOTS |
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| PhenixRising Guru Joined: 07/11/2023 Location: United KingdomPosts: 2069 |
Two different scenarios 1) Emergency Stop - Dynamic braking 2) Planned power-down - Typically to reset a servo-drive During a simple "panic-abort" the PID gets: Command_Position = Current_Position (instant deceleration) |
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