20 second UPS?

[MorrisonHiker]

I'm on self-powered.

I think that explains why you never have the issue. It seems to only happen on TBC.

Honestly, I'm thinking about going back to Self-powered as I don't see much benefit from TBC.

Here, we get like 1.5 cents per kWh so we bank it instead of getting a credit for it. If I was making 40 cents per kWh, TBC would make more cents/sense. lol.

 

[miimura]

Personally, I would buy a used UPS off eBay without batteries and put in some high C-rate LiFePO4 batteries rather than the expensive SuperCap system listed above. I will do the LiFePO4 conversion when the AGM batteries in my UPSes need replacement.

So I finally got around to doing this. I found some used A123 26650 batteries on e-Bay and bought two 11S strings and cut off the circuit board that was on the original pack. The cells were drained to nearly zero but they did take a charge normally and I cycled them a few times and manually balanced them. I took a 8S section of one and hooked it up to my old SmartUPS 700. The float voltage from the built-in charger is 28.0 VDC and it charges at up to 4A at very low SOC, dropping the current as the voltage rises. That is well within the max voltage of these batteries. Normal charging should be up to 3.6V/cell or 28.8 VDC for the 8S pack. I left them for several days with the UPS supply holding the float voltage and it doesn't go anywhere, so that's stable. However, one of the cells droops more than the others, leading to about 220mV imbalance after 48 hours. Eventually, the cells that drop, would push others higher, over their max voltage. A balancing BMS would prevent this by bleeding off the high cells and forcing the pack voltage into the lower cells. The lower pack voltage actually allows a larger voltage range because most of the balancers have a fixed balance activation voltage depending on the target cell chemistry. The LiFePO4 BMS balance boards I've seen start balancing at 3.6V or 3.65V.

The load test was very interesting. I set up a load that lit up all the load meter LEDs on the UPS. That was about 385 W according to my Kill-A-Watt. When I pulled the UPS power cord, the battery pack voltage immediately dropped to 25V and slowly dropped to about 24V before I stopped the test after 4 minutes. The amperage draw, according to my clamp meter, was 23-24A. That is only about 70% conversion efficiency. I stopped the test because the batteries were getting hot. In another test at lower power I ran it all the way down to the UPS low voltage shutdown which happened at about 19.8V. The specs say these cells can take 50A max continuous discharge, but that would clearly require some active cooling or significant heat sinking, which is not practical to implement in this kind of UPS application. If I were going to put this into service, it would require a 8S2P pack as a minimum in order to ensure the batteries remained at a safe temperature. Either that or ensure that the connected wattage was lower. Doubling the cells would extend the runtime even further, so it is not really the high power, short runtime solution that I thought it would be.

 

[mspohr]

So I finally got around to doing this. I found some used A123 26650 batteries on e-Bay and bought two 11S strings and cut off the circuit board that was on the original pack. The cells were drained to nearly zero but they did take a charge normally and I cycled them a few times and manually balanced them. I took a 8S section of one and hooked it up to my old SmartUPS 700. The float voltage from the built-in charger is 28.0 VDC and it charges at up to 4A at very low SOC, dropping the current as the voltage rises. That is well within the max voltage of these batteries. Normal charging should be up to 3.6V/cell or 28.8 VDC for the 8S pack. I left them for several days with the UPS supply holding the float voltage and it doesn't go anywhere, so that's stable. However, one of the cells droops more than the others, leading to about 220mV imbalance after 48 hours. Eventually, the cells that drop, would push others higher, over their max voltage. A balancing BMS would prevent this by bleeding off the high cells and forcing the pack voltage into the lower cells. The lower pack voltage actually allows a larger voltage range because most of the balancers have a fixed balance activation voltage depending on the target cell chemistry. The LiFePO4 BMS balance boards I've seen start balancing at 3.6V or 3.65V.

View attachment 349276

The load test was very interesting. I set up a load that lit up all the load meter LEDs on the UPS. That was about 385 W according to my Kill-A-Watt. When I pulled the UPS power cord, the battery pack voltage immediately dropped to 25V and slowly dropped to about 24V before I stopped the test after 4 minutes. The amperage draw, according to my clamp meter, was 23-24A. That is only about 70% conversion efficiency. I stopped the test because the batteries were getting hot. In another test at lower power I ran it all the way down to the UPS low voltage shutdown which happened at about 19.8V. The specs say these cells can take 50A max continuous discharge, but that would clearly require some active cooling or significant heat sinking, which is not practical to implement in this kind of UPS application. If I were going to put this into service, it would require a 8S2P pack as a minimum in order to ensure the batteries remained at a safe temperature. Either that or ensure that the connected wattage was lower. Doubling the cells would extend the runtime even further, so it is not really the high power, short runtime solution that I thought it would be.

Probably best to use a balance charger rather than rely on the built-in APC charger which is designed for sealed lead acid batteries. Don't know how you would defeat the built-in charger, though, without disconnecting the battery from the unit which defeats the purpose. I faced a similar situation with a small UPS and ended up deciding to just buy a replacement SLA battery which was cheap and should last another 4 years.

 

[miimura]

Probably best to use a balance charger rather than rely on the built-in APC charger which is designed for sealed lead acid batteries. Don't know how you would defeat the built-in charger, though, without disconnecting the battery from the unit which defeats the purpose. I faced a similar situation with a small UPS and ended up deciding to just buy a replacement SLA battery which was cheap and should last another 4 years.

If I was to deploy it I would put a balancing BMS board as described earlier in this thread and put a JST connector so I could easily check the cell voltages periodically, just for peace of mind.

I did just buy a SLA battery for the larger UPS at my office because I wanted it to just work and I needed a little more runtime since we don't have any other battery or generator there.

 

[Sylvia Else]

So, my thought was, I wonder if there is a market for a super capacitor UPS? IF you could build it out of components that would last 20 years, you could build a fairly small and light unit since it would only need to bridge a 20 seconds power gap in a Powerwall (or any generator) household. Thoughts?

If we take a typical UPS power output of 750 watts, and require a 20 second runtime, then that is 15kJ of energy that would have to be stored in the capacitor. A typical maximum voltage for supercapcitors is 2.7 volts. Sometimes they're packaged together to offer other voltages, but 2.7 seems to be the basic voltage. To store 15kJ in a capacitor at a peak voltage of 2.7 volts requires about 4100 Farads.

However, it's not practical to get all of the energy out - perhaps you go down to half the voltage. The energy is proportional the square of the voltage, so now we're talking about 20kJ, and about 5500 Farads.

But drawing 750 watts at half of 2.7 volts implies a current of 555 amps. A brief examination of datasheets suggests that this not going to be achieved with a supercapacitors amounting to 5500F, so a practical implementation will likely need a greater capacitance but using a smaller proportion of the energy in each, thus requiring a lower peak current.

I looks to me as if the capacitors alone are going to run to four or five hundred dollars, retail, and mass 1.5 to 2 kg. Add to that the transformer, and power electronics required to generate the output from a varying voltage input, and you're not talking about something trivial to build, and expensive components are apt to destroy themselves in this kind of application if you make a mistake.

Changing the batteries in my UPS is not difficult or messy, and they can be hot-swapped so that the computers do not have to be powered down.

 

[mspohr]

If we take a typical UPS power output of 750 watts, and require a 20 second runtime, then that is 15kJ of energy that would have to be stored in the capacitor. A typical maximum voltage for supercapcitors is 2.7 volts. Sometimes they're packaged together to offer other voltages, but 2.7 seems to be the basic voltage. To store 15kJ in a capacitor at a peak voltage of 2.7 volts requires about 4100 Farads.

However, it's not practical to get all of the energy out - perhaps you go down to half the voltage. The energy is proportional the square of the voltage, so now we're talking about 20kJ, and about 5500 Farads.

But drawing 750 watts at half of 2.7 volts implies a current of 555 amps. A brief examination of datasheets suggests that this not going to be achieved with a supercapacitors amounting to 5500F, so a practical implementation will likely need a greater capacitance but using a smaller proportion of the energy in each, thus requiring a lower peak current.

I looks to me as if the capacitors alone are going to run to four or five hundred dollars, retail, and mass 1.5 to 2 kg. Add to that the transformer, and power electronics required to generate the output from a varying voltage input, and you're not talking about something trivial to build, and expensive components are apt to destroy themselves in this kind of application if you make a mistake.

Changing the batteries in my UPS is not difficult or messy, and they can be hot-swapped so that the computers do not have to be powered down.

I don't think your assumption of 750w is typical. Maybe if you're running a bank of servers. I only have a few small crucial loads totaling 24 watts (cable modem and a few RPi servers). This makes the problem much easier.

 

[JohnRatsey]

I've put one of these mini UPSs https://www.amazon.co.uk/gp/product/B075QZQSS1 on each of my cable modem and router so that I can keep internet connectivity should there be a power outage. However, as I don't have the backup gateway the PW 2 will shut down.