If this is your first visit, be sure to check out the FAQ by clicking the link above. You may have to register before you can post: click the register link above to proceed. To start viewing messages, select the forum that you want to visit from the selection below. |
|
|
|
Thread Tools | Display Modes |
#16
|
|||
|
|||
APC question
J. P. Gilliver (John) wrote:
In message , Andy But the remaining time value kept changing. I think it is only somewhat accurate. Or, like time to finish a multi-file move or copy, it might change as the load changes. Might be worth having Task Manager open and seeing if there's any (inverse) correlation between CPU load and projected time left. The metrology on a UPS is only approximate. ******* http://batteryuniversity.com/learn/a...tate_of_charge "BU-903: How to Measure State-of-charge Explore SoC measurements and why they are not accurate. Voltage Method ... This makes the *voltage-based* SoC method *impractical* for a battery in active duty. === UPS discharge activity... " My experience with lead acid, over a wide temperature range (trying to charge my car battery at -20C), is that the voltage method can be off by 50%. In other words, when I think the battery is "full", it might only be "half full". The terminal voltage is temperature sensitive. Even the density is temperature sensitive, so if you use a hydrometer on a wet battery, you must use the temperature table to correct for the temperature of the battery when you checked the composition of the liquid in it. And the temperature is not the air temperature either. It's the temperature of the positive battery terminal. In a car, the battery could be heated by the trip you just took, needing 12-24 hours to track ambient (air temp) again. You charge car batteries, to prevent them from "freezing". A healthy car battery, the liquid might be 12 molar sulphuric. The freezing temperature might be -55C in that state. A bad car battery, one where you've let it discharge, the liquid is getting closer to tap water. Consequently the freezing point is only a bit below 0C. If the car is outside and it's -20C, now the battery can freeze. The liquid becomes a solid, expansion forces take effect. So if a person owns a lead acid car battery near the end of its life, and the temperature at night will be dropping to -20C, then you try to charge it back up before the cold weather comes. As you charge it, the liquid becomes more acidic. The temperature issue also causes problems for the charging circuit. Whether charging the battery, or float charging it to a certain voltage, the desired terminal voltage is also temperature sensitive. At -20C or +20C, the correct float voltage is going to be different than the 13.2V at room temperature (for a car battery, your UPS may use 6V batteries). In summary, voltage and lead acid are loosey goosey. If you think you know what you're doing... you don't. The alternative would be a fuel gauge chip. https://www.ecnmag.com/article/2012/...y-fuel-gauging "An improved solution over the voltage-based method is the Coulomb counting technique. In this technique, the charge flowing into and out of the battery is integrated to get an accurate estimate of the remaining capacity. This technique works well, if the battery]s initial state-of-charge is accurately known." A problem with that technique, is the battery load may vary from 10mA to 100A. And this may require a large dynamic range from the design. That's probably easy to do with sigma-delta (if someone was starting such a design today). Older designs would be more crappy (flash converter or SAR). The processor in the UPS, may have a 10-bit ADC for example. Not a lot of anything to work with. No 24-bit gear in those, although the silicon cost to do 24-bit, wouldn't be that high. Silicon is "almost free", and only adding pins to the microprocessor package costs money. On a UPS, the electricity is relatively free, so the power to run a measurement circuit wouldn't be an issue. In any case, the UPS has to turn off "everything" at some point, when the battery is too low. Paul |
Ads |
|
Thread Tools | |
Display Modes | |
|
|