APC smart UPS sc1500 problem - Page 1
Forum section: electronics / repair

author: andreroest
author_rank: Newbie
timestamp: July 09, 2018, 03:41:37 pm
content: I have an APC UPS which worked well until last year.The batteries went down and I replaced them by a bigger set of batteries which I connected external.The problem is now that when I switch on the UPS (with or without 230V supply it starts to run with a green led (flashing) and all the battery leds green.After a number of seconds the battery red light flashes and the relais switches off the batteriy and every two seconds a buzzer sounds.I checked the connection to the battery which is alright as well as the battery voltage which is 25VDC.I want to use this UPC as a net backup so it is not connected to a PC.Any idea how to solve this problem?

author: shakalnokturn
author_rank: Super Contributor
timestamp: July 10, 2018, 07:16:18 pm
content: Is the wire leading to your external battery pack thick enough?I'm guessing excessive voltage drop could cause the UPS to measure the batteries as under-voltage when running.Check minimum voltage on startup running on batteries at the UPS PCB rather than across the battery pack terminals.

author: killcode
author_rank: Contributor
timestamp: July 11, 2018, 01:25:19 am
content: That sounds like undervoltage condition as shakalnokturn said.After you sort that out, you'll also need to make some changes to battery constant if you have a bigger battery and want the full run time.Serial protocol info here:https://networkupstools.org/protocols/apcsmart.htmlit's pretty straight forward to change the battery constant to something reasonable - SC1500I default battery constant should be 8F by the table I have:Enter Y (UPS should respond SM);Enter 1 (one, not el; wait 4 seconds);Enter 1 (one, not el; UPS should respond PROG);Enter 0 (zero, not oh; UPS should respond with current constant);Write down the existing value so that if something goes wrong, you can at least put it back to that value;Enter + (plus) or - (minus) to increment/decrement the value;Enter R to reprogram constant value (UPS should respond Bye);Enter Y (UPS should respond SM);Enter 0 (zero, not oh; UPS should respond with the new constant);exit terminal.You can also tweak the battery voltage measurement to ensure it doesn't float them too high.

author: BradC
author_rank: Super Contributor
timestamp: July 11, 2018, 02:00:04 am
content: Just to supplement what killcode said.If you are more than doubling your battery capacity you want to tell the UPS it has external batteries rather than winding the battery constant right up. You want the battery constant to be as close to the default value as possible to give the UPS the best chance at accurately estimating your runtime. Any SmartUPS can be told it has external batteries, even if it isn't capable of that out of the box.Here's how I do it.Do what killcode said to get into programming mode, reset the battery constant to default (as if the UPS has seen much use it won't be default anymore).Now, use the 'j' command to get an estimated runtime based on your current load. That'll be in minutes. I find a resistive load at 30-50% of the UPS capacity is the best way to manage this, and it can then be used for the subsequent runtime calibration.Do some basic calculation given the size of your new external battery and figure out what the runtime *should* (or is likely to be). I do this by factoring in the discharge current and using peukerts constant to get an estimation of battery capacity at that load. That allows me to calculate how long the UPS should last. You can just estimate, but double the battery capacity will give you more than double the runtime.Change the number of external batteries with the '>' command. After each change, check the estimated run time. Do this until you get as close to the runtime as possible by altering the pack count.Now you can tweak the battery constant to get it right.So the process might be :- Enter programming mode- 'j' to get the runtime- '>' '+' to add a battery pack- 'j' to get the runtime- '>' '+' to add a battery pack- 'j' to get the runtime- '>' '-' to remove a battery packNow the runtime is slightly less than what you are after, so we bump the battery constant a couple of counts- '0' '+' '+' '+' '+'- 'j' to get the runtime.....lather, rinse and repeat until you get to your estimated runtime.then 'R' to save.This way you can get your initial runtime estimation spot on. Then you do a runtime calibration and the UPS will do the final adjustment for you.I added 55AH to an SU2200, SUA3000 & 17AH to an SU1400. Using this method (and a fairly accurate estimate based on corrected battery capacities) I got the actual runtime within 5% of the calculated runtime first go on all 3 units. The UPS corrected that marginally after a runtime calibration. The after-calibration battery constant values were within about 3-4 counts of where I started.As for the undervoltage issue. The UPS tests its battery condition by looking at the instantaneous drop in voltage when going from float to load. *Any* additional resistance in the battery cables over and above what comes out of the factory will completely screw up that evaluation leading the UPS to indicated the battery is toast. That can be calibrated out using constants 5 & 6 (constant 5 appears to affect the path resistance measurement and 6 appears to affect the dead battery indicator level) but you are seriously on your own there. There is literally zero documentation on what those constants do and I spent days playing with them to figure it out before some kind soul gave me the factory values to put in after my SU1400 corrupted its eeprom one day.I just use mega sized cables for the external batteries *and* keep the internal batteries.