Troubleshooting the APC UPS and other UPS's

This page was originally hosted at but the domain has expired. A copy is now hosted here as the information is VERY useful if you have APC UPSes.

Troubleshooting the APC UPS and other UPS's
APC Smart UPS troubleshooting, battery problems, bug fixes and troubleshooting for most UPS systems.

APC Smart UPS Voltage Gain Bug:
If you have a older APC Smart UPS and are having many strange problems with batteries, freeze ups, etc. chances are it's because of a bug that I have identified and confirmed in most older APC Smart UPS systems. There is a easy fix but please read this document in full to understand it.

APC UPS units are well built power inversion systems. However, problems do arise and in most cases it's not the direct fault of the UPS system but instead the consumable batteries.
This problem and technique applies to all UPS systems, regardless of make.
Top APC (and non-APC) UPS Symptoms:
  • Cooling Fan (if present) turns on and off constantly or does not shut off
  • Loud continuous beep from UPS system
  • Battery light comes on
  • UPS System shuts down with no LED's, no response and a continuous beep (common to the Smart UPS)
  • Internal relays "click" on and off repeatedly (common to the APC BG1500 Pro)
  • Serial communication and/or SNMP Card "freeze" and communication is lost
  • Unit must be unplugged and battery disconnected to reset
Most would attribute many of the above to a internal UPS fault or other UPS related failure. However, 95%+ of the time the above symptoms are related to battery issues. Even if you put new batteries in and continue to have the above problem, it's still the fault of the batteries and I will explain why.
Most lead acid batteries ship semi-charged, the charge level varies between them.
Most of the larger APC (and non-APC) UPS systems use higher voltages to reduce the size and weight of the transformer (thinner windings), reduce the size of DC battery cables and reduce the circuit board trace sizes (higher voltage means lower current). Common voltages include 24V, 48V and on some UPS systems even 120 VDC and above. The small UPS systems use a 12V (single) battery and rarely have the above problems, here is why:
While the larger voltage UPS's use higher voltages of 24 VDC and above, the batteries are still 12 VDC. So, to accomplish the voltage required the batteries are wired in a series configuration such as: - [-12v+]-[-12v+] + 24
We will use for example the APC Smart UPS 1400:
- Battery Voltage of 24VDC
- Unit uses (4) 12V batteries in (2) banks of 12 Volts, each of the 2 banks are in parallel, together both banks give 24 VDC.

- System by default maintains a charge/float voltage of 27.60 VDC, divide that by 2 and you get 13.80 VDC per 12VDC battery. Within range on most sealed lead acid batteries. (13.80/6 = 2.3 VDC per cell)
- As the battery pack (4 batteries) ages, some of the batteries may wear more then others which bring the serial configuration out of sync. In other words, 1 of the 4 batteries may actually have less voltage. So what happens is the voltage the unit is looking for cannot be maintained and the UPS system continues to charge even though the other batteries may be fully charged.
Now as a battery reaches full charge its current draw drops off and because the batteries are in series the other battery with a lower voltage in the serial array never can reach it's full charge. The UPS system senses the final voltage is reduced and when it cannot bring it up to the specified voltage, the unit triggers either a "Bad Battery" alarm or the unit shuts itself down to prevent damage.
The "Bad Battery" condition is triggered when:
  • [Most common] The battery pack stops drawing current before the "full" voltage threshold is reached. This will happen when a battery legitimately goes bad and it will no longer take a charge because of plate sulfation, loss of electrolyte or plate damage. But in other cases and even for new batteries this alarm can be triggered because one battery becomes full in the serial chain preventing the others from charging and the final battery pack output voltage is lower then the "full" voltage threshold. This is known as current controlled charging. Remember, as a battery charges it's current flow is reduced. When current tapers off but the voltage is still too low a "Bad Battery" alarm is triggered. Some older APC Smart UPS systems have a bug in the voltage gain parameter sometimes causing "Bad Battery" conditions to be triggered falsly because the voltage gain parameter is set too high. Luckily, there is a fairly simple fix.
  • The battery continues to draw the same amount of current and the voltage does not change within a preset time.
  • Too much current draw indicating a internal battery short or wiring short.
  • Battery temperature (for units equipped with a sensor) is too high. Often a bad battery or internal battery short.
    Sometimes the thermistor will also go bad and report incorrect temperatures.
NOTE: APC UPS systems including the APC Smart UPS, the APC BR series and APC Back UPS are very sensitive to battery packs (banks) that are un equalized. Sometimes, the system will not even report a "Bad Battery" condition but instead exhibit erratic behavior such as unit turning on and off, relays clicking, fans turning on/off, beeping, and system freezes. To fix this apply the battery bank equalization charge as described below. If using a APC Smart UPS also apply the voltage gain fix.
Many times people will install new batteries and it will work but sometimes it still triggers a "Bad Battery" alarm and they fault the UPS System. However, when you order new batteries nothing says that the voltage of all of them will be the same and so when you connect them you will still be out of range or not synchronized.
Equalization Technique:
Read this very carefully a few times to make sure you fully understand this. Failure to do so could destroy your UPS system and/or batteries.
  • Get new batteries (or try it with your current batteries)
  • Wire all of the batteries in parallel together (+ goes to +, - goes to -)
  • Use a volt meter to confirm your batteries are in parallel, if right you should read around 12 VDC (if charged)
  • If your batteries are not charged, charge them in the parallel configuration above using a regular battery charger
  • Now, we have to apply a equalization charge of 15VDC. Look for a wall transformer (or other source) that outputs 15VDC, if using just the internal battery pack look for one with at least 250mA output.
  • Connect the 15VDC source to the parallel wired bank and let it sit for 2-4 hours
  • After this is done disconnect the 15 VDC source, wire the batteries back in to the normal serial configuration, test the output is around 26-27VDC and connect it to the UPS system.
  • Your problems should now be gone!
Some battery chargers also have a equalization feature that will increase voltage to 15VDC for 2-4 hours. Only apply the 15 VDC equalization charge to fully charged batteries or this process will not work.
If using a 48VDC unit or above, the reading are different but still apply a 15VDC equalization charge with all batteries in parallel. Remember, make sure the batteries are in parallel to do the manual charge and equalization, not series!
In short, battery banks go out of "sync" and need to be equalized, even if they're brand new. Manufacturing processes often cause small voltage fluctuations between batteries and the UPS systems do not like this.
In some cases, the Smart UPS systems have trouble even after the above steps. From thorough testing and experimenting I have found that older model APC Smart UPS systems are operating at too high of a battery voltage gain causing all types of problems. The good news is that the APC Smart UPS charge controller is powered by a on-board microcontroller, and thus we will change it so that it's running at the correct setting, a big bug APC apparently fixed with newer revisions.

APC Smart UPS Battery Voltage Gain Bug and Fix:
Older model APC Smart UPS have a bug in the voltage gain, it's flat out wrong. It cooks batteries, causes premature battery failures, and all sorts of other problems including the system freezing up and shutting down AC power to the load. Even serial communications is lost in some instances requiring complete AC and DC disconnection. Almost all of these issues are in older Smart UPS systems running at around 2.3 VDC per cell voltages. Follow these instructions to reprogram the system voltage gain and solve your problems.
The following only applies to the APC SmartUPS. The below instructions are for the APC SmartUPS 1400 or any SmartUPS with a battery voltage of 24VDC. On 48VDC units, use 54.24 instead of 27.20. (or 2.26 VDC per cell)
  • Connect your serial (APC) cable to the UPS and your computer
  • Make sure the APC monitor service if installed is stopped otherwise the serial port will be in use.
  • Open hyperterminal (On Windows start->run hypertrm)
  • Create a new connection on the proper com port and use 2400 baud, 8-N-1 Xon/Xoff)
  • Press Shift+Y you will get SM
  • press 1 - wait a second - press 1 again
  • you should see PROG
  • type B
  • A value of 27.XX should be read back. On older Smart UPS systems with the mentioned bug you will see 27.60, 27.67 or in some cases even higher. Newer revisions use 27.30, but even that is not our perfect setting and can cause issues. We're going to reprogram the system's EEPROM all together for the perfect setting.
  • Press the - key (minus) then press B
  • Repeat this step above until you see 27.20 (If you're on a 48VDC unit select 54.24)
  • Confirm the setting of 27.20 by pressing B again
  • Press Shift+R to exit
  • Close hyperterminal
  • Reset your unit (Disconnect battery and AC)
  • You're done! If you didn't equalize your batteries, do it and the unit will function even better.
NOTE: Based on reported information and personal knowledge it seams as though these older units only became real problematic as they aged. I believe that the abnormally high voltage gain combined with component rating drift (due to age) also contributed to the erratic behavior mentioned. The lowered voltage gain I specified above will also help compensate for this drift.
By following the above instructions your unit will try to maintain a battery voltage of 13.56 volts (per battery) which eliminates many false alarms and will also greatly increase battery life and solve almost all of your problems. Combined with the equalization technique your system will last reliably for many years on new batteries. After the above change your unit is basically brought up to the newer revision. The voltage gain fix is the most major difference. 13.56 VDC is the lower range float voltage for most SLA (Sealed Lead Acid) batteries and also in range for flooded-cell lead acid batteries. Running each battery at 13.56 VDC (2.26 VDC per cell, or 27.20 in series) will significantly extend the life of the batteries but when using this voltage it is recommended to apply a 15VDC equalization charge every 6 months to maintain very long life. In many cases, UPS battery life's of up to 5 years!
There is actually a whole science behind float voltages and my highly tested advice does vary but in this case, 2.26 VDC per cell is best because of reduced charging temperatures, reduced battery gassing/electrolyte evaporation and better charging across a wide variety of temperatures. For extended battery life and reliability, there is a very slight decrease in battery capacity but is unnoticeable. I could get in to a whole other discussion on the physics behind lead acid batteries but please just take my advice, and you will be very satisfied with the outcome.
I also chose this voltage also because it allows us to use both SLA and regular car batteries (flooded cell) together, something the APC was not designed for but can now handle with this fix.
Although unconfirmed, it is my belief that APC was aware of this voltage gain problem and is why they were exchanging units with newer revisions. It would also explain why some APC Smart UPS use 27.67 VDC and others use 27.30 VDC voltage gains. Now that your unit is out of warranty, it's the only option to fix your unit.
The above information should fix about 95%+ of APC related issues. The equalization technique works with ALL UPS systems using multiple batteries in serial, basically any voltage 24VDC or above.
I have seen the strangest problems in UPS systems of all different manufacturers, it turns out almost always relating to the batteries being out of sync and/or the voltage gain bug. This is information you cannot find anywhere as APC and other sources just tell you to replace the battery and if it fails still replace the unit, not always our favored choice. You should change the voltage gain too even if the equalization technique solved your problems because it will also prevent future problems.

Get tremendous run time from your APC Smart UPS XL System using 2 or more automotive batteries
  • Apply the Voltage Gain Fix above (factory voltage too high for these batteries)
  • Purchase 2 12V car batteries (Walmart has them from $55/each)
  • Purchase 2 battery cables with battery post connectors (Walmart $5/each)
  • Purchase 2 battery post connectors (Walmart $1.54/each)
  • Purchase the external battery pack connector (Made by Anderson Power Products) available from powerwerx ($6). Make sure you match the SB connector to the external SB connector on the UPS. On a SU1400 it's the SB120 plug. [Buy from PowerWerx]. Also purchase a connector with the same gauge as the battery cable.
  • Put the batteries side-by-side and connect + to - using the battery post connectors and some heavy gauge wire (#8/#6 or #4), then chop off the end of the battery cable (not the battery post, the other end), put it in the SB plug, crimp (or hammer the front nearest the wire). Do this for both the +/-, note the polarity on the plug and match to the right terminal. Should look like this - To UPS [Bat A] + to - [Bat B] + to UPS
  • Make sure your voltmeter reads around 25 VDC.
  • Temporarily take out of series, put in parallel, apply the 15 VDC equalization charge for 4 hours.
  • Put back in to series, confirm you have 25 VDC, connect to UPS.
  • You now have a UPS system that has a very long run time. About 150-200Ah compared to the 32Ah internal battery pack. You could add about another 1 (or 2) of these banks, I would not push it. On the 2nd or 3rd bank make sure you parallel these banks and don't put them all in series or you will blow your UPS through the roof. It's also recommended you connect a DC breaker or a fuse for protection against shorts. On the SU1400XL the proper fuse/breaker size would be 60 Amps.
NOTE on the SB Plugs: Some older UPS systems have a slightly older revision of the SB plug, it's the same except for a plastic channel that prevents the 2 from mating properly. All you have to do is cut out the plastic center. If you encounter this, you should easily see the issue and fix it.
NOTE on wire: Make sure you use 6 gauge wire or heavier for the SU1400XL. #8 gauge is adequate but the SB connector for the external pack is only available for 6, 4 or 2 gauge wire. The SU1400 unit uses #10 gauge wire internally but it's a very short distance and the insulation is probably rated to a higher temperature. A typical way of cutting the cost of expensive copper wire.

Did this help you? (Fred)

Need Help? I will be glad to assist via e-mail. If your problem is not related to the above or is a internal board fault such as bad capacitors or damaged components I can repair the board for a small fee if you send it to me.

© 2011
Note: Fred Schrank is not associated with this blog, only the original website from which the article was retrieved.

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