Notes : APC SmartUPS Battery Float Voltage Calibration
Too high a battery float voltage is a problem I've seen on many of my APC SmartUPS units, and one that appears to be rather common. Sealed lead acid batteries do not tolerate too high a standby voltage, and suffer from dramatically shortened lifespan when run at elevated voltages and temperatures. In the worst case, overcharging results in elevated temperatures and rupture of the battery case. Outside of warranty, APC is of no assistance, suggesting only that I purchase a new unit. (Helpfully, they offer a discount with the trade-in.) Nonetheless, I'd like to not think of $7-800 purchases as being for disposable items.
I've long sought a manner to adjust or recalibrate the battery voltage on my SmartUPS SUA1500 RM2. It would also be nice to be able to adjust the battery charging voltage to use other types of batteries; e.g., flooded lead-acid marine or car batteries. This post describes, using information from a variety of sources, an approach to recalibrate the battery voltage and other measurements on SmartUPS devices by using the PROG mode available by serial connection. It also suggests a method for modifying the UPS circuit board when PROG doesn't work, as with my SU700 units.
Excessive charging seems to be a matter of drifting calibration: Within the period of 2 to 3 battery changes, the battery float voltage rises beyond the battery specifications. Most sealed lead-acid batteries specify a float voltage of 2.25-2.30V, or 13.5-13.8V per battery.
It is my understanding that older BackUPS units could be adjusted with a potentiometer on the board. Calibration of newer models, and the SmartUPS line is factory set in the unit's micro-controller. APC is of no assistance with units that are out of warranty. I've recently found information of how to recalibrate the battery float voltage through reprogramming the battery gain, and some information about modifying the UPS hardware, described below. I can't take credit for discovering methods demonstrated, but hope it will be useful to another to have a coherent description.
Calibrating by serial cable
An excellent description for the APC SmartUPS protocol is available from the Network UPS Tools Protocol Reference. This describes how one may communicate with the UPS in standard use.
A post to the NUT users mailing list by an Alexander Talos describes setting several additional calibration variables by using PROG mode: Hidden Programming Mode. In PROG mode, notable adjustments include battery voltage gain, load gain, line voltage gain, and on-battery voltage output.
PROG mode may be used by connecting to the UPS with a serial console (2400bps, 8N1, no flow control), and sending "1", twice, at least a second apart . The UPS will respond "PROG" once you've succeeded. You can leave PROG mode by sending "R" (response: "BYE"), which will put the UPS in dumb mode, followed by "Y" (response: "SM") to re-enter smart mode.
Once in PROG mode, sending the character corresponding to a setting will lead to the UPS printing the current value of that measurement (e.g., 27.67 for battery voltage). Settings are programmed by the next values received. For multi-character values, the appropriate number of characters will be read. For gain parameters, the gain of this value may then be adjusted by sending one or more "-" or "+" characters to decrement or increment the gain. At each press of "-" or "+", the UPS will reply with the current gain value.
"B" may be used to calibrate the battery voltage gain. On my unit, the stock setting of "E9" resulted in about 27.35V at the batteries. Adjusting this to "E6" lowered the voltage to my desired 27.30 (2.275V/cell). Note that my goal was to get the voltage correct at the batteries, not necessarily have the UPS report the same voltage as my voltmeter. After adjustment, the UPS still reports a voltage of 27.67. Perhaps some other calibration measure may be used to change this inconsistency, but I don't know about it.
I am unsure of how universal this procedure is across the APC SmartUPS series. Apparently, battery gain is adjustable on the SU2200, and I have tested on my SUA1500. However, with my SU700, adjusting "B" doesn't seem to change the voltage I measure at the battery.
Here's a list of other settings:
- B: Battery voltage gain.
0=maximum gain, FF=minimum gain. Adjust with the batteries fully charged, and an accurate voltmeter connected. Factory setting was "E9" on my SUA1500, and "E6" gives me the desired 27.30V battery voltage. "FA" reported for SU2200 in above post. - L: Line voltmeter gain
Apparently a setting of 0 will lead the UPS to not detect any input power. Factory setting is "DD" on my SUA1500. "EA" reported for SU2200 in above post. - P: Power load gain
Factory setting is "D6" on my SUA1500. - o: On battery output voltage
Factory setting is "120" on my SUA1500. It doesn't appear adjustable on this unit: "+" or "-" responds "NO".
Some other values are described in Alexander's post, above. I do not know of additional documentation for PROG mode, and am afraid to try every combination for fear of bricking the unit.
Modifying the circuit board
Another avenue for calibration is to modify the circuit board. Before finding the above PROG mode documentation, I was headed this route. It is likely that the calibration drift at fault is caused by drifting component values, so why not change them back? According to a DanielD67 at diyforums.net, "...how to adjust the voltage on Smart-UPS 1000: the charging control circuit is IC14 APC2020 and on pin 13 is a resistive divisor of +24VFET formed by R118-100k and R119-22K1. Reducing R118 by adding another 5M6 you can reduce the charging voltage at 27.4V." (Battery Float Voltage Adjustment?)
As adjusting "B" in PROG mode did not work with my SU700 unit, I'll see what I can do to adjust its voltage, and report back here.
Last Modified: 17 Nov 2009