Worrisome battery heat failure on Back-UPS RS 1500G

[cbarn]

Hi all - not looking for warranty support, just looking for information and a bit of reassurance.  I have two Back-UPS RS 1500G units, one in my home office, the other in our family room.  These are obviously older units (2017), and the batteries have been replaced with genuine APC.  The office unit was definitely overdue, last replacement was 2020.  It also has the expansion unit for additional capacity.

Yesterday morning when I came into the office the UPS fan was on, and a check of APCUPSD showed it had successfully run a self test, starting at 100% battery and at 29% load.  Oddly the battery was down to nearly 80%, which is far below what I'd expect for a self test, but as it reported OK I didn't worry about it.  It was back up to 100% in reasonable time.  But the fan never shut off - it continue to run into the evening.  At about 10pm I ran another self test manually, which it immediately failed with a replace battery warning.  I shut the unit down, disconnected the expansion unit, and pulled the battery.  It was VERY hot - not so hot I couldn't handle it, but not comfortable.  The expansion unit wasn't noticeably warm to the touch.  Two hours later the battery was still warm, with a hot spot at 104F, and now 12 hours later it's still 90F, 20 degrees above ambient.  FLIR photos attached, you can see two distinct hot spots.

Obviously the battery is toast, and replacements are on the way (meanwhile have swapped in the newer battery from the family room UPS, office+network slightly more important than the entertainment gear, although kids might disagree!) - but I'm very curious as to what may have happened here aside from simple age (6 year old battery).  And, what would've happened had I not noticed the failure immediately?  I'm assuming that continuing to run would've continued heating the battery - what protection circuits are present in the RS 1500G, would it have eventually noticed and shut down power delivery+charging, or would it have caught fire?

Science and tech nut looking for science and tech elucidation ... thanks!

[Teken]

The typical failure mode seen by end users varies depending upon many environmental factors.

Regardless, generally speaking if the RBC was left in place as is. The RBC would have continued to be charged by the system.

In turn the RBC would continue to heat up, expand, and one of several outcomes would be seen upon discovery / maintenance / operations.

Self Test: The self test as designed and performed by APC is based on 1980’s technology and methodology. As such this very crude test will never tell you the true state of the RBC until it reaches a terminal state as you saw. 🤦‍♂️

Meaning you can see self test passes when in reality the RBC is toast.

Other scenarios are the RBC will continue to heat up, deform, and depending upon what type of chassis design (Plastic vs Metal) frame. The case will be deformed and the only method to remove the RBC is to take the UPS Housing apart to remove the individual cell / RBC Cartridge.

If the RBC has deformed greatly it’s near impossible to remove it without damaging the housing. A metal case can be massaged back into shape vs a plastic case will simply break apart when removing the deformed RBC / Cell.

In other cases the RBC will begin to vent as the internal heat generated exceeds the design limits of the battery. Should this condition persist and the environment lacks proper ventilation an explosive scenario would exist if an ignition source was present.

The above scenario is extremely rare as most consumer hardware have / use very small RBC cartridges.

The most common outcome is you will hear a hissing noise accompanied by a sharp odour which of course is toxic if ingested in large quantities or long term exposure.

The system only incorporates a limited amount of hardware protection and none of them will engage until the system sees a terminal condition.

Internal Fault: Any electrical fault seen by the system will cause the unit to disable itself. The most common faults seen are relay weld, over load, output short, etc.

When these conditions exist the system will appear be on but cannot support the connected loads.

RBC: The most common problem seen by users are the system begins acting strange. That can be random beeping, dropping load, to turning off randomly. As noted, a reasonable person would initiate a manual self test in the belief if it passes the problem lies elsewhere.

When in fact the root cause (IS) the RBC! 

The system has no ability to detect any individual cell problems vs a lithium based product where a BMS is incorporated to obtain feedback from the cells.

Depending upon the BMS it can monitor each cell for individual voltage, temperature, and balance them.

All lead acid APC UPS no such monitoring is present. The RBC compartment is monitored by a single thermistor taped to the side of the battery compartment.

As such it can never tell if a single cell is heating up due to high internal resistance which perpetuates an endless over charging condition when present.

In an enterprise environment where the site is highly regulated (ISO Standards) basic to advanced tests and inspections are performed daily, weekly, monthly, yearly.

These visual and hands on technical validation tests can range from simple things such as these:

RBC: Visually inspect all RBC installations date and physical condition for smell, deformation, heat, acid leaks, corrosion on terminals, wiring, fuses.

FAN: If a fan is present is it operating as expected in its normal operations. All Online (Double Conversion) units the fan is always on and running. The vast majority of newer systems incorporate a dual stage fan and only engage the high output during specific conditions outlined later below.

The vast majority of Line Interactive / Standby models the fan only comes on during these conditions:

Charging, Battery Only, >75 Load, High Temperature, in some instances during extended AVR periods.

Much more to discuss and share if interested.

Questions Ask . . . 👍

[JamesOConnor · Accepted solution]

While VRLA batteries rarely burst into open flames without an external spark, leaving a swelling, overheating battery powered on in a residence carries serious risks of toxic gas release, chemical leakage, and board-level heat damage.

What Happened

The unit experienced thermal runaway:

1. Internal Short Circuit: At six years old, internal cell separators break down and water dries out, dropping overall pack voltage.

2. The Charging Trap: The UPS interprets low voltage as a drained battery and continuously pumps current into it to reach float voltage.

3. The Heat Loop: Depleted, shorted cells convert charging energy directly into heat. As lead-acid cells heat up, their internal resistance drops, causing them to draw more charging current, escalating the cycle until distinct hot spots form.

UPS Safeguards & Fire Risk

The RS 1500G has limited protections:

• Thermal Fan: Triggers when air inside the chassis heats up, explaining the continuous fan operation.

• Charger Cutoff: Firmware eventually trips a charger fault if voltage isn't reached after extended charging, but it measures internal ambient board heat, not direct battery casing temperature.

What Happens If Left Unattended

• Plastic Fusion & Swelling: The battery swells and melts, physically wedging itself inside the UPS body and destroying the chassis.

• Gas Venting: Safety valves pop open, releasing flammable hydrogen gas and highly toxic hydrogen sulfide (a strong rotten-egg odor).

• Power Termination: The thermal loop typically ends when an internal lead connector melts open or board fuses trip, cutting off the charger before open flames occur.

Immediate Action Required

1. Isolate the Overheating Battery: Keep the warm pack on a non-combustible surface (like concrete or tile) until it reaches ambient temperature.

2. Do Not Reconnect: Do not attempt to charge or reuse the hot pack.

3. Set a Hard Replacement Schedule: VRLA batteries must be replaced every 3 to 4 years. Internal resistance rises steadily with age, and built-in self-tests only check short-term voltage drop under load, failing to predict thermal runaway.

[cbarn]

Thanks James and Teken - I've replaced the batteries in the UPS itself and in the expansion unit, and I'm putting them on a 3-year replacement schedule unless self-test or endless fan indicates a need to do it sooner.  The expansion unit is a voyage of discovery on its own - no fan, really no circuitry to speak of at all (I think there may be a fuse where the negatives all come together but wouldn't bet on it), just four batteries tied together to create a pair of 24V outputs with a common negative.

[Teken]

The expansion pack doesn’t incorporate any smart detection as one would expect. First generation units simply incorporate a fuse on the positive terminal.

Last generation of these units simply incorporate a protection fuse and a resistor to identify an extended battery pack is installed.

Even in 2026 SE / APC continues the trend in designing, producing, and selling 1980 technology at ridiculous prices which offer no meaningful or useful information / metrics! 🤦‍♂️👎

Questions Ask . . . 👍