Smart UPS 750 (LCD Display) almost blew up

Anonymous user · ‎2021-06-30

Hi there,

I have two Smart UPS 750 in use (the newer LCD version) + a UPS 5k RM also with a display.

Last week one of the Smart UPS 750 devices started smelling burned and when I located where the smell came from, I noticed that the case had at least about 70 or 80 degree (far beyond being able to touch it, so measured it with an IR thermometer). I disconnected everything, including the battery connector and the batteries + the unit took several hours too cool down. Also, the fuse in the house which served the UPS blew.

The batteries were bulged and the battery cases started to crack.

The red battery light turned on one or two days ago. I know that the batteries CAN expand but I've only experienced that when ignoring the red battery light for an extended period of time.

The UPS and battery is quite some years old but I never worried about the battery because it worked just fine, had its specified battery holding time and the battery replacement light wasn't on until a few days ago.

So my question: although the device isn't most certainly under warranty anymore: Is this supposed to happen? I mean if I weren't there, the UPS might have burned the house down if the fuse in the house didn't blew. Shouldn't a UPS protect against power outages, not producing them?

Can I just put new batteries in (the defective ones were genuine APC batteries!) or do I have to replace to unit? Is there some kind of compensation from APC? I mean, batteries shouldn't just heat up like this and burn your house down, isn't that clearly a quality issue? If the light would have come on earlier, I would have replaced the batteries in time and I guess none of those things would have happened.

Thanks.

BR

BillP · ‎2021-06-30

Batteries eventually will fail no matter who manufactured them (CSB batteries are high quality by the way, I have several 12V 17Ah GP series batteries which are over 18(!) years old and still retain around 40% capacity). How and when they fail is very much dependent on the operating temperature, charging voltage and number / frequency / depth of discharge cycles. These parameters are essential and usually specified by the manufacturer. Batteries in smaller APC units like the the 750VA version have to endure elevated temperatures due to the small space shared with heat generating components like the MOSFETs and the transformer. Most SLA batteries are rated for operating temperatures between 20-25 degrees celsius, each degree above that temperature shortens useful battery life by a certain amount. Therefore it is very important to operate the UPS in a cool environment, because the internal heat generating components are going to heat up the batteries well above the room's ambient temperature anyway. Though I have never owned an SMT750 LCD ups, I do know that the previous generation (SUA750) ups had temperature compensated battery charger. The problem is that all SUA750 units I have seen, have their float voltage set too high for such a small UPS size (27.5 volts), so even with the help of temperature compensated charger it cannot reach the optimal life span of the batteries. Eg. when room temperature is 20 degrees celsius, batteries are sitting at 30-32 degrees celsius inside the UPS case. For such temperatures, float charge should be reduced to 26.8-27.0 volts at maximum for optimal battery life, which is not done in these UPSes. 27.5 volts would be perfectly fine if the batteries were sitting at 20-25 celsius.

If you are not someone who is really technically knowledgeable about these devices, the best you can do is to observe the normal operating temperature of the ups, and check it regularly. If it starts to elevate for seemingly no reason while the ambient temperature stays the same, it is very likely that the batteries are going bad and the UPS has not yet had the chance to detect the problem by itself. If you don't want to bother with this, you can just replace the batteries every 2-3 years, preventing the problem altogether.

See Answer In Context

BillP · ‎2021-06-30

Hi Lu -

Batteries can expand when they experience a thermal runaway event. I certainly understand why you would be alarmed by this experience. This can be caused by operating in an overly warm environment, not replacing bad batteries promptly, or using very old batteries. Typical battery life is 3-5 years is a good environment ( 20-25°C). Swollen or venting batteries can be very concerning, but are not a fire hazard. In very extreme situations you could find some small leakage of electrolyte. You can find the battery safety datasheets here: http://www.apc.com/us/en/who-we-are/sustainability/battery-safety/battery-compliance.jsp

In reading your post, I'm having a hard time understanding the exact sequence of events you experienced. Was the replace battery light already on when you found that the batteries had swollen? If so, how long had it been illuminated for? Are you saying that the UPS tripped the input breaker in your house? if so, at what point did that happen? You mentioned burning - was there any visible flame or smoke?

Given that you have safety concerns, it might be most effective if you contact your local support team directly for assistance rather than the forum. They will be able to answer your questions more quickly and effectively. It will be helpful to have the exact model and serial number of your UPS handy, as well as whether or not the battery was the original battery or a replacement. You can find your local contact information here: http://www.apc.com/us/en/support/contact-us/index.jsp

Liam

Terry_Kennedy_apc · ‎2021-06-30

I'm just another customer, so treat my advice accordingly.

A number of things combine to make this sort of thing more common than we'd like:

The manufacturers of the batteries (like CSB, etc. - not APC) operate on pretty thin margins and have a lot of competition. So their products are engineered down to a price point - it is hard to find a "premium grade" battery.
APC UPS charging voltages tend to drift out-of-spec over long periods of time. This almost always (due to circuit design reasons too complex to go into here) is a drift to a higher voltage. That shortens the life of the batteries. My experience is that you'll get 3 years out of the original new UPS + batteries, 2 years on the first replacement batteries, and only 1 year on the second replacement batteries, unless you adjust the voltage (a technical task, will void your warranty, etc.).
On all but the lowest-capacity models, there are strings of batteries in series. This might be as few as 2, or as many as 16 batteries. All the UPS sees is the voltage across the whole string of batteries (some more advanced models like the Symmetra RM have a sense line mid-string to try to get a better idea of what is going on in the battery pack, but it is still mostly guesswork). If one of those batteries fails in a high-resistance mode, it will become a point heat source which will damage nearby batteries. If a battery fails in a low-resistance mode, the remaining batteries in the string will see a higher than expected voltage and they will all begin to overheat, which will damage them as well as any nearby batteries.
Many APC UPS models have a series-parallel design, where more than one series string (see above item) are connected in parallel. This includes any of the expandable units (Matrix, Symmetra, any UPS that accepts an extended runtime pack) as well as some fixed-configuration models where all of the batteries are internal to the main chassis. Even if the UPS detected a problem early on (hard to do, see the above item) and completely cut power to the batteries, the remaining parallel battery strings are still connected to the bad string and will attempt to discharge into the bad string in an attempt to equalize voltage. In larger systems, we're talking about well over 100 Amps trying to go through what may well be a small 7AH 12V battery. Most of the modular APC units only have a single battery bus. Some models have battery packs with individual status reporting (Matrix, Symmetra) so the UPS can report which battery pack is bad. But with only a single battery bus, the only option the UPS has is to increase the battery bus voltage in order to charge a pack that is reporting low (even if it is fully charged). That subjects the other packs to a slight additional stress which, over time, can cause damage to the other packs. By the way, this is why I always recommend replacing ALL batteries at one time rather than only some packs as they fail.

I have quite a few APC units here, including a Matrix 5000 that is over 20 years old and still working. I change the batteries pro-actively every 3 years and verify the float voltage at that time.

BillP · ‎2021-06-30

Batteries eventually will fail no matter who manufactured them (CSB batteries are high quality by the way, I have several 12V 17Ah GP series batteries which are over 18(!) years old and still retain around 40% capacity). How and when they fail is very much dependent on the operating temperature, charging voltage and number / frequency / depth of discharge cycles. These parameters are essential and usually specified by the manufacturer. Batteries in smaller APC units like the the 750VA version have to endure elevated temperatures due to the small space shared with heat generating components like the MOSFETs and the transformer. Most SLA batteries are rated for operating temperatures between 20-25 degrees celsius, each degree above that temperature shortens useful battery life by a certain amount. Therefore it is very important to operate the UPS in a cool environment, because the internal heat generating components are going to heat up the batteries well above the room's ambient temperature anyway. Though I have never owned an SMT750 LCD ups, I do know that the previous generation (SUA750) ups had temperature compensated battery charger. The problem is that all SUA750 units I have seen, have their float voltage set too high for such a small UPS size (27.5 volts), so even with the help of temperature compensated charger it cannot reach the optimal life span of the batteries. Eg. when room temperature is 20 degrees celsius, batteries are sitting at 30-32 degrees celsius inside the UPS case. For such temperatures, float charge should be reduced to 26.8-27.0 volts at maximum for optimal battery life, which is not done in these UPSes. 27.5 volts would be perfectly fine if the batteries were sitting at 20-25 celsius.

If you are not someone who is really technically knowledgeable about these devices, the best you can do is to observe the normal operating temperature of the ups, and check it regularly. If it starts to elevate for seemingly no reason while the ambient temperature stays the same, it is very likely that the batteries are going bad and the UPS has not yet had the chance to detect the problem by itself. If you don't want to bother with this, you can just replace the batteries every 2-3 years, preventing the problem altogether.