@petemate Thank you for the interest. The name Faston is very old, with newer names being used such as "quick connect" or the stolen name "spade lug" and similar names. Faston is a brand name if I remember right, just like a tissue in the US is called "Kleenex", and to vacuum in the UK is to "Hoover".
Lead acid batteries have similar failure modes like other battery types including build up of dendrites internally. Over time, the many discharge and charge cycles will migrate the plate lead out to the plate surface and build the dendrites toward the opposite plate, until it breaches the separator which causes a fast discharge path. If you try to continue to charge with the low resistance dendrites, the bad cell will get very hot, as well as the other cells that are now getting over-charged (the bad cell usually gets a much smaller voltage on it with dendrites). There is usually lots of hydrogen created at this point too, so the whatever valve is being used will vent the gas - and makes things dangerous as well. The small gel cell batteries are also called VRLA for Value Regulated Lead Acid - the safety vent allowed the batteries to be used in many more applications.
If the UPS design is a good design, the charging current won't be allowed to get so high that it causes a thermal event - but caveat emptor: I have seen some that definitely provided too much current. A good mature design will even detect the battery voltage is too low or is even decreasing, and shut down the charging and cause an alarm. The designs that we had would do a monthly battery load test by dropping the load on the inverter on purpose just briefly and measure how low the battery voltage dips, paying attention to the level of the load.
Having said that, I have had a substantial part of my consulting business looking over UPS fires. The industry has convinced some of the safety agencies (UL, CE, etc.) to only require a breaker or fuse on the positive terminal, with the idea that the only other connection is the negative terminal which goes to ground. But batteries leak, and many UPS systems have a metal grounded chassis. So if you have an electrolyte leak mid string (say around the 6V cell midpoint), you will have a 6V short circuit around the bottom cells to the grounded negative terminal. As with other slow cooking failures, this battery baking is a slothful thing to do and is quite preventable. The larger UPS systems are required in nearly all countries to have a breaker path on both battery terminals, especially as you get into the 50KW to 1MW+ size systems.
Another thing that makes the smaller lead acid based UPS less dangerous is that the lead dendrites have a rather high resistance, due to the fact that it is lead. The idea is to keep the battery temperature below ~ 70 deg C or so (which is very hot to the touch). If the battery is a UL rated battery, then the plastic enclosure is coordinated to have a melting temperature above 105 deg C. and purposely has a very low flammability rating (94V0). Lithium metal, on the other hand has fairly low resistance comparatively and can get into trouble much easier. For safe lithium battery systems, the BMS implementation and/or built-in thermal trip is very important. This is where solar systems can get into trouble as well.
About 20 years ago, there were many in the industry who had good ideas on making true smart battery systems, but the marketing guys were rightfully not interested. This is because most end users are not interested in safety (or security, on IoT smart devices). People will go online an buy a cellphone charger without paying any attention that is does not have a UL/ETL/CE mark. They are just happy that they found one for 1/3 the price of the regular ones.
Home
Help
Search
Login
Register
