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The place where I work has a fairly large UPS comprising of rows of sealed lead acid batteries (Haze HZS-7.5HR). The batteries are now 6 years old and are not holding charge very well.

The service technician report states, among other things that ".. the batteries may go into thermal runaway and therefore become a danger to the site."

Is this statement fair, and if so, why would older batteries - which I assume have less energy then new batteries be more likely to suffer from thermal runaway?

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I am afraid your service tech is right, older sealed lead acid batteries are more prone to develop thermal runaway. This article summarizes this nicely.

Older batteries do have higher internal impedance, and therefore tend to dissipate more internal heat under the same charge-discharge conditions as new ones. However, if the UPS is well designed and has proper system of sensors, the runaway should never occur, and near-faulty cells should be marked for replacement long before the run-away can occur.

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When they don't hold a charge very well, it means the risk of one cell becoming shorted increases which lowers the string voltage and turns the cel from a capacitor into a resistor and this can make one cell runway with self heating with sufficient current from I^2R for R_shunt losses rather than ESR series losses.

Of course you can reduce the charging current with a large series R, ask if the units have temp compensated Charging voltage and se if there is a "de-sulphation pulser add-on device" that draws current every 20 to 50us and dumps it back into the battery in <50ns to breakdown lead sulphate crystals. That works in some cases but dendrite lead growth from undercharged or overcharged battery cells can cause whisker shorts.

Good batteries come in clear plastic poly-carbonate cases so you can see what's going on.

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