[Nut-upsdev] Re: [nut-Patches][303751] Checking UPS Temperature

[Arjen de Korte]

> One disadvantage of handling it through a script is that is will not
> be done by default. Most users probably don't know about the problem
> of burning batteries, as it is not very common.

I forgot to add one thing to this. The fact that this not very common, is
that a UPS should be protected for exactly this kind of situation. There
are two ways to fry a (sealed) lead acid battery. Either by passing a very
high current through it (short circuiting for instance) or by overcharging
it with a modest current for a long time. The first is obvious and doesn't
happen unless the case is opened and the battery terminals are shorted.
The second may happen when the battery is charged to a voltage above 2.47
V/cell (13.65 - 13.80 V for a 6 cell battery). Until that voltage is
reached, charging current should be limited to about 1/10th of the rated
capacity. If you do that, the cells will prevent overcharging themselves,
since the current will drop to essentially zero after a couple of hours.

However, when one cell fails (which is guaranteed to happen for every
battery pack, unless it is replaced before that happens), the remaining
cells can be overcharged if the float voltage is not reduced. There is a
simple way to detect this. Within 8 - 12 hours charging with 1/10th of the
capacity of the battery, the float voltage of 13.65 - 13.80 V must be
reached. When that voltage is reached, charging current should go down to
essentially zero within another 8 - 12 hours. If either of these doesn't
happen, the battery is dead and charging must be stopped.

It surprises me that the UPS in question got a safety approval (but it
might not have it). Even if the charge circuit failed, an independent
secondary circuit should have prevented the 'meltdown' of the battery
pack.

Best regards, Arjen
-- 
Eindhoven - The Netherlands
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