Mark Rogers wrote:
On 07/01/10 11:10, Steve Fosdick wrote:
In a modern unit I would expect both the converters to be switch mode designs and, from what I remember, one of the difficulties of switch mode PSU design is that one generally wants the control electronics powered from the low voltage DC which isn't available until the unit has started up thus necessitating some extra components for a "startup mode". My guess is that in the case of those cheap UPSs they miss out the extra components and use the battery to power the control electronics until the input SMPS has got going in which case if the battery is dead it won't start.
Interesting...
The implication of this is that if I charge the battery outside the unit then I can recover the unit.
Alternatively, if I can provide the necessary DC supply to the control electronics then the UPS may recover itself.
Any hints as to whether this can be done and/or how? I appreciate, by the way, that the whole point of a UPS is that there are high voltages kicking around, so this is a completely theoretical interest and I won't blame anyone if I subsequently kill myself...
Yes you can sometimes "jump start" a dead ups by providing the battery voltage at the battery terminals. generally it is 24 volt as the most common configuration is 2 12v batteries in series. But smaller or larger units may vary here. Also some will trickle charge the cells when plugged in but not switched on so if you leave them plugged in for a few hours they may then start. Be prepared however that if you do it with the batteries in circuit then they will draw a reasonable current from your "jump" supply.
That said, at the point the cells have dropped below the nominal capacity you can pretty much assume it is because you have a dead cell or something so the battery is probably shot.
Usual disclaimers apply, Yes there is mains voltage present and above that the batteries can vent explosive gases that you can then ignite by arcing terminals etc....know what you are doing before attempting this.