Can you daisy chain UPSs?

[Rajahal]

Just a hypothetical, I don't have any need for this.  I was just wondering if it is possible to daisy chain UPSs to gain longer battery life.  Say you have 5 identical UPSs, each of which has a 10 minute battery.  If you plugged them all into each other in a line, would they power your server for 50 minutes?

 

If this would work, how would it manifest?  Would each UPS battery drain in parallel at 1/5th the normal rate, or would each battery drain serially?

[Joe L.]

Just a hypothetical, I don't have any need for this.  I was just wondering if it is possible to daisy chain UPSs to gain longer battery life.  Say you have 5 identical UPSs, each of which has a 10 minute battery.  If you plugged them all into each other in a line, would they power your server for 50 minutes?

 

If this would work, how would it manifest?  Would each UPS battery drain in parallel at 1/5th the normal rate, or would each battery drain serially?

serially

[Rajahal]

So it would work?  Interesting.

[kizer]

APC says.

http://emea-en.apc.com/app/answers/detail/a_id/397

 

 

We do not recommend this configuration for the following reasons:

 

 

1) It is not UL tested. Therefore, should something go wrong and damages be caused to your connected equipment, APC would not honor the claim. (Underwriters Laboratories Inc. [uL] is an independent, not-for-profit product safety testing and certification organization)

 

2)This configuration does not provide any extra surge protection. The UPS is designed to remove any possibility of a surge reaching the output receptacles. If a surge, strong enough to damage equipment was received, the first UPS in the chain would sacrifice itself to protect its load. This would mean that power would be removed from the 2nd UPS in the chain and force it to battery.

 

3)Whenever connecting a 2nd UPS into a 1st UPS, the chance of Overloading the 1st UPS is greatly increased. The amount of receptacles in a UPS are restricted. This is due to the power limitations of the UPS itself. Although, the amount of receptacles has been increased, the overall Watt capacity of the 1st UPS remains the same. The capability of the 2nd UPS, will be inversely affected by that of the 1st UPS. Therefore, the overall Watt capacity of the configuration is no greater than that of the 1st UPS in line.

 

4) In most cases, daisy-chaining UPSs does not allow for extra run time. If you are using a UPS that outputs a step-approximated sine wave when on battery, as soon as the fist UPS goes on battery, the second UPS will also go on battery because it will see the step-approximated sine wave as distorted or bad power. Both units will discharge together, and will not provide any extra run-time to the load.

[Rajahal]

Ah, I figured there was some reason it wouldn't work.  Thanks kizer.

[Rajahal]

^ is this on topic spam?  Weird.

[bubbaQ]

You will also run the risk of overheating the second UPS if it is fed anything other than a sine wave from the first, it will cause a lot of efficiency loss as it tries to clean up that input, or feed it through a transformer, and that loss creates excess heat.

 

Modern computer power supplies OTOH can accept square-wave input with no problems.  There can be a loss in efficiency, but not nearly the magnitude as the UPS.