Daisy-chaining 2 x UPS in serial...

[BillP]

It is said that it's a bad idea. I have read the APC documents listing the reasons why:

http://www.apc.com/site/support/us/en/faq/?page=content&country=ITB〈=en&locale=en_US&id=FA157424&redirect=true

http://www.apc.com/solutions/display.cfm?id=59BFAC22-5056-AE36-FE722E8CC1311295&ISOCountryCode=gr

But all of those reasons seem irrelevant:

1. Daisy-chained UPSs do not provide any extra surge protection.

OK, but that’s not my objective and I’m not trying to achieve extra surge protection.

2. Whenever connecting a second UPS to another UPS, the chance of overloading the first one is greatly increased.

I understand that, but I’ve measured my device loads carefully, and they total a maximum 300w. So if you add around 200w (measured) for the 2nd UPS when it is in bulk charging, that’s still around only a third of the 1st UPS capability.

3. In most cases, daisy-chaining UPSs does not allow for extra runtime. If you are using a UPS that outputs a step-approximated sine wave….

OK, but the SUA1500I doesn’t - it’s pure sine wave, so the scenario there shouldn’t be an issue.

I have 2 x SUA1500I that are currently connected to both Source inputs of a AP7721 Transfer switch. It works great, in the sense that one gets used until flat and then the other kicks in. The only issue is the second is already partly flat because of the continuous fan in battery mode despite zero load, but that's another discussion. The problem is the duration simply isn't long enough to cope with extended power cuts, so I am in the process of adding a pure sine wave 3KVA 24v inverter/charger with a 2 x 200ah battery bank.

http://powerwalker.com/index.php?lang=en&page=product&item=10120209

So the question arises how best to utilise the existing UPSes, and I am wondering:

1. Given that the inverter is pure sine-wave, I should be OK connecting the SUA1500I to it, correct? I know many people who do similar. With a modified sine-wave inverter the UPS will NOT function properly - been there, done that. But with a pure sine-wave inverter it should be OK?

2. I am not really planning to do a serial or daisy-chain topology, but given the factors mentioned above, is there any real reason why it would actually be a problem to connect 2 x SUA1500I in serial?

Just trying to understand if there is actually a real issue here, or the advice is just being cautious and providing a general rule that could be broken in specific circumstances....

[voidstar_apc]

On 4/3/2016 12:35 PM, Ben said:

1. Given that the inverter is pure sine-wave, I should be OK connecting the SUA1500I to it, correct? I know many people who do similar. With a modified sine-wave inverter the UPS will NOT function properly - been there, done that. But with a pure sine-wave inverter it should be OK?

Connecting a UPS to a pure-sine inverter should work, but in practice it depends on the UPS model and sensitivity. Due to how the inverter works, the zero crossings jitter in phase which can trip some UPSes. I saw this in my lab the other week so I just connected the piggy-backed UPS directly to line, however if I wanted to keep that configuration, I'd try lowering the sensitivity.

[BillP · Accepted solution]

Thanks for your reply. Inverter arrives in a couple weeks so will give it a go and report back.