[Tfug] FTGH: APC Back-UPS Pro 650

[Jeremy D Rogers]

> > On 4/20/07, Sean Warburton <> wrote:
> >> I'm sorry! I didn't know a series/parallel discussion would ensue...Here
> >> was
> >> how I found it: the positive went somewhere into the unit, and the
> >> negative
> >> went directly to the positive in the second battery. the negative from
> >> the
> >
> > Umm.. so they were in series, not parallel. So if you added more in
> > series, you would increase the voltage. That doesn't sound like a good
> > idea.
>
> Actually, it is a good idea. UPSes are rated by power rating (VA, or KVA) A
> formula for power is P=IV, where I is current and V is voltage. Another
> formula for power is P=I^2 R
>
> As someone else in the thread pointed out, if increase teh voltage, you
> lower the current ( I ). Then, I^2R (power) losses are reduced.  As a
> result, power loss due to heat (resistance ( R ) in the wiring, connections
> and circuitry) will be less for higher voltage in the battery bank.

Yes, I understand that if the electronics are designed from scratch,
higher voltage is more efficient and there is great benefit...

> Low end UPSes typically use 24V battery storage. However, when you look at
> the 2KVA-3KVA range, they use 48V battery packs to reduce lossses. In higher
> capacity UPSes, they design the battery voltage to be even higher.
>
> Another way to look at it is: If your UPS uses a higher battery voltage, the
> transformer used to boost the voltage to 115VAC will be smaller and more
> efficient. The transformer (or inverter really), will not have to boost the
> voltage as much, and pay for that difference with a high current.

Right, but I'm not talking about the design from scratch.. Sean was
asking if he could just double the number of batteries connected in
series to his existing UPS. What I meant is that I wouldn't expect the
components on the input/battery side that are used in these devices to
happily work with a sudden doubling or tripling of the input voltage.
I agree completely about the efficiency and so forth, but do you
really expect the electronics that are designed to be as cheap as
possible to maximize profit margins, to be able to handle such a large
swing in input voltage?

If so, nifty! I don't claim to know much about the inner workings and
design of these things, I just find it surprising to expect that the
device would have no problem handling twice the input voltage.
JDR