UPS : longer runtime &12VDC tap

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hello group; a post with 2 linked questions about UPS:

i have a mobile application requiring UPS online supply, for a "small"
load (approx 250W) that needs to run for 1 hour. more than half of that
power is drawn by 12VDC equipment.

question A: can i extend a UPS's runtime by simply adding similar
batteries? in general, what do the chargers know about voltage and
capacity of the batteries? if there are more batteries, same voltage,
will it only take more time to charge? (and then, to discharge...)

question B: is it OK to tap the 12VDC of the batteries directly before
the inverter or will something (charger or inverter) be thrown off by
such a "leak"? by doing this i hope to drop the 120VAC - 12VDC
transformer that adds significant weight to the setup (which needs to
be as light a possible).

(safety note: this setup will never run unattended; it powers equipment
that is constantly monitored)


Re: UPS : longer runtime &12VDC tap

krousson wrote:

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Look here for a thorough discussion of this:

Using an External Battery for a UPS -- Belkin 1100VA

Re: UPS : longer runtime &12VDC tap

thanks for the pointer, roby.

the points about emissions and heat are well taken, but probably not an
issue (i plan to use gel cells, and the system will undergo testing and
monitoring for overheating issues). between the lines i understand
there should be no problem in adding cells in parallel to augment the
AHours as long as the inverter and charger are able to sustain the
constant usage.

however it's still not clear for the direct DC connection. my guess is
that it should not be a problem (the power that will go on DC would
have anyway gone through the inverter and back to DC again) but i would
like to know if others have done this (or even if such a product (UPS
with 12V supply "side port") exists).


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Re: UPS : longer runtime &12VDC tap

krousson wrote:
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Please don't top-post.  See the links below.

While I appreciate that English is not your native language (and
you handle it much better than I can handle yours), things are much
more readable if you capitalize the first letter in a sentence, and
the personal pronoun 'I'.

 Some informative links:

Re: UPS : longer runtime &12VDC tap

krousson wrote:
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This device does AC-DC-AC conversion.

APS750 750W 120VAC from 12VDC battery.

(User manual)

A sample article where a guy sets his up. This is a slightly
different model than the one above.

Battery for Powerverter $139 12V 75AH Maintenance free
Read the documentation before trying to find a substitute.
A car battery may not live long if used for this application.

12V*75AH = 900Watt-Hours. Divide by 1.2 for loss thru conversion.
900/1.2 = 750Watt-Hours. Your 250W load could run for 3 hours from
the battery. With the Powerverter, you can recharge later, when
you return from the field.

As for the DC-DC part of your problem, this device should work.
The top device here, converts 10-19VDC to 12VDC. The output
is isolated, which means no DC path between input and output.
That is important, as you have multiple paths in your system,
and isolation can prevent accidents. The isolation feature doesn't
have a very large range, and is not intended for "lifting" the
output, but rather for avoiding grounding problems and the like.
The only feature missing in the description, is cutoff to prevent
this DC-DC inverter, from discharging the battery too far. I don't
see mention of that explicitly in the information provided. When the
alarm goes off on the Powerverter, then would be a good
time to switch off the DDH150 as well.

You can avoid the question entirely, about interaction between the
DDH150 and the output stage (inverter) of the APC750. When returned
from the field, set the APC750 to "charge only". In this mode, leave
the DDH150 switched off. When in the field, disconnect the AC input
to the APC750. Set the ACP750 to "Auto/Remote" and it will start
inverting and producing 120VAC. Turn on the DDH150 to convert
battery power to regulated 12VDC. By doing that, you never have
to answer the question.

If you don't like that answer, use an AC to DC device, plugged into
the APC750, and then everything is automated for you. There is the
additional conversion loss from going AC to DC, but with a 3 hour
run time on the battery, it might not matter.

The APC box charges the battery with up to 20 amps while
running from line voltage. The description of "3 stage battery
charger", in my interpretation, means the charger has a
bulk mode (first part of the charging process), an
equalizer mode (high voltage charging for recovering/reconditioning
a weak cell), and a float mode (constant voltage equal to battery
voltage applied, to keep battery in fully charged state - not
trickle, as I think that is slightly different).

Tripplite has pre-sales support, and probably
also has an engineering department. Perhaps
sending them a drawing of the application, would
allow them to comment.

Frequently asked questions.

Pre sale inquiries.

Some battery terminology in the following links:

In terms of battery technologies, they have different
operating temperature ranges. While there are battery
technologies with denser storage capacity, they may not
be usable over a wide temperature range, or may not be
able to take shock/vibration. At least lead acid have
known characteristics, and you can depend on them.


Re: UPS : longer runtime &12VDC tap

I doubled the batteries on my APC SmartUPS 750 in expectation of
doubling the run time.  If I didn't use APC's software, the runtime
more than doubled, as expected.  However, when I used APC's Powerchute
software, there was no increase in run time.  Despite doing the
hardware-firmware run time recalibration with doubled batteries, the
software had a mind of its own about how long the UPS could run with
the load I had on it.  Different makes/models of UPS probably react
differently.  If the load is under constant supervision (and perhaps is
not a computer), the UPS software is a non-issue.

Recharging 4 batteries took longer than 2, (but if I remember correctly,
not twice as long.)

krousson wrote:
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