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SmokChsr
Who let the magic smoke out?
Premium Member
join:2006-03-17
Saint Augustine, FL

SmokChsr to CajunWon

Premium Member

to CajunWon

Re: Home UPS powered by car battery

I didn't see a specific response to this particular point, but your #14 wire is too SMALL.. You'll need at a minimum #10, if you have extended the leads from the UPS at all. The Voltage drop on the wire along with the dropping voltage of the battery could very easily account for the early shut off. Also make sure you have good solid connections at every point in the path.

Keep in mind that when you are trying to draw 0.7 amps @120V for your laptop that equates to at least 7.0 Amps (if their were no losses {which there are}) at 12V from the battery. In the real world you are probably drawing between 11-15 times the amount of current at 12V than your load at 120V.

CajunWon
join:2005-12-30
Cary, NC

CajunWon

Member

said by SmokChsr:

when you are trying to draw 0.7 amps @120V for your laptop that equates to at least 7.0 Amps (if their were no losses {which there are}) at 12V from the battery. In the real world you are probably drawing between 11-15 times the amount of current at 12V than your load at 120V.
Didn't know of this DC/AC Amp multiplier, would be nice to learn more of this aspect through on-line reference. My uneducated thoughts are that (given the same load requirement) there should be no more Amps delivered from a large battery than the oem battery if 6' of 14ga wire introduces negligible line loss.

btw: still using this setup which has worked well through a few power outages and many brown-outs.

SparkChaser
Premium Member
join:2000-06-06
Downingtown, PA

SparkChaser

Premium Member

said by CajunWon:

Didn't know of this DC/AC Amp multiplier, would be nice to learn more of this aspect through on-line reference.

All SmokChsr See Profile was saying is that the power will at least stay the same. 120V @0.7A= 84 watts, so 84watts/12V =7A that's at 100% efficiency, which it won't be.

electric6
@ameritech.net

electric6 to CajunWon

Anon

to CajunWon
said by CajunWon:

6' of 14ga wire introduces negligible line loss.

btw: still using this setup which has worked well through a few power outages and many brown-outs.
Some suggestions.
Do not waste time on the little throw away apc models. Get at least SU1000 or SU1400. These go for about $50-100 on ebay. Then get a web management card for the smart slot so that you can easily monitor what the ups is doing.
The battery thresholds are not user programmable, but you can specify external batteries and extend the run time to hours. SU1400 and bigger have internal fans, but for your load it does not really matter.
User bigger AWG wire, 6' is a long piece. Use fine stranded wire that bends easily.
AGM type batteries are sometimes available at reasonable price, a couple of 12V 100Ah AGM batteries would be a lot safer than wet cells.

SmokChsr
Who let the magic smoke out?
Premium Member
join:2006-03-17
Saint Augustine, FL

SmokChsr to CajunWon

Premium Member

to CajunWon
said by CajunWon:

Didn't know of this DC/AC Amp multiplier, would be nice to learn more of this aspect through on-line reference. My uneducated thoughts are that (given the same load requirement) there should be no more Amps delivered from a large battery than the oem battery if 6' of 14ga wire introduces negligible line loss.
Yes the Current draw would be the same on either battery. Not having looked up this particular UPS, normally the OEM will use very short leads to the battery to minimize the line loss. When connecting longer leads to an external battery then you would want to increase the wire size to compensate for the line loss.