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Registered Member #191
Joined: Fri Feb 17 2006, 02:01AM
Location: Esbjerg Denmark
Posts: 720
car batteries are suitable for cars, designed for huge crank current and what not. hence the name. For your application, better look for UPS batteries.
Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
He'll get way more battery for way less cost if he goes for the car batteries. The UPS batteries are just sealed lead acids. (though meant to be indoors) I dont see why a safe arrangment using car batteries isnt plausible.
Registered Member #1792
Joined: Fri Oct 31 2008, 08:12PM
Location: University of California
Posts: 527
There are computer power supplies designed for low voltage DC input, e.g.
wrote ... Bear in mind that fusing and switching are harder at DC due to non self-extinguishing arcs. PSUs with active PFC may get confused by a DC source
I took apart a UPS recently (36V battery bank) and the fuses were of the same form factor of car fuses. That might be worth looking into for fuses, if you're staying in the vicinity of 12V inputs.
Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
Mattski wrote ...
wrote ... Bear in mind that fusing and switching are harder at DC due to non self-extinguishing arcs. PSUs with active PFC may get confused by a DC source
I took apart a UPS recently (36V battery bank) and the fuses were of the same form factor of car fuses. That might be worth looking into for fuses, if you're staying in the vicinity of 12V inputs.
i think the normal car fuses (the colored plastic type) are meant for up to 35Vdc, which sounds like its reasonable for the purpose you suggest Mattski, with a voltage drop they wont even be out of spec at 36V.
EDIT: (ATC, APR, ATO, or the APX maxi types are often rated at 24Vdc or 42Vdc.)
Registered Member #3989
Joined: Thu Jul 07 2011, 05:10PM
Location: In a van down by the river.
Posts: 52
Ash Small wrote ...
Probably a stupid idea, but it 'should' be more efficient that the inverter/psu method.
four 12 volt (actually 12.6 V) batteries in series with centre tap gives +/- 25.2 V,
A string of eight electrolytics across 25.2 V gives 3.15 V across each electrolytic, use diodes to parallel.
A string of five electrolytics gives 5.04 V, again use diodes to parallel,
diodes from two batteries in series gives 12.6 V.
Current drain from batteries is equal.
Is 12.6 V, 5.04 V and 3.15 V close enough to 12, 5 and 3.3?
EDIT: I've not taken diode voltage drop, etc. into consideration here.
Well I think two batteries in series and eliminate the psu alltogether is a cool idea, like a gigantic laptop!! Prolly run for a long time too, my computer and lcd will run a long time on a little inverter and one car battery, hwhats the inverter get like 80% efficiency max? Wouldn't stright clean dc from batteries be about as close to 100% efficiency as you can get? Just add some solar cells and be good to go.
Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
If you regulate with diode drops from 12.6V down to 3.3V, at 30 Amps, you will waste (12.6 -3.3) = 9.3V x 30A = 279 watts wasted for 99 watts passed, thats 26% efficient.
I dont like linear regulation schemes for this reason.
The math speaks to me like this: if you have the highest voltage down regulating (12.6-3.3V) and at the highest current (30A) then you will always be wasting enormous energy (way more then your actually passing), especially compared with a lower V drop with very low current. ((like 12-12.6) and then 0.1 A)
Registered Member #3989
Joined: Thu Jul 07 2011, 05:10PM
Location: In a van down by the river.
Posts: 52
Patrick wrote ...
If you regulate with diode drops from 12.6V down to 3.3V, at 30 Amps, you will waste (12.6 -3.3) = 9.3V x 30A = 279 watts wasted for 99 watts passed, thats 26% efficient.
I dont like linear regulation schemes for this reason.
The math speaks to me like this: if you have the highest voltage down regulating (12.6-3.3V) and at the highest current (30A) then you will always be wasting enormous energy (way more then your actually passing), especially compared with a lower V drop with very low current. ((like 12-12.6) and then 0.1 A)
Would the efficiency improve much with four 6volt golf cart batteries in series? The cells in them are huge and would last much longer anyway I bet.
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
Patrick wrote ...
If you regulate with diode drops from 12.6V down to 3.3V, at 30 Amps, you will waste (12.6 -3.3) = 9.3V x 30A = 279 watts wasted for 99 watts passed, thats 26% efficient.
.
I think you've missed something here Patrick. You regulate the voltage with a string of capacitors (capacitive divider), the diodes are just to parallel the capacitors.
similar circuits (with diodes) are used to charge 6V batteries (from a 6V supply) while they are connected in series for 24V, but without the capacitors.
I don't know the tolerance allowable on the 12V, 5V, 3.3V etc (and - voltages), but I assume there is some tolerance. (eg, I imagine the tolerance on the 5V rail is ~4.5-6, etc).
(I'm sure this will work with a resistive divider, but I've had a drink now, so I may need to contemplate the capacitive divider concept in the morning . I might try it tomorrow, as I have some low voltage electrolytics and diodes lying around.)
Registered Member #1225
Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253
If you regulate with diode drops from 12.6V down to 3.3V, at 30 Amps, you will waste (12.6 -3.3) = 9.3V x 30A = 279 watts wasted for 99 watts passed, thats 26% efficient.
Yea, and that is especially bad considering he is using batteries.
About the diode bridge, i'm sure DC wouldn't stress it too much. I know which 2 diodes are conducting alternate with AC, but they still have to be rated for the RMS current plus some headroom, so while DC may stress it a bit more dissipation wise, i think they could handle it. If you feel like taking the PSU apart, you could just bypass the diode altogether.
Registered Member #3271
Joined: Mon Oct 04 2010, 02:29AM
Location: Canada
Posts: 159
Backyard Skunkworks wrote ...
Looking to put a couple of the servers at my house on extended battery backup after some extended power outages.
I've got a good source of car batteries, a U-Pull-It yard selling all for $25/each including core fee.
Look for "deep discharge" batteries for marine use or sealed batteries for UPS use.
Car batteries are great for a large, short burst, high current to start a car. OTOH just leave your lights on for a few hours and it will be dead. They rely on the car alternator to keep them topped up most of the time. They are not that good for steady discharge mode.
The other point to consider is the emission of hydrogen on the recharge cycle. Not a good idea in an enclosed place or your basement. I have found that a house CO monitor is quite sensitive to H2 (may vary with models) and can monitor when I recharge a car battery in a closed garage (not attached to the house) in winter. Finally if you have several units do consider the sulfuric acid fumes that will corode stuff around if you do not have good ventilation.
Overall I would go with a sealed UPS units if at all possible.
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Best car batteries for server backup?
. I might try it tomorrow, as I have some low voltage electrolytics and diodes lying around.)
