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Registered Member #3792
Joined: Sun Mar 27 2011, 06:07PM
Location:
Posts: 136
I got 2 mini - itx boards that i'm planning to use as servers. Both of them already have a converter that converts the output of a 12V wall adapter to the required 12 / 5 / 3.3 V for ATX. Along with the servers, the modem that will be used for the internet connection is also 12V.
Is it possible to connect them to a car battery directly? The adapters of the servers have rated output between 12 and 14.5V so i don't think that the 13.25V float charging voltage is going to harm them. But i'm not sure what is going to happen if it drops below 12V.
The modem is also a headache. Specified voltage is 12V with a true (measured) output of 12.20V. Highest input voltage is the float charging voltage of 13.25V. Would this 1.05V voltage difference harm it in any way?
Of course, filtering and reverse voltage protection are a must and are already considered. Car battery is not connected to a car so voltage spikes and voltage regulation do not apply.
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
When the voltage drops below ~11.5V, the battery is already mostly discharged, so you can use a circuit which disconnects the load when the battery voltage reaches maybe 11V. As for the modem, it *should* (as in 98% cases) be all right with 13.25V, but you can never be 100% sure.
PS. The 13.25V float charging voltage is pretty low, usually it is around 13.6 - 13.8V.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Those converters feed the 12V supply straight through. And car batteries like an occasional topping charge to 14.4V for good health. I don't think the hard drives would appreciate that.
Registered Member #3792
Joined: Sun Mar 27 2011, 06:07PM
Location:
Posts: 136
@Dr. Dark Current: After a bit of reading about ATX voltage specifications, it seems that the 12V line can go as low as 11.6V and i'll be stopping the battery from further discharge at 11.8V (don't know if Wiki is correct about the 11.8V as the lowest battery voltage). The 13.25 float voltage is a bit low but, as Wiki mentions, for preservation voltage is maximum 13.2V.
Maybe i could use a low - dropout regulator for the modem.
@Steve Conner: Thankfully, no HDD's are used. Instead, they are replaced by flash memory cards. That way, maximum power draw dropped to 10W (servers are based upon Intel Atom mini - ITX boards).
Registered Member #162
Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140
I'm not sure about Automotive lead-acid batteries but we repair UPSs using the Yuasa type valve regulated lead acid batteries, chargers and inverters, based on such batteries; For long standby use/life, 'float' charging, battery voltage = 13.65v @ 20C -20 mV/C (approx, there is loads of data on the Yuasa website, I'm sure there are other good manufacturer websites) It is normal to discharge a '12v' battery to 10v in ups service, a warning status signal to all connected devices is wise I haven't tried myself but a SEPIC converter looks ideal for your use.
Automotive lead acid batteries may release hydrogen and/or oxygen so need good ventilation.
Registered Member #3792
Joined: Sun Mar 27 2011, 06:07PM
Location:
Posts: 136
After some tests with the equipment it seems that i'll have to ditch the whole "car battery UPS". Batteries are getting charged by the UPS (since the one reads 12.62 and the other reads 12.58 two weeks after i disconnected them from the charger) but manipulating the low voltage output is nearly impossible.
It would be easier to connect the wall adapters to the UPS output directly. I'll lose some energy (since we convert low - to - high --> high - to - low) but operation will be guaranteed. Backup time is already 9+ hours so losing a bit is not a big deal.
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Car battery server UPS
