I wish to design a system that allows multiple Uninterruptible Power Supplies to operate from a single 12V battery while ensuring that there are no unexpected undesirable consequences.
I already have multiple Uninterpretable Power Supplies that must only supply power for less than 20-25 seconds, normally, since the LP fueled back up generator normally supplies all connected devices / circuits within 14-17 seconds of AC – utility failure. The total is comprises 3-4 sec. sense (loss of voltage), generator starts within 5 seconds, and RPM sense (up to speed) and load transfer occurs within 7 – 8 seconds.
Since power loss, sense, start, & transfer are not (cannot)" be instant <1 sec. not just the computers, but the DSL modem & 2 set top boxes (need 3+ minutes to boot), the professional VTRs (used mostly as play sources) – DVD recorders when being used, ect. have to have continuous power.
Over the years I've gone through hmm 3 or 4 sets of two each 12V UPS batteries X four or five UPS X $19 – $26 batteries = a LOT of MONEY!
I realize/understand each UPS has it's own battery voltage sensing and charging so using a single (with sufficient ampacity/capacity) and therefore only one UPS can be allowed/used to charge the single battery. Unless someone can recommend a BETTER blocking diode yet one that has sufficient forward current capacity than the NTE 6087 Si. Schottkey Barrier Dual rectifier (diodes). They have the following characteristics-specifications:
Prv – 45V Max, Ifsm – 150A, Ip – 30A, and Vf .82@ 30A
I'd place the cathode on the B+ comming from the UPS(s) which will NOT be maintaining the battery.
Unless the backup/standby generator FAILS to start, no UPS should/would 'ON' longer than 30 seconds. To keep the 12 V DC leads from the HEAVY duty battery I will be using #8 wire <4' for one and #6 >8' for the other combination of UPSs supplied by the second EAVY duty battery. I plan to use standard #14wg Romex and standard AC plugs-receptacles to route the AC (line and/or UPS) to the location nearest the equipment needing UPS backup until the generator can take the load.
Cautions, suggestions, corrections, etc. invited and WELCOME.
Best Answer
Sounds reasonably OK (without a detailed analysis).
The selected diode sounds very acceptable for the periods of use that you envisage.
The battery will be essentially fully charged at all times so the under 1V drop should be acceptable.
It would be possible to arrange a MOSFET as a "diode" so that there was even less forward drop, but it is probably not worth the additional complexity.
A caution: It is quite possible that individual UPS's will not tolerate well operation with no battery in charge mode. If they do not "see" a battery this may affect their ability to drop into operating mode when needed.
It may be better or necessary to disable the UPS charging circuitry in some manner in all but one UPS and to connect the systems directly with no diodes.
If direct connection is not viable it may be possible to place a very small battery on the diode side of each non-charging UPS so that it sees this battery for charging purposes. This battery need not be in especially good condition. As soon as the UPS is loaded and the "local" battery voltage starts to drop the diode connected main battery will take up the load.