The Answer



Prius FAQ


Web Links



















Why the UPS isn't the ideal solution

Elsewhere I exhibit some satisfaction over selecting the Lucent UPS as my "inverter solution."  It was cheap, and in many respects fulfills all the project requirements.  But, it has quirks.  Some bad quirks, some very silly quirks.  The bad quirks make it unusable for a non-geek.  The silly quirks make me want to stick pins in the designer doll.


The UPS must be turned ON ALL THE TIME.  It requires an input voltage before it will start up in battery mode.  Why is this an almost fatal flaw?  Because if the Prius isn't connected before the UPS battery runs down, you can't start the UPS!  Why isn't it an actual fatal flaw?  Because you can always use another UPS to provide startup "priming" power.  That means you always must have a 230V UPS available on standby.  Grrr.  Another reason this misfeature is a problem is that the UPS isn't 100% efficient.  It requires about 200W just for internal operations.  If the UPS were protecting computers or other devices that needed continuous power, it wouldn't be an issue, but for normal household lights and appliances, where a brief outage is insignificant, this power is simply wasted.  Not the end of the world, perhaps, but still an expense.


After running a test to determine how quickly I would have to rush home to connect the Prius in case of a power failure, I noticed that the batteries, whose voltage I was monitoring, didn't seem to be fully charging.  I initially ascribed that to the age of the batteries, but after checking the charging voltage I realized that it was the charger itself that was at fault.  I called tech support, who advised me that "Oh yes, the UPS must be delivering power to the load for the batteries to charge."  How much?  "About 30%!"  So, in order to properly charge the batteries, the UPS must not only be connected, but supplying power.  But if it is supplying power, then it won't deliver power long enough for me to get home and plug in the Prius!


If you think about the scenarios implied by these quirks, you will see that this is not a set-it and forget-it system!  If the Prius is present and available to be connected before the UPS battery runs out, most of this can be ignored.  But "worst case," we must consider that it will require a number of hours to get home and get the Prius connected.  To make this work, therefore, we first must make a choice:

Do you have very reliable power in general?  If you normally have no outages or only very brief ones a few times a year, it's a bit profligate to keep this monster UPS running all year long.  It would be cheaper to provide a smaller UPS for your computer and resign yourself to trickle charging and a startup procedure for the big UPS in a true emergency.  On the other hand, if your utility power is relatively flaky, if you have a number of computers that each have a UPS, if you have other devices that can't tolerate an unscheduled outage, then running one big UPS is no more wasteful than what you're already doing, and you might as well take advantage.  In my situation, I have both somewhat flaky power and a number of computers, so I've selected this case.  If you decide to leave your UPS off all the time, you can "trickle charge" the UPS batteries with an external supply.  You will need a simple supply of "raw DC" that can supply a few mA at about 270V. 

If you leave the UPS on all the time, and know that your Prius will never be more than a few minutes away (or perhaps you have more than one), you might be able to disregard this step.  However, I have provided for a "priming" UPS supply, one that can provide a few hundred watts of power at 230V to start up the main UPS.  Since the large UPS refuses to start up unless it's connected to the AC line, this can be used to fool it into believing it is when AC is unavailable.  This is a very undesirable solution since it will require maintenance of a rarely used device, and one that should be unnecessary.

Finally, there's the requirement that the UPS be delivering power in order to charge the batteries.  In many typical situations, e.g., one person works, spouse is at home, the spouse should be drilled in shedding UPS load if the Prius can't be connected immediately, to preserve the UPS battery charge.  Big nuisance.

This page and these quirks are dictated by the particular UPS that I selected, or, more accurately, that selected me.  I continue to look for a more appropriate UPS or a commercial inverter.  I've written up specifications for the ideal inverter in case any manufacturers care to take up this project.

Site 2005/6


  Created 08 Oct 2005