Using Automotive Type Battery in UPS

[Anonymous user]

Thinking about using an almost new 12 volt auto battery to replace the sealed traditional battery in my APC 725.  Bought this 725 years ago and the sealed battery that came with it only lasted a few months, maybe two short power outages in that time. Since then the 725 has been used as a power strip only.

Thoughts?

Bob

[Brad_C]

You can. A couple of gotchas. I'm not familiar with this specific unit, but I'll make some educated guesses.

The differing cell chemistry will likely result in you boiling the automotive battery. Frequent water top ups will be required.

The UPS likely does not have any external cooling, or if it does it'll be designed to last exactly as long as the original battery before overheating and doing damage. This applies to being on battery and also charging a battery. So while you can use a much bigger battery of a different chemistry you'll likely see issues with :

- Overheating

- Battery damage or accelerated aging due to plate corrosion

- Runtime estimation issues due to differing cell chemistry

- Super long re-charge times

So yes you can. No you probably shouldn't.

[MarkC_apc]

I've been using smart UPSs for off-grid inverters for several years - powered by golf cart batteries at the 48 VDC level.  This is for emergency use and the power levels are about 1/2 of the rated WATTAGES.  My observations:

1.  The use of FLA batteries (48 VDC) requires external charging as the charge system in the smart UPS is much too low to adequately charge the flooded type batteries.  A FLA type charger should be used.

2.  Most automotive type - at least the FLA type - do best when charged at higher voltages than the UPS can provide.  UPS chargers are designed for "float" voltages to charge the included sealed batteries (SLAs).  The higher charger voltages will "gas" the acid to keep mixed.

3.  These UPS (at least the older SU, SUA, SMTs) are not as efficient as built for purpose off-grid inverters are not intended for continuous service, but make extremely clean power and do not overheat in extended use when kept below rated wattages.  

I would however, guess that using an external "smart" charger that can adequately charge whatever "auto" battery you have - you will result in a long run time that could damage your (backup?) style UPS if you run it at high loads continuously.  However, in normal backup service for 10-20 minutes, it should not be a problem.  Since it is an old unit, try it out - time to get a new one anyway if you need reliable backup.  BE SAFE and do not reverse polarity - or you will need a new one!

NOTE - not to confuse the issue - your system is 12 VDC system - I mentioned 48 VDC as that is the power voltage level of my off-grid systems.  You would of course use a 12 volt charger!

[Anonymous user · Accepted solution]

Thanks for the replies. I connected the system and let it set for a day (no smoke) and then gave it a try.  But I could only get 80 volts from the UPS (via inverter) when I unplugged the wall cord.  Tried several loads and a couple more auto type lead acid batteries but no joy.  Something inside the UPS must be out of whack.  So I will surmise the concept is OK in a situation where the power only flickers off and back on after a few seconds or minutes.  I would not want to go away from the unit and have to power go out for hours tho.

Thanks for the input.  I will think about this some more.

Bob

[wayne_i_apc]

I'll "second" most of the previous comments about the "gotchas" of using an automotive battery to power a UPS, it can work, but maintaining a proper charge, and possible overheating the UPS by extending it's runtime with the larger battery are likely the biggest concerns.

Regarding the 80 volts you are measuring-- I'm not sure about your particular UPS model, but IF it produces a stepped-sinewave output you need to use a "True RMS" reading voltmeter to properly read the output voltage, see the APC FAQ at http://www.apc.com/us/en/faqs/index?page=content&id=FA158838&actp=search&viewlocale=en_US&searchid=1....

A quick test to convince you the UPS IS delivering approximately correct output voltage would be to plug an incandescent light into the UPS, then kill utility power to the UPS and watch for a change in brightness of the light when the UPS switches to battery. If the voltage to the light were to really drop to 80 volts, the light output would drop by something close to 1/2...

[Anonymous user]

The multi-meter reads 120V when the UPS is plugged into the wall and drops to 80 volts or so when I pull the wall plug.  I did have a 100 watt bulb connected to the outlet on the UPS and the light diminished quite a bit when the plug was pulled. Plug the cord back in and the light (and meter) work correctly.  The voltage does vary a bit with a constant load, it would vary between 80 and 90 volts with the light plugged in.

The commercial power in this town is much more consistent than it was when I bought the UPS so it's not as critical to have battery power backup these days.    

Thanks for your input.

Bob

[Anonymous user]

Bob, there is a gotcha related to the UPS project itself.  The automotive battery will work, but, it can delivery much more current than the small sealed 12V 3.6/7/9A, whatever your UPS uses.  The problem is that when the engineer was designing the UPS, he needed to be careful with the maximum current the unit could suck from the battery.  More current, more problem with the transistors (that needed to be bigger), power and heat dissipation, etc.  In some cases when the battery is vastly powerfull, the engineer must project a current monitor that limit the circuit operation, give it a physical limit, if not, it could burn out in smokes.  Then, the engineer takes in consideration the maximum current the sealed small battery can supply and develop the circuit around that number. It means, the circuit will run based on the max current from the battery used, and everything works fine.   When you super dee duper increase the battery current capacity, you are trashing this detail from the engineer project and allowing the circuit to accelerate and use a 900HP turbo engine, instead of the lawn mower 5HP running at full throttle.   May be the UPS you have already has current limit protections, so in this case you are safe.  Just to be on the safe side of the river, don't let your ups running for long in case of black-out at the streets.    Keep both eyes on the acid level (it will evaporate) and have it ventilated, means "air flow" to outside. Ventilate doesn't mean just move air inside and keep it close to the baby born nose.

I myself already thought to do it in one of my APC large unit, but that think uses two batteries 12V@9A in series.  Then you see, that my unit is said to be 950W or 750VA, but think; 12+12 @9A = 24V with 9A during one full hour, or at least what the manufacturer want you to think. In pure DC power, 24V x 9A = 216W. Considering "time", we need to think in Joules. One Watt = one joule per second. So, 216W x 3600s = 777600 Joules.  Now, 950W / 85% of productivity of the unit = 1117W really consumed from the batteries.  1117W / 24V = 46A.  9/46 = 0.19 of an hour = 11 minutes.  Other way, 777600 Joules / 1117W = 696 seconds = 11 minutes.  All math is correct. All of this, IF the batteries could supply 46 amperes for 11 minutes, and then juices dries out... what I doubt.  But two automotive batteries (even the smaller I could find) would do it, with pleasure and love.  Or a very small 24V truck battery.

[Anonymous user]

I gave up on this UPS, the voltage never came up over 80 volts so it went to the recycle transfer station.  I never had much of a load on this UPS, my desktop computer and two monitors so wasn't too worried about depleting the battery.  The only time the computer was on was when I was in the house or gone for less than an hour or so.  I never ran out of battery when the unit was new so figured I'd catch the power outage before the battery gave out.

Thanks for the input.

Bob 

[Anonymous user]

On 6/18/2017 7:29 PM, Bob said:

The multi-meter reads 120V when the UPS is plugged into the wall and drops to 80 volts or so when I pull the wall plug.

You'll have to use a true RMS meter to get accurate voltage measurements from the inverter's output. It's a stepped squarewave and this will cause average-responding meters (the most common type) to read  incorrectly (usually low -- 80 or 90 VAC).

The UPS was in all probability working fine.

Speaking as someone who has done this exact modification with a few different APC UPSes, I can say that even with the difference in battery design, it doesn't seem to matter much. I've used maintenance free lawn and garden batteries and none has lasted any less than three years. Most managed at least five. And while there's a fuse inside the UPS that should blow if things get really out of hand, I always provided for a second one, installed very close to the battery.

Do be careful not to load the unit excessively. At anything like full load, the inverter just isn't designed to run for any longer than the stock battery can manage. I'd recommend no more than a 30-40% load, and ideally less than that.

[Anonymous user]

So why would a light bulb dim when you disconnect it from the wall and plug it into the UPS that is running on the battery?  If the meter is reading lower on the UPS battery (RMS) I would think the light bulb would not know the difference and still shine at the same bringhtness....

[Anonymous user]

If you were using a true RMS meter to measure the UPS output, I stand corrected.

If not, I could only wonder if your line voltage is somewhat higher than normal. Mine is (~126 VAC) and as such, incandescent light bulbs I've had running from a UPS have dimmed slightly when the unit goes to battery.

I've never seen a partial failure of the inverter's output stage that left the unit functional. Usually the damage is truly impressive and results in magic smoke being let out. Or at the very least, extremely unhappy noises from within and completely incorrect output characteristics.