UPS Batteries and Amp Hour rating.

[Brcobrem]

Hi,

I hope this post is not completely off topic here. But because I install UPS's to regulate and supply backup AC power electronic equipment, perhaps you will let this one slip on in. Here goes. . .

The typical lead-acid battery in most of the *smaller* 120v AC switching UPS's is commonly rated 12v 7Ah, 12v 8Ah or 12v 10Ah. Some of them are 6v, but most of them are 12v.

One UPS manufacturer decided to make purchasing "off the shelf" a little more challenging by using a not-so-standard 12v 9Ah battery. Note the *9Ah* mentioned here.

My thinking is that in non-critical runtime environments, I can just use a commonly available 12v *8Ah* battery and all I will lose is a little runtime when the UPS goes "on battery" (compared to the 12v 9Ah battery). I'm not certain how much runtime would be lost, but my guesstimate would be that if a 12v 9Ah battery would typically run a piece of equipment for 20 minutes, perhaps the 8Ah battery would run that same equipment for 15 minutes.

My thinking is also not to use a 12v 10Ah battery, because perhpas the charging circuity was not designed to handle charging a higher capacity battery, or perhaps it would just take much longer time to recharge the battery when the power does come back on. I would primarilty be concerned about overloading the charing circuitry.

I will greatly appreciate any commments, criticisms or suggestions that you may have on this topic.

Regards,
Brcobrem

 

[tallgirl]

Are you sure you can find the 8Ah batteries in a geometry that is compatible with the 9Ah ones? That's always been the hardest problem I've had when replacing UPS batteries. The enclosures are practically designed around physical size of the battery the manufacturers are planning to use.

The worst problem I can think of is that the output voltage from the battery will decrease faster as the charge remaining in the battery decreases. Since you'll be using a greater percentage of the total capacity, the voltage decrease (which is perfectly normal -- happens all the time) will be even greater.

 

[dereckbc]

My thinking is also not to use a 12v 10Ah battery, because perhpas the charging circuity was not designed to handle charging a higher capacity battery

Wow never worked with anything that small, most of the systems I work with are 500 to 2000 AH batteries series up to make 310 VDC.

It just takes longer to recharge. Really makes no difference what current the rectifiers will supply because the batteries will take all the current a rectifier will supply when the battery is discharged up to a point of the battery internal resistance. So lets say in one system you have a rectifier that will supply 5-amps, and another will supply 10-amps. When either rectifier is connected to a discharged battery, they will both supply their full current capability of 5 and 10 respectively. The 10-amp rectifier will just get the job done in 1/2 the time theoritically.

 

[Brcobrem]

Are you sure you can find the 8Ah batteries in a geometry that is compatible with the 9Ah ones?

In this particular instance, the batteries are the same size and have the same connector tabs.

The worst problem I can think of is that the output voltage from the battery will decrease faster as the charge remaining in the battery decreases.?

If I understand correctly, then you are confirming what I was taking a guess at: When the utility AC power goes out, and the UPS then switches to "on battery", the UPS will keep the output AC voltage constant (as required to run the 120v equipment), but just just lose some runtime becuase the battery will deplete faster. In other words, the 8Ah battery will provide less 120v runtime than the 9Ah battery will provide.

Please let me know if I am not correct in my understanding now.
Thank you very much for your reply tallgirl !

Regards,
Brcobrem

 

[Brcobrem]

Wow never worked with anything that small, most of the systems I work with are 500 to 2000 AH batteries series up to make 310 VDC.

Yep, those do sound like the big boys. The biggest ones I see in the small business scenario are 12v 17Ah (4each in parallel). They are about the size of a large motorcycle battery. Still pretty heavy to lift when you pack a couple of them into a UPS and an auxiliary battery pack and have to get it from the trunk then into someone's office.

It just takes longer to recharge...

Thank you ereckbc. That makes sense. Sort of like auto batteries if I am understanding correctly: The car's alternator doesn't burn up just beacuse you installed a 75 cold cranking amp battery versus a less expensive 55 cold cranking amp battery (ie. it would just take longer to recharge a depleted 75 compared to the 55 as you point out.). Additionally, you'll get more starter cranks (ie. "runtime") with the 77 becuase of it's higher amperage capacity.

Thank you to very much for your reply. Please let me know of I did not understand correctly.

Regards,
Brcobrem

 

[tallgirl]

In this particular instance, the batteries are the same size and have the same connector tabs.

Then it's likely that the batteries either have a more liberal amp hour rating (heh), are made for a deeper discharge, or have thinner walls allowing more electrolyte, etc. to be put inside. Since volume goes by the cube of the dimensions, a tiny increase in all three dimensions can result in a large increase in volume. I'd like to take one each of those batteries and cut them open to see the internal difference.

If I understand correctly, then you are confirming what I was taking a guess at: When the utility AC power goes out, and the UPS then switches to "on battery", the UPS will keep the output AC voltage constant (as required to run the 120v equipment), but just just lose some runtime becuase the battery will deplete faster. In other words, the 8Ah battery will provide less 120v runtime than the 9Ah battery will provide.

Within the specifications of the UPS, the UPS should try and keep the output voltage constant as the battery's output voltage decreases. The current from the battery will have to increase since the volt-amps need from the battery is, more or less, a constant for a given volt-amps output. As the current increases, the voltage drops even further due to circuit resistance (our dear friend Ohm and his law). At some point the UPS should decide that the voltage from the battery is too low and give up.

So, yes, an 8Ah battery will provide less run-time, but it's not like you'll get 1/9th less runtime (9Ah versus 8Ah), probably something closer to 1/5th less since voltage drop is related to the amount of charge remaining, and you're reducing the charge remaining faster. I'd have to sit down and do a bit of calculus to get a better estimate.

My recollection, and it's been a while, is that "amp hours" is measured to when the output voltage drops to 10.5 volts under some constant load. If you want a better idea, see if you can find the specifications the manufacturer used to calculate the amp-hours for each battery. That might give you a clue.

Please let me know if I am not correct in my understanding now.
Thank you very much for your reply tallgirl !

Nope, your understanding is correct.

The only remaining issue is that the UPS charging circuit is designed for a specific rate of charge, and that's typically linked to the battery's capacity. Since your battery has an 11% smaller capacity, rather than, say, 50% smaller, I can't see there being a major problem. But then, I'm not the UL, UPS manufacturer, or anyone else who could say there isn't an actual problem.

 

[robbietan]

you might also want to check the rating of the battery charger. no use installing a *9Ah* battery if the charger is rated to charge up to *8Ah*.

 

[zbang]

In the grand scheme of things, the difference is noise, especially after a year or so. Also, unless the UPS mfg is buying lots of batteries, their 9ah is probably an 8ah with a new label. (IIRC, almost all sealed lead acid batteries come off of 3 or 4 production lines.)

BTW, what mfg is this? I'd like to avoid them

 

[megloff11x]

It's been my experience, as with everything else being made in overseas sweat shops, it's cheaper to buy a new UPS than a new battery.

I had a nice old cordless drill and when the batteries gave up, it was cheaper to buy a new cordless drill with two new batteries, charger, and case (and more powerful too) than to get two new batteries for the old drill.

 

[zbang]

It's been my experience, as with everything else being made in overseas sweat shops, it's cheaper to buy a new UPS than a new battery.

I dunno. I spent $50 to replace the batteries in a $350 UPS; I'd call that a fairly good deal. Also, AFAIK, APC and Leibert manufacture in the USA.