Uninterruptible Power Supply questions

NEVER EVER connect batteries in parallel!

And you must keep in mind, that some cheaper devices are constructed in such a (lousy) way, that the components used will only survive for the backup time provided by the batteries.

(Double the capacity, your UPS will run longer, parts may reach a higher temperature and fail.)

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Kind regards,
Gerard Bok

Because.....?

Because no two batteries have exactly the same voltage. But they do have a very low internal resistance (well, inititally at keast. Connect them in parallel and both the internal resistance and the temperature rise sharply ;-)

The resulting current is delta U/R or a small voltage over a very low resistance. Which will still yield a huge current!

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Kind regards,
Gerard Bok

Probably wise to add, that batteries may explode when connected in parallel.

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Kind regards,
Gerard Bok

So all those 24V truck systems out there are rolling death bombs?

LOL!

JazzMan

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Well, around here, some are. But for a totally different reason :-)

It seems that you are right though. I am totally amazed that trucks in deed seem to run their batteries in parallel. And even more by the fact that several references claim that available power equals the power of both batteries simply added together.

Learning every day, I guess :-)

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Kind regards,
Gerard Bok

Hi...

Huh? Any I've ever seen have been two 12's in series, not in parallel :)

Take care.

Ken

The big trucks use parallel/series arrangements to get the needed voltage and current requirements.

JazzMan

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Because in theory one battery will always have a higher voltage and be trying to charge the other. In practice I've done it with small gel cells and had no problems, but I wouldn't do it as a permanent installation.

That would be a series system right? Two 12V batteries in series is perfectly acceptable, two in parallel is not.

Dual battery systems use diode based isolators to keep one battery from interacting with the other, I suppose you could use something like that with a UPS.

I've tried much the same with an APC Smart UPS with puzzling results. When I ran it as a dumb UPS, with no monitoring software, I got twice the run time, as you would expect. When I ran the paralleled batteries along with APC's Powerchute software I could not extend the run time, (despite lots of recalibration). It was as if Powerchute knew the maximum capacity of two batteries and would not recognize/accept/deal with 4, (despite the fact that it monitors battery voltage, seemingly as an indicator of when to shut down a system).

I've no idea whether this is relevant to your UPS.

I think the problems warned of here in running them series/parallel are misplaced. To quote from GE's "The Sealed Lead Battery Handbook" 1979, page 91:

"With sealed lead batteries there are no major problems with parallel c> >

Thanks for the responses so far...

Yes extending the runtime may be bad for the components and surely they are only designed to last so long. You can buy an invertor for a boat or camper and that is rated to run for longer periods of time. The invertor in the UPS is huge! I think as long as I'm well under the 700VA peak output I'll be okay to extend it a bit.

Yes hooking up batteries in parallel does sound a bit scary to me... I think parallel is the wrong description.. 2 in parallel and in series with another identical set. See figure 3 in the link below.

Maybe this is only good for 6 volt batteries? I'm sure I've heard of 24volt truck systems too.

Here's a link to some of the DIY projects I read earlier..

Here is a link talking about some higher end UPSes that can accept addon batteries.

Maybe there is a DIY project out there that you can perform.

It looks like in this example though 24v and 36v systems are used though.

I don't think I'm willing to go this far for a home UPS, I just thought maybe this would be an opportunity to buy a larger capacity battery replacement instead of the identical replacement ones and have the same run time. The way I figure it.. I'm replacing 2x6volt 12ah batteries.. with

1x12volt and higher aH rating.. Unless someone here can say for sure that 2 in series provides a larger capacity of run time.. That's what I'm trying to figure out. There must have been a reason the UPS mfg went with 2 6volts instead..

There are people that have built their own DIY UPS too -->

I should also note that performing this with a car battery or deep cycle marine battery is possible but dangerous because of the hydrogen released from the battery. Should not be in a sealed room like a bedroom etc...

Just get two larger AH capacity batteries. If the UPS is set up in such a way as not to recognize the larger capacity try running a bypass around the logic function circuitry and just let the "brain" act as a charge regulator.( I refurbished an old UPS for a friend awhile back and we modded it by using two sealed wet cell batteries( ones designed for use on four wheelers and such.) The batteries did have to go external but it gave him about four-fold the run time and the wet cells are much more durable. You could of course just use sealed lead acids of greater capacity, the main advantage of course being that they'll last more cycles since you won't discharge them as deeply during each AC off cycle.

if you are OK with using external batteries ,

a car battery is probably your best deal...

just be SURE to properly fuse it in case something fail or the wires short.... ....

Mark

That's perfectly reasonable, but isn't a large cell internally equivalent to small cells of the same voltage in parallel?

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I use an APC 1200 UPS and have it connected to two external car (actually marine/starter) batteries. This is a 24v system. This has worked reasonably well for me but for the past 7 years, but I have used batteries that require water....and when I forgot to refill them, they would dry out. I am now using a maintenence-free battery, and will see how long it lasts.

Also, I originally used a deep cycle battery but I think a regular starting battery would be better. We seldom have blackouts so the battery would almost always be at max charge. This is what normal car batteries are designed for. Of course, your situation may be different.

If I were you, I would just get a large car battery and try it....they only cost $35-45 at Costco. I can't imagine a UPS failing because it was only designed to run for a few minutes.....but who knows. I have run my UPS for well over 10 hours straight with no problems...but it has a cooling fan that kicks in. cv

Mark wrote: