Electronics > Projects, Designs, and Technical Stuff
Pitfalls on substituting larger Ah battery pack (with same voltage) on UPS?
elecdonia:
--- Quote from: TheMG on July 30, 2020, 01:11:16 am ---
--- Quote ---I'm measuring an unequal division of this 27.4V between my 2 new batteries. One battery measures 13.16V while the other battery measures 14.24V.
--- End quote ---
Normally the voltages will equalize after the batteries have been on float charge for long enough. I've worked with large UPS systems that have as many as 40 batteries in series, and on every maintenance check they're always within about 0.2V of one another or less. No resistors or anything fancy going on to balance the voltages. The UPS doesn't even know what the individual battery voltages are, only the total series string voltage.
That is, of course, provided that all batteries are identical and manufactured in the same batch, and that they are all healthy. Also, batteries of poor quality could also have manufacturing inconsistencies that lead to unequal voltages.
Give it a few days and see if they start to equalize. If they don't and there remains a big difference between the two, it may be an indicator of a faulty battery.
--- End quote ---
That's helpful information. Thank you!
After a couple of weeks I will retest the float voltage on my two batteries. If a substantial difference in float voltage persists then I will update this thread.
BTW, these are brand-new Interstate Battery brand SLA batteries, 12V, 12Ah. They were purchased at the same time.
-E
pqass:
Thank you everyone for your input.
I've summarized the issues identified so far and my response below.
18AWG WIRE TOO SMALL:
I've since upgrade the wiring to 10AWG solid; I don't have the equivalent stranded wire.
I will get another Anderson grey, gender-less 50A connector to match the one that came with the 18Ah pack to allow for easy separation rather than needing a screw driver.
See attached photo of current draw while on-battery. It actually draws more than my original estimate of 5A+losses.
ADD FUSE:
There is an existing 100A fuse between the pack batteries opposite where the wires are connected. I will lower it to 25A.
HARDER ON CHARGER:
Using a clamp meter on the positive battery wire, with the UPS on-line, I see it toggling between 160mA (5sec) current draw and 510mA (<1sec) charge. This should not abuse the 18Ah pack nor the charger.
Each battery in the 18Ah pack was charged individually with a smart automotive charger to full before I connected it to the UPS 10 days ago. Each battery is still virtually at the same voltage (13.26V and 13.27V, while connected to the UPS and measured during its 5s draw interval). This pack is not new and I haven't tested it until UPS cut-off but it will do at least 1 minute on battery which is all I need. When it does eventually go, I'd rather replace it with the larger 18Ah pack where currently the 10A on-battery draw will be less hard on it vs the originally spec'd 7Ah pack; thus should increase the time between replacements.
LONGER CHARGING TIME:
As per TheMG, the charge time for my UPS is normally 16hrs (on a dead 7Ah pack) but will be extended to 32hrs (on a dead 18Ah pack). This is not a concern as I don't intend to run on-battery for long; most of the load will be shutdown via apcupsd after about 1 minute; currently the front panel says I have 7 minutes while 20% loaded (150W). So, I should get at about 14 minutes on a new 18Ah pack. My primary motivation is to save money in the long run; not extend on-battery time.
HEATSINK THERMAL MASS (OR THERMAL BUDGET, IN GENERAL) SIZED TO THE AMOUNT OF ENERGY IN THE PACK:
The UPS has a rear exhausting 30mm fan (it's off when on-line, it's on when on-battery), unattended load is 20W, attended load 120-150W (which will be shut down within a minute); the UPS is rated 780W max. I purposely bought more than my real need so I shouldn't be approaching the thermal budget even if I extended the on-battery time by double.
Last year when I disassembled the unit, the transformer core (4" square 2" thick beast) was quite hot to the touch. This appears to be steady state for this UPS. I don't remember the size of the heat sink for the output transistors. But I have since mounted a 50mm fan sucking out air from the top vent.
The case temp of 45 degrees was measured with a infrared thermometer on the side of the case where the transformer is located. The case temp of 30 degrees was measured on the side of the case where the original pack would be. Keeping the pack external should extend its life.
SHOULD SHROUD WIRES; possible mains line voltage on battery terminals:
Battery negative is grounded; reads 0 ohms to back ground screw.
james_s:
It shouldn't be harder on the charger, the charger is current limited, the battery cannot draw more current than the charger is willing to give it.
With fusing 25A for 10AWG ought to be fine, just remember that the fuse is to protect the wire, or whatever is weakest. The fuse should always be the weakest link in the chain so that if anything goes wrong, the fuse blows before anything else does.
TheMG:
--- Quote from: james_s on August 11, 2020, 12:04:37 am ---The fuse should always be the weakest link in the chain so that if anything goes wrong, the fuse blows before anything else does.
--- End quote ---
When it comes to UPS, the manufacturers normally choose the fuse ratings to protect the wiring and battery packs, nothing else. The components in the inverter circuit will almost always burn open before the fuses in the event of an actual UPS fault. I have yet to see a UPS actually blow its battery fuses. Usually one or more MOSFETs/IGBTs in the inverter will dump its magic smoke in a spectacular fashion.
Some of this is due to the fact that typical designs use many transistors connected in parallel to achieve the desired current capability, but usually not all of the transistors will fail short at precisely the same moment, and the fault current through one or two transistors is insufficient to blow the fuses.
james_s:
Well, attempting to protect semiconductors from failure with a fuse is rarely successful, but what I really should have said is that the fuse is to fail before anything likely to start a fire. A short circuit that burns up the wiring can cause it to catch fire, melted wiring can short out and result in an even more direct short across the battery which can rupture. If the mosfet acts as a fuse that's sub-optimal but it probably won't result in a fire, just lots of smoke.
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