Electronics > Projects, Designs, and Technical Stuff
Pitfalls on substituting larger Ah battery pack (with same voltage) on UPS?
pqass:
I have an APC UPS (specifically a "Back-UPS XS 1300 LCD") that has started chirping which usually means battery replacement time. It takes two 12V8Ah lead-acid batteries in series (https://www.upsbatterycenter.ca/apc-back-ups-xs-1300va-lcd-bx1300lcd-compatible-battery-pack).
However, I happen to have two surplus 12V18Ah batteries (from an APC Smartups 1500, https://www.upsbatterycenter.ca/apc-smart-ups-1500va-lcd-120v-w-network-card-smt1500nc-compatible-battery-pack).
Both packs are 2 x 12V (24V) nominal, but differ in Ah. My load is about 120W.
I've extended the [short] internal red/black battery wires with additional 12in of 18AWG stranded wire (heater cord) to the larger 18Ah pack now sitting next to the UPS.
The only difference having the larger 18Ah pack I see is:
a. tolerating a lower charging rate to the larger pack, and
b. getting a longer on-line time bonus.
Was this wise? Any pitfalls?
FYI: The UPS seems to eat packs every 3.25 years. This latest was my 4th in 13 years!
Also, the UPS seems have a case temp. of 45°C which I think may be the cause of their early demise.
NiHaoMike:
18AWG sounds really thin for such high current. Should use wires at least as thick as the stock wires, preferably thicker. Other than that, you might want to add a large capacitor where the old battery was in order to keep the high frequency impedance low.
Also check the float voltage, some APC units have it too high or too low.
pqass:
18AWG was the only stranded wire I had to hand. It should be good to 10A.
With a 120W load, 1A is drawn at 120VAC. At 24VDC, it should be 5 times that, plus losses.
So it should be good on a temp. basis.
The short internal wires seem to have more insulation so it's hard to tell if it's 16 or 14AWG; it's pretty stiff though.
The large pack is floating at 26.7V now; 13.4V per battery seems right.
How large a cap?
elecdonia:
I doubt that changing the Ah of the batteries will make much difference as long as both batteries are the same Ah. Lead-acid batteries are charged to a specified terminal voltage, not by current. Larger Ah batteries can accept larger maximum charging currents than smaller batteries. But that isn't an issue here because you are increasing the Ah of the batteries. It might be an issue if you wished to use batteries with a smaller Ah than the originals.
I have a related question: Is there any need for a pair of resistors (one across each battery) to equalize their "float voltage" when fully charged?
I have two APC "Back-UPS BP1100" units that I recently restored. Both are from the early 2000's and both required replacement of several small electrolytic capacitors on their main PC boards. My BP1100 units contain a pair of 12V 12Ah SLA lead-acid batteries wired in series.
Both of my BP1100 units have a "fully charged" DC voltage across both batteries of exactly 27.40 V. This corresponds to 13.7V per battery. This is within the specified range of 13.5-13.8V "fully charged for standby use," as labeled on the batteries themselves.
However 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.
Does this overcharge one battery while undercharging the other?
Or is this difference too small to matter?
Will it gradually go away after operating the UPS for a week or two? (I haven't checked this yet)
I'm requesting comments/recommendations from forum members who have worked with UPS units containing multiple 12V batteries connected in series.
-E
duak:
A tangential caution. The UPS's charger may not be able to supply enough current to charge the larger battery after a major discharge. It it's not current and/or temperature limited, could it overheat? Bigger batteries have a lower ESR and the charger may not be able to handle it.
A friend of mine that runs a cabinet shop picked up a walk behind forklift for a song, but didn't get the right charger with it. Even though it was rated for the voltage, it was too small and always struggled to supply even 1/20 C for that battery. (500 AH?) The charger was of the ferroresonant type with intrinsic current limiting and is supposed to be as reliable as an anvil. The transformer finally croaked so I cobbled up something with another transformer until he decides to get a proper charger.
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