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Please excuse the possibly simple question but this is all a bit new to me.

I have an APC Smart UPS 1000 which has a very large/tall 2700uF 40V capacitor on the battery (24V) connector. Not knowing how good all the capacitors are and having already ‘melted’ a set of new batteries, I have exchanged all of the electrolytic capacitors on the board as this is recommended to resolve overcharging problems.

The problem is that the new 2700uF 40V electrolytic is TINY compared to the original.

Does it matter? Surely a 2700uF 40V cap is still a 2700uF 40V cap no matter how big it is? Or is there a reason why the old cap would have been spec’d To be BIGGER?

OK, a bit more information which, hopefully, will get me closer to a solution...

Thanks for your answers.

The original caps are EPCOS B41848-A7278-Z which is TDK, from what I can find online. The problem is that I can not find a spec sheet.

If anyone knows the specs for these caps, can you let me know please?

Or

I have found some Nichicon GU series caps at RS which match the snap pin dimensions. They are 50V rather than 40V but, otherwise, are 2700uF.

Does anyone know enough about APC Smart-UPS design to be able to say whether they are suitable?

Or

Point me in the direction of somewhere where I can buy the correct/matching caps?

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    \$\begingroup\$ Ripple current capability is a important spec that tends to be related to physical size. \$\endgroup\$ – Unimportant Feb 6 '19 at 11:01
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    \$\begingroup\$ ESR depends on surface area. Greater the surface area between plates, smaller the ESR. So larger electrolytic capacitor will have lower ESR. There may also be differences in tolerance of breakdown voltage, but this is difficult to predict. \$\endgroup\$ – Indraneel Feb 6 '19 at 14:14
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Yes, in your case it matters very much. Not so much the actual size, because caps are getting smaller as manufacturing technology gets better. However, There is a reason the original capacitor in the APC UPS unit is so large. Look at where it's placed in the circuit. The extra surface area in this capacitor reduces ESR and allows the capacitor to handle a lot of ripple current. Also, the size of the capacitor allows it to disperse heat better and remain cooler. This increases the life of the cap. Now, if you replace the original cap with a capacitor that is much smaller in Physical size there is a good chance it can't handle the ripple current (which generates heat inside the capacitor) and it will overheat and fail very early. Also, the lower ESR of the Epcos cap allows for smoother switching from grid power to battery power. The smaller cap may work for a while but it is likely to overheat and fail early. If you want the perfect cap to replace the old one you need this cap and it is available at both Mouser and Digikey. TDK/EPCOS B41607A7278M009 This cap will upgrade the unit. It can handle more Ripple current, heat, has a longer service life and has a lower ESR than the original 2700uf cap. Good luck with your project.

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Surely a 2700uF 40V cap is still a 2700uF 40V cap no matter how big it is

No. Those are only two of the important parameters that define electrolytic capacitors.

The others are: maximum operating temperature (which can have a huge impact on life), loss tangent/DF/Q, ESR, and (as @Unimportant stated) the ripple current rating.

Capacitors are always improving and getting smaller, so it is possible your new ones are suitable. But you will need to get the datasheets for both and verify this.

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