Please help me reduce ripple and HF harmonics at 25-60 amps

[LikesElectronics]

Hi everyone,

Recently I acquired a broken down SOLA 1500 sinewave UPS which I was able to repair. Since I already have a UPS for my PC I decided to use it solely as an inverter. I replaced the internal 24v battery pack with 2 external deep cycle marine batteries connected to the unit with 2 80cm 300A high grade welding cable to minimise losses.
Problem is; when the unit is running under a light load, it immediately sounds the “low battery” alarm. It can run for hours like this, but the thing that makes me uncomfortable is when looking at the battery voltage on my CRO, I can see very pronounced “dips” approx every 9mS; alternating between 2 distinct “dips” of 4-5 volts and 2 volts with apparent high frequency harmonics. Either my batteries are damaged, they were bought as new factory seconds with cosmetic defects, or these types of batteries simply cannot supply the heavy surges of power needed, fast enough. Under load, they steadily supply around 24.6 volts which is probably why the UPS is complaining. However, I do not like these dips in voltage since I can hear the unit struggle when a large monitor is switched on, which does not happen with the internal batteries. I doubt they are defective though, so I’d like to build a capacitor and choke filter close to the UPS unit to “support” the batteries. An LC filter would help, I suspect, however I have never built one even remotely as powerful as this unit needs.
Could anyone please point me to a schematic or describe a simple filter that would be cost-effective? How many turns of what gauge wire would I need and would I be able to salvage the inductors found in wrecked PC power supplies?
If you read through all this; thank you. I’d be grateful for your help and post an update with any success or failure stories.

[Nagwind]

Hi

Just saw this letter/website...

Harmonics and loss of capacity? Firstly: Check the capacitors. I'm talking about the large storage electrolytic capacitors... On both both the primary and the secondary side of the UPS. DC ~ AC and AC ~ DC etc.

NB! Those responsible for storing DC before it gets converted back to AC are very critical. A safe rule is that the may be a bit larger in value. They are often the reason why a UPS blows up. I'm long this business...

Hope it may be of help to one or some...