Does inverter-caused Hum harm the battery?

DIY
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Per the Subject… I have a small off-grid system consisting of 500-ish watts of solar, a SmartSolar charge controller, a Phoenix 12/500 inverter, and 315AH of LiFePO4 12v battery. There is also a 12 V DC power supply (set for just under 13.5v) that feeds the system at night, preventing the battery from discharging below about 50% SoC so it can serve as a UPS. All is working as expected, but some background before the question:

The initial system build had a much smaller battery, about 21AH of tired AGM. It was enough to keep the system stable, and to cover for short mains outages at night. But I found that there was a LOT of AC ripple on the 12v bus, caused by the Phoenix inverter when it fed a non-resistive (0.8 power factor) load. That ripple fed into all of the DC-powered parts of the system, causing a lot of hum on the audio components. The official Victron “solution” to this was thicker wires and a bigger battery, which at the time was outside my cost envelope. My solution, instead, was to add a filter - a large inductor in series with the positive feed to the inverter, with a large capacitor directly across the inverter input terminals. The parts were in the proverbial “tin box” in the garage, so zero cost. That reduced the ripple at the battery to negligible levels, and cured the hum. Measurements at the battery showed nearly zero AC current.

Fast forward to today. Now that I have some 300AH of really stiff Lithium battery in place, I thought I’d try removing the filter and see if the hum returned. Good news, it did not; all is still as quiet as with the filter, and I also eliminated the small energy loss of the filter itself.

But.

As expected, the AC load on the battery has returned. With about 136 watts of load on the inverter (at 0.8 PF), I see 5.5 AMPS of AC current at the battery terminals, as measured by a clamp meter. Because the battery is so stiff, that doesn’t translate into any significant AC voltage (so no hum), but I am concerned what impact the pulling and pushing 5 amps into and out of the battery at 60 hz, 24/7/365 days will have on the battery life.

Will an AC “load” shorten the life of a LiFePO4 battery? Alternatively, if I put the filter back in, will that shorten the life of the input caps in the Inverter (since they will be taking the brunt of the non-resistive load-induced ripple)?

2

In a word, No.
The Battery will cope with this, due to the low impedance.
However, I would like to point out that the AC current from the battery to inverter will look like a half wave rectified AC, and this is superimposed on a DC current too. So you won’t actually be ‘pushing’ any current back into the battery. At the low point of the complex current wave form - coincident with the output current waveform zero crossing, the battery current will still be powering the inverters internal dc loads. As the Ac current reaches the maximum - on either positive or negative cycle, then the battery current will also increase to maximum, giving an AC current superimposed on the dc current one would expect.
It would be possible, as you did, to add a filter with a large inductor and reservoir capacitors to remove this AC current component, but this would add cost and volume to the product. The filter capacitors would also be subject to a high ripple current - this would have a negative effect on the capacitor lifetime.