Hi guys,
Can anyone please tell me what the yellow half drained battery warning light on my 1990 SW20 means?

Photo, perhaps?  Our SW has a red battery light . . .

If its a JDM car then it may be the battery fluid level light.

Yep sounds like battery fluid level light. If it has just come on then too up the electrolyte with distilled water to just above the plates.

If you have a battery without a sensor then just unplug the sensor from the harness to take off the warning light. The plug is 10cm or so from the sensor.

I couldn't find a replacement battery with a hole for the sensor.

Cheers

Thanks guys. Im in the middle of a cluster conversion and my new cluster doesn't have this light. So I can leave it out I guess?

Should be OK, since there's nothing for it to connect to on the new cluster. The only problem will be if the wire goes to pin on the connector that the new cluster expects to be getting a signal from - but that's just part of what you'd expect with a cluster change.