As you can see from the battery picture below.

How do I know when I need to replace the battery?

When I unscrew those yellow lids I can still see liquid inside the battery - do I replace the battery if I see low levels of liquid inside?

Or is there another way to tell?

 

 

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batteries will last at least 2 years minimum if maintained, the liquid you see is water, although in a charged state will be hydrochloric acid.
to test a battery you either need a hydrometer to measure the specific density of the acid or a battery tester. a good battery will have ay least 12.5v but that is no way of checking its cranking capacity.
the level of the acid needs to be above the lead plates you see but not all the way to the top of the lids, bout half inch below.
some batteries will have an inspection hole with a ring stating condition of the battery. green = good, black and white are the other 2 but i cant be sure now which means dead and whcih means recharge/replace?

I replace the battery basically when its dead, i.e will no longer hold its charge

If you see low levels of liquid (electrolyte) inside the battery, top up the levels with water up to the indicator line, DON'T overfill them

hi, i dont know about perth but most battery/auto places here will put a meter on it, load it up and test it for free.
a charged battery will have 12.6 volts or more in it and running should be about 13.8 volts.
the water u add to the battery is distilled or demineralised ,for the level there should be a little tab inside the battery to mark it (usually around 10mm-15mm above lead cells)

Im gonna say it anyways but you do know you have to use Distilled Water (or rain-water) to fill your battery.

Thanks for the tips

What happens if you dont use distilled water, and instead put in tap water?

Your supposed to use distilled water, as it doesn't have the minerals that tap water does. The minerals in tap water stick to the lead plates inside the battery and decrease the life of it or something.

Ive used tap water in the past to top up though and never had any issues. All comes down to mineral content in the water i guess.