You know, the time when you do your preventive maintenance.
Unless you have sealed AGM, Lithium or other such batteries you need to check the water at least once a month. Even if you do have sealed batteries, there are other things to check.
Back to the water, this is a rehash, but in case you missed it, keep a supply of distilled water and a box of baking soda handy. A rubber apron, gloves and safety glasses should be in your kit. Oh, you’ll need a wrench and a plastic funnel, as well. Keep the water up to the split ring. Use the baking soda to neutralize the acid that accumulates on top of the battery.
Make a note if a battery or cell uses more water than the rest. That battery may have a problem. If water consumption goes up all around, you could have a charge controller problem. I had a charge controller kill some fairly young batteries, so I know it can happen.
Keep the batteries clean and dry. After a washdown with the baking soda, give them a good wipe or they will be white and crusty from the soda. I find that a blow nozzle and compressed air can make cleaning a breeze, but not everybody has their batteries where they can get away with that.
You may need to clean the terminals, too. Acid fumes leak around the terminal posts and can make a mess of things. This can cause resistance, heat and wear on the terminals. Wearing safety glasses, I use a fine rotary brush in a cordless drill to scrape off the crud and polish things back up. A dab of Vaseline or even axle grease on the terminals and posts can help limit a recurrence. Since you may be stirring up some lead dust, you might want to add your Covid mask to your safety gear. Replace or repair cables, if needed.
Even if your terminals are all clean and shiny, take a wrench to them. You’ll be surprised how they can get loose. I was at a guy’s house the other day. He isn’t big on maintenance and as he began his biennial battery check it took gallons of water to top the Trojans up and the terminals would spark under load because several were loose. Batteries need water because they turned the old water into explosive gas. Do you really want sparks around that??? Don’t wait two years to check this stuff. Batteries should last longer than two years, but not if you don’t maintain them.
My electric jalopy gets checked monthly, but it lives a hard life and had one terminal loose and two that were getting right colorful with corrosion. It does not take long. On the other hand, my boat uses an identical battery set and there are rarely issues with it. The difference, I think, is that the boat is a much cleaner environment. No dirt to accumulate with acid. The jalopy got its batteries cleaned, but with the work it does it will soon be a mess, again. Both the boat and the jalopy are part of the solar power storage. Despite the hard use, the jalopy batteries are pushing 6 years of service.
In the area where you have your charge controllers and inverters, you won’t likely find any corrosion, but those cables get loose, too. I keep the appropriate tools right there at the control panel so I don’t have to go hunting down the right Allen wrench or crescent wrench or screwdriver. Infrared, non-contact thermometer guns are helpful tools for a number of occasions in the solar biz, including checking terminals for poor connections by looking for heat. Yes, you can use a finger to detect heat, but there are places I personally don’t care to stick my fingers!
I have one set of batteries in one of my experiments I need to get to because it looks like a cat peed on them. I’m not really looking forward to cleaning that up. I can’t really prove it was Smokey, my big, half-wild tomcat, but he HAS been walking funny, lately. Add that lesson to your list of safety rules: Never pee on a 48 volt battery pack.