Can I safely put 4 12v 100ah agm batteries in parallel?

Can I safely put 4 12v 100ah agm batteries in parallel?

Searches have turned up everything from no more than 2 to multiple (no real number given).

One site diagrammed 4 in parallel with each battery fused in addition to a load fuse. I think the idea being if a battery shorted it would blow its fuse, protecting the other batteries.

I can't afford LiFePo4 drop in batteries and I don't have the experience (yet) to build my own.

I also don't want larger capacity batteries. Handling larger batteries would be a challenge for me.

Yes, you can put (4) agm batteries in parallel. I wouldn't bother with fuses between each battery, seems like a lot of fuss to me and not necessary.

In some locations wearing a seatbelt in cars isn't compulsory. You might well get through your whole natural life just fine, but you could also find yourself launching through the windscreen to end up a crumpled bloodied mess on the road, dead.

It's rare for a lead acid battery to develop a shorted cell but it does happen. In that case the other batteries in the set will push as much current at the faulty battery as they can and it will draw to bring the voltage to equalisation. That can cause all sorts of nasty problems. I fit fuses / breakers between all parallel batteries in high capacity systems. As a percentage of cost it's not that much really.

Well, think about this as you do your money math. Your 4 100ah AGM batteries would be replaced by 2 lithium. While the money math is still slanted toward AGM, then factor in that 4 of them will weigh 320-360 pounds and 2 lithium about 60. Also that the lithium will outlast the AGM by about a factor of 3. So while the AGM are less of an outlay initially, you'll be replacing them in a few years. People will likely now jump in and tell you how long they have had their AGM batteries, but remember that if you look, you can find people who had a dog that lived to 17-18 years too, but 13-14 is about average, and I am talking about averages, not the exceptions. I was not thrilled about paying $615 for a battery, but I put it on a credit card and it is now half paid for. By Dec 1 it will be all paid for, and I should see something like 8 years with it. For example, a V-Max Tank 100ah deep cycle AGM will run you $260. You need 2 of them to perform like a 100ah Ruixu lithium. That costs about $650 these days. That extra $130 buys you 4-5 more years of use. And for MANY pound less, something that matters more in mobile applications, as you don't want to be driving around with that extra weight. Just something to think about.

That strayed off topic some but I wanted to toss that in there just to give you an option to think about, especially when you said lithium was too expensive. In the long run, they are less expensive.

I didn't add a lot of detail to original post as I didn't want to get into AGM vs lithium. You don't have to convince me that, over time, lithium is cheaper, weighs less and takes less space.

I am remodeling a 200 sq ft. room on the back of the house. Space and total weight won't be a problem. I can handle moving around 75 lb batteries but these old, wore out joints would have trouble with 100+ lb batteries. Upfront cost is a consideration.

The room has grid power. I will either go with an 'all-in-one' solution or an ATS with separate charger and inverter. Other than a freezer, the room only needs to power a ceiling fan and a few lights. I put a watt meter on the freezer for a month and it averaged 1200 watts/day. The room is rarely used at night.

We have lost power for extended periods before and that is the main reason for the solar. It is my insurance policy that I hope to never need. I want to be able to plug in my fridge and possibly the microwave and/or induction plate during any outage.

I expect the 800 watts of panels will easily power the room most days and batteries should have no problems at night. I don't see the batteries getting a lot of use. I want to start with 4 100ah AGM configured to 24 volts. I suspect the setup is overkill and I likely won't need more than that, however, I want to know that I can add another 4 batteries if it turns out we draw more power than I anticipate or if I decide to add more function to the room.

If I want to expand capacity, I would make two sets of two new and two 'old' batteries in parallel then connect them in series. That would double my AH and keep it at 24 volts for the equipment I have.

I could start with a 48v system, but I'm not sure I want to do that when the smaller system will probably handle what I need/want. I just want to make sure I can easily add capacity should the need arise.

I'm still in the planning stages with this room. It will be a while before I'm ready to buy. I may look a building a small LiFePo4 battery to learn on. Maybe by the time I'm ready for batteries I will be comfortable building what I need. I'll also keep looking at prices. If I feel I can go lithium for 25% more than AGM as eddie1261 suggests then I will do it. Nothing on credit though. If I ain't got the cash I don't get the goods, period.

You can safely connect many LA batteries in parallel as long as they are in good condition and they are the same capacity and type.
Don't mix old and new batteries.
But the wiring is important: let's say 4 batteries: you should connect your + output cable on the first battery and the - output on the forth battery. Then the power will be distributed evenly for all batteries.
If you connect your + and - output cables to the same battery then this battery will deliver 70% of the current the next 50% and bat 4 only 25%.
Your first battery will die first.

Use same cable type for all connections.

It's better to start with 48V. Less current= thinner cables, less voltage drops, more stable input to inverters.

Btw: I started with 10KW deep cycle batteries @ 48V (8*110Ah) and 2 year later I added 20KW lifepo4 32*200A for my home.

I wished I never bought those lead acid. It looks cheaper to start but in the long run it is not. On top you get next problems:
Gassing or , dryout on overcharge, heavy, large voltage drops for heavy loads, bad for inverters; unstable outputs, low life cycle and 30% power loss for every charge cycle. You charge 100Ah and you get back 70Ah.
Not really suitable for solar because of sluggish charge: the last 30% takes hours. Needs equalization every month.

The advertised capacity is usually +- 60% in reality. If you discharge > 50 DOD then expect max 300-400 charge cycles.
For a 100Ah battery you have hardly 30Ah available.
For a 100Ah Lifepo4 you get at least 80Ah, a much longer lifetime and very stable output .