As the batteries age, you do have the possibility of a cell going bad and shorting out. This is dangerous, as it can happen abruptly.
While the battery can short a cell, it's usually not the kind of short most folks think of. Most internal shorts start out as a slow self discharge of that cell and increase as more shed material builds up. While a chunk of a grid can fracture, and can do so quickly & abruptly even this still does not create a huge current demand. With a shorted cell any sort of battery explosion will take some time as thermal runaway is not instant. Usually it's just one cell and the battery now becomes essentially a 10V battery as opposed to 12V where the shorted cell self discharges itself. First the cell itself has to self discharge then the battery will begin being
over-charged.
The result will be a very large bang as the good battery explodes from too much current draw resulting in the battery overheating.
It is not the good battery that will blow up it is the gassing/shorted battery that can
potentially blow up. Again, this is only well after the battery has shorted and the shorted cell has been continually overcharged. With a bank simply in-parallel the shorted battery will most often just deplete the entire bank. However if the bank is connected to a charger trying to push teh voltage to 14+V then the shorted battery will continue heating as it is over charged compared to the others. Most internal shorts are noticed because all batteries are being discharged faster and one cell begins overheating and gassing. Most owners notice the smell then the heat. Explosions are extremely rare and I see them just as often, perhaps more often, in single starting batteries because the construction of the grid is cheaper and weaker than a typical deep cycle or AGM or GEL product.
Yes it can happen but is extremely rare, and it can also happen to single starting batteries, not just paralleled batteries, especially when they are improperly charged or pushed well beyond their useful life..
Scared the daylights out of me, as I was only about 10 feet from the battery at the time.
This is why proper battery containment is important. Far too many boats out there lacking sound and safe battery containment or compartments as well as using incorrect chargers..
I would strongly suggest if you wire two batteries in parallel, you put a breaker between them.
In order to properly size a breaker to trip into a shorted cell the breaker will routinely trip in a nuisance manner during starting etc.. As I mentioned the explosion takes time to heat the battery and off-gas enough to cause an explosion. There will rarely, if ever, & I have never seen a single case where I could size a breaker to trip, into a shorted battery, without nuisance tripping under typical bank use. I have experimented with this at length..
Breakers are relatively cheap, and will save you from having two ruined batteries, when one decides to short out.
While breakers are cheap they will not protect paralleled batteries from one another in an internal short situation. I don't know where this myth got started but it is simply untrue when based on typical usage of a boats banks. If all you ever pull off the house bank is perhaps 15A then you theoretically could fuse between batteries but most boats have the capability of pulling hundreds of amps. A typical 1kW starter motor for example can pull as much as 450A+ during cranking. Windlass motors can pull over 300A. Thrusters can pull over 700A. Electric winches can pull over 400A. Inverters can pull over 400A.
I have physically tested this theory with actual shorted batteries every time I get one. Even taking a fully charged 400Ah AGM bank and instantaneously paralleling it with a single shorted G-27 battery there is not enough current developed to blow or trip even a 40A ANL fuse. However windlasses, inverters and engine starting can easily place enough demand on that fuse to blow it.
Beyond experimenting with just plain Jane lead acid I ran some experiments for an ABYC committee I am on using LiFePO4 cells/batteries. Even with lithium batteries we could not develop enough current between cells top trip a fuse that was sized to fully handle typical system loads.
Technically, it is better to use an isolator that is designed for that purpose, as it will prevent one battery from depleting the other one. But worse case, as least put a breaker between them.
A house bank that is used as a single large bank should be wired in parallel not through an isolator and it does not need fuses or breakers between each battery as they only that add more voltage drop & connection points but won't serve the desired purpose the way folks far too often assume it will.
If you want to eliminate parallel batteries on your boat then wire large 2V or 6V cells in series. Of course series wired batteries bring with them an entirely new issue and that is one of cell imbalances.
As has been mentioned previously any bank wired in parallel, or series, should be done with batteries of the same brand, model, date code (actual manufacturing date) and age. If you can use an impedance tester to better match the batteries when you buy them, they will last a good long time. Proper bank wiring is also key to long life. Taking the pos & neg off one battery in a parallel bank is a recipe for intra-bank imbalances and shorter bank life.
