Although I will not tell you to not wire them in parallel, there are some dangers you should be aware of. If one battery shorts out, the other batteries will dump gobs of current into the shorted battery. After a short time there will be a significant explosion. If the batteries are not contained in an explosion proof container, it could do serious injury or cause a fire. I had this happen in my motorhome. I was about 10 feet way when the battery exploded. Fortunately the battery compartment contained the explosion, but if the batteries had been exposed, I could have been injured from flying plastic. It may be wise to isolate each battery with a 50 or 100 amp fuse. This way if a battery shorts out, it just pops the fuse, rather than causing other damage.
When this happens the most dangerous time is when the system is charging and the charger is attempting to push voltage above static resting voltage. This is one reason I really dislike leaving unattended vessels with large 80A plus inverter chargers running with no one on-board. IMHO this is a very unsafe practice regardless of series, series-parallel or parallel banks. The
smallest charger to maintain charge and supply DC dockside loads is always the safest choice. An 80A + charger is always the least safe choice for unattended charging.
The risk of internal shorts and large chargers also highlights a
failing of the charger industry in not allowing for, or designing for, a temp sensor for each battery. This type of sensing system is not hard to do, and it was a topic of conversation just two weeks ago, over lunch with a few of the
industry Guru's..
A member here had a short happen three weeks ago with a 6 battery 900Ah parallel string. IIRC he had two 40A chargers running (80A) and the shorted battery got to 159F. Once charging stopped the battery began to cool and it was then disconnected from the string...
If it's just the bank, not charging, then an internal short usually just depletes the entire bank through the short in one cell. It does cause that shorted cell to gas and
explosions are usually a result of non-ignition protected devices in the battery compartment. In a situation where it's unattended heat can cause connections inside the battery to melt and arc too but this type of explosion is much, much rarer. I had it happen on a commercial guys work boat when he left a rolling 80A shop car charger attached to a single battery in a 22' aluminum work boat at a boat yard dock. The battery had shorted internally and then the charger dumped what ever it could into for a period of time until it exploded.
The current derived between parallel batteries is limited by internal resistance and voltage differentials so a bank
not charging is at considerably less risk of a dangerous explosion than one that
is charging. One charging, especially with a huge charger, is at a significant risk of thermal runaway. Of course this can happen, when charging, in a series packs too. When not charging, a short in a series only pack will have no current fed to it, but during charging it will have charger potential feeding the short.
It would be extremely hard to design a fusible system where by the fuses would trip if one battery became a 10V battery, as opposed to a 12V battery, without also tripping during normal use. I actually tried this in my shop, with a shorted AGM battery, and no matter what I did I could not get a properly sized fuse to trip, between batteries, under internal battery shorting conditions.
All that said shorted batteries are very rare when the batteries are taken care of and tested regularly. Even when ignored these events are still quite rare. The highest incidence I see, of internal shorts, occurs in flooded 4D & 8D batteries and I have not been able to get a good explanation as to why other than;
"Our 4D & 8D product are not designed as cycling batteries. Deep cycling this battery probably led to an early plate failure."...