Any battery that is low enough that the plates are exposed is a problem. 3 conserves you trouble think about a circumstance when the existing battery you have goes down and you are in the middle of no place.

What You Need To Know About Your Rv Batteries Koa Camping Blog
Batteries in parallel problems. Such a configuration is called 4s2p meaning four cells in series and two in parallel. Laptop batteries commonly have four 36v li ion cells in series to achieve a nominal voltage 144v and two in parallel to boost the capacity from 2400mah to 4800mah. If you have a wet cell battery are filling it for the first time and are using an old style battery charger non smart charger and short the terminals while you are filling it yes it is possible to hook up the charger backward and reverse charge it. For secondary rechargeable batteries the stronger battery would charge the weaker one draining itself and wasting energy. Negative to negative and positive to positive. How to connect lithium batteries in parallel and series.
Personally i would set the charging to the higher rate for the bank. However once a month or so id switch out each battery and charge it individually on a smart charger such as a c tek to equalise the bank. Understanding about repairing batteries can assist you with this issue. If all the batteries or the majority are down it is simply time to add distilled water. Some packs may consist of a combination of series and parallel connections. Recondition batteries in parallel all you require to do is to follow these basic steps.
If one battery is only down a little and the plates are all covered it is possible the battery is beginning to fail. To join batteries in parallel use a jumper wire to connect both the positive terminals and another jumper wire to connect both the negative terminals of both batteries to each other. If you connect rechargeable batteries in parallel and one is discharged while the others are charged the charged batteries will attempt to charge the discharged battery. Actually yes but not without help. Series connection increases battery voltageparellel connection increases battery capacity. You can connect your load to one of the batteries and it will drain both equally.
Reversing the polarity on a battery can happen only a couple of ways. Having three batteries in parallel does prove slightly more problematical when charging. Learn about adding water and good battery maintenance tips here. With no resistance to slow this charging process the charged units can overheat as they rapidly drain and the discharged battery can overheat as it attempts to charge at well above its design.