Quote Originally Posted by moetorola View Post
Yes, the both batteries voltage wise would be at some voltage since they are parallel but not necessarily capacity wise.

Batteries of different age, manufacture date for a few examples. It is never a good idea to charge two 12volt batteries the are in parallel. They should be monitored separately.
While the voltage may be the same one battery may reach 100% charge while the other is at 80%. Or vise versa, while one battery has reached 100% the other 80% the charger may over charge the fully charged battery. It really depends on charger being used at the end of the day. The old cheap metal style chargers with no regulation voltage based charging only, I would not trust em.

It is frowned upon in the industry I work in. When in nearly every case the batteries are left unattended and some cases forgotten about. This method may be fine for charging up, but for float charging long term, as most of us do today, not recommended.

Luckily with todays chargers having individual banks, each battery is monitored separately we don't have to worry about this. Except in parallel.
Sorry but that is wrong. It is a common misconception that the charging current will divide equally between 2 batteries in parallel which creates the misunderstanding that the weaker one will be overcharged before the better one gets fully charged. However you can even charge batteries of radically different sizes because the current divides in proportion to their capacity so both charge at the same rate and reach full charge at the same time.

When charging in series where the current is identical in each battery your argument is valid but it doesn't apply to parallel operation. Due to frequent misunderstanding of the difference between parallel and series, the rules for matching gets applied mistakenly to parallel.

You don't say what industry is having problems in your experience but I've worked in telephone exchanges with a 2,500 sq ft room full of backup batteries in parallel on standby float. It is standard practice for off-the-grid solar systems.