12.8 - 12.9 volts is a fully charged battery
You cannot use a 12 volt charger to charge 2.6 volt batteries. The 2.6 volt batteries will become hot, very quickly, and explode.
yes
No!
you only need two 12 volt batteries to make 24 volt. connect them in series
Yes, if the two 6 volt batteries were connected together in series.
how Can you charge multiple 12 volt batteries with a car?
Depends on the battery size. If your boat is a 12 volt system and you wire two 12 volt batteries in series you then have 24 volts. If the batteries are 6 volt batteries you will be fine. If you have a 12 volt system on your boat and want to use two batteries then wire two 12 volt batteries in parallel. You will still have 12 volts but will have double the amperage available and this will extend the running time of the batteries.
Two 6 volt batteries in series, yes. In parallel, no.
Depends on the amperage of the batteries you are using. Both setups will give you 36 volts, but you will normally have a longer run time with six 6 volt batteries in series than three 12 volt batteries. That is why golf carts use 6 volt instead of 12 volt batteries.
Yes they do make 9 volt batteries that are rechargeable. There is the Energizer Recharge, NiMH and the HiTech brand carries 4 types of rechargeable 9 volt batteries.
You must hook them up in series. You will need four 12 volt batteries to do this.
Duracell and Energizer are two reputable manufacturers for 9 volt batteries. These brands are very reliable and dependable brands. Both Duracell and Energizer offer rechargeable 9 volt batteries are well as Alkaline batteries.