The logic behind 2 batteries paraller is simple if one is weak or die , takes the other with it.
than you always buy both the same, identical and chance is, they last much longer.
I have seen trucks and power boats hard wired from manufacture.
Select to expand quotesamsturdy said..
Felix, I have two batteries of similar size connected through a rotary switch so I
can use one or the other or both. One is the house battery and the other the starter.
Often when I'm motoring I will charge both together. Is it true that if the cables from
switch to batteries are different lengths, the battery on the shorter length will charge
first ??.
The resistance difference between the cables would be so small that it would make no difference at all, if one battery charge faster it could because it is used more so more discharge or in lesser condition and have a greater internal resistance .
To Test a battery you should measure the voltage between +/- while using it, if it goes below the nominated voltage the battery is close to his life end, measuring the specific gravity of the acid give a good indication of charge but not of condition of the battery.
The best is to use a load bank but it is very expensive tool.
I used to service heavy cleaning equipment driven by batteries, one of my client told me once: I really care about the batteries, check them weekly and add water when needed, I always use boiled water 
..............
The basic rule is: series connect increase the voltage and parallel increase the capacity (Amp. per hour)
Select to expand quotefelixdcat said..samsturdy said..
Felix, I have two batteries of similar size connected through a rotary switch so I
can use one or the other or both. One is the house battery and the other the starter.
Often when I'm motoring I will charge both together. Is it true that if the cables from
switch to batteries are different lengths, the battery on the shorter length will charge
first ??.
The resistance difference between the cables would be so small that it would make no difference at all, if one battery charge faster it could because it is used more so more discharge or in lesser condition and have a greater internal resistance .
To Test a battery you should measure the voltage between +/- while using it, if it goes below the nominated voltage the battery is close to his life end, measuring the specific gravity of the acid give a good indication of charge but not of condition of the battery.
The best is to use a load bank but it is very expensive tool.
I used to service heavy cleaning equipment driven by batteries, one of my client told me once: I really care about the batteries, check them weekly and add water when needed, I always use boiled water ..............
The basic rule is: series connect increase the voltage and parallel increase the capacity (Amp. per hour)
Thanks mate. A neighbour has a big boat and has upgraded his battery bank
ensuring that all his cables are the same length. He was the one who told me
this theory.
Select to expand quoteKarsten said..fishmonkey said..Karsten said..PhoenixStar said..
... the older battery will be slower to charge, will draw current from the newer battery...
Not sure I follow.
Notwithstanding what the OP chooses to do, I'd like to understand this issue.
If the two batteries' positive and negative terminals are tied together with short, thick wires, their voltages will always be exactly the same.
Current will only ever flow from a high to a low voltage. If the voltages are tied to be the same, no current will flow.
For the sake of our collective understanding, if I'm missing something here, I'm happy to learn.
...connecting them doesn't magically make their voltages the same. the voltages of the batteries will equalize and settle at an intermediate point (the physics of it isn't a simple equation)....
In the nicest possible way, I must disagree.
I'd like to clarify this because there may be Seabreeze forum members/visitors that read this thread in future looking for info on the parallel connection of house bank batteries - it's a common question.
Excluding the moments after initially connecting the positive and negative terminals of two flooded SLA's together (parallel) with a short, thick cable, the voltages will thereafter be exactly the same, no matter internal resistance differences or how many charging-usage cycles they go through.
The combined voltage will not be an "intermediate point" voltage, but for all practical purposes the same voltage - eg. if one is 12.6v, the other will be 12.6v. If one is drawn down to 12.2v, the other will also read 12.2v - it has no choice.
Reason: Resistance of reasonable thick copper cable - for example, gauge AWG 4 at 400 mm length (and AWG4 is not all that thick)- is approx 300 micro-Ohm. Application of Ohm's law shows that the two batteries will never maintain a voltage differential across such a minute resistance.
a bit of confusion here. i was not suggesting that there would be a persistent differential voltage between the batteries. what i meant was that the battery with higher voltage would discharge some of its energy into the battery with lower voltage. so the final voltage would end up somewhere between the two starting voltages.
fishmonkey, you should complete your explanation, it's 100 % correct,
final voltage would be somewhere berween but not for long, chemical proces in bat. is ongoing,newer stops
and charging - discharging each other continue until they both dead.
Select to expand quoteCharriot said..
fishmonkey, you should complete your explanation, it's 100 % correct,
final voltage would be somewhere berween but not for long, chemical proces in bat. is ongoing,newer stops
and charging - discharging each other continue until they both dead.
+1 for that statement
I work for a company which installs stand alone solar/battery setups. Big and small. The last one consisted of 24 series connected batteries weighing 235Kg each (approx 1200AH each). We avoid connecting batteries in parallel. It can be done and I actually have 5 x 160Ah batteries connected in Parallel on my boat. But I went to great length to avoid all the pit falls and issues of connecting batteries in parallel.
In about a months time we will be replacing the batteries on a remote resort that had 4 banks (24 series connected batteries) of batteries connected in parallel where 2 of the banks failed. Yes, when the batteries were all connected to the main bus, the voltages were the same but when we monitored the batteries, the charge current for each set was different and so was the discharge rate from each bank. The different discharge rates were like 12/60 amps. And these were identical batteries. When we isolated the 4 banks from each other, they all had different voltages. So you don't need to be a rocket scientist to work out what they were doing to each other when they were connected to the main bus in parallel again.
If you think all AGM batteries can be simply paralleled up or the same with flooded lead/acid, then I think you need to look bit further in to what you are doing. My opinion comes from hands on experience and not something I read or heard about. The decisions we make with the design of our installations have to last at least 10 years other wise we are out of business.
But its your boat and you can do what you want. And I suppose it's your money as well, but please be careful. Batteries can unleash an amazing about of power in an instant. 
Great update Crusou, simply each cell has mind its own , if one goes, takes whole lot with it.
and we talking professional installation , where all bat. are the same and electronic control.
we lucky, we use mostly bat. conected in parallel / there are 6 volts banks, but approach
will be different , I can advice on that if need it/. working over over 40 years with
all types of batteries inc. a few years in army running petrol and diesel gen. sets.
Bat. are very friendly, do not need to worry about different age, capacity, length of wires
and all sort of funny suggestions, just disconnect interconnection when you leave the boat.
If your system has solar charge and auto- night lights, there is no idle, and batteries
have nothing else to do, just talking among each other, I would leave them connected
permanetly but often, You must monitor them individually under load.
Talking to the neighbour who upgraded his battery bank. He said he'd bought a device
supposedly invented by an Aussie that rids/prevents battery sulphation. Cost about $250.
Anybody know this device ??.
