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Topic Review (Newest First)
11-27-2006 11:19 AM
DeepFrz One of the most informative threads I have read on paralleling batteries. Thanks everyone.
11-15-2006 11:37 PM
This discussion has produced a conclusion...

Gongrats all, on a helluva thread. Much dialog (knowledgable to boot) on a much needed subject. After reading and digesting it all I've come to my own conclusion and will implement it forthwith in my old 35'. I agree with the premise that one large bank (four or six Trojan 105's) is a better setup, with a completely separate engine bank. One master on/off for each bank. In cases (hopefully not) that either goes flat I'll simply carry a heavy duty set of cables for emergency jumpering.
I believe it will be easier to monitor one large house bank (using a Link2000 or similar) than wiring 2 separate house banks, associated switches, wiring, yadayadayada, and monitoring both, not to mention remembering to switch back and forth...I've got enough other things to remember, like where I stashed my spare Racors...
Cheers, and thanks!
11-14-2006 11:41 PM
CharlieCobra Helluva thread.
11-14-2006 08:07 AM
dave.verry Dog,

Thank you for bring up this point, I was remiss in not explaining the difference.

Correct, there are two additional single points of failure, the connections between the 6V blocks (discounting the wire and crimps). The first configuration has redundancy, the second does not, so the first is preferable. Increased reliability can be accomplished by feeding both strings to the same terminal point, instead of one string to the other.

At some point the amount of redundancy in your system depends on how much you want to take it. At the extreme you can have two totally independent systems duplicating everything including loads (electronics, lights, refer, etc.). At some point you have to decide how much redundancy to have.

Don’t get me wrong, I have spent a lot of time on this thread arguing for paralleling batteries as a means to improve reliability and capacity, and would advocate doing it the first way, but in the end it comes down to personal preference.
11-13-2006 07:52 PM
sailingdog The only problem I see with this Dave is that the second configuration is a bit more prone to failure than the first.

From what I see... if any battery in the first configuration dies, you have a 12V and a 6V battery, which may or may not be connected, depending on the failure. On the second one, if a battery fails, you will definitely have a connection between the 6V and the 12V battery.

However, I am a bit rusty on my electrical circuit theory, since its been over 20 years since I've used it regularly.
11-13-2006 08:31 AM
dave.verry Bill is correct, in essence you are making a 12V battery.

There are two ways, actually. The first is as Bill described it, putting two 6V batteries in series (positive to negative, creating one “string” of 12V, and connecting the two positives and the two negatives of the strings together [the posts not connected to the other battery in the “string”]) the second is to parallel two 6V batteries together, twice (positive to positive, negative to negative) so you have two of these assemblies, and then connect those assemblies positive to negative to form the 12V “string”.

Use whichever way best fits your boat and how the batteries and wiring fit.
11-12-2006 10:52 PM
btrayfors Ralph,

I'm assuming that you have a 12V system, and that you want to use four 6-volt batteries for the house bank.

Essentially, you make two 12-volt batteries by connecting two batteries in series (positive to negative). THEN, you connect the two 12-volt batteries in parallel (positive to positive).


1. Connect the first battery's positive pole to the second battery's negative pole. This will give you 12 volts, measured between the two unconnected poles.

2. Do exactly the same thing with the third and fourth batteries. Now, you have two 12-volt battery banks, each consisting of two 6-volt batteries in series.

3. Now, connect these two banks in PARALLEL, by connecting positive to positive, negative to negative. You now have one large 12-volt bank consisting of four 6-volt batteries in series/parallel.

This combined bank will have twice the capacity in AH of each individual bank. If you're using golf cart batteries with a nominal 20-hour rate of 220AH, then you will have a 440AH house battery bank.

If you have any doubts or uncertainties about this, ask a knowledgeable friend.

11-12-2006 09:44 PM
ralphhurley Dave,
I'm trying to use four 6 volts to replace my house bank. When you say " parallel" do you mean that the batteries are connected pos to next battery's neg pole? If so, does that continue on to battery 3 and 4?This is as opposed to series which would connect all four pos poles and all four neg poles seperately?
10-10-2006 08:18 AM
Paralleling does not cause premature failure!


Not true. When the cells are connected together with cable they will be at the same voltage in all conditions. As they are charged the charger will impose the voltage and the batteries will accept the charge dependent on their capacity and charge state. If they are exactly matched (an impossibility, I know, but just for the sake of argument) they will accept the same current, if the charger can deliver 30 amps, both will have 15 amps going in. As the charger reaches its upper voltage limit (boost or float charge) the current will taper off in both batteries until the current counterbalances the self discharge of the batteries (full charge).

The same happens if the batteries are not matched. The difference occurs in the amount of current each battery accepts curing the charge cycle. If one battery has twice the capacity of the other (through wear-out, or even if two batteries of different capacities are used), and using our 30 amp charger, the larger batter will accept twice the current of the smaller one (20 amps for the larger, 10 amps for the smaller). The same taper will occur as the final voltage is reached and both batteries will reach full charge at about the same time.

In the discharge mode, a similar thing happens. As you put a load on a battery, the internal resistance causes the output of the battery to drop immediately. This drop is small so if you try and measure it you will need a voltmeter to several decimal places, preferably measuring 1/100 of a volt @ 12volt range (this would be called a 5 and ˝ digit meter). The important point is that this voltage drop allows the current of parrelled batteries to be shared in spite of their relative capacities! Taking out two examples the exactly matched batteries will exactly split the load current equally, and our mismatched batteries will have the current from the larger one at twice that of the smaller one.

This equates to the load currents of parrelled batteries as having the same relative current based on their capacity. Both batteries will discharge at the same rate (but with different currents) and have the same state of charge at the same time, and reach the end charge state (weather it’s 50%, 25%, 100%, or what ever) at the same time!

So, parrelled batteries charge at the same time and discharge to the same relative state of charge at the same time. Each batter will do the same relative work based on its own capacity. If one fails faster then the others, it would have failed in the same time if exposed to the same duty even if used independently. Paralleling batteries does NOT cause them to fail prematurely.

BTW, this conclusion is based on many, many scientific papers I have been using in the course of my work, and that I have reviewed while and after I have been posting on this thread. I will make them available to any who ask.
10-10-2006 05:18 AM
neilchristophers All good reading and fair comment from btrayfors with regard to switching banks at 3am so we must be better off with one bank ! as for parallel or series if this is a option series must be better as two cells in parallel will have different voltages and the higher one will allways be doing more work so will fail earlier, then leaving the job up to the other one on its own which will now fail before its time . Not what we want. Neil
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