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Old 17-08-2014, 07:57   #1
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One More Time: 6V or 12V?

Hi Everyone,
This question must have been discussed hundreds of times, so please bear with me and help me decide this puzzle (one more tine). I currently have a house bank of 4x Trojan T105 Batteries which are all wired in-series. By my count, this gives me a 12V System with a total capacity of say 225 Ah; however, my usable capacity from all those Amps is only about 90 Ah. The time has come to replace these Trojans (again) and I'm thinking: why not replace them with 12V Deep-Cycle (Wet-flooded) Batteries and wire them together in parallel? Assuming I can fit 4x 12V Batteries @ 150 Ah, this would instantly increase my usable capacity to say 240 Ah. That's a whopping increase from what I have now, and for what sounds like a reasonably higher cost. I realize that 12V Batteries of a similar size as 6V Batteries must have thinner plates, but leaving the boat every year for 3-6 months has turned me into a recidivist abuser who must replace batteries at an alarming rate. What's wrong with a 12V house bank that's actually made up of 12V Batteries? Are we stuck using 6V Golf Batteries because that's all there was way-back-when when we first started mass-producing cruising boats...?


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Old 17-08-2014, 08:07   #2
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Re: One More Time: 6V or 12V?

Check your connections, if you have a 12v system and 4 x T105s you should have a bank of 450 AH, so a usable capacity of say 225 AH if you don't go below 50%. There should be a mix of series and parallel connections.

If you are replacing them often how are they being charged when you are away from the boat? Solar would be a good way of keeping them trickle charged.

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Old 17-08-2014, 08:19   #3
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Re: One More Time: 6V or 12V?

In short: If you wire batteries in series you increase voltage. (6V+6V=12V). If you wire Batteries parallel you increase the current (12v/100A+12V/100A=12V200A). You want 12V for all your consumers I guess. You can mix too, (6V+6V=12V)x2. But your question suggests that you ask a marine electrician for help and installation.
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Old 17-08-2014, 08:25   #4
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Re: One More Time: 6V or 12V?

Quote:
Originally Posted by paulw View Post
Hi Everyone,
This question must have been discussed hundreds of times, so please bear with me and help me decide this puzzle (one more tine). I currently have a house bank of 4x Trojan T105 Batteries which are all wired in-series. By my count, this gives me a 12V System with a total capacity of say 225 Ah; however, my usable capacity from all those Amps is only about 90 Ah. The time has come to replace these Trojans (again) and I'm thinking: why not replace them with 12V Deep-Cycle (Wet-flooded) Batteries and wire them together in parallel? Assuming I can fit 4x 12V Batteries @ 150 Ah, this would instantly increase my usable capacity to say 240 Ah. That's a whopping increase from what I have now, and for what sounds like a reasonably higher cost. I realize that 12V Batteries of a similar size as 6V Batteries must have thinner plates, but leaving the boat every year for 3-6 months has turned me into a recidivist abuser who must replace batteries at an alarming rate. What's wrong with a 12V house bank that's actually made up of 12V Batteries? Are we stuck using 6V Golf Batteries because that's all there was way-back-when when we first started mass-producing cruising boats...?


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same issues for us. We are 24 VDC. I made CAD models of the two vastly different boxes and also of the proposed batteries. I assembled combinations in 3-D space. I chose the highest AH that could be crammed into each box. One is 6V in series, one is 12 V wired in parallel & series. Keep the wires way oversize & short. Batteries within a box are identical.
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Old 17-08-2014, 11:57   #5
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Re: One More Time: 6V or 12V?

If you have 4 Trojan T105 then they are in 2P2S.

2P = 2 parallel
2S = 2 series

If you had 4 in series than you have 24V. Very unlikely.

You will have 450Ah of which 225Ah are usable if you have solar to charge them every day to 100%. If no solar, then you need an awful lot of motoring and a good alternator to get them full at least once a week.

To be able to help you here are a few questions
1 Do you live on board at least part of the year or do you just do weekend trips?
2 How do you charge the batteries?
3 How do you check if they are full or what state of charge you have?
4 How are the batteries connected to each other and where do the positive an negative cables attach? Draw a diagram and post it here.

As far as lead acid batterie go Trojan T105 are the best you can have. Very durable and decent price. If you have to replace them often then you have a serious charging problem that will even quicker kill any other type of lead acid.
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Old 17-08-2014, 18:36   #6
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Re: One More Time: 6V or 12V?

There are so many errors in the original post that I'm suspecting we have a troll here.

If, indeed, the OP has that pathetic level of misunderstanding of his DC system, it's no wonder he's replacing batteries so often.

I'd suggest some reading of a good basic electric reference (Charlie Wing's book, Calder, some of the hundreds of posts on DC systems on this Board and others, etc., etc.), then a re-posting of the question with sufficient details to allow a responsible answer.

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Old 17-08-2014, 20:44   #7
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Re: One More Time: 6V or 12V?

Bill +1
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Old 17-08-2014, 21:12   #8
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Re: One More Time: 6V or 12V?

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Originally Posted by paulw View Post
I realize that 12V Batteries of a similar size as 6V Batteries must have thinner plates, but leaving the boat every year for 3-6 months has turned me into a recidivist abuser who must replace batteries at an alarming rate. What's wrong with a 12V house bank that's actually made up of 12V Batteries? Are we stuck using 6V Golf Batteries because that's all there was way-back-when when we first started mass-producing cruising boats...?

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Others have pointed out your error in calculating capacity. I'll add that there is "no free lunch" for lead acid capacity.

Each LA cell is a couple of volts. 3 cells make 6 and 6 cells make 12.

The capacity @12V is basically related to plate size and density. Denser (heavier) is more tolerant to repeated deep cycles.

So basically total capacity is about battery weight - no free lunch.

You have probably the best batteries going for LA. AGM may be more tolerant to leaving alone but if you are consuming Trojan batteries frequently you have a charging issue not a battery issue.

Solution could be as easy as a 20w solar panel to keep a float charge on, or not...
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Old 17-08-2014, 22:28   #9
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Re: One More Time: 6V or 12V?

Thank you so much for pointing out that my 4x 6V Trojans in effect make up 2-pairs of 12V Batteries with a nominal capacity of 450Ah instead of 225Ah! I don't know how I missed that, I guess things get fuzzy when you haven't been on board for a couple of months... For the past 8-yrs, we've been part time sailors in the Carib, but we're hoping to move on board permanently (or 6-9 months p/yr) by next Jan. I also do have a Solar Array, a Wind Generator, a Sterling Smart Battery Charger, a Link Monitor etc. etc. Finally, I don't see anybody raising red flags about the principle of switching to 12V Batteries, except that they may be somewhat bulkier (and/or may be overkill).


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Old 17-08-2014, 22:40   #10
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Re: One More Time: 6V or 12V?

I think you are misinterpreting the posts. No one is advocating 12V batteries and at least 2 have stated you have the "best' solution going.

Your stated goal in post one was to increase capacity by switching to 12V and now you know that is not possible so why do you want to switch?
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Old 17-08-2014, 22:55   #11
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Re: One More Time: 6V or 12V?

Why do you say that it "is not possible" to switch to 12V batteries and increase the capacity of my house bank?


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Old 17-08-2014, 23:18   #12
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Re: One More Time: 6V or 12V?

Because the 6V golf cart batteries already have very high power density. Going to 12V batteries will not increase the stored energy for a given space/weight. 6V golf cart batteries are about as good as you can do with lead acid batteries as far as power/weight/space is concerned.
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Old 17-08-2014, 23:44   #13
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Re: One More Time: 6V or 12V?

Research LiFePo4 batteries. As far as usable A/Hrs are concerned, seems to be an alternative. The reports of little drop off in voltage under load is a huge benefit, especially when matching usable amp hrs... Oh and make sure not to confuse Boeing and other lithium tech with LiFePo. From my extensive reading and research, the technology from one to the next Lithium is vastly different.

Price wise, the difference is not as drastic as the "on paper" seems to be. SOC and volt drop gets the 2 much closer comparatively speaking

Any naysayers, please post links, I am in the learning phase and appreciate any knowledge
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Old 18-08-2014, 00:30   #14
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Re: One More Time: 6V or 12V?

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Why do you say that it "is not possible" to switch to 12V batteries and increase the capacity of my house bank?


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Read again - the standard "cell" for LA batteries puts enough plates in a cell to produce about 2 volts. The capacity of that cell (amps) is determined by how big and dense the plates are.

To make 6 volts you need 3 cells. To make 12 volts you need six cells. It doesn't matter how many "boxes" those cells are in.

With some nuances the weight and size of a battery determines it's capacity for a given voltage 6, 12, 24, 48 - whatever.

Quote:
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Because the 6V golf cart batteries already have very high power density. Going to 12V batteries will not increase the stored energy for a given space/weight. 6V golf cart batteries are about as good as you can do with lead acid batteries as far as power/weight/space is concerned.
What he said...
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Old 18-08-2014, 11:13   #15
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Re: One More Time: 6V or 12V?

Paul, when you say you are changing these batteries at an alarming rate, how often is that?

If you have solar and it is left connected when you are away then the batts should last a long time or do you isolate them when you leave the boat.

Finally, if you have a link 10 then presumably you have some idea of your daily electrical needs. Base your battery needs and charging around this. No point in having more batteries if you can't easily charge them.

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