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Old 11-01-2019, 12:22   #91
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Re: 4 six volts better than 2 twelve volts

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Originally Posted by MarknLori View Post
Can anyone explain why it is a problem to connect two twelve volt units in parallel? When I say twelve volt units I am referring to a pair of 6 volt batteries connected in series.

Many people do that. Many people don't. With batteries matched by brand, capacity, age and health, it is generally not a problem until one of them goes north on you. Me, I do not like batteries in parallel and have never liked batteries in parallel and will never wire batteries in parallel. It's kinda like green eggs and ham. Except I will never try it.


You do get a bit of extra juice out of one larger bank than two smaller ones, due to Peukert Effect, but this is mostly only an issue with high discharge current. At C/20, Peukert effect effectively does not exist. Just sayin.



But you can have your cake and your ice cream, too, by wiring two series banks that can be switched into parallel configuration, or kept isolated, as desired, with an A/B/Both/Off switch of decent quality.
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Old 11-01-2019, 12:32   #92
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Re: 4 six volts better than 2 twelve volts

Maximum three strings in parallel is a good guideline. After that may need a BMS to avoid balancing issues, but rarely done with lead chemistries.
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Old 11-01-2019, 12:41   #93
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Re: 4 six volts better than 2 twelve volts

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Originally Posted by Beau.Vrolyk View Post
The Trojan deep-cycle batteries are dramatically better than the much cheaper batteries you can buy in auto parts stores and discount shops.
Yes 12V batts sold in such places are crap. And Costco 6V GCs aren't great. But the Deka / Duracells are excellent, and Sam's is a discount place.

An exception that proves the rule.


> you really DO need a deep-cycle battery for a typical cruising life-style when you only want to run your engine or generator once or twice a day

yes. Also, when regularly away from mains for more than a few days at a time, solar is necessary for longevity to finish the long tail, regularly try to get to 100% Full. Running ICE sources early AM helps.

Firefly is the lead exception, will let you stretch out the PSOC lots longer without murdering the bank.

Lots more expensive per AH, and also as it happens available in 12V!

Another exception that shows the "rule" is just a guideline to help those with a more limited understanding of the details.
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Old 11-01-2019, 12:50   #94
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Re: 4 six volts better than 2 twelve volts

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Where do you get these $100 Trojan T-105s? They are $200 each here in the Pacific Northwest, My bank is now ten years old and I need to replace them this spring.

Florida
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Old 11-01-2019, 13:22   #95
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Re: 4 six volts better than 2 twelve volts

Probably anywhere with **lots** of golf courses.

Try buying them in a remote Pacific island, cost as much as Rolls / Surrette do in NA.

Transportation costs after the first leg from the factory are a huge proportion of the final consumer cost.

That's why getting a big bank online is very rarely good value.
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Old 11-01-2019, 14:14   #96
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Re: 4 six volts better than 2 twelve volts

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Transportation costs after the first leg from the factory are a huge proportion of the final consumer cost.

That's why getting a big bank online is very rarely good value.
True for sure... and that's why we are not buying Fireflies here in Australia!

And FWIW, T-105s run around 250 AUD each here...

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Old 11-01-2019, 14:30   #97
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Re: 4 six volts better than 2 twelve volts

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You don’t get anything for free, double the volts is half the Amps and obviously the same the other way, 6V batteries only have half the amps when you connect them as 12 Volt for your boat or anything else.
In effect not getting anything for free, so the battery weight will be the same, for a given power output.
I use 2 small 12Volt sealed batteries ( about the same size as a car battery ) for small loads and 1 x 12Volt battery for the Engine, with a dual charger,
Charging is more important, so Shore power ( I have it but never I never use it ) charge both sets from the Engine and both from the Solar panel ( just one 148W )
That panel keeps everything topped up even in the UK with no little sun.
Most important is to reduce DC loading with modern equipment and LED lighting.
This is electricity 101. In series you add the volts. In parallel you add the current. You are not getting half the current - you are getting the same current, twice the voltage and the same wattage either way. There is really no difference between the six volters and the twelve volters except the series connection is external to the battery case. The power is the same. The big advantages are: easier handling of the weight, more flexibility of installation layout and a battery that is built for heavier discharge & more recharges.
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Old 11-01-2019, 14:34   #98
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Re: 4 six volts better than 2 twelve volts

2 6 volt batteries have more plate area than 1 12 volt battery
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Old 11-01-2019, 14:39   #99
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Re: 4 six volts better than 2 twelve volts

I also vote for the GC-2 size golf cart batteries, and also confirm that when you wire two in series you don't get any more Amp-Hours, only more voltage (i.e. 12 volts from the 2 times 6 volts). Typical Amp-Hour ratings for golf cart batteries are about 220 Amp-Hours per battery. There is one more benefit that I don't think has been discussed, and that is that if one cell fails, you can change one GC battery and that is a lot less expensive than changing one 8D size battery.
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Old 11-01-2019, 16:30   #100
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Re: 4 six volts better than 2 twelve volts

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Originally Posted by gonesail View Post
tough to beat 6v. trojans wired for 12 volts. .
In America maybe

In Australia they worked out quite a bit more expensive than the 12v 220ah agm's we purchased.
Not to mention the extra work with 6v trojans
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Old 11-01-2019, 16:50   #101
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Re: 4 six volts better than 2 twelve volts

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2 6 volt batteries have more plate area than 1 12 volt battery

LOL Sometimes yes, sometimes no. Sometimes an apple is bigger than an orange. Sometimes an orange is bigger than an apple. Depends on how big each one is. Would you say that an 8D has less plate area than a pair of GC2? You would be wrong. If you said 2 GC2 had more plate area than a typical gp31 you would be right, I'm sure.



The reasons why you might prefer GC2 over all other form factors are well established. Mostly it depends on space available for installation, and possibly your preference for series but not parallel interconnections. Maybe breaking the weight down into more manageable pieces. Might also have something to do with price, since Duracells from Sams Club are so darn cheap and last a good long while. A general shoot from the hip prononcement that two 6v batteries (what size?) has more plate area than a single 12v (what size?) battery contains no logic and is not a logical reason to prefer 6V batteries.
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Old 11-01-2019, 18:40   #102
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Re: 4 six volts better than 2 twelve volts

Questions: a T-105 is rated for 217amp hours and if you had two in series, how many amp hours can you draw from those two batteries without harming the batteries? When recharging such batteries, what would be the ideal amp rate?
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Old 11-01-2019, 18:49   #103
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Re: 4 six volts better than 2 twelve volts

Yes the market conditions down under apparently make going AGM cheaper than FLA?

Full River is OK, not a super-respected brand in NA compared to Lifeline & Odyssey, is apparently one of the better value one down under.

The supply channels from China are certainly more dominant there too, US suppliers pretty unaffordable.

So no real point on cross-arguing about batt type value, state where **you** are in the title and save time.
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Old 12-01-2019, 02:10   #104
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Re: 4 six volts better than 2 twelve volts

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Questions: a T-105 is rated for 217amp hours and if you had two in series, how many amp hours can you draw from those two batteries without harming the batteries? When recharging such batteries, what would be the ideal amp rate?
In series to 12V, the bank would be rated at 217-Ah capacity. Most makers suggest no more than 50% for best longevity.

Ideal charging rate depends on the specific battery (brand/type/etc.); whatever the maker (Trojan, in this example) suggests, probably on their website.

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Old 12-01-2019, 08:55   #105
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Re: 4 six volts better than 2 twelve volts

40A charger would be fine, 20A a bit slower, bigger a waste of money unless you might expand later.

AGM requires much higher rates for longevity, but when going right up to 100% don't finish that much earlier, 5-7 hours usually.
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