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Catalina/Capri 25/250 Sailor's Forums

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A question of batteries

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Turk
Admiral

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USA
736 Posts

Initially Posted - 03/09/2006 : 22:10:30

I want to add a second battery, well actually, two batteries. I started looking at marine 12 volt deep cycle batteries that would fit in the existing battery tray (model 24) and a second tray that I would add. The battery I was looking at had 80 amp hours of capacity. I also checked out the golf cart 6 volt batteries. They were rated at 158 amp hours. Hmmmmm, from my electrical knowledge I believe the difference is that the 6 volts would produce double the amp hours for the same size battery. Is that correct? My question is, I've been told that tying 2 - 6 volts together in series to produce 12 volts would provide more power than 2 - 12 volts in parallel producing 12 volt.. If both have similar amp hour capacity, why is this true?

Turk

www.turk.mysailboatblog.com
2003 C250 WK #663
Northeast Ohio
Mosquito Lake/Lake Erie

Edited by - on

dlucier
Master Marine Consultant

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Virgin Islands (United Kingdom)
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Response Posted - 03/10/2006 : 05:50:48

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by Turk
 My question is, I've been told that tying 2 - 6 volts together in series to produce 12 volts would provide more power than 2 - 12 volts in parallel producing 12 volt.. If both have similar amp hour capacity, why is this true?<hr height="1" noshade id="quote"></blockquote id="quote">

Basically...

When batteries are connected in paralell, amps are additive and voltage remains constant.

When batteries are connected in series, voltage is additive and amps remains constant.

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OLarryR
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Response Posted - 03/10/2006 : 05:55:33

Not an electrical wiz, so I leave it to others to learn from as far as any benefits running batteries in series...but...why would you want to do this ? Just get marine batteries with high amps. The main problem with going in series is that you then have no possibility of using one battery as a backup for the other.

I must admit that I do not practice real good battery management but that may improve. i only bought my boat in the fall and have yet turned my attention to better battery management. But what i can tell you is that when I had the old motor and it would not start half the time, I had to keep trying to start it every 10 minutes or so since by hand it rarely started. When the motor gave me trouble the 50% of the time it did not start after the first few tries, i then would shift the 3 way switch from "Both" to only one battery, so at least if I got to the point of running it down, I still had the backup battery with near full charge.

I think the real benefit of being able to use one battery and keeping the other a backup is when at anchor and staying overnight. With just one battery in use, the drawdown on amps overnight could be 5, maybe even as much as 10 amps depending on what besides the anchor light you have on. If you have two 12 volt batteries and run one down a lot or it develops a bad cell, at least you can switch to the other battery. With two 6 volt batteries, you have no backup. In the rare case (but does happen) of developing a bad cell, you can only play your radio with a 6 volt battery - it's not going to start the motor. Then again, if your motor starts easily by hand, then maybe not quite so bad.

I guess the battery costs are driving this decision or is it that you have a need for seeking more power out of two batteries and striving to max out load carrying ability ? Still...think it better to get higher quality 12 Volt batteries with loads of amps for you versus going the 6 volt route and then have to rely on both for all utilities that require 12 volts..with no backup.

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Turk
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Response Posted - 03/10/2006 : 07:04:56

Maybe I should explain myself. I have a 2003 5 HP nissan, no electric start and no alternator for charging. I do have a 65 watt solar panel and regulator that has no problem charging one battery fully in a day or two. The power from the batteries are strickly for on-board lights, new auto tiller, wash down pump, XM radio and depth meter. I'm worried the auto tiller may drain more power than I have over a long weekend.

I understand the principle of hooking batteries together in both parallel and serial. My question is why am I finding on the net that 2 - 6 volts will outlast 2 - 12 volts when they are hooked together producing 12 volts. Is it that the 6 volt batteries are better designed (larger plates) for deep cycle operation than 12 volt batteries? Maybe I'm misreading this and maybe there are much higher amp hour golf cart batteries available than 12 volts - but I don't think so. Several souces have said, go with 2 - 6 volt golf cart batteries. By the way, the cost of 12 and 6 volt wet cell batteries are almost the same.

Edited by - Turk on 03/10/2006 07:06:17

OLarryR
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Response Posted - 03/10/2006 : 07:46:30

Turk,

I talked to our electrical guys and ...this is sort of what I digested. Power being greater for 2-6 Volt batteries in series versus 2-12 Volt batteries in parallel is sort of true in regards to flooded batteries.

Taking the parallel arrangement, If one battery is stronger than the other, the stronger battery does the work or carries the load. Similarly, if the batteries are charged in parallel, the good battery benefits from the charge more so than the weaker battery. the weaker battery tends to continue getting weaker.

In the series setup, they indicated that both batteries are forced to do the work and when they get charged, both get charged because the charge goes thru both of them, whereas if I understand it correctly, in the parallel setup the charge does not go thru...or benefit a weaker battery as much.

I do know that for solar panel setups, they mention about putting diodes in the lines so that in parallel setups, one battery does not drain the other battery but this discussion I had today with the electrical guys not thows a different light on the subject for me regarding paralllel setups and flooded batteries....maybe the solar panel if used for both batteries may not charge one of the batteries that well unless perhaps a separate solar panel was hooked up specifically to each battery.

I still do not understand this fully but they indicated that with sealed batteries this is not an issue. Why ?....I am not sure.

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dlucier
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Response Posted - 03/10/2006 : 07:56:09

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by Turk
 My question is why am I finding on the net that 2 - 6 volts will outlast 2 - 12 volts when they are hooked together producing 12 volts...<hr height="1" noshade id="quote"></blockquote id="quote">

I don't think it really matters whether one has two 6V batteries in series or just one 12V, if each system's output is 100Ah, then each system will output 1A for 100 hours, 2A for 50 hours,...etc.

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dlucier
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Response Posted - 03/10/2006 : 08:44:40

Truthfully, I've never really understood the "two 6V batteries in series" concept as applied to the way the usual small boat sailor uses electricity.

Typically, electricity is used on our boats for light loads like lighting, radios, instruments, autopilot, or some other low power device. To power this low current load, we use a battery (a deep cycle) that is designed to provide relatively low amps for a long period of time.

Now, I'm not an expert on golf cart batteries, but it is my understanding that they are designed for powering high amperage motors for short periods of time, something like 70-80 amps for an hour or two. This designed output would fall somewhere between a starting battery (high amps/short duration) and a deep cycle (low amps/long duration).

Now, if I were using an electric outboard motor to move my boat, I might choose a golf cart battery setup, with its higher amp/lower duration output, but with my small loads, it seems the battery of choice would be one that is designed for this output, the deep cycle.

Can someone resolve this golf cart vs deep cycle battery conundrum for me?

Edited by - on

Happy D
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Response Posted - 03/10/2006 : 16:21:07

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Hmmmmm, from my electrical knowledge I believe the difference is that the 6 volts would produce double the amp hours for the same size battery.<hr height="1" noshade id="quote"></blockquote id="quote">I don't think they make 6 volt batteries in a group 24 size but I could be wrong. All the 6 volt batteries I've seen are huge, and heavy. That's why they have so many amps.

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Turk
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Response Posted - 03/10/2006 : 18:32:06

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by Happy D
 
<blockquote id="quote">quote:<hr height="1" noshade id="quote">Hmmmmm, from my electrical knowledge I believe the difference is that the 6 volts would produce double the amp hours for the same size battery.<hr height="1" noshade id="quote"></blockquote id="quote">I don't think they make 6 volt batteries in a group 24 size but I could be wrong. All the 6 volt batteries I've seen are huge, and heavy. That's why they have so many amps.
<hr height="1" noshade id="quote"></blockquote id="quote">

No, Sam's club has 6 volt group 24 batteries for $48 ($7 for core exchange in Ohio) They also have group 31 and up.

Also, from what I've read, golf cart batteries are generally built with thinker plates than Marine deep cycle batteries. My source is Don Casey's book. It is this characteristic ((the thickness) that makes for a good deep cycle battery.

The ability to charge more evenly when set up in series makes sense (if it's true) and would give one a more reliable power source over time. I like that. I am planning on buying 2 batteries to start off so the idea of a weak battery would not apply at first. I think I am getting convinced to go with 6 volts in series.

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dlucier
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Response Posted - 03/10/2006 : 21:23:02

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by Turk
 ...Sam's club has 6 volt group 24 batteries for $48...I think I am getting convinced to go with 6 volts in series.<hr height="1" noshade id="quote"></blockquote id="quote">

Turk,

What is the Ah rating for the golf cart battery?

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Turk
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Response Posted - 03/11/2006 : 06:59:43

<blockquote id="quote">quote:<hr height="1" noshade id="quote">
What is the Ah rating for the golf cart battery?
<hr height="1" noshade id="quote"></blockquote id="quote">

The 6 volt golf cart batteries have a 158 AH rating.

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dlucier
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Response Posted - 03/11/2006 : 09:29:00

Turk,

First, like you originally posted, I too, am just trying to figure this thing out.

Two 6V 158Ah batteries in series gives a capacity of 12V at 158Ah.

Two 12V 105Ah batteries in paralell gives a capacity of 12V at 210Ah.

Is this correct?

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Turk
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Response Posted - 03/11/2006 : 14:36:10

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by dlucier
 Turk,

First, like you originally posted, I too, am just trying to figure this thing out.

Two 6V 158Ah batteries in series gives a capacity of 12V at 158Ah.

Two 12V 105Ah batteries in paralell gives a capacity of 12V at 210Ah.

Is this correct?
<hr height="1" noshade id="quote"></blockquote id="quote">

No. The 12 volts are 80 Amp hours. Most group 24 12 volts are around 80.

Both scenarios on paper would produce the same amount of amp hours.

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dlucier
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Response Posted - 03/11/2006 : 17:17:19

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by Turk
 No. The 12 volts are 80 Amp hours. Most group 24 12 volts are around 80.

Both scenarios on paper would produce the same amount of amp hours.
<hr height="1" noshade id="quote"></blockquote id="quote">

Ahh, you're right, group 24 Exides...I was thinking group 27.

Geez, I've been Googling the 6V/12V battery thing every which way and I'm amazed how little information concerning this is out there.

Anybody got a good site?

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ClamBeach
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Response Posted - 03/11/2006 : 17:49:00

The heart of the 6V versus 12V battery debate simplifies down to weight.

Heavy duty 'golf cart' (industrial) batteries have solid lead plates rather than the 'sponge' plate composition of starting or semi-deep cycle batteres.

Solid lead plates last far longer under the stress of deep discharge cycles but produce less amperage per pound of lead.

More lead also equals more weight per cell and at some point a 12V battery with enough solid lead plate to produce the amperage required simply becomes too heavy to be managable.

Easy to envision... instead of (2) 200 lb batteries you'd have to wrestle (1) 400 lb battery. That's pretty serious weight to muscle around.

Here's an excellent site: http://www.windsun.com/Batteries/Battery_FAQ.htm

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OLarryR
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Response Posted - 03/11/2006 : 18:20:15

Turk,

I did a quick search on Yahoo putting in the keywords "Battery series paralllel" and one of the first links that came up was the following which may be useful.
http://sierranevadaairstreams.org/owners-guide/understanding/battery-config.html

I have to get off the PC now and did not have time to see what addl links could prove useful. So suggest you the above keywords and check out addl links.

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