8Ds versus Diesel Truck Batteries

[FlyWright]

I did something similar in replacing my massive 8D start with a single G31 to start both engines, but I'm only cranking small Perkins 85hp 4.236's. Since my port alternator is dedicated to this start batt, I always start my port engine first and verify the alternator is online before starting the stbd side.

If I need ever need help with more juice for the start, I have a 6xGC house bank that can be paralleled with the twist of the switch that's located outside the ER. So far, it's never been needed.

I'm a big fan of G31 start batts, too. Nice improvement, Tom! On to #499!

 

[Alaskan Sea-Duction]

I don't know, but two 31s are about as heavy as an 8D. They take up the same space as an 8D. Spins my engine just fine. She fires off in less than a full turn......

 

[ghost]

8d is about 160 pounds, 31 about 60

My back can make two trips of 60 just fine. Dragging 160, not fine.

 

[tiltrider1]

I have two 8D's in parallel. I talked to Dyno batteries and asked them which way I should go. They told me to stick with the 8D's, said they were better batteries and would cost me less in the long run. Like I said earlier, you can always find a young buck to haul them off the boat for you.

 

[tiltrider1]

I spent a little more time talking batteries with Dyno. The first point they wanted to make was that the conversation was of like kind, in other words apples to apples. The first thing they wanted to point out, you can buy an 8d lead acid battery as cheap as $150 or up to $400. The cheap battery will weigh 79lbs and the expensive one will weigh 149lbs. The next important number is the number of plates that are in the battery. The cheap one has 17 and the expensive one has 31. Finally how thick are the plates. Often the 31 plate has thicker plates than the 17 plate battery.

Keeping all things the same you will need 3 group 31 batteries to replace one 8d battery. While one group 31 battery can start your engine there was a whole long conversation about performance drop off due to age, temperature, depth of discharge etc. once you go to 3 group 31s cost will be higher than 1 8d.

I am not saying that you can’t use group 31s. There is a performance difference but the difference might not be of value to everyone. I am saying to consider which performance requirements you need before making the decision.

As always there is no one right decision.

In case anyone is unfamiliar Dyno is a battery manufacture.

 

[Alaskan Sea-Duction]

Til interesting post. I have 3208T/As. These engines are used in industrial machines of all kinds. Same engine, yet you will see two group 31 for start batteries. I also have a 2009 truck with a 6.4L diesel. It uses two group 27 start batteries.


So why is the marine industry or at least the used boat folks insisting 8Ds are the only proper battery for a marine engine that is identical to an older dump truck or bus or excavator using something other than 8D?

 

[Baker]

8d is about 160 pounds, 31 about 60

My back can make two trips of 60 just fine. Dragging 160, not fine.

"Good" group 31s are 75lbs....at least that is what my Odysseys weigh. Less good ones I have seen down to 50lbs or less.

 

[tiltrider1]

Keeping apples to apples I am going to be talking in terms of Dyno's commercial grade specs.

A group 31 battery is capable of producing 850cc(cold cranking amps). There is nothing cold about this. Its a bench test for how many amps can be extracted from the battery in 30 seconds while not drawing it below 9 volts.

Now this is enough power to start a normal Cat 3208 in normal conditions. Yet, a Cat 3208 normaly starts with in seconds so its fair to say that one group 31 commercial grade battery is all you need to start 2 Cat 3208 engines.

But, what if we spec a different engine, or what if we want a battery that can handel below normal tempratures, or what if we don't trust the boat owner to buy a commercial grade battery.

What if we have two tired DD8v71's in a boat, half way between Ketchikan and Bella Bella were the batteries are 7 years old and the engines have been run out of fuel and now we need to prime and start and everthing is 36 degrees. How much battery do we need now. Honestly I don't know but you get the idea.

What if you are Ocean Alexander and you know that money for batteries is insignificant and the last thing you want is your reputation tarnished because some one said you didn't put enough battery in the boat, what do you sepc then.

That said, if you are going to buy the cheapest battery possible, ignore the maintence for as long as possible, and demand it start your tired old DD8v71 in cold weather I would suggest you buy 2 8D's and hook them up in parallel. On the other hand if you own two Cat 3208's and you live in LA and you only use the boat for harbor cruises then one group 31 should do it.

Its the age old argument about how safe is safe enough.

I am parinoid, I had one 8D to start 2 455 gas engines, clearly that was over kill, but I never failed to start my engines, parinoia curred. Now I have 2 8D's to start 2 Cat3208's obviously still parinoid.

 

[ghost]

Also worth noting, If my 31 dies, I have 3 other ways to start engines. 1 is parallel to the house bank, second is to start the dc gen which puts out 200 amps dc at its peak, third is the other 31 that is isolated that can also be put into the circuit. There are still some failure scenarios that can remove more backups than you might think, but knowing your backups is the biggest step to take.

 

[ghost]

...and the dc gen can be hand cranked to start

 

[FF]

"Its a bench test for how many amps can be extracted from the battery in 30 seconds while not drawing it below 9 volts. "

YES! Many starters are rated at 9v , and will spin fast enough at 9v to start a cold engine.

So for a boat with a huge house bank , using the house for starting 100% of the time might make sense.

Even 50% down the house , with a lot of plate area, starts in most temperatures just fine.

Using a GP 31 for a thruster or noisemaker would give a safety factor , if the batt could be easily moved for a jump start.

 

[ranger58sb]

Keeping apples to apples I am going to be talking in terms of Dyno's commercial grade specs.

A group 31 battery is capable of producing 850cc(cold cranking amps). There is nothing cold about this. Its a bench test for how many amps can be extracted from the battery in 30 seconds while not drawing it below 9 volts.

Not all 31s are created equal. Ours produce 1150 CCAs, and weigh 77.8 lbs each.

-Chris

 

[Wxx3]

I use a pair of group 24’s to start my Lehman 135. The engine jumps to life on starting. Why would you need more than a pair of 31’s to start both engines? Once one is started, you have your alternator brining up the volts to start the other engine.

i have a single g31 to start engine. 5 years old now (wow, time flies) and the boat can sit for months and it still starts right up. In fact, so fast that after a prolonged time or oil change, I let it crank an extra 5 seconds with Stop pushed to build oil pressure before start.

 

[tiltrider1]

Not all 31s are created equal. Ours produce 1150 CCAs, and weigh 77.8 lbs each.

-Chris

This means your battery has more plates then other G31’s, since your battery weighs in at 77lbs it suggest the plates are of decent thickness yet they are much thinner than what you would find in an 8D battery that produces 1250cca. While your g31 is probably adequate it won’t have the longevity of the 8D. It’s also possible that a better battery might be a g31 850cca with thicker plates for more longevity.

I am not suggesting any one answer is better. I am trying to point out how misleading battery information can be and also how there can be so many different answers for what looks like the same problem.

Batteries are like boats, it’s always a compromise.

 

[FF]

Its every ones horror but "stuff happens" and a dead start batt does happen.

A small load for days is the worst.

The usual thin plate start may not be recoverable if run to 3-4v , and will have lost much 50%??? of its capacity from one killing.

The heavier duty 8D does also suffer but will recover a better percentage of its ability if killed.

 

[ranger58sb]

While your g31 is probably adequate it won’t have the longevity of the 8D.

Didn't mean to imply ours were better (although they are considered one of the better AGM brands), just that G31s vary.

FWIW, our oldest bank lasted 12 years in a combined start/house bank. Odyssey PC-2150s, TTPL design.

-Chris

 

[Baker]

Keeping apples to apples I am going to be talking in terms of Dyno's commercial grade specs.

A group 31 battery is capable of producing 850cc(cold cranking amps). There is nothing cold about this. Its a bench test for how many amps can be extracted from the battery in 30 seconds while not drawing it below 9 volts.

Now this is enough power to start a normal Cat 3208 in normal conditions. Yet, a Cat 3208 normaly starts with in seconds so its fair to say that one group 31 commercial grade battery is all you need to start 2 Cat 3208 engines.

But, what if we spec a different engine, or what if we want a battery that can handel below normal tempratures, or what if we don't trust the boat owner to buy a commercial grade battery.

What if we have two tired DD8v71's in a boat, half way between Ketchikan and Bella Bella were the batteries are 7 years old and the engines have been run out of fuel and now we need to prime and start and everthing is 36 degrees. How much battery do we need now. Honestly I don't know but you get the idea.

What if you are Ocean Alexander and you know that money for batteries is insignificant and the last thing you want is your reputation tarnished because some one said you didn't put enough battery in the boat, what do you sepc then.

That said, if you are going to buy the cheapest battery possible, ignore the maintence for as long as possible, and demand it start your tired old DD8v71 in cold weather I would suggest you buy 2 8D's and hook them up in parallel. On the other hand if you own two Cat 3208's and you live in LA and you only use the boat for harbor cruises then one group 31 should do it.

Its the age old argument about how safe is safe enough.

I am parinoid, I had one 8D to start 2 455 gas engines, clearly that was over kill, but I never failed to start my engines, parinoia curred. Now I have 2 8D's to start 2 Cat3208's obviously still parinoid.

It still isn't apples to apples. Not all batteries of the same type are created equally. And my Odysseys have 1100CCAs....hence my point. They also weigh 75 pounds where as other 31s weight 50lbs. Generally, the more lead the better.

 

[Baker]

Not all 31s are created equal. Ours produce 1150 CCAs, and weigh 77.8 lbs each.

-Chris

Sorry Chris....I didn't see your post until after I posted mine. We said the exact same thing!!! And likely have the same battery...haha