8D vs Type31

Can someone tell me what the difference is between a 120AH 8D and a 120AH Type 31 other than size and weight? Just about to do a battery purchase and it seems to me Type 31 would be the way to go to get more AH’s in the same space as my 8D’s are taking up now.

I don't know where you are getting your AH numbers. Most 8Ds if they report AH capacity at all, report 200 amp hours. Group 31s are usually 100 amp hours. Neither are particularly good house batteries if that is what you are after.

Two golf cart batteries wired in series give about 220 AHs, are true deep cycle designs and take up the same room as an 8D.

David

DavidM said:

I don't know where you are getting your AH numbers. Most 8Ds if they report AH capacity at all, report 200 amp hours. Group 31s are usually 100 amp hours. Neither are particularly good house batteries if that is what you are after.

Two golf cart batteries wired in series give about 220 AHs, are true deep cycle designs and take up the same room as an 8D.

David

Click to expand...

Thanks for the quick reply. My boat came with two 8D 120AH LA batteries. My previous boat was equipped with Type 31 120AH LA batteries. I guess the answer to my question is 120AH is 120AH whether a large or small battery. I have to change out my two 8D’s which are in two banks and used for starter or house since each bank is the same. I was thinking 3 or 4 Type 31 AGM’s in each bank.

I know this subject has been discussed a lot but was mainly interested in the size and weight difference between the 8D and the Type 31 yet producing the same AH.

I suspect that 120 AH on your 8D label is bogus. No 8D will be 120 AHs. It is almost twice the weight as a Group 31. Weight equals capacity.

So it sounds like you have an opportunity to upgrade your batteries and do it right. A Group 31 starting battery will start anything up to 6 liters. If you have a bigger engine then maybe an 8D is warranted. And a Group 31 won't kill you getting it in and out next time. Get rid of those 8Ds and just do it once and be done with it.

But set up your house battery bank with two GC batteries wired in series. They will last much longer than a run of the mill 8D.

David

Get rid of the 8D. I am assuming you have a diesel engine? If so buy 2-Group 31 and tie them in parallel. (Positive to positive). No worries. I did this a few years ago on the stbd engine and then the port side a few weeks ago. No more 8Ds!!

8D batteries are huge and heavy and have a lot of amp hours. If this is what you had replace each 8D with a pair of GP31s in parallel. If you are old like me the GP31s are so much easier to man-handle in and out of the boat. 8Ds and GP31s are good starting batteries and are not deep discharge design. Not very good as house batteries. Take a look at the newer OSGR starters because for less than a couple hundred dollars you can buy a new offset gear reduction starter that doesn't need the amps of an 8D. You may be surprised that you can get by with just 1 GP31 in place of the 8D.

Every LA battery can be judged by its weight of lead. If your 8D is heavier than 2 grp 31 , it will have more guts .


This assumes you are comparing deep cycles or starts , not the silly "marine" rated batts that are best at what they are created for
.Hours pulling a 20 ft boat with a 12v trolling motor and having enough left to start an engine.

JamieM said:

Thanks for the quick reply. My boat came with two 8D 120AH LA batteries. My previous boat was equipped with Type 31 120AH LA batteries. I guess the answer to my question is 120AH is 120AH whether a large or small battery. I have to change out my two 8D’s which are in two banks and used for starter or house since each bank is the same. I was thinking 3 or 4 Type 31 AGM’s in each bank.

I know this subject has been discussed a lot but was mainly interested in the size and weight difference between the 8D and the Type 31 yet producing the same AH.

Click to expand...

As David has said, 120 Ah for an 8D isn't correct. More like ~245 Ah, IIRC.

That said...

One Group 31 battery with decent cranking amps (CCA, MCA) will start many diesels. Two in parallel will start many more. Three in parallel, especially if they provide very high cranking amps, will start most anything. You can compare youe engine's minimum cranking amp requirements to battery cranking amps... so your looking for the right balance of those features.

And then that said...

If your boat has combined start/house banks (like I think you're describing) with "one 8D per engine and approx half of your house load" -- then it's much like ours. Note that engine starting is all about cranking amps, and that house loads are mostly about capacity (Ah).

Three decent dual-purpose (sometimes labeled "marine" G31s will give you enough cranking amps and slightly more capacity (approx ~300 Ah total for a bank of 3, versus the ~245 Ah 8D you have now) so that'd be a slight improvement. And moving each individual G31 is not as backbreaking as moving a single 8D.

If it helps, I eventually replaced one of our 3xG31 banks with 4x 6V deep cycle (not dual-purpose) "golf cart" batteries, increasing that bank capacity to ~440 Ah. Had to be sure those batteries provided enough cranking amps to start that engine.

If it helps more, the 3xG31 banks have been Odyssey AGMs. Cost an arm and a leg each, but they provide huge cranking amps, decent Ah capacity, and have lasted 10-12 years each bank. AGMs also don't off-gas as much as flooded batteries, and they're basically low maintenance (no periodic watereing, etc.).

If it helps even more, the GCs are Lifeline AGMs. Much more capacity (Ah), not as much cranking amps (but sufficient for our purposes, according to Lifeline), ditto arm and leg, ditto AGM advantages... only had them a couple years now so longevity verdict still out.

-Chris

The problem with G31 batteries is really not the battery, its that people try to replace their 8D starting battery with a single group 31.

I checked recently and a Group 31 Does Not meet the CCA recommendations for my Cummins 6BTA engines.

That is not to say that it would not work, but it does not meet the Manufacturers Minimum recommendations.

Two Group 31’s in parallel would meet those recommendations.

ksanders said:

The problem with G31 batteries is really not the battery, its that people try to replace their 8D starting battery with a single group 31.

I checked recently and a Group 31 Does Not meet the CCA recommendations for my Cummins 6BTA engines.

That is not to say that it would not work, but it does not meet the Manufacturers Minimum recommendations.

Two Group 31’s in parallel would meet those recommendations.

Click to expand...

Consider replacing your starter with a modern OSGR type starter for about $135. Your required CCA requirement will go way down and you may get by with 1 GP31.

So to settle this. If you have a diesel truck, how many batteries do you have under the hood. My guess, like me you have 2 batteries in parallel.

Old rule of thumb was 1 cca/ cu in for gas, and 2 for the diesel. I used 1 Crown grp31 w 1100cca for a season, to crank my 2007 5.9 QSB with zero problems.
A charge relay problem, causing my thruster batteries to not get charged made me combine all 3 grp 31s. So thrusters and start are now combined. You can notice a bit of a difference when starting, but 1 will do. Of course if you have a tired engine or starter more batts would be better.

Alaskan Sea-Duction said:

So to settle this. If you have a diesel truck, how many batteries do you have under the hood. My guess, like me you have 2 batteries in parallel.

Click to expand...

I have three diesel trucks, both ford and dodge.

All three of them have two large starting batteries in parallel.

My advice is worth exactly what you pay for it.


https://www.elreg.com/blog/why-are-gear-reduction-starters-replacing-direct-drive-starters/


If a older direct drive starter uses up to 50% more power than a modern OSGR starter then if you replace your starter you will need half the battery power.


A new OSGR starter for a cummins 6BTA costs about $135.


If you still want 2 batteries, buy even smaller batteries for the smaller CCA requirement.

obthomas said:

My advice is worth exactly what you pay for it.


https://www.elreg.com/blog/why-are-gear-reduction-starters-replacing-direct-drive-starters/


If a older direct drive starter uses up to 50% more power than a modern OSGR starter then if you replace your starter you will need half the battery power.


A new OSGR starter for a cummins 6BTA costs about $135.


If you still want 2 batteries, buy even smaller batteries for the smaller CCA requirement.

Click to expand...

Yes, that is interesting, Thanks for posting it!

Check out the Firefly 31 batteries made by Caterpillar. Read some reviews and hit up some user reviews. These batteries behave much like a Lithium battery and can be taken down to 80 %, some folks report lower. They were designed to be used and abused as Caterpillar was having problems with batteries in their big equipment that sat unused for months then wouldn't start. And vibration was also an issue. So they design and built this new type of battery which doesn't require a complete charge and still keeps going on like the Ever Ready battery:

https://www.emarineinc.com/Firefly-Oasis-12V-Group-31-Battery

This is from an article written by Jeff Cote, a systems design engineer and owner of Pacific Yacht Systems:

"Carbon Foam Batteries. The Firefly Oasis carbon foam AGM battery was developed by Caterpillar to replace the FLA batteries that they were using in their work vehicles. They found that the old battery technology could not, consistently, withstand constant vibration, inconsistent charging or long periods without charging. This new technology is based on the sealed valve-regulated design of a conventional lead acid battery but the negative plate has been replaced by a light-weight, sulfation-resistant, microcell material. One of the biggest benefits of this technology, for boaters, is that the battery can be left in a partial state of discharge (i.e. extended cruising, sitting on a trailer or at a dock without power) and it will not lose any permanent capacity. You simply have to charge the battery and it will return to 100% of its original capacity

If you are looking for the maximum capacity with the smallest footprint that weighs the least, then Lithium batteries (LFP) batteries may be the best choice for you. They are by far the most upfront-cost option but if you use your boat a lot and plan on keeping it for a long time, they will pay for themselves and offer the best value in the long term. A unique characteristic of LFP batteries is that they don’t go through the three stages of charging: bulk, absorption and float. The battery is bulk charged until full (almost no absorption stage required), at which time float voltage and currents are mandatory to prevent over-charging. Note there are very few chargers or alternator regulators that are capable of safely and efficiently charging LFP batteries. If you are designing or implementing a LFP battery system, you will need to choose charging components that are compatible with this technology.

Cost Per Cycle Calculation. This year at the Vancouver International Boat Show, we featured a battery display in our booth that compared the total usable amp hours, weight and cost of FLA, AGM and Carbon Foam batteries. If you are trying to spend the least amount of money possible to replace your battery bank, it would seem to make sense to go with FLA batteries but if you do the math, you can see the value in AGM and Carbon Foam.






Adding Batteries Means Upgrading Your Charger. Another challenge we see is that boaters increase the size of the battery bank but do not increase the size of the charger. One of the most common causes for a battery to fail prematurely is overcharging or undercharging. When sizing a charger, the general rule of thumb is that the charge rate should be at least 10 percent of the total amp hours of the battery bank. For instance, a 400Ah deep-cycle battery bank would require a 40A charger. With large battery banks (800Ah or more at 12VDC), it is worth considering adding a large charger or even a second charger, in parallel, to ensure that the batteries charge at the right rate of charge to prolong battery life.

For FLA, AGM and Carbon Foam AGM batteries, we recommend a smart three-stage charger. These smart chargers detect voltage and resistance from the battery prior to charging and determine which stage to charge, either bulk, absorption or float. They can be left for months at a time and will reliably monitor the state of your batteries. As a reminder, you will also have to change the settings on your charger and your battery monitor to reflect the new battery chemistry.

When a boat has no space for additional batteries, we often recommend upgrading from FLA to AGM. More usable battery capacity with AGMs can translate to increased battery capacity if you keep the same number of batteries or alternatively you can reduce the battery bank size and still have the same amount of usable capacity. If you have been contemplating replacing your battery bank, these new batteries just make sense. Although the upfront cost is higher, the value proposition is unbeatable because of the combination of increased depth of discharge and faster charging.

About the author: Jeff Cote is a systems design engineer and owner of Pacific Yacht Systems, a full service shop delivering marine electrical and navigation solutions for recreational boats. Visit their website and blog for info and articles on marine electrical systems, projects and more: www.pysystems.ca

"With large battery banks (800Ah or more at 12VDC), it is worth considering adding a large charger or even a second charger, in parallel, to ensure that the batteries charge at the right rate of charge to prolong battery life."


A different concept might be 2 chargers , each used for 1/2 the battery bank.


Should a cell go bad only 1/2 the batts will be killed by over charging.

Yup. Chevy Duramax.

Alaskan Sea-Duction said:

So to settle this. If you have a diesel truck, how many batteries do you have under the hood. My guess, like me you have 2 batteries in parallel.

Click to expand...

I start two Perkins 4.236 engines from a single Group 31 Costco/Interstate battery somewhere in the 900-950 CCA range. I've never had a problem with the 1st or 2nd engine start. I also have a house bank of 6 GC batts for 660AH that I can parallel if needed. Never have found the need except for once when the boat was launched after being on the hard for over a month with chronic interruptions in shore power. The culprit turned out to be a shorted diode on the port alternator that was slowly draining the start batt when the shore power was off.

I agree that the 8D must be mislabeled. I'm used to seeing about 220AH or more from an 8D. I replaced 2 8D house batts (~450AH) with my 6xGC bank in the same boxes/foorprint and increased my bank size by ~50% to 660AH. Total house bank cost around $600...money well spent.

Alaskan Sea-Duction said:

So to settle this. If you have a diesel truck, how many batteries do you have under the hood. My guess, like me you have 2 batteries in parallel.

Click to expand...

This doesn’t settle much, two batteries in parallel under the hood is likely because of space limitations...

OK, i'll be the outlier here. No plans to change my two 8D engine start batteries to something else. No cost advantage and the space and weight are not issues. I do get a strong back to do the lifting, just as I do for the 70# Grp 31 AGMs for the thruster.

RSN48: the discussion is WRT dedicated starting batteries. I have a pair of 6 year old group 24’s in parallel and they are working fine. Perhaps $120 a piece. I may replace them this year as they are starting to get a bit tired. I will do a load test first. Personally, I think firefly batteries overkill in this application.

WRT Jeff Cote: my neighbour got a Jeff Cote designed solar system on his boat that included expensive Italian flexible solar panels. They failed after short period of service. The manufacturer did not honour the warranty. So while Jeff Cote was not necessarily to blame, he system he designed failed, so I don’t consider his knowledge to be perfect by any means.

Jim

Alaskan Sea-Duction said:

So to settle this. If you have a diesel truck, how many batteries do you have under the hood. My guess, like me you have 2 batteries in parallel.

Click to expand...

I had only 1 in my old Dodge/Cummins pickup. A group 31.
I never had a problem starting it even in Ct winters.

Well...

I was mistaken in a previous comment.

On the Cummins 6 cyl 5.9L engines the minimum CCA recommended is 950.

I checked and many of the Group 31 batteries on the market exceed that rating, some significantly.

So... if a person wanted to save the weight, space, etc... you could replace a 8D with a Group 31 in Cummins 5.9L applications.

DavidM said:

Two golf cart batteries wired in series give about 220 AHs, are true deep cycle designs and take up the same room as an 8D.

David

Click to expand...

Same footprint but taller -- golf cart batteries are a great replacement for 8D, but you need the vertical clearance. A side benefit is that a single-golf cart battery weighs about half what an 8D weighs, so much easier to lug around.

JamieM said:

Can someone tell me what the difference is between a 120AH 8D and a 120AH Type 31 other than size and weight? Just about to do a battery purchase and it seems to me Type 31 would be the way to go to get more AH’s in the same space as my 8D’s are taking up now.

Click to expand...

I do not think there is a lot of difference in the 12v requirements of your 34 Pilot and my 30 Pilot. Did your Pilot come factory equipped with two banks of one 8D each? Mine came with only one bank of two group 34 Odyssey AGM batteries in parallel. It now has two banks, the two Odysseys as a start bank and two yellow-top Optima AGMs as a house bank. I cannot imagine me or anybody else getting an 8D in or out of my cramped engine compartment. These are batteries I, who is the person doing ALL maintenance here, can handle.

When we purchased our 4788 (2 x Cummins 6bta) we knew the two 8D starters needed replacement. We also replaced the ancient charger with a new smart charger. These engines have airflow pre-heaters. We replaced the 8D FLAs with Grp 31 AGMs plus a third for the generator. We went thru our first full season with zero issues including cruising thru this winter so far. We also had a bad surprise need to replace the 4 expensive Full River AGM house batteries due to improper setting of the Magnasine. For the house we went with 8 Costco GC FLA batteries PLUS a Battery Watering Systems filling set up. We reset the Magnasine accordingly and I think my total water consumption has been, maybe, a third of a gallon for all eight batteries since May 2019. When cruising I typically run the genny about one hour most mornings or evening for cooking and hot water (and charging).

Firefly Batteries

rsn48 said:

Check out the Firefly 31 batteries made by Caterpillar. Read some reviews and hit up some user reviews. These batteries behave much like a Lithium battery and can be taken down to 80 %, some folks report lower. They were designed to be used and abused as Caterpillar was having problems with batteries in their big equipment that sat unused for months then wouldn't start. And vibration was also an issue. So they design and built this new type of battery which doesn't require a complete charge and still keeps going on like the Ever Ready battery:

https://www.emarineinc.com/Firefly-Oasis-12V-Group-31-Battery

This is from an article written by Jeff Cote, a systems design engineer and owner of Pacific Yacht Systems:

"Carbon Foam Batteries. The Firefly Oasis carbon foam AGM battery was developed by Caterpillar to replace the FLA batteries that they were using in their work vehicles. They found that the old battery technology could not, consistently, withstand constant vibration, inconsistent charging or long periods without charging. This new technology is based on the sealed valve-regulated design of a conventional lead acid battery but the negative plate has been replaced by a light-weight, sulfation-resistant, microcell material. One of the biggest benefits of this technology, for boaters, is that the battery can be left in a partial state of discharge (i.e. extended cruising, sitting on a trailer or at a dock without power) and it will not lose any permanent capacity. You simply have to charge the battery and it will return to 100% of its original capacity

If you are looking for the maximum capacity with the smallest footprint that weighs the least, then Lithium batteries (LFP) batteries may be the best choice for you. They are by far the most upfront-cost option but if you use your boat a lot and plan on keeping it for a long time, they will pay for themselves and offer the best value in the long term. A unique characteristic of LFP batteries is that they don’t go through the three stages of charging: bulk, absorption and float. The battery is bulk charged until full (almost no absorption stage required), at which time float voltage and currents are mandatory to prevent over-charging. Note there are very few chargers or alternator regulators that are capable of safely and efficiently charging LFP batteries. If you are designing or implementing a LFP battery system, you will need to choose charging components that are compatible with this technology.

Cost Per Cycle Calculation. This year at the Vancouver International Boat Show, we featured a battery display in our booth that compared the total usable amp hours, weight and cost of FLA, AGM and Carbon Foam batteries. If you are trying to spend the least amount of money possible to replace your battery bank, it would seem to make sense to go with FLA batteries but if you do the math, you can see the value in AGM and Carbon Foam.

Adding Batteries Means Upgrading Your Charger. Another challenge we see is that boaters increase the size of the battery bank but do not increase the size of the charger. One of the most common causes for a battery to fail prematurely is overcharging or undercharging. When sizing a charger, the general rule of thumb is that the charge rate should be at least 10 percent of the total amp hours of the battery bank. For instance, a 400Ah deep-cycle battery bank would require a 40A charger. With large battery banks (800Ah or more at 12VDC), it is worth considering adding a large charger or even a second charger, in parallel, to ensure that the batteries charge at the right rate of charge to prolong battery life.

For FLA, AGM and Carbon Foam AGM batteries, we recommend a smart three-stage charger. These smart chargers detect voltage and resistance from the battery prior to charging and determine which stage to charge, either bulk, absorption or float. They can be left for months at a time and will reliably monitor the state of your batteries. As a reminder, you will also have to change the settings on your charger and your battery monitor to reflect the new battery chemistry.

When a boat has no space for additional batteries, we often recommend upgrading from FLA to AGM. More usable battery capacity with AGMs can translate to increased battery capacity if you keep the same number of batteries or alternatively you can reduce the battery bank size and still have the same amount of usable capacity. If you have been contemplating replacing your battery bank, these new batteries just make sense. Although the upfront cost is higher, the value proposition is unbeatable because of the combination of increased depth of discharge and faster charging.

About the author: Jeff Cote is a systems design engineer and owner of Pacific Yacht Systems, a full service shop delivering marine electrical and navigation solutions for recreational boats. Visit their website and blog for info and articles on marine electrical systems, projects and more: www.pysystems.ca

Click to expand...

After running AGM batteries and having them discharged flat by boat yards, I made the decision to go to Firefly carbon foam batteries. After a year using them I can only say they are the best. A bit pricey compared to AGM, half the price of a Lithium battery.

With the Firefly batteries one does not float the batteries as is typical with FLA and AGM. Firefly recommends fully charging, then let them run down partially before charging.

Great battery the Firefly.

Craig

comparing the large 8D to a group 31?? i took out an 8D starting batt,and put in 2 group 31`s in paralell! lotsa (1400 amps) to easily start my 6 cylinder volvo 70B! get the marine connectors...i got mine from a comany in seattle called dyno, check their website to see what their marine connectors look like, or call them...i have 6, 265 amp each, 2 makes a 12v, 265 amp set! i have the marine connectors, that i can bolt on the wires! they are flooded batts, last about 10 yrs for me, i go north toward alaska to fish, and put approx 300+ hours on each year!...clyde

Conepatus said:

After running AGM batteries and having them discharged flat by boat yards, I made the decision to go to Firefly carbon foam batteries. After a year using them I can only say they are the best. A bit pricey compared to AGM, half the price of a Lithium battery.

With the Firefly batteries one does not float the batteries as is typical with FLA and AGM. Firefly recommends fully charging, then let them run down partially before charging.

Great battery the Firefly.

Craig

Click to expand...

What is the expected life span of a Firefly?