Start Battery Reality Check

[Diep]

I've got a pair of Cummins 6BT and an Onan 8kw genset. Each has its own independent battery and alternator. All the starter runs are 2/0 cable.

The best I can tell the 6BT requires 950CCA/1200MCA and the Onan requires 360CCA.

Right now the 6BTs are started by lifeline 8D deep cycles and the Onan has a 4D deep cycle. The 8D offers 1350/1675 amps and the 4D 1100/1360.

We are thinking about switching the batts to starting type. Keeping with Lifeline (for example) the 3100-HC offers 1175/1400 amps which is still over the requirements for the 6BTs but not by as much. Is there any reason not to cut a little closer to the minimum requirement for a dedicated staring battery? Each will still have its own battery and the ability to parallel if needed.

The 4D for the genset seems like extreme overkill. The wire runs are pretty long for this right now so I don't know if that is a factor. Thinking about sizing the battery way down, locating it much closer to the gen, and adding the capability to parallel with the main start batts.

I'm looking for a reality check. Should the start batteries be sized with big margins if each engine has its own dedicated?

 

[tiltrider1]

The rating is when the battery is brand new, as the battery gets older it will produce fewer CCA. Eventually it won’t produce enough to start the engine. The cheaper battery will get there sooner. Do you want to replace the new battery sooner or later?

You can take a different approach. You could replace the 8D with two Group 27 batteries. This would give you more CCA in an easier to handle package.

The 4D is a bit over kill. However, on most boats the Genset is the last backup plan. So, Having enough battery for maximum cranking is not much over kill.

 

[Jay N]

WESTERLY's start battery for her 6BT Cummins: 1998-2006 Interstate FLA Grp 31 750CCA, 2006-2019 Optima Grp 31 AGM 900CCA (replaced 2020 by same).

Have never had a main engine start issue, even after rolling engine over to prime new injectors. Emergency backup is from the House Bank, or from the generator (5kw NL with U1 370CCA battery).

I find engine manufacturer's CCA requirements to be conservative. There is a school of thought that the CCA requirement assumes a normal battery service life including CCA decline. Capacity testing will signal end of service life.

 

[tpbrady]

Agree Jay. Our bigger piece of iron, 6CTA, had 2 FLA 4Ds which we replaced with 2 Gp 31s FLA. The engine hardly completes a revolution before it starts. When one of the 4Ds failed, it still started, but just a little slower.

Tom

 

[SteveK]

I have mostly had twins and mostly had two batteries paralell used for both. The theory being two batteries is more than one engine needs and once the first is started (the one charging the start) the second engine is sure to start as well.

 

[JackConnick]

I have 8D FLA start batteries for my Cummins 5.9BTs. I've been looking at the new 12V Odyssey Extreme G31 Starting / Deep Cycle Battery - 1150 CCA, 103 Ah.
The group 31s have the same CCA.

I think a group 27 starting battery would be more than enough for your genset.

 

[mbevins]

I have 1 8D only that is used to start both DD 6-71s and the Gen. As back-up I have a parallel switch to my large house bank. Never in 10 years have I needed to use the switch. I always start the engine with the alternator that feeds the 8D first , that is only out of caution. In reality it doesn't matter.

I really think we over think this type of stuff sometimes.
I will add that I did this when I reconfigured my batteries for an inverter install. I decided I needed more house supply than engine redundancy.

 

[ranger58sb]

We are thinking about switching the batts to starting type. Keeping with Lifeline (for example) the 3100-HC offers 1175/1400 amps which is still over the requirements for the 6BTs but not by as much. Is there any reason not to cut a little closer to the minimum requirement for a dedicated staring battery? Each will still have its own battery and the ability to parallel if needed.

The 4D for the genset seems like extreme overkill. The wire runs are pretty long for this right now so I don't know if that is a factor. Thinking about sizing the battery way down, locating it much closer to the gen, and adding the capability to parallel with the main start batts.

I'm looking for a reality check. Should the start batteries be sized with big margins if each engine has its own dedicated?

Assume your house battery bank is completely separate? In many cases where starting batteries seem oversize, the original battery architecture has combined start/house functions...

If isolated to start functions, and especially if you have a momentary battery parallel switch for engines, I think a single Lifeline GPL-3100HC for each engine offers sufficient headroom. I'd also compare the Odyssey Extreme ODX-AGM31 (previously 31-PC2150), 1150 CCA, with your preferred terminal option.

Indeed, the 4D for an 8 kW genset is way overkill. We have a 21.5 kW Onan, and a G24 starting battery for it (Odyssey Extreme ODX-AGM24). Works fine. Previous boat had an 8 kW Kohler and we used an Odyssey Extreme G34 for that, nearly same CCA as the current G24. A pair of jumper cables in lieu of an installed battery parallel switch can be a reasonably easy/inexpensive back-up for that.

-Chris

 

[Datenight]

When reconfiguring the battery system in the Green boat I replaced the 8-D with a group 31 for the Perkins 6-354T. Always started instantly.

Rob

 

[rslifkin]

Keep in mind that boats aren't typically started as cold as a car. So you'll never actually be down to the temperature where CCA is rated, usually closer to the MCA temperature at the coldest. And a diesel will need somewhat less power to start in warmer conditions. That said, the bigger battery will have less voltage sag and crank the engine a bit faster as a result.

The group 31s are most likely fine as starting batteries, especially if your engines never crank for more than a few seconds before starting.

If you're worried about having it too close to the limit but want to reduce the cost, space, and weight of the starting battery setup, I'd consider either 2x G31 or G27 in parallel for each engine (to have extra power on tap), but eliminate the dedicated generator start battery and put in a switch to let it use either of the engine start batteries.

 

[mvweebles]

Here's a thread off SBMar from a guy with 6BTAs who is asking about swapping pairs of G27s (ignore the question is for LFP). Tony Athens, acclaimed Cummins guru, responds halfway down that the G27s are the right setup for these engines.

6BTA Starting battery

I currently have two group 27 lead-acid starting batteries per motor. I would like to reduce weight and save space and so am considering a single 12V 100AH Lithium Ion Battery (link below)...

www.sbmar.com

Unless these batteries also serve as house bank (as posited by @ranger58sb ), I'd ditch the 8D/4D batteries. They're unnecessarily expensive, heavy, and bulky.

BTW - a single G31 starts my Perkins 4.236 (75hp) with ease. A single G27 should easily support your generator,.

 

[Bacchus]

I have a GP27 for gen start and replaced the on/off batty sw with a 1-2-all Sw that had the same foot print and bolt pattern. The 2nd pisition was wired to my house for emergency starting - never needed it but also handy to top off gen start charge from house charger.

 

[Diep]

Thanks for all the opinions and experience!

In the bigger picture the main engine starts currently feed the house panel as well. It seemed obvious in this context that they would benefit from being big. We don't like that set up and plan to add a dedicated house bank. I didn't want to muddy the water too much looking for thoughts to start roughing out the start side of a new architecture.

Interesting idea from rslifkin. With the way the boat is set up now I think a totally independent gen start battery is absolutely essential. However, in a new architecture with dedicated house bank and two engine starting banks, maybe it would be OK to start the gen with main start batteries. A large house bank could back this up as well. Still makes me a bit nervous, I would probably keep a (small) gen start battery.

 

[NoRain]

I have 8D FLA start batteries for my Cummins 5.9BTs. I've been looking at the new 12V Odyssey Extreme G31 Starting / Deep Cycle Battery - 1150 CCA, 103 Ah.
The group 31s have the same CCA.

I think a group 27 starting battery would be more than enough for your genset.

Jack, I’ve used a parallel pair of Odyssey PC2150 (now called ODX-AGM31) as the start bank serving both of my Cummins 450Cs (6CTA8.3) for over 9 years now. Plenty of oomph.

 

[Comodave]

I have twin 6CTAs in my boat. 2 group 31s on each engine. Works great. Formula did a nice job on the cables. In order to parallel the batteries they made custom cables. Each cable goes fron the positive post on one battery to the positive post on the second battery. Then to the battery switch. The negative cable is similar. This way there is only one connection on each battery terminal.

 

[tiltrider1]

My turn to ask a battery question.

My battery box is 20x23. Height is not an issue. I just pulled two 8D’s out but I’m going back with something much more manageable. The 8D’s were 1350CCA each and lasted 15 years. While NO issues i have lost confidence due to their age. I prefer no maintenance AGM to FLA.

I can fit 3 Group 49s (900CCA each) or I can fit 4 Group 48s (750CCA). I am also willing to evaluate other ideas.

I am stuck with stock alternators as I have Cat 3208’s, which have only enough room for the stock alternator. This eliminates the idea of paralleling LFPs.

 

[rslifkin]

Interesting idea from rslifkin@. With the way the boat is set up now I think a totally independent gen start battery is absolutely essential. However, in a new architecture with dedicated house bank and two engine starting banks, maybe it would be OK to start the gen with main start batteries. A large house bank could back this up as well. Still makes me a bit nervous, I would probably keep a (small) gen start battery.

Knowing that you currently have a combined house/start setup, I agree. That's how my boat was originally. Batteries were port, stbd, and gen. Port and stbd each fed half of the DC panel. Once I split off a proper house bank, I got rid of the gen battery, so the batteries are now house, start 1, start 2. For starting power, each engine and the generator have a selector switch so they can use either start battery (and nothing draws from the start batteries when the engines aren't running, so if one goes dead it's because it failed, not because of accidental discharge). Normally I leave the port engine on 1, stbd and gen on 2. That setup has worked well since it was installed about 7 years ago.

I figure the gen takes less power to start than the mains do, so even a weak start battery may get it running. And the chances of both start batteries failing at the same time is very low, so it's unlikely that I won't be able to get something started. Worst case it's time to pull out the jumper cables to start something from the house bank (I don't have a pre-wired way to do that on my boat).

 

[BruceK]

Re the genset battery. It usually has to feed glow-plugs before feeding the starter motor. It may only be charged by the genset alternator when the genset runs, that may be frequently, or rarely. Don`t undersize it. Onan`s recommendations of a 6D seem excessive to me, but I follow them, or a near equivalent.
The genset battery may be salvation on many boats. An alternative source of charge maintenance is a good idea, be it the boat`s charge system, a dedicated solar panel, or otherwise.

 

[JackConnick]

Re the genset battery. It usually has to feed glow-plugs before feeding the starter motor. It may only be charged by the genset alternator when the genset runs, that may be frequently, or rarely. Don`t undersize it. Onan`s recommendations of a 6D seem excessive to me, but I follow them, or a near equivalent.
The genset battery may be salvation on many boats. An alternative source of charge maintenance is a good idea, be it the boat`s charge system, a dedicated solar panel, or otherwise.

I have all 3 start batteries on a Victron Phoenix 3/50 charger for maintenance on shorepower. I accidentally ran down the Group 24 gen SLA battery (ignition switch got flipped).I have a NL 5.5 KW genset.
I just ran the mains for about 5 mins with the Phoenix on the inverter and started the gen easily.
A 6D is silly big, sorry. But I guess I don't know how big a gen you're firing.

 

[JackConnick]

My turn to ask a battery question.

My battery box is 20x23. Height is not an issue. I just pulled two 8D’s out but I’m going back with something much more manageable. The 8D’s were 1350CCA each and lasted 15 years. While NO issues i have lost confidence due to their age. I prefer no maintenance AGM to FLA.

I can fit 3 Group 49s (900CCA each) or I can fit 4 Group 48s (750CCA). I am also willing to evaluate other ideas.

I am stuck with stock alternators as I have Cat 3208’s, which have only enough room for the stock alternator. This eliminates the idea of paralleling LFPs.

Group 31 Odyssey is your answer. 1050CCA 13"L x 6.8"W x 8.9"H

NorthStar Batteries ODX-AGM31M | Fisheries Supply

Save More on Your NorthStar Batteries #ODX-AGM31M ODY ODX-AGM31M 12V G31 AGM 103AH 1150CCA BATTERY at Fisheries Supply. Excellent Customer Service, Ready to Ship. Marine Supplies Since 1928!

www.fisheriessupply.com

 

[BruceK]

I have all 3 start batteries on a Victron Phoenix 3/50 charger for maintenance on shorepower. I accidentally ran down the Group 24 gen SLA battery (ignition switch got flipped).I have a NL 5.5 KW genset.
I just ran the mains for about 5 mins with the Phoenix on the inverter and started the gen easily.
A 6D is silly big, sorry. But I guess I don't know how big a gen you're firing.

It`s a 7.5KW Cummins Onan with 3 cylinder diesel. Don`t disagree 6D seems excessive but, last time I checked the Onan Manual that (or similar capacity) was specified. This one gets charged like yours, plus a splitter off the stbd alternator. Prior boat genset (Onan 6.5 3 cyl Kubota)battery only charged from its alternator, I fitted a dedicated small regulated solar panel.

 

[SteveK]

My turn to ask a battery question.

My battery box is 20x23. Height is not an issue. I just pulled two 8D’s out but I’m going back with something much more manageable. The 8D’s were 1350CCA each and lasted 15 years. While NO issues i have lost confidence due to their age. I prefer no maintenance AGM to FLA.

I can fit 3 Group 49s (900CCA each) or I can fit 4 Group 48s (750CCA). I am also willing to evaluate other ideas.

I am stuck with stock alternators as I have Cat 3208’s, which have only enough room for the stock alternator. This eliminates the idea of paralleling LFPs.

You have 2700CCA now and looks like you are replacing with 2700 or 3000 CCA. If you chose G31 would be 3 as well. OEM says to have a minimum of 1800. So I think you are on the right track.

 

[ranger58sb]

Indeed, the 4D for an 8 kW genset is way overkill. We have a 21.5 kW Onan, and a G24 starting battery for it (Odyssey Extreme ODX-AGM24). Works fine. Previous boat had an 8 kW Kohler and we used an Odyssey Extreme G34 for that, nearly same CCA as the current G24. A pair of jumper cables in lieu of an installed battery parallel switch can be a reasonably easy/inexpensive back-up for that.

It`s a 7.5KW Cummins Onan with 3 cylinder diesel. Don`t disagree 6D seems excessive but, last time I checked the Onan Manual that (or similar capacity) was specified.

??

Our Onan 21.5 kW MDKBE is a 4-cylinder, and the manual specifies only a minimum of 625 CCA.

The ODX-AGM24 we installed last season spec's at 840 CCA. Works fine.

-Chris

 

[Iggy]

I have twin 6CTAs in my boat. 2 group 31s on each engine. Works great. Formula did a nice job on the cables. In order to parallel the batteries they made custom cables. Each cable goes fron the positive post on one battery to the positive post on the second battery. Then to the battery switch. The negative cable is similar. This way there is only one connection on each battery terminal.

I did almost the same thing. My 4D was dyeing and there rated about 1400CCA. I went with 2 group 24s totaling 1600CCA. So much lighter to install!

Dave, not sure how you got one connection to each terminal? To balance the the two batteries, I came off of opposite ends of the two batteries with the POS and NEG cables.

 

[Bacchus]

I believe he is talking about using "Flag Terminals" they are available for post or stud terminals. If you use the pos from on batty in parallel and the neg from the other they are balanced. When I paralleled by 2 8D house battys that's exactly what I used.

Battery Terminal – Stud Top – Flag Locking - 1/0 (one-ought) - Custom Battery Cables

Use flag connectors to connect multiple batteries to the same cable Made of cast copper alloy, tin plated for corrosion resistance

custombatterycables.com

 

[Iggy]

I believe he is talking about using "Flag Terminals" they are available for post or stud terminals. If you use the pos from on batty in parallel and the neg from the other they are balanced. When I paralleled by 2 8D house battys that's exactly what I used.

Battery Terminal – Stud Top – Flag Locking - 1/0 (one-ought) - Custom Battery Cables

Use flag connectors to connect multiple batteries to the same cable Made of cast copper alloy, tin plated for corrosion resistance

custombatterycables.com

Oh! Thanks