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Engine Start Battery Options

I'm in the market for fresh engine start batteries, and looking at two options.  I have researched this subject in the Forum archives, and found some excellent info...especially Don's more recent posts on his battery system re-do.  But I'm still unsure which way I want to go, so looking for input.  I don't want to get into debates about battery type, or brand.  I'm just interested in weighing the pros and cons of 2 different setups.

My current battery arrangement is two O'Reilly AGM8D house batteries (installed Dec. 2013, haven't been abused, still in good shape) and two Optima Model 34/78-980 Red Top start batteries, which were in the coach when we bought it in Nov. 2013 - actual install date is unknown.  The Red Tops have been working fine up to now, but are starting to get tired, hence my search for replacement.

I am looking at two options.

Option #1:  Replace the Red Tops with 2 fresh AGM group 34 batteries of similar CCA rating (at least 800 CCA).  I will decide on the actual brand later - don't want to go off on that tangent right now.  This option provides total CCA of 1600 (two fresh fully charged batteries wired in parallel), which has always been sufficient for my C8.3 Cummins.  My engine manual calls for a minimum of 1280 CCA at 32 degrees F, and minimum of 1800 CCA at 0 degrees F.  In our 5 years of ownership we have only started the engine perhaps once or twice in below freezing temps, and both those times I simply used the boost switch + block heater = no problem.

Option #2:  Replace the Red Tops with a single O'Reilly AGM8D.  This battery is rated at 1450 CCA, which is less than the dual Red Top combo.  However, it still meets the Cummins CCA requirement for 32 degrees, and if it was colder than that I would just use the boost switch.  Where the AGM8D has a big advantage over the Red Tops is in its capacity: 245 Ah versus 100 Ah (50Ah x 2).  I am thinking this would come in handy when dry camping.  I could use the boost switch to connect both battery banks and increase my useable capacity (staying above 50% drawdown) from 245 Ah to 367 Ah.  Yes, I know, I would have to closely monitor voltage to avoid getting in a bind, but I do that anyway.  We have always been able to get along with our two 8D house batteries, but I usually have to start the generator in the mornings to use the microwave.  I'm thinking the extra 122 AH of capacity might JUST allow us to avoid using the generator in the mornings and wait for the sun to come up and start powering our PV battery charging system.

The price of the two different options (depending on what brand "small" batteries I buy) could be close.  The AGM8D battery weighs twice as much as the (2) group 34 batteries, so there would be a weight penalty, but it also seems like the coach would be better balanced (side to side) if I used the "big" 8D battery.  I THINK the AGM8D would fit in the existing start battery rack, but it would be very tight in the width dimension.  The start battery cables would hook up just fine either way.

One thing I wondering about (with Option #2) is the problem of mixing old and new batteries.  If I were to combine (with boost switch) both battery banks to extend our dry camping time, I would have one new battery tied to two old batteries.  Would this be a problem, or harmful to the new start battery?  If all three batteries were fully charged (normal condition at end of driving day) then I would think when they were combined that they would just equally "share the load" as they were drawn down overnight.  But I could be wrong...

One other thing - if I decided to use the AGM8D as a start battery, I would probably replace our (OEM) diode isolator with either a manual switch or some kind of "smart" combiner, and connect the alternator directly to the start battery.  This would give me more control over the way the batteries are being charged, especially when in extended dry camping situations.

Welcome any thoughts/ideas/comments from the Collective.  I highly value the wisdom and experience of those who have gone before!

Re: Engine Start Battery Options

Reply #1 –
I replaced the shot three 34 series when we purchased our used U300 nine years ago with threeAutozone 34 series Duralast batteries for the chassis/engine batteries. They lasted for 8 years until one was a bit low so I replaced all of them with 31-950 series Duralast batteries. Almost exactly $300 total for all three. After asking for a discount, Autozone dropped the price 10%. They have 950 CCA or 2850 CCA total. What may seem like overkill in summer is a good resource to have available at high altitudes and sub feezing weather in wintertime.
https://www.autozone.com/batteries-starting-and-charging/battery/duralast-battery/97512_721426_25698

Using a boost switch makes the solar system less efficient as it has a constant power draw. It's not a latching solenoid/relay. This heats the solenoid and will decrease it's lifespan even though it's rated "continuous duty." Plus, anytime you use anything to combine banks and only have deep cycle batteries on the house batteries, the conventional or AGM batteries on the engine side will see reduced life.

We shut off the inverter at night. Not a big deal, just flip the switch. Even with only 400ah on our six 68ah AGM deep cycle house batteries, we have plenty in the morning to run the microwave and latte maker. I never use the boost switch. I consider it only for use in emergencies.

The start batteries don't lose much in dry camping for a week so I don't worry about them. If longer while plugged in at home, I  tie both banks together lighter plug to lighter plug for charging. Voltage stays 13+V on both banks that way.

Pierce

Re: Engine Start Battery Options

Reply #2 –
I like the idea of using an 8d agm for start with a battery combiner to connect to house
batteries. The only problem I see is that the combiner would disconnect at 12.7 volts which means that the start battery would not share the load at any voltage below that. You could use a manual switch as well, but would have to keep eye on voltages so start battery does not get too low.

Re: Engine Start Battery Options

Reply #3 –
Yes, if using a battery combiner, one would need a dash switch in addition to the combiner's normal "automatic" feature.  But,  there should already be a switch and wiring to the OE combiner (solenoid). 

OR

One could just add the combiner and leave the OE boost switch.

Re: Engine Start Battery Options

Reply #4 –
Quote from: Pierce & Gaylie Stewart –
1.  I replaced all of them with 31-950 series Duralast batteries.

2.  Using a boost switch makes the solar system less efficient as it has a constant power draw. It's not a latching solenoid/relay. This heats the solenoid and will decrease it's lifespan even though it's rated "continuous duty".

3.  Plus, anytime you use anything to combine banks and only have deep cycle batteries on the house batteries, the conventional or AGM batteries on the engine side will see reduced life.

4.  We shut off the inverter at night.

5.  The start batteries don't lose much in dry camping for a week so I don't worry about them. If longer while plugged in at home, I  tie both banks together lighter plug to lighter plug for charging. Voltage stays 13+V on both banks that way.
Pierce,

Thanks for your comments, to which I would make the following responses:

1.  The Duralast Group 31-950 batteries sound like a good deal, but they are 13" long, where my battery rack is sized for Group 34 batteries, which are 10" long.  31's would not fit without some modification to the rack, which I was trying to avoid.

2.  Good point on the boost switch drawing constant power.  I could avoid that by doing what Brett said in his comments: add a manual switch.

3.  In my Option #2, all three batteries would be identical in size and type and capacity.

4.  We can't shut off our inverter cuz we have a residential fridge.  I could add a separate smaller dedicated inverter for just running the fridge, but so far haven't seen that as a high priority.  Our Magnum MS2812 isn't too bad a power hog when in standby mode.

5.  I don't worry about our starting batteries either.  We have a Trik-L-Start that keeps them topped up all the time.  If I used the AGM8D as a combo start/house battery, when dry camping I would need some other means of recharging it after a big draw down.  A manual switch connecting both battery banks would serve that purpose by allowing either our solar panels or our generator to recharge all 3 batteries simultaneously.

Re: Engine Start Battery Options

Reply #5 –
Something else occurred to me whilst pondering this question.  The "standard" starting battery configuration on my coach is two 800 CCA AGM batteries connected in parallel, which yields (optimally) 1600 CCA to the starter.

Cummins says under "normal" temperature conditions (above freezing), my C8.3 engine only requires 1280 CCA to spin the starter.

If I engage my boost switch, I am adding two AGM8D batteries which connected in parallel put out (optimally) another 2900 CCA.

So the total starting power of both battery banks could be (optimally) 4500 CCA!  :o

Is it possible to send TOO many amps to the starter?  What can these diesel starters safely handle without melting down?

Re: Engine Start Battery Options

Reply #6 –
Quote from: Chuck & Jeannie –
Is it possible to send TOO many amps to the starter?  What can these diesel starters safely handle without melting down?

In a word, NO.  No such thing as too little voltage drop.  The starter will not draw any more amps if hooked up to 2 or 20 batteries, but the more batteries (amps) the lower the voltage drop and the faster it will start and the longer the starter will last.

Re: Engine Start Battery Options

Reply #7 –
You could have a million 12 volt batteries in parallel and it would not make any difference to the starter motor. The voltage drop would be less and the starter would love you for that.

The more voltage drop you have, the harder the starter has to work and since it takes longer to start, the hotter it gets and is more likely to overheat and fail.

It's just like trying to run a roof AC unit on a coach that is supplied by a long extension cord if you don't have enough CCAs to start your coach. You never can have too many CCAs.

Pierce

Re: Engine Start Battery Options

Reply #8 –
The single O'Reilly "AGM8D" (This is the part# that O'Reilly uses if you are trying to look it up on their website) that is used in our coach for starting is still spinning it up just fine after 5 years or so. It was probably abused some by not having a charging set up while camped on shore power for long periods for the first 3 years or so of its life (no solar on the roof or shared charge set up), but it does the job without complaint. Usually starts the coach within a couple of seconds. We have the electronic version of the Cummings 8.3, which because of the intake grid heater and ECM, is probably at bit more power hungry at start up. The key is to have good electrical connectivity. When I changed the battery cable set up with new 4-0 marine cable and bus bars, there was a notable improvement in the starter's enthusiasm for cranking the coach. When this 8D reaches the end of its useful life, I will likely by another one. It fits lengthwise the OEM battery tray perfectly where the Red Tops sat sideways. I like the simplified wiring of this set up compared to extra connection points in the multiple parallel OEM method. That said, I wouldn't mind having two of these in parallel for starting :D  Of course, I have four of these O'Reilly AGM8D's for the house batteries and a heavy duty Blue Sea magnetic relay to act as a boost switch. I am embarrassed to admit that I accidentally started the coach with the new boost switch engaged but with the heavy duty Blue Sea chassis battery disconnect in the off position. This meant that there was NO help from the chassis battery, but the starter turned over as it normally does and started the coach no problem. I never intended to do that as it stresses other components in the set up beyond what I would like to see, but it is nice to know that it is possible... at least with the medium duty Cummings 8.3 ISC. Obviously, it would probably be a different story with the bigger ISM or M11.
Don

Re: Engine Start Battery Options

Reply #9 –
And the 9 liter 6V-92TA won't do well in winter with a single 8D or two 34 series batteries.

P

Re: Engine Start Battery Options

Reply #10 –
Quote from: Don & Tys –
Of course, I have...a heavy duty Blue Sea magnetic relay to act as a boost switch.
Don,

What switch are you using?  (I could look it up in your old posts, but feeling lazy)

I was just looking at the Blue Sea Systems ML-ACR automatic charging relay.  The way I read it, this device could be used to replace my diode isolator and my boost solenoid.  Does that sound right to you?

BLUE SEA SYSTEMS ML-Series Heavy Duty Automatic Charging Relay | West Marine

Re: Engine Start Battery Options

Reply #11 –
I just replaced my 3 engine batteries with AGM Deca.  I was told that red tops are not made in the USA anymore.

Re: Engine Start Battery Options

Reply #12 –
Chuck,
I went with the Blue Sea 7713 Heavy Duty Remote Battery Switch rather than the ACR version. I don't fully remember why I went with that version as the cost is similar, but I am sure I had a reason ::) I know part of it had to do with my Sterling ProSplit R Zero Voltage Drop Battery Isolator and 12 Volt - 210 Amp Sterling Alternator-to-Battery Charger installation. Potentially, I suppose it could confuse the battery monitors somewhat (I also have one on the chassis battery). Anyway, removing the battery isolator sounds good, but I would have to study the implications to be comfortable with that set up. If and when some part of that battery menagerie needs updating, I will give it some thought. Too busy planning other upgrades and changes to worry about it as the coach DC systems are working harmoniously together at this time...
Don    
Quote from: Chuck & Jeannie –
Don,

What switch are you using?  (I could look it up in your old posts, but feeling lazy)

I was just looking at the Blue Sea Systems ML-ACR automatic charging relay.  The way I read it, this device could be used to replace my diode isolator and my boost solenoid.  Does that sound right to you?

BLUE SEA SYSTEMS ML-Series Heavy Duty Automatic Charging Relay | West Marine

Re: Engine Start Battery Options

Reply #13 –
Thanks, Don.  Your whole DC setup is MUCH more complicated than our coach, so any resemblance is purely coincidental!

Re: Engine Start Battery Options

Reply #14 –
Quote from: Chuck & Jeannie –
If I engage my boost switch, I am adding two AGM8D batteries which connected in parallel put out (optimally) another 2900 CCA.

So the total starting power of both battery banks could be (optimally) 4500 CCA!  :o

Chuck,

The flaw in this is the boost solenoid and related wiring will limit the amperage that you will really have.

Mike

Re: Engine Start Battery Options

Reply #15 –
Mike,

What is the limiting amperage (solenoid) or wiring (what gauge and length)?

I would have though that it was over 250 amps for an instant (i.e. starting) use.

Re: Engine Start Battery Options

Reply #16 –
Quote from: wolfe10 –
What is the limiting amperage (solenoid) or wiring (what gauge and length)?

Both,as neither will give you the full 2900 cca potential through them.
250 amps for a short time yes
Just because you have a bigger bucket full of electricity don't mean you have the means to use its full potential. The solenoid and related wiring is the choke point, like the small end of a funnel.
Mike

Re: Engine Start Battery Options

Reply #17 –
The ACR combiner uses the stock boost wiring to operate its switch in the rear and uses the boost switches original location. An additional wire would need to be run ,which we did not do,  to light up the switch to positively show the rear switches actual position. 

You have a no load manual connect/disconnect as part of the switch

Re: Engine Start Battery Options

Reply #18 –
Chuck, you asked:
One other thing - if I decided to use the AGM8D as a start battery, I would probably replace our (OEM) diode isolator with either a manual switch or some kind of "smart" combiner, and connect the alternator directly to the start battery.  This would give me more control over the way the batteries are being charged, especially when in extended dry camping situations.

Welcome any thoughts/ideas/comments from the Collective.  I highly value the wisdom and experience of those who have gone before!"


And the debate (Isolator vs, Manual vs Gadget) rages on..............
I fail to understand why we continue to complicate this very simple issue:

From a technical and engineering standpoint, these three options are personal choices, with pro's and cons to each. As such, each owner must decide what kind of talents they possess and to what degree of complexity they want to commit, in terms of reliability and bragging rights: 

In my opinion, there are three easily defined and distinct groups:

If one possesses perfect Engineering and Operating skills (as well as focus):  install a "Bank 1, Bank 2, Both, OFF" manual switch (or redesign to a single battery bank design, as Wyatt has done): Because
    • You understand electrical engineering theory, design and the operation of your coach components..............without reservation.
    • You either have or will replace your current battery instrumentation and install a battery instrument system designed with appropriate accuracy, precision, monitoring and alarms.
    • You recognize proper, improper and inconsequential operating conditions without fail
    • You remember to operate manually, without fail (can't have memory lapses)
    [/list]
    If one is a Gadget Guru: install any one of the dozens of automatic battery bank combiner gadgets: Because:
      • You believe the advertisement hype.
      • You believe that the failure rate of Gadgets is lower than Isolators
      • You believe that reliable and unreliable Gadgets are easily sorted out
      • You believe that the Gadget engineering has been thorough and complete for all batteries and operating conditions
      • You believe that Gadget complexity improves system reliability
      • You do not realize that improper Gadget operation can instantly ruin a Gadget and, left uncorrected, can quickly and irreversibly damage entire sets of batteries
      • You feel that partial or complete Gadget failure will be easily detected and corrected
      • You do not believe that Gadgets risk your battery investment
      [/list]
      If one is a KISS advocate: Continue to use the current ISOLATOR/BOOST SW design:  Because
        • You understand that while it may be more expensive than a manual switch, it is the next least expensive and (by far) poses the least operating risk (highest operating return value) of the three options.
        • You understand that an Isolator has ZERO, meaningful, negative operating characteristics.  And, if you don't understand that, you keep asking until you do.
        • You understand that an Isolator and BOOST Switch design is the most elegant option in coach lifetime application (all owners/all operating conditions).
        • You understand that an Isolator is many orders of magnitude more user-friendly and is far less prone to operator error induced damage to battery sets.
        • You understand that Isolator/BOOST SW failure characteristics are far easier to detect, understand and correct than a Gadget's will be.
        [/list]
        HTH,
        Neal

        Re: Engine Start Battery Options

        Reply #19 –
        Every new coach has a combiner as far as I am told.  No isolators.

        Boost switch requires 3amp load to stay on. Combiner consumes zero.  Magnetic.  Silver contacts  100k? Cycle rating?

        Isolator consumes power.  Hence the fins. Combiner consumes zero.

        Versus a separate charger for the engine batteries?