How much is enough!

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I would go along with Bigfish except that you should consider lithium batteries if you can afford the extra initial cost. They will serve you better in the long run.
Also, I would use a DC-DC charger even if you use AGM batteries. It ensures a good charge and will care better for your batteries than direct from the alternator. Some DC-DC charger incorporate solar input which simplifies the installation at no extra cost.
And yes, don't skimp on cabling sizes.

Sorry but I disagree with using a DC-DC just because.................( it's there to buy )
Most of the newer alternators will put out 100Amps @14.2V and it's hard to find a Dc-Dc that compares to that .
The only real use for a Dc-Dc is when you have a ''smart alternator'' that the ''smart'' can't be turned off .
But......
Retired and out of the game for a few years now, and being honest, I don't know if the chosen vehicle has a smart system or not and if it has , can it be turned off.

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Hi Noggins.

Sure, a 100A @14.2 alternator will pump the charge in really well. And therein lies the problem.

AGM batteries are usually rated for a charge rate of no greater than 20% of rated capacity. That means that a 100Ah battery should be charged at a maximum rate of 20A. If it has been discharged down to 12v then the alternator is likely to pump in as much as 40A which is not good and will diminish battery life.
A DC-DC charger rated at 20A will satisfy the requirement nicely. Furthermore, such chargers have an algorithm with several charging stages which further enhances the charging process. And of course, it acts as an isolator so eliminating that cost and complication.
The ability of these chargers to fully perform with inputs below 12v ensures that losses in the input cabling (and there will always be some) do not interfere with providing the battery with exactly what it needs for best performance and life.
It really is a no-brainer.
As a bonus, DC-DC chargers are available with solar input eliminating the need for a separate MPPT regulator.

I would have mentioned lithium but poster specifically said deep cycle. I have considered Lithium but my bloody Fullrivers refuse to die!! Hahaha.
Also noggins. your alternator may put out 100 amps but it also has to run lights, air con, radios, fans etc.etc. The smart alternator in my vehicle cannot be turned off. Not very often I see over 13.5 volts and this is monitored via the OBD2 port with an Ultragauge. Majority of new cars have a smart alternator as standard now . I use a 50 amp dc/dc charger and am very happy with its performance as its output can be varied depending on your needs. I also use one in the camper as well. Dc/dc charger will definitely look after your battery better than an alternator. The dc/dc argument is a shitfight to say the least...just do whatever works for you without bagging others who run a different system. So long as everything runs well and is reliable...your on a winner!

cheers.

The 100A alternator does not fry your crank battery, so why should it fry your house battery?
Answer, charge rate is determined by the voltage offered and alternator voltages are not high enough to fry batteries.
That means that as the battery voltage comes up, the difference between the alternator voltage and the battery voltage reduces and the charge rate also reduces as a direct result.
Internal resistance protects the battery.
Cheers,
Peter
OKA196 motorhome

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Peter,

As I clearly said, battery manufacturers express limitations on current rates when charging batteries. For AGM it is typically 0.2 of the battery Ah capacity. Exceeding these recommendations will decrease battery life. (Or "fry" it to use your pejorative)
Crank batteries being of flooded construction ar capable of accepting much higher charge currents directly from the alternator.
Your proposals fly in the face of science and the recommendations of battery manufacturers.
My expression is from the manufacturers.

As an example, the following is from Century Batteries:...........
"Deep Cycle AGM:- AGM batteries should be charged using a charger with a mode switch for AGM type batteries. It is recommended to use a charger, current limited to 20% or 0.2C of the batteries capacity at a charging voltage of between 14.6 – 14.8V following deep cycle use, or 13.6V – 13.8V for standby use. Charging should be paused if the battery or ambient temperature exceeds 50°C."
See the whole Century page here Similar expressions from other manufacturers.

Certainly the charge current will diminish as the cell voltage rises but it is the high current which precedes that does the harm.
Internal resistance of these batteries is in milli-ohms and far too low to "protect the battery". It is in fact what permits the high currents. The resistance of the cabling is much higher but still too low to offer protection.

I have used a standard AGM as a crank battery in the OKA (the same one) for 9 years.
The alternator output voltage is about the same as the recommended float voltage of the battery and does it no harm because at that voltage you will not achieve excessive charge rates.
That's "practice" not "theory".
Cheers,
Peter
OKA196 motorhome

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Peter,

We were not talking about AGM's as cranking batteries. We were discussing charging AGM's as auxiliary batteries.
And you are still referring to alternator voltage which is not the point. The concern is excessive charging current which is produced by the magnitude of the differential between battery and alternator voltages.

A cranking battery (flooded or AGM) does not discharge down to 12v or less as the result of a normal start therefore upon subsequent charging its voltage differential to the alternator is small enough to limit the charging current to an acceptable value. But I am genuinely interested as to why you use an AGM as a cranking battery as opposed to a customary flooded start battery. Is it located in the engine bay?

On the other hand, an auxiliary battery regularly discharges down to 12v (sometimes even lower!) so if directly connected, the differential to the alternator is great enough to cause charge current excessive to the manufacturer's specification. In this application, a DC-DC charger is very appropriate for battery health.
OEM alternators and their regulators are designed for flooded cranking batteries not AGM deep cycle batteries on auxiliary duty. I have experienced this to my sorrow.

This is understanding not misinterpreting.

And as a matter of interest, in practice it is borne out by the observation of a pair of volt/amp meters on my dash for my cranking and auxiliary batteries. Observation of their information is enlightening.

Allan, at the risk of repeating myself, I don't think an alternator will give excessive charge rates due to the relatively low voltage offered, irrespective of the SOC of the battery.

I had several experiences with flooded batteries (house batteries in our previous F350 motorhome and crank batteries in the OKA) failing in relatively short time frames (2 years). I came to the conclusion that they were falling apart internally as a direct result of corrugations.
In 2009 I installed a Fullriver 120A HGL standby battery as a crank in the OKA (no, it is not under the bonnet). It struggles a tad in very cold weather without the benefit of glow plugs (but would happily crank the Perkins for 30 seconds without a break), but did the job extremely well for 9 years until replaced by another (unknown brand) AGM, simply because I was buying batteries for the house and got a good deal. This one is working a tad better than the Fullriver.
Cheers,
Peter
OKA196 motorhome

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Yes Peter, I would agree wholeheartedly that corrugations are a killer for batteries, as is heat. The engine bay is the worst place to locate a battery and a few auto makers have engineered alternative locations from time to time but most opt for the convenient spot.

A problem in engineering (and elsewhere) is that the ideals of physics do not always prevail due to all sorts of reasons. That is why it it is unwise to proclaim that it "will happen" or it "won't happen". All I can point to is what an "authority" or manufacturer may proclaim and add my own experiences.
Which is why you have experienced one thing and I the other. And I certainly do not dispute your experiences or their validity.

I also have destroyed several AGM's when positioned under the bonnet and after repositioning to the cabin and adding dc-dc charging I am celebrating their performance. Maybe their distance from the alternator and the accompanying cable resistance would limit the charge current to a non-destructing value but I put in dc-dc charging anyway so have not experienced the former. Maybe a bit of cable resistance is benefitting your auxiliary batteries. And if your cranking battery is not in the engine bay then it would have a longer cable from the alternator which may be of benefit. See the "maybe's" creeping into this?

I do observe that the current from my alternator to the cranking battery sometimes is as high as 40A initially after an engine start but soon drops to a lower value to settle at just a few Amps for a while, then zero until the next start. It does not take more than a few minutes to replenish the battery. The auxiliary battery ammeter on the other hand sits right at the 40A maximum of the charger for a couple of driving hours until settling down. The 40A is shared between two paralleled AGM batteries so is within their specifications. In truth, I have not ascertained their balance but I assume it is reasonable as they are identical in all respects. Anyway, it works which is as you say, "practice not theory".

It just goes to show that there is more than "one way". And I am a great believer in "what works, works".
If I was starting afresh I would be tempted to use a single lithium bank for everything including cranking and carry a little lithium jump-starter for emergency. Ahh, dreams and experiments!

Allan B how do you intend to do this "If it has been discharged down to 12v then the alternator is likely to pump in as much as 40A" with an auxiliary battery in parallel with the main vehicle start battery?
Many here need to do some technical study on batteries and associated equipment, and not make rash statements on cable size when they are clueless to the distance the cables are going to run and current associated.

Why when somebody asks a question the threads have to dribble into diatribe?

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Dave B18,

I really do not understand your question. As Pauline would have it..... "Please explain". lol

Allan this is the problem here, all are close to pretty clueless on low voltage electrics and how they should be setup and operated in an RV setup. I realise you all have poor electrical setups and most crap to non optimum gear, and then add the lies and fantasies and all we get is more crap RV electrical systems. Really useless replying when people outright lie and think they know, don't listen comprehend or do research. Not that it is that easy to do research on good RV systems as what so many think here is good is crap and so do the large majority think what is good is bordering on crap to non optimum. How different Australia is to the good ole' US of A where the van dwellers and RV people want the utmost in reliability and prepared to spend money on the right gear and get it right first time. In Australia majority of RV people do things or buy things 3 times before they get it close to right.

Dave B18...I,ve heard some shit in my time but you absolutely take the cake. You have jumped on here called anyone who gets more than 4 years out of an agm a liar. Now you say that all on here are clueless about 12 v electrics. You then slam everyone for having piss poor electrical setups in their rv, trailers etc. Where do you find the time to inspect everyone's vehicles. Dead set wanker....mods can chew me out if they want but we dont need self opinionated dolts like this slamming all the forum members as incompetent!! ...and that what he has done!

Why dont you do everyone a favour and just piss off? Its obvious you are way to intelligent to be talking to mere mortals. Why waste your time on here when we have no idea?

You are what would be called a "sexual intellectual"

Bigfish little brain, jealousy and lousy education are a hindrance in life as you have found out. Perhaps if you had spent more time getting educated would have been substantially beneficial to you.
Well sadly you are all clueless as I can see it in the replies. There is not one reply to the OP that is on the money or close to correct - all just diatribe.
Thankfully not a bother to me. I will leave you all in your electrical misery and pain which you all think is normal to good.
Thankfully unlike you, I never have wasted emotion over anything - never!

Thanks for the laugh..

Dave B18, Bigfish is quite correct in his observations of your behaviour here. By all means query challenge and debate things but there no need for what looks like an out of place outburst and then personal offensive remarks from you. If this behaviour continues Sir you'll likely earn yourself a short term ban from the site. Please desist with such inappropriate forum behaviour.

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Dave,

The OP asked for suggestions on the solar panel size and the battery size to suit his setup. I thought the first two responses from Bigfish and myself were pretty much on the ball given the load data offered. Ignore the side arguments.
I have not seen a response to the OP from yourself. Would you care to give a response now and thus perhaps enlighten us all?

I think he,s moved on Allan... sad eh?....lol

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Very disappointing Bigfish. lol again!
I was keen to read his response to Trevor's questions.

Thanks I’ll have a look.

Thanks. I have done a fair bit of reading about where you place your solar panels. I like your idea and there is science to back it up. I might get a portable solar panel as I have seen them on windscreens etc. some of the new “ flexible “ ones look good.
Thanks for your thoughts. Trevor

Hi Trevor,

Folding panels are more efficient than the “blanket” panels, and are also generally cheaper, they just take up more space.

Macca.

Thanks for your information.

Thanks very much for your information. Putting a 40 litre fridge in not a 60 now. Would a 300 watt solar panel be overkill?

As others have alluded to you can’t have too much solar!!! Bear in mind a Solar panel that is genuinely 300W will be a large panel, approx 1.7m2 in size.

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Trevor, Bigfish is right.
If it is figured that say 200watt is satisfactory for a sunny day then you would be wise to allow for the many not-so sunny days and plan to have a bit up your sleeve.

Bigger is also better so that you can camp in the shade a bit more.

Our system recharges perfectly by midday if I park it in the sun facing north but when we stayed in a lovely leafy campground in Vic it couldn't keep up - each day the battery went a bit flatter (only 10% or so - we moved on before it was a problem). We had periods of direct sunlight but it wasn't all day.

It is for that very reason I have chosen 200 watts on the roof and 240 watts portable on long cables. It works well for my needs.