Mike DiD replied:

My Remco 80amphour AGM Auxiliary is designed to be charged at a Cyclic Voltage (i.e. not permanently connected to a charger) of 14.4 to 14.7 volts.

My Alternator puts out 14.4 volts. That's all that matters.

Just because the Alternator is also charging a Wetcell Main battery, it has no effect on the way it charges the Auxiliary.

In differences during discharge don't matter - the Isolator separates them during discharge.

If one cell fails in a battery then there will be a major problem - that may lead to the "my AGM battery blew up my Wetcell battery" stories. If one cell fails in a battery connected in parallel with another you will have problems - if if both batteries are IDENTICAL size, chemistry, age and state of charge.

Mike

Mike DiD replied:

Just to clarify -

If a cell failure occurs with the 12 volt batteries connected in parallel you have a problem - it will be no better or worse if the batteries are identical or different. (of course if the higher resistance battery fails it will get more current from the lower resistance battery, but both batteries have equal probability of failing).

If you want to protect against meltdown in case of cell-failure, you need to put a Temperature monitor on both batteries and open the Isolator if either one overheats - I've never heard of an Isolator with that feature - because it is very unlikely. (Don't bother patenting it - it's now been published here)

"Seeing as its unlikely you will have two identical batts both at identical state of charge etc, "
- SPOT-ON, those who argue that you MUST have identical batteries, forget that the reason for having identical batteries (which is important in a few specific situations) is so that they maintain identical capacity. BUT the whole point of 4WD Dual Battery installations is to allow for vastly different state-of-charge in the two batteries (Start full, Auxiliary discharged).

Mike

Mainey (WA) replied:

Ron,
Various qualified people advise AGM batteries absolutely must not be paralleled with conventional lead-acid batteries. You must change all or none - including the starter battery, unless the AGM batteries are charged from a completely separate source.

Member - Norm C (QLD) replied:

Ron, I am a long way from a battery guru. But I took lots of advice, including from this forum before I did my set up. All I can say is, it works.

I have a normal wet cell starter. Also in the engine bay I have a standard wet cell 105AH DC, 'isolated' from the starter with a solenoid.

I also have two 120AH AGM DCs which are normally carried on the CT, But I have recently set up so I can put one in the back of the 'Lux if preferred.

With my normal set up, all the batteries are in parallel when the engine is running. When engine is off and trailer remains connected, the 3 DCs remain in parallel. Id I unplug the Anderson plug, the 2 AGMs remain in parallel, but the other DC and the starter are isolated apart. I rarely leave the Anderson plug connected for long without the engine running.

I've had this set up for about a year. Not long enough to prove battery life, but it certainly works. Batteries all charge well and have given no trouble.

As stated many times before on this forum, the only real trouble with putting an AGM in parallel with a normal wet cell is that the AGM will accept much higher charge currents. If you only do short trips, the AGM might suck up most of the charge current at the expense of the starter battery. If this is a worry, use a voltage sensing isolator like a Redarc or similar.

I say again, I am no expert, but this set up works for me.

Raymond from Wanderin 4 Wheelers replied:

Hi Ron
The GU had the normal wet cell battery supplied by Nissan and we had a AGM fitted as secondary battery for two years using a smart relay and had not problems. The AGM's need abit of heat shielding as they did not like the heat of the turbo.
Ray

waynet replied:

Hi Ron173
Definately go for the AGM battery and change your start battery to an AGM when it dies. You will not cause any damage to either battery by charging them at the same time, get your supplier/installer to check the charging voltage is within the battery manufacturers specifications. AGM atteries can be damaged by high charging voltage, excessive heat and excesive heat.

Happy camping

Wayne

Sand Man (SA) replied:

Hmm,

Lot's of interesting replies here Ron. I'll try to keep mine simple.

If you have a good quality Isolator, you will not have any trouble with different battery types. The Isolator, in effect, will keep the two batteries electrically separated from each other.

My installation has an Exide Extreme (wet cell) as the Primary battery and an Exide Orbital (AGM) as the auxiliary, both managed by a good quality Isolator.
(No, I don't have shares in Exide)

In addition, I have a 75AH Thumper in the back of the Jack, which is connected to the Auxiliary battery by its own "In-Car" charging unit.

All happily live with each other and are charged off the vehicle alternator when travelling.

Mike DiD replied:

So based on the number of happy users of Wetcell + AGM, it seems we can bury the myth of "you can't have a wetcell and an AGM battery together".

I assume this myth started when someone put in an AGM Auxilairy when they already had an aging Wetcell Starting Battery - then one cell shorted out in the Starter from old age and the AGM pumped huge amounts of current into the Starter and cooked it. My 80 AmpHour AGM has 1500 amp short-circuit current capability ! (less will flow if only one cell is shorted).

If the Starter had been an AGM and one cell shorted, you would have had exactly the same result.

Mike

Mike DiD replied:

I've just noticed that the title of this thread could throw people off - after all, an AGM is still a lead acid battery, just the acid is held in a glass mat rather than a liquid that can spill or that you can measure the SG to determine state of charge.

There are three ways of storing the acid in lead acid batteries -
- Wetcell, Gel or AGM

This has nothing to do with the plate thickness - which is the major factor that differentiates between Starting, Hybrid(Marine) or Deep Cycle design. (Plate chemistry and acid concentration can also be used to improve deepcycle ability.)

One reason AGMs are becoming so popular is because they can inherently work well as Deep Cycle AND still be a good Starter Battery. I have the manufacturers data for my Remco 10inch battery - it works as an 80 amphour (20hour) Deep Cycle, but is also rated for a starting current of 300amps for 5 minutes or 550amps for 5 seconds. Even if you discharge it to 0% remaining as a Deep Cycle, it will still have 80% of it's new capacity (here 64 amphours) after 200 discharges.

Mike