AGM / Lead acid compatibility

Submitted: Wednesday, Mar 01, 2006 at 11:06
ThreadID: 31280 Views:6023 Replies:9 FollowUps:14
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G'Day all,

I have a question for the battery gurus out there, I have a standard lead acid wet starting battery, running a dual battery system with a wet deep cycle auxiliary battery.

Question is,... the deep cycle is ageing and nearing replacement, I'm pretty deadset on going AGM to replace it, but will this be ok with my standard wet starting battery??

I heard 'you shouldnt put a wet in parallel with an AGM', so when my vehicle is running and my 'isolator' is charging both batteries, they must be in parallel or else I'd have a 24v system, so how true is this statement?

Or are my worst fears true..... that to go AGM I need to buy one for the vehicle too?

Rgds

Ron
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Reply By: Mike DiD - Wednesday, Mar 01, 2006 at 11:46

Wednesday, Mar 01, 2006 at 11:46
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
AnswerID: 157792

Follow Up By: Ron173 - Wednesday, Mar 01, 2006 at 12:22

Wednesday, Mar 01, 2006 at 12:22
Mike, G'Day,

Was happily reading your reply, ie reading exactly what I wanted to hear, until the last paragraph..............

now I am wondering what the chances of a cell failure are? and if my batteries are NOT identical will this risk still be there?

Seeing as its unlikely you will have two identical batts both at identical state of charge etc, so is this risk about as likely as winning lotto?

Ron
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Reply By: Mike DiD - Wednesday, Mar 01, 2006 at 12:36

Wednesday, Mar 01, 2006 at 12:36
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
AnswerID: 157807

Reply By: Mainey (WA) - Wednesday, Mar 01, 2006 at 15:06

Wednesday, Mar 01, 2006 at 15:06
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.
AnswerID: 157847

Follow Up By: Ron173 - Wednesday, Mar 01, 2006 at 15:25

Wednesday, Mar 01, 2006 at 15:25
Mainey,

yes I heard that, and is the reason I decided to ask on here to get a few opinions cos everyone seems to tell me different.

I dont know where to get the official. The guy trying to sell me batteries would obviously want to sell me two.

Might just get another deep cycle wet n save up for two AGMs together, or wait till July for tax cheque!!

Would be good to hear it official from one or more of these qualified people.

Although that said, it cant be that bad or else we would hear more about it, I reckon there'd be lots of people on here running an AGM with a wet.

Rgds

Ron
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Follow Up By: Mainey (WA) - Wednesday, Mar 01, 2006 at 17:30

Wednesday, Mar 01, 2006 at 17:30
Ron, I’ve no university certificate on my wall, so I'm not 'qualified' :-)

Umm, maybe I can sell you a new Fullriver AGM deep cycle ?
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Follow Up By: Ron173 - Thursday, Mar 02, 2006 at 13:18

Thursday, Mar 02, 2006 at 13:18
Thats a possibility, where are you? I'm nsw (northern)

Ron
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Follow Up By: Mainey (WA) - Thursday, Mar 02, 2006 at 13:26

Thursday, Mar 02, 2006 at 13:26
In WA, can source for you in NSW/Qld at prices as shown in 'trader add'
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Reply By: Member - Norm C (QLD) - Wednesday, Mar 01, 2006 at 16:00

Wednesday, Mar 01, 2006 at 16:00
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.
AnswerID: 157859

Follow Up By: Ron173 - Wednesday, Mar 01, 2006 at 16:17

Wednesday, Mar 01, 2006 at 16:17
Norm,

thanks for that, sounds good, i wouldnt have prob of sucking all charge as my isolator will only charge it once starter full.

Thing is someone will now come along and tell us its a NO NO

Your advice is much appreciated and its always good to hear others setups.

It'd be good to get someone who'd give you it in writing, then if it stuffs up you got some ground to stand on.(which in my worst nightmare could be a vehicle fire)
Might try local ARB thats ARB just for the record LOL! and see what they say.

Or if I bought an AGM from a dealer n got him to put it in, he'd be knowingly fitting it with a wetcell.

Rgds

Ron

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Follow Up By: Mike DiD - Wednesday, Mar 01, 2006 at 18:10

Wednesday, Mar 01, 2006 at 18:10
"my isolator will only charge it once starter full. "

How does your Isolator do that ? The only way to determine that your Starter battery is full, when it is connected to the constant voltage of an Alternator, is to check when the current drops to around 2 amps.

Or do you mean that the Isolator will charge the Auxiliary once the Starter battery voltage rises above 13 volts - which could be when the Starter battery is 20% full (depending on the charging curent).

Mike
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Follow Up By: Ron173 - Wednesday, Mar 01, 2006 at 20:01

Wednesday, Mar 01, 2006 at 20:01
yes once the voltage goes up it opens the aux to charge.
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Follow Up By: Member - Norm C (QLD) - Wednesday, Mar 01, 2006 at 21:16

Wednesday, Mar 01, 2006 at 21:16
Ron, I've spent a lot of money at ARB. A good company, but don't expect them to be experts at dual battery systems. They will have their own product and know how to do it well. Move them beyond that and I'm not so sure. Don't expect them to be battery experts, particularly a product they don't normally sell.

You can do this yourself without too much drama. Or see a good auto elec. Problem is some of them have not heard of AGMs either. They only recommend what they are familiar with. First auto elec I spoke to about AGMs said ' Yeh I've done a lot of those Gel batteries'. Huh.

If you get advice from Collyn R, Mike DiD and a few others on this forum, I think they know what they are talking about.

In the end you have to back your own judgement. Use the right size cable, use good circuit breakers or fuses and you will be OK.

Search this forum and spend a couple of hours reading. Yes, there is some conflicting info, but this is one of the most discussed subjects and there is heaps of good info available.
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Follow Up By: Ron173 - Thursday, Mar 02, 2006 at 13:50

Thursday, Mar 02, 2006 at 13:50
I spent a lot at ARB too, opposite lock promote and fit their own systems, thought ARB might too.

Anyway from all replies here it would seem that its no bother to run an AGM with a wet cell, so will prob just go with it.

Rgds

Ron
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Reply By: Raymond from Wanderin 4 Wheelers - Wednesday, Mar 01, 2006 at 16:11

Wednesday, Mar 01, 2006 at 16:11
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
AnswerID: 157862

Follow Up By: Ron173 - Wednesday, Mar 01, 2006 at 16:21

Wednesday, Mar 01, 2006 at 16:21
That sounds good too.

I'm beginning to wonder if there is a valid reason why people say not to do it.

Lots of people claim it shouldnt be done yet theres two setups here with no probs.

Heat not a prob for me, aux in rear of vehicle (not in passenger area so no vent issues)

Thanks

Ron
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Follow Up By: Charlie - Wednesday, Mar 01, 2006 at 17:37

Wednesday, Mar 01, 2006 at 17:37
I'm running a standard wetcell cranking battery and an optima yellowtop behind the backseat with no problems at all.As long as your alternator curent is compatable with the battery what can go wrong ?
Regards Charlie
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FollowupID: 412210

Reply By: waynet - Wednesday, Mar 01, 2006 at 21:46

Wednesday, Mar 01, 2006 at 21:46
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
AnswerID: 157942

Reply By: Sand Man (SA) - Thursday, Mar 02, 2006 at 02:29

Thursday, Mar 02, 2006 at 02:29
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.

Bill


I'm diagonally parked in a parallel Universe!

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AnswerID: 157983

Reply By: Mike DiD - Thursday, Mar 02, 2006 at 08:12

Thursday, Mar 02, 2006 at 08:12
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
AnswerID: 158009

Reply By: Mike DiD - Thursday, Mar 02, 2006 at 08:39

Thursday, Mar 02, 2006 at 08:39
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
AnswerID: 158015

Follow Up By: Ron173 - Thursday, Mar 02, 2006 at 13:54

Thursday, Mar 02, 2006 at 13:54
Mike,

thanks for input, like you say I'm inclined to agree we can bury the myth from all the helpful replies here.

think I'll go get an AGM, then when starter is stuffed get AGM there too.

Rgds

Ron
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Follow Up By: Mike DiD - Thursday, Mar 02, 2006 at 14:03

Thursday, Mar 02, 2006 at 14:03
I found the best price for a Remco 80 amphour at BatteryWorld - althought price varied quite a lot from store to store.

Mike
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