Dual Battery systems and AGM batteries

Submitted: Friday, Mar 20, 2009 at 10:16
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I know nothing about auto electrics and consequently have become very confused with the advise I have received.

I currently have a dual battery system and plan to change over the second battery from a normal acid cell battery deep cycle to an AGM deep cycle.

I was told that I would need the Alternator to be charging at at least 14volts. I went to an auto electrician to check the charge rate and it was 13.83 to the starter battery and 13.79 to the deep cycle. However the Auto electrician said that would be plenty to charge a AGM and not to worry!!!!!!!!!!!!!!

Any advise?
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Reply By: Rossc0 - Friday, Mar 20, 2009 at 11:21

Friday, Mar 20, 2009 at 11:21
Hi,

Don't know which AGM your looking at or have but Optima recommend 13.65 to 15 volt for alternator charging for their 12V batteries.

Which ever one your looking at will have something similar on the specification sheet for the battery.

You may want to increase the size of the cabling to the auxiliary battery to reduce the voltage drop.

Cheers
Ross
AnswerID: 355110

Follow Up By: TerraFirma - Friday, Mar 20, 2009 at 16:18

Friday, Mar 20, 2009 at 16:18
Ross, On your profile it says youd rather push your Fords than drive a Nissan or Toyota, thats a bleep er.! By the way how often do you Push the Ford..? LOL..
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Follow Up By: Rossc0 - Friday, Mar 20, 2009 at 17:50

Friday, Mar 20, 2009 at 17:50
Touch wood (pats self on head) never.

It's just an extension of the old saying "I'd rather push my Ford than drive a Holden".

Cheers
Ross
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Reply By: TerraFirma - Friday, Mar 20, 2009 at 12:04

Friday, Mar 20, 2009 at 12:04
You will be fine with 13.8 volts, don't worry
AnswerID: 355117

Follow Up By: Hughd - Friday, Mar 20, 2009 at 16:07

Friday, Mar 20, 2009 at 16:07
The voltage is not a problem, but I understand it takes a lot more time to re-charge aa AGM than a car battery or marine battery.
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Follow Up By: TerraFirma - Friday, Mar 20, 2009 at 16:15

Friday, Mar 20, 2009 at 16:15
Not necessarily, AGM batteries are known for their rapid charging properties. It's a big feature tick for AGM technology. Also don't forget they will hold their charge longer also,so your topping up times to full charge will be fine. The only thing to remember is that they like all batteries don't like being discharged below 50% of their capacity on a regular basis.

If you plan on discharging to well below 50% regularly you can buy a full deep cycle AGM battery as apposed to a Hybrid AGM or general purpose battery.

I have some 6-7 AGM batteries now doing all sorts of things, they are fantastic and reliable.
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Follow Up By: TerraFirma - Friday, Mar 20, 2009 at 16:23

Friday, Mar 20, 2009 at 16:23
AGM, or Absorbed Glass Mat Batteries
A newer type of sealed battery uses "Absorbed Glass Mats", or AGM between the plates. This is a very fine fiber Boron-Silicate glass mat. These type of batteries have all the advantages of gelled, but can take much more abuse. We sell the Concorde (and Lifeline, made by Concorde) AGM batteries. These are also called "starved electrolyte", as the mat is about 95% saturated rather than fully soaked. That also means that they will not leak acid even if broken.

AGM batteries have several advantages over both gelled and flooded, at about the same cost as gelled:
Since all the electrolyte (acid) is contained in the glass mats, they cannot spill, even if broken. This also means that since they are non-hazardous, the shipping costs are lower. In addition, since there is no liquid to freeze and expand, they are practically immune from freezing damage.

Nearly all AGM batteries are "recombinant" - what that means is that the Oxygen and Hydrogen recombine INSIDE the battery. These use gas phase transfer of oxygen to the negative plates to recombine them back into water while charging and prevent the loss of water through electrolysis. The recombining is typically 99+% efficient, so almost no water is lost.

The charging voltages are the same as for any standard battery - no need for any special adjustments or problems with incompatible chargers or charge controls. And, since the internal resistance is extremely low, there is almost no heating of the battery even under heavy charge and discharge currents. The Concorde (and most AGM) batteries have no charge or discharge current limits.

AGM's have a very low self-discharge - from 1% to 3% per month is usual. This means that they can sit in storage for much longer periods without charging than standard batteries. The Concorde batteries can be almost fully recharged (95% or better) even after 30 days of being totally discharged.

AGM's do not have any liquid to spill, and even under severe overcharge conditions hydrogen emission is far below the 4% max specified for aircraft and enclosed spaces. The plates in AGM's are tightly packed and rigidly mounted, and will withstand shock and vibration better than any standard battery.

Even with all the advantages listed above, there is still a place for the standard flooded deep cycle battery. AGM's will cost 2 to 3 times as much as flooded batteries of the same capacity. In many installations, where the batteries are set in an area where you don't have to worry about fumes or leakage, a standard or industrial deep cycle is a better economic choice. AGM batteries main advantages are no maintenance, completely sealed against fumes, Hydrogen, or leakage, non-spilling even if they are broken, and can survive most freezes. Not everyone needs these features.
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Follow Up By: Rossc0 - Friday, Mar 20, 2009 at 17:53

Friday, Mar 20, 2009 at 17:53
If your worried about the time it takes to charge then get a bigger (more amps) alternator.

Lowest I've ever seen the voltage on my auxiliaries is 11.5 volts and they were back to full charge in about 30minutes of driving. Using 100amp alternator.

Cheers
Ross
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Reply By: Best Off Road - Friday, Mar 20, 2009 at 20:36

Friday, Mar 20, 2009 at 20:36
The auto elec is an auto elec, not an AGM Battery expert.

13.8 is not enough. They need 14.1 to 14.4, as printed on top of my Lifeline AGM.

Here is not the place to ask such questions, you'll get varying and possibly unreliable information.

I suggest you ring Federal Batteries in Sydney, they know their stuff.

I'd also suggest that if your alternator is only putting out 13.8 it is sick.

Cheers,

Jim.

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Follow Up By: Member - Phil G (SA) - Saturday, Mar 21, 2009 at 22:51

Saturday, Mar 21, 2009 at 22:51
Gday Jim,
Is that voltage on your battery for cyclic use or standby use?
What temperature is written on the battery?

Reason I ask is because alternator voltage will vary because of temperature compensation. At hot temperatures the voltage drops, so you don't overcharge the battery. Mine gets down to 13.5V at 45 degrees C. Back up to 14.2 when it's cold.

Cheers
phil
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Follow Up By: Member - JohnR (Vic) - Saturday, Mar 21, 2009 at 23:43

Saturday, Mar 21, 2009 at 23:43
Jim, I read that too - the requirement for 14.1--14.4. Anything under that won't be charging properly.

Phil, I reckon my 100 Series doesn't charge properly for the 350ah of AGMs I have, I always need to use the battery charger for that or a good days solar.
Cheers,
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Follow Up By: Member - Phil G (SA) - Sunday, Mar 22, 2009 at 09:10

Sunday, Mar 22, 2009 at 09:10
Gday John,

Your situation is different to mine and others.
You have a lot more battery - 2 under the bonnet and 350Ah in the back; and the Karavan is a long way from the alternator. And you need a very good earth back to the engine block.

Any idea how much voltage drop you get when the batteries are down and the engine's running at say 2000RPM? My 100Ah AGM is much closer to the alternator, and with 8Ga wiring, I was getting up to 0.6V drop under those conditions. I have just upgraded the wire to 4Ga, but haven't been away to see what difference its made.

The other issue is that your AGMs are in a cool spot in the Karavan. Your alternator is is a hot spot under the bonnet, so temperature compensation will drop the voltage which is appropriate for a warm battery under the bonnet but not for a cool battery in the Karavan.

All charging voltages are with a battery at 20 degrees - a lot less than underbonnet temperatures.

Cheers
Phil
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Follow Up By: Member - Mike DID - Monday, Mar 23, 2009 at 08:58

Monday, Mar 23, 2009 at 08:58
All Battery manufacturers specify two charging voltages -

Boost voltage - to get as much charge into the battery as quickly as possible - leaving this voltage on after the current has dropped below 0.01C will overcharge and damage the battery eventually.

Float voltage - a charging voltage that can be used continuously.


Page 16 of the Lifeline Technical Manual recommends 13.8 volts Float at 20 deg C and 13.2 volts at 50 deg C (typical Engine Compartment temperature).

For Boost it recommends 14.5 at 20 deg C and 14.0 at 50 deg C.
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Follow Up By: Best Off Road - Monday, Mar 23, 2009 at 12:39

Monday, Mar 23, 2009 at 12:39
I was away and working from memory.

Now back and had a look at the battery (Lifeline)

Charging Voltage 14.2 Volts at a temp of 77 F (25 C).

Float/Stanby Voltage 13.2 - 13.4 at 77 F (25 C).

Comparing what Mike has put up from the Tech Manual, it is clear that temperature has a big bearing on this.

Jim.

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Follow Up By: Member - JohnR (Vic) - Monday, Mar 23, 2009 at 13:08

Monday, Mar 23, 2009 at 13:08
Phil, you are right, I actually have a dual battery system under the bonnet for the two batteries there, and then a separate system that is hard to satisfy in the Karavan. Too much temperature compensation. I have reduced the likely voltage drop to the rear with pretty well, welding size cables positive and negative, but Toyota's temperature control.

What would you guys suggest for that? The box for the batteries in the Karavan would vary a bit :-)))
Cheers,
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Follow Up By: Member - Phil G (SA) - Monday, Mar 23, 2009 at 22:36

Monday, Mar 23, 2009 at 22:36
Gday John,
I don't expect you can do anything more than you have already.
To fit a 20amp Arrid twincharge would work if you only had 100Ah of battery but 350Ah is a big ask.

A couple of hours a day on a Christie's 80AMP generator at 14.7 Volts might be an alternative, but I don't speak from experience on this one.

Tongue in cheek......why not use that diesel heater to warm the AGMs so they'll take a bit more charge :-))

Cheers
Phil
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Follow Up By: Member - JohnR (Vic) - Monday, Mar 23, 2009 at 22:55

Monday, Mar 23, 2009 at 22:55
G'day Phil, actually the Arrid I would like to know a bit more about or an equivalent. I wonder if I should crank up the battery charger in the boot at times.

I sometimes think the heat absorbent black face on the battery box will keep them hot enough. Some guys have actually got some temperature monitoring inside the box, to get their chargers to compensate. For what, I am not convinced so far ;-))) I don't know our charger is quite that sophisitcated, it is only the Mark I Xantrex 20 amp. Some sales people will sell you anything to gain you a half penneth of loss.
Cheers,
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Follow Up By: Member - Phil G (SA) - Monday, Mar 23, 2009 at 22:55

Monday, Mar 23, 2009 at 22:55
Jim,
Temperature has a heap to do with this battery stuff, and is not recognised by most. Just a few bits of info from the Remco AGM tech info:

- Battery Life at 40degrees is half as long as at at 25 degrees.
- Battery charging voltage (2.4V/cell): 15.0V at 5degC vs 13.8V at 45degC
- Self discharge rate is 3% at 20degC vs 10% at 40degC
- And Battery capacity is half at zero degrees than it is at 25 degrees.

Keep them cool, and keep them charged and they will do you well!
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Follow Up By: Member - Phil G (SA) - Monday, Mar 23, 2009 at 23:06

Monday, Mar 23, 2009 at 23:06
John,
I wouldn't be convinced either! Temp compensation is great for a hot environment, but your problem is that your Karavan batteries sit in a cold environment.

As far as battery temp goes, I have a probe on the AGM in the canopy and it is never more than ambient.
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Follow Up By: Member - JohnR (Vic) - Monday, Mar 23, 2009 at 23:26

Monday, Mar 23, 2009 at 23:26
Phil, lots of people don't know that they have things like battery control solenoids in their circuitry that steal their current every hour they are switched in too. I was talking with a fellow a few short hours NNE of you who didn't know why his batteries went flat over four days and wouldn't listen to me until I made him stop talking and listen. Then he understood.

My battery controller in the Karavan will take 14-16 amps a day, so it is best to keep it charged :-( You can take it out but then you have it wired to deplete to whatever the batteries can disgorge.
Cheers,
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Reply By: Maîneÿ [wa] - Sunday, Mar 22, 2009 at 12:48

Sunday, Mar 22, 2009 at 12:48
bks,
different AGM producers/manufacturers have different specifications for their batteries, this is because some are actually better made with superior materials than some others.
So you will need to know which AGM battery it is your going to buy before you ask very specific technical questions, as you can see by the replies so far, not all are the same answer as the Auto elec, however that does not mean some may be incorrect, just as some may be a bit more correct than others!

Not sure how the "charge rate was 13.83v to the Starter battery and 13.79v to the Deep Cycle" was determined by the sparky, as I would think both batteries would be running in parallel and if were correctly set-up would be the same numbers. Maybe a fridge was running?

Mainey . . .
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Follow Up By: Member - Phil G (SA) - Sunday, Mar 22, 2009 at 13:22

Sunday, Mar 22, 2009 at 13:22
Gday Mainey,

Re: "charge rate was 13.83v to the Starter battery and 13.79v to the Deep Cycle"

Thats just simply voltage drop in the cable and isolator and connections. You will get some voltage drop no matter how good the setup. 0.04V drop is very little and suggests the deep cycle battery is close to fully charged.
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Follow Up By: Maîneÿ [wa] - Monday, Mar 23, 2009 at 18:03

Monday, Mar 23, 2009 at 18:03
I've just checked the Voltage in my three batteries, all are *exactly* the same Voltage - when engine is running.

Initially the Cranking battery was 12.7v
The two AGM DC's were 12.9v each (they are wired in parallel anyway)

However, when I started the engine, all three batteries instantly levelled out, initially to 13.62v then started to rise till I stopped looking @ 14.38v.

(these numbers were taken with a hot engine compartment, the vehicle was under the carport, so the solar system had no effect on battery charge numbers)

Mainey . . .
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Follow Up By: Member - Phil G (SA) - Monday, Mar 23, 2009 at 21:45

Monday, Mar 23, 2009 at 21:45
Gday Mainey,

That's what I'd expect, considering all three of your batteries were *fully* charged when you measured, and I'd guess your vehicle was at idle. You could try the same measurements again when your AGMs are say 50% discharged, and use a multimeter to 2 decimal places, rev the vehicle to 2000rpm and see how much voltage drop. A DC clamp ammeter could be used to see how much current is going into the AGMs from the alternator at this time.

I've been measuring battery voltage with a Digital MM for the past 20 years. These days I have a digital meter to 2 decimal places in the cab with sensing leads on each pos terminal, which measure voltage continuously while driving. I use the small voltage differences to tell me how many amps are going into my AGM (this is how OEM vehicle ammeters work), and to determine how well the AGM is recharging.

Just to let you know how my current system works, I run 3 batteries in the LandCruiser. I have a pair of Exide Extreme N70 under the bonnet, joined by 2Ga cable and a marine switch. I use both for cranking and winching, but can also isolate them if I need to run the fridges for an extra day or two. My 3rd battery is a 100Ah Remco AGM in the cool of the canopy. Its connected via fusible links, 4Ga cable, a Rotronics isolator and has 4Ga earth back to the chassis, body, and engine block. I don't have a need for solar, because I'm a desert traveller and never stay put for more than 3-4 days. My 110amp alternator recharges the lot over several hours. I'm very dependant on my alternator, so I carry a spare for desert trips.

Cheers
Phil
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Follow Up By: Maîneÿ [wa] - Tuesday, Mar 24, 2009 at 11:15

Tuesday, Mar 24, 2009 at 11:15
Phil,
the original poster stated: "charge rate was 13.83v to the Starter battery and 13.79v to the Deep Cycle" and also "Yes its pretty apparent that he (auto elec) only measured the charge going to the batteries when they were full. I had been driving for about 1/2 hour on a hotish day before I took it into the auto electrician"
So yes I believe his batteries were also *fully charged*
However, mine only went 3 Klm or maybe even 4 klm (only 5 minutes) before I measured my battery Voltage with the engine idling.

Mainey . . .
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Reply By: Member - Mike DID - Sunday, Mar 22, 2009 at 19:52

Sunday, Mar 22, 2009 at 19:52
If you have a look at the manufacturers data for AGM batteries you will see that they will charge fully at 13.8 volts - this is one of the advantages of AGM batteries over wetcell batteries.

At 20 degrees air temperature, a modern Alternator should start charging, from a cold start, at 14.4 volts to replace the charge used in starting, then after 15 to 30 minutes drop back to 13.8 volts to avoid overcharging the battery.

This approximates the 2nd and 3rd stages of a 3 stage charger. The 1st stage isn't needed, because normally a car battery is never discharged more than 10%.

AnswerID: 355499

Follow Up By: bks - Monday, Mar 23, 2009 at 12:31

Monday, Mar 23, 2009 at 12:31
Yes its pretty apparent that he only measured the charge going to the he batteries when they were full. I had been driving for about 1/2 hour on a hotish day before I took it into the auto electrician.

It is all just too complicated Now I'm told that you should not put AGM baTTERIES IN THE ENGINE BAY BECAUSE OF THE HEAT.

Easiest just to replace the existing acid battery with another acid battery.
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