Were my AGM batteries being overcharged.

Submitted: Wednesday, Apr 05, 2017 at 20:10
ThreadID: 134606 Views:5241 Replies:9 FollowUps:5
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My dual battery system until today consisted of a 220AH AGM battery with a dual battery charging system. I normally charge with a 240v projecta 7 stage smart charger. I charged the battery on the weekend. Today i installed a projecta 25amp dc - dc charger. I have wired this into the system so that i can choose to use the dc - dc charger or charge straight from the alternaror. I have noted that using either the 7 stage charger or the dc - dc charger shows the battery as near full charged and only puts in a few amps until charging complete. If I connect directly to alternator, roughly 12 amps is being supplied, even though the battery is near or at capacity according to the smart charger and dc - dc charger. My thoughts are that if I continued to charge just using the direct current from the alternator, that the batteries would indeed be overcharged. Am I coming to the right conclusion here. If I am, i cant help but think that direct charging is harming the battery (if taken to full charge) and the dc - dc charging will charge it correctly.
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Reply By: IvanTheTerrible - Wednesday, Apr 05, 2017 at 20:18

Wednesday, Apr 05, 2017 at 20:18
What voltage is at the battery when each charger is of and are both set to agm?
AnswerID: 609955

Reply By: Battery Value - Wednesday, Apr 05, 2017 at 20:41

Wednesday, Apr 05, 2017 at 20:41
Hi kcandco,

Ivan x2.
You took current readings (with clamp meter?) but what about the voltages?

BTW you can't over-charge an AGM battery in a hurry unless you apply a very stiff 15V or more.
But the alternator could still exceed the specified max charging rate which is about 55A in this case. Unlikely though with an almost fully charged battery.

To shed some light on this we need to know the battery voltage together with the current readings when charging by DC/DC (or mains powered) and then by alternator.

Regards, Peter
AnswerID: 609957

Reply By: Member - Phil G (SA) - Wednesday, Apr 05, 2017 at 21:02

Wednesday, Apr 05, 2017 at 21:02
I expect that 12A from your alternator will drop off pretty quick if your AGM is nearly charged. I also installed a projecta IDC25 today! But in my caravan. They switch to float when charging current drops to 3.8A - my guess is that you are not far off float and the float voltage is not a lot different from the common alternator voltage on a Toyota with a warm motor. The delivered voltage will not kill your battery.

But following on from Peter's good info above, an alternator can kill an AGM because the Alternator can deliver more amps than the battery can accept (AGM batteries usually have an Initial Charge Current written on the side - the 55A that Peter referred to). It can do this when the AGM is discharged down a bit, and your wiring is very good! I'm also assuming your 220Ah battery is not in a heated environment.
AnswerID: 609958

Follow Up By: Battery Value - Thursday, Apr 06, 2017 at 06:35

Thursday, Apr 06, 2017 at 06:35
Hi Phil,

interesting to see the IDC25's absorption to float switch over is based on current taper.
What happens if you run a fridge and other things which add to the battery's absorption steady state current resulting in >3.8A? Does the IDC25 apply an overriding absorption time limit?
FollowupID: 879850

Follow Up By: Peter_n_Margaret - Thursday, Apr 06, 2017 at 14:29

Thursday, Apr 06, 2017 at 14:29
There is no way that an alternator direct charge can hurt an AGM unless the voltage is also excessive, and that is most unlikely.
The battery will take what it needs according to its voltage and its rate of acceptance.
I have been using a Fullriver 120Ah HGL AGM as the sole crank battery in the OKA for 7+ years now. That is more than double the life that I have ever had from any other crank battery in this vehicle.
The Alternator can provide up to 85A at 14.3V. No harm can be done.
OKA196 motorhome
FollowupID: 879873

Follow Up By: Member - Phil G (SA) - Friday, Apr 07, 2017 at 22:38

Friday, Apr 07, 2017 at 22:38
Peter, not talking about cranking batteries.
Read Bantam's response below.
I killed a Remco 105Ah AGM in the cool environment of a canopy by charging it with an alternator. The battery had initial charge current of 25A written on the side. I often measured 45 plus amps going into it. I had excellent wiring to achieve this. It lost capacity by venting. The supplier refused warranty for any of their AGMs that were charged from an alternator without any current restriction.

Like Bantam said, one of the big benefits of a DC-DC charger is that it limits the initial charge current.
FollowupID: 879931

Follow Up By: Member - Phil G (SA) - Friday, Apr 07, 2017 at 22:46

Friday, Apr 07, 2017 at 22:46
Battery Value: "What happens if you run a fridge and other things which add to the battery's absorption steady state current resulting in >3.8A"
Gday Peter, At this stage I only know what I read in the manual. But I like to measure everything. Will see with time
FollowupID: 879932

Reply By: The Bantam - Thursday, Apr 06, 2017 at 09:24

Thursday, Apr 06, 2017 at 09:24
I think you are drawing conclusions based on the behavior of different charging systems ..... not on any real solid facts.

First and above all, find the spec sheet on line for your battery ....... I've been hearing people sprout unreliable generalisations about AGM and other batteries for years ....... you need good hard reliable information from the manufacturer of the battery ...... not anybody else especially a manufacturer of chargers.

Yes some AGM batteries do have very much limited "maximum initial charge current" ..... some do not .... get your facts from the manufacturer.

some of these facts may be stamped on the side of the battery ..... but get the manufacturers spec sheet and read it. ... pretty much all the respectable manufacturers have comprehensive spec sheets on line ...... many have some very good and very readable technical advice notes too.

secondly much of what we realy need to know about lead acid family batteries is from terminal voltage ....... what is the terminal voltage delivered to the battery in question when connected direct to the alternator and after, the battery has been charged for some time without load ...... the completed charging voltage ..... how does that compare to the manufacturers spec sheet for the battery.

Another matter you may not have considered is the age and condition of the battery ....... some batteries with age will continue to draw quite significant charge currents even when "fully charged" .... this can be a sign of poor health.

All that said ....... thousands of AGM batteries are slowly murdered ( actually not that slowly) every year, because they are charged at currents way above their maximum initial charge rate.

If you have a battery that has a manufacturer specified "Maximum Initial Charge Current" of 20 amps and you have it directly connected to an alternator capable of delivering 120 amps ....without some mechanism to limit charge current... you have a problem ..... and the battery will have a short life.

In my opinion the single best reason to use a DC to DC charger with most AGM batteries is to control maximum charge current.

AnswerID: 609971

Reply By: RMD - Thursday, Apr 06, 2017 at 12:38

Thursday, Apr 06, 2017 at 12:38
How are you determining the charge into the battery is 12amps???
Is it via an ammeter directly in the line just before the battery OR are you measuring the alternator output charge/flow rate and presuming that is going into the battery???

What is the normal full voltage reading of your alternator when not charging any aux batteries and only running the vehicles battery/electrics?

I have a 105 AH AGM Full river which is 7 years old and all it's life it has been on a solar charge regulator AND the vehicle alternator voltage of 14.2v. Which ever is the highest charges the battery. Solar reg is set at 14.2v also.

AnswerID: 609977

Reply By: Tony F8 - Saturday, Apr 08, 2017 at 20:42

Saturday, Apr 08, 2017 at 20:42
My 80 series is coming up to 20 years old, in all that time I have only replaced the start and auxiliary batteries 3 times, the last change was 3 years ago, so that equates to about 8 years between changes, the auxiliary has always been Full River, running a simple bi-directional marine VSR. If the alternator was going to overcharge them, I don't think I would get that life out of them. Try not to over think something that has been working well for many years.
AnswerID: 610042

Reply By: Batt's - Saturday, Apr 08, 2017 at 22:28

Saturday, Apr 08, 2017 at 22:28
I'd be wondering why would you spend the money and time on setting it up and then want to bypass it when it will do a better job unless you don't trust it and expect it to break down.
AnswerID: 610047

Follow Up By: kcandco - Saturday, Apr 08, 2017 at 22:41

Saturday, Apr 08, 2017 at 22:41
Hi Batts
The reason I have done this is that in a camping situation where I am doing limited / no driving, I could charge at 44 amp (direct from alternator) rather than restricting myself to 25 amp with the dc - dc unit. It is actually my intention to use the dc - dc unit all the time, but if I find myself with low batteries, I could, and i emphasise the word could charge at 44amp instead of 25. I guess it also gives me that backup if the dc - dc charger dies which I think is highly unlikely, and it only cost me $29 to install a selector to change circuits, and actually I should add that a solar panel will probably be added hopefully not to far away.
FollowupID: 879963

Reply By: kcandco - Saturday, Apr 08, 2017 at 22:32

Saturday, Apr 08, 2017 at 22:32
Hi Everyone and thank you for your replies. Sorry I have not been able to respond till now. I have recently installed a BatteryCheck battery monitoring system on the battery which is comprised of 2 x 6v 220 AH agm batteries in series. They came from a forklift when near new, and are now about 3 - 4 years old. The batterycheck is installed on the negative terminal of the battery and then the negative of the batterycheck goes to earth. It uses bluetooth to send info you my iphone, which has an app specially for this. Volts, amps, state of charge, battery health and battery status ie charging / discharging are displayed. I have had my projecta 7 stage smart charger hooked up and the readings are nearly identical so i think the batterycheck is fairly accurate. Since installing the dc dc charger, I have only done minor testing, but noted that when charging direct from alternator approx 12 amps was showing on battery check. When switched to dc-dc charger this dropped to 2- 3 amps approx. Sorry but i am not sure of the voltage at the time but think it was 13.8. I will do some more testing. The max amp is not stated on the spec sheet but it does say a recommended charge rate of 44amps at 14.22v applies, untill current drops below 4.4 amps, then charge at a max of 15.3v. I will check out the actctul batteries tomorrow for max amp details. It will be interesting to see how it all works at easter while camping for 4 days.

regards Kevin
AnswerID: 610048

Reply By: The Bantam - Sunday, Apr 09, 2017 at 10:17

Sunday, Apr 09, 2017 at 10:17
just a couple of thaughts.

Even with poorly selected components and poor if any design input, some people will get good life and results out of their systems, either because of the way and the circumstances under which they are used, or because of the system being installed well by accident.

Many battery systems are installed with light cabling ...... this is not necessarily a disadvantage.

If you have a battery with a limited " maximum initial charge current" use of a smaller size battery cable linking it to the main charge system my be sufficient to bring it close to the charge limit and/or minimize the time that the battery is over it's charge limit.

In automotive electrical it is very rarely the current carrying capacity of the wire that is the predominant design consideration ..... voltage drop is.

10mm2 cable will carry 100 amps all day and not get warm, it will easily carry twice that short term ....... but roll out enough of it at 100 amps and several volts will go missing.

Limiting charge currents by introducing a resistance between the voltage source and the battery is a very old and common thing. Sometimes it is an actual fixed resistor, sometimes it is a light bulb, sometimes it is components in the charger and sometimes it is the cable connecting the battery that is used as the current limiting resistance.

This series resistance will cause more voltage drop and more current flows .... as the battery charges and charge current reduces the voltage drop will redeuce till almost nothing at end of charge.

Yes this can be designed with fairly basic maths ..... but often it is purely by accident.

Thus many people DO have charge limitation with their batteries but don't know it.

AnswerID: 610061

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