A.G.M.BATTERIES

Hi Sandman, I know nothing about calcium batteries. Your comment about 4 level charging is interesting and make me wonder what happens in the normal operation of chargeing when in a vehicle. Does the alternator have some multi-step charging system ? I think not, so why should we use a special charger when the battery is not in the vehicle?

Please note I am not sugesting that there is anything wrong with your answer I am just interested in whats going on.

I don't have sufficient knowledge on vehicle alternator operation to give you an indicative answer Kiwi, but I would guess that yes, a vehicle alternator will drop into a "float charge" during the final mode of charging.

What I do know is that an "ordinary" battery charger will "boil" the crap out of an AGM battery if left on for too long. In fact, any battery. The multi-stage chargers address this by the float charge stage where it "trickle" charges the final bit to fully charge the battery without over charging and permanently damaging the battery.

They are well worth the investment IMO.

Stationary batteries need an equalising charge step to avoid stratification of the acid - all the dense acid stays down the bottom and less dense acid stays on top.

In the interests of long battery life, equalising charge should only be applied once a month. An equalising charge applies a higher voltage which causes strong gassing to stir up the acid. Imagine a battery that stands 70cm high and its terminals have been welded on - Equalising is the only way to stir up the acid.

You don't need Equalising Charge in car batteries, because the acid is being continuously moved around while driving.

Mike

Gel cell and AGM batteries generally do not need equalising – you can not “stir up the electrolyte” as it is absorbed in a gel paste or a glass mat (hence AGM.)

Engineers who develop batteries (as distinct from auto electricians) have ascertained that these higher voltages are indeed necessary to charge batteries to 100% of their rated capacity. Auto alternators have cheap simple regulators built into them. Marine electricians know better, they fit smart regulators to their alternators and those are able to charge batteries to nearly 100% capacity. These smart regulators cost as much to buy as a complete auto alternator complete with its regulator. Part of the smart regulator is a temperature probe to glue on the side of the battery to to monitor its temperature – without this probe it is too dangerous to run an alternator hard enough to charge a battery to 100%. 70% is more than enough to prevent sulphating and provide nearly full cranking current so this is what they are designed to do. The question you should be asking is why do auto electricians claim an auto alternator can charge a battery to 100% of its rated capacity – may be it is because the TAFE colleges use the KISS principle and don't want to spend too much trouble explaining things where the maths get too complicated so they have their own definition of “fully charged.”

PeterD

Thanks Sandman, MikeDID & disco 1942,

I have an electrical / electronics/engineering background and suspected what disco1942 has stated.

To my knowledge, battery fluids will not stratify unless they are (a) not being used & (b) are held at a constant temp. Also it wood be extremely difficult for fluids to actually 'move around' in modern batteries as the seperators between the plates are so close and tightly packed with the 'active ingedients'. Bit different in the old days where telephone exchanges had rows of huge open topped glass boxes with bits of wood hammered in between the plates to stop them bending!

Hi Kiwi Kia

Just simply, the car alternator doesn't have anything special and
relies on the fact that by using a constant voltage (14.2 in my Patrol)
the battery doesn't get fully charged in typical use.
(but can on very long trips)

Using a special charger outside the vehicle does a better job
and will allow the battery to come closer to fully charged
quicker.

As stated in other replies the AGM and in particular the
Spiral wound AGM versions are less critical in these respects.

Robin Miller

"The question you should be asking is why do auto electricians claim an auto alternator can charge a battery to 100% of its rated capacity "

- nope, the question you should be asking is "why are replacement regulator manufacturers and 3-stage charger manufacturers claiming that standard Alternators only charge to 70%" - because then the answer is obvious.

If buyers believed that the existing Alternator charged to 90% or more, manufacturers would have no hope of convincing people to spend huge amounts of money for little benefit.

Have a search for where the "Alternators NEVER charge to more than 70%" myth started.

Mike

Robin

You measure 14.2 V from your alternator in the back yard when testing. What voltage does it output after a half hour of running when things have heated up under the bonnet. I think you are forgetting the temperature built into the regulator. A warm battery requires less charging volts than a cold one. My alternator starts off at 14.4 to 14.5V and 20 minutes down the road the temperature compensation has reduced it to 13.8V or less depending on the outside air temperature.

PeterD

Hi PeterD

No not forgetting anything - I didn't consider that relevant to the general answer
Kiwi was asking.
Some units have temp compensation specific to battery which requires temp sensor on it, some also have hit and miss approach- sounds like yours is in that cat. Many have nothing except a defacto type caused by inherrent volts drops
in the chip substrate used when hot.
And almost none have what you might call a 3 stage charger.

My own is actually constantly monitored with a digital guage in car, and is 14.2
changing little to 14.1 + - 0.1 when warm.

I'm not in the "car batteries only charge to 70% camp" but by design their
typical use is to less than full charge on your normally commute trip.

Robin Miller

Mike

There is no myth regarding the charging capabilities of alternators. The charging regimes for deep cycle batteries has been developed by people who understand what is required to get the maximum capacity from a battery for deep cycle use.

In automotive use batteries work over a much smaller charge/discharge level. In automotive applications it is quite permissible to not charge a battery to 100% of its capable capacity. you just need sufficient charge levels to prevent sulphation. If you spend 40 minutes loading a vehicle with the interior light and the radio on you will draw far more from the battery than you will whilst starting the motor. The equipment required to charge a battery to 100% of its rated is quite expensive and when this is not required in automotive use why not use simpler equipment that is far cheaper and is still able to provide satisfactory operation. That keeps the cost of our vehicles down.

Just because an automotive alternator is eminently suitable to maintain auto batteries do not assume it is suitable for other applications – including deep cycle batteries. Yes I know people survive with car fridges in the bush on automotive batteries and I also see members of these forums complaining that they are not completely satisfied. We can do better and if we use the correct techniques we will destroy fewer batteries.

PeterD

PeterD

We may have a communication problem - I certainly would not suggest that an Alternator will charge a Wet DeepCycle battery reliably (especially in a Camper Trailer on long cables). They take a long time to charge.

The comments above were only intended to apply to normal starting batteries.

Mike

Mike DID I pose that question in post Site Link having had an email from a C/T manufacturer. I think it is an important question for a lot of owners

Mike,

I am trialling the C&D [Dynasty] High Rate series UPS 12-270 [Its an AGM type] as a starter battery. So far I cannot get its voltage past 12.57V from the alternator but can achieve the specified 12.84V from a 3 stage mains charger.

When I connect the OEM wet-cell it returns an s.g of 1.300.

Is this something I should be concerned about? The test started 4 mths ago, I am checking if this battery can crank reliably, I expect its primary function as a DC should be ok.

eng

I wouldn't use that battery in that application. The data sheet is here:

Site Link

It's upper temperature spec is too low for under bonnet use and the maximum charge current is 15A which your alternator could easily exceed.

An sg of 1.3 is quite high IIRC, is your system overcharging?

Mike Harding

Thanks, already have the pdf for the battery. There will actually be 4 units in the rear canopy used as DCs, so the temperature + charge current shouldn't be a problem [fingers crossed].
The test is whether it would function reliably as a cranker if the OEM should fail in the field.
The s.g reading is from a cheapy hydrometer[sp?]....will have to check the alternator voltage tomorrow..................off to work

ps: I am not a convert to AGMs.......very much a sceptic

eng

My Remco AGM is being used indoors - it's permanently connected to a 15 Amp power supply set to 13.8 volts - it's happy with that and draws aboout 50mA.

Lead Acid batteries are self-regulating when connected to the appropriate Float Voltage.

What would a 3-stage charger do differently ? If it noticed the current going above 2.5 amps (assuming a 25amp setting for Stage) it would automatically change to Stage 2 Absorption Voltage (14.5) to recharge the battery as quickly as possible then change back to Float Voltage when current dropped below 2.5 amps. 3-stage chargers are no longer rocket science.

Mike

Mike H

"equalisation V at 25C (14V4 to 14V8) " - this sounds way too low for an Equalising voltage - should this be Absorption Voltage ?

Mike

Nope, not according to the data sheet:

Site Link

Select "UPS 12-310" they even have a version in Spanish should you feel so inclined :)

Mike Harding

Since it's an AGM, the Equalising charge certainly isn't necessary to prevent acid stratification.

I guess they see that applying a voltage higher than Float occasionally, will ensure boosting all cells in battery.

They actually call it "Equalizing and Cycle Service Charging Voltage", so it is actually the Cyclic Voltage.

Mike

>I guess they see that applying a voltage higher than Float occasionally,
>will ensure boosting all cells in battery.

That was my supposition too - keep in mind this battery is intended for UPS applications where it may never see a discharge cycle and may be on float for years so I suppose an occasional boost will ensure all the cells remain at the same state of charge in that situation. Beyond that I don't know anything about the chemistry side of batteries so can't comment. I just do what it says on the box :)

Mike Harding

Sorry for delay, pulled a double shift......
Alternator voltage is 14.23V, I think its safe to conclude its not overcharging w.r.t voltage, just leaves the over-current as you have pointed out.

I will split the 2 batteries to test further.

eng

Mike

I am wondering if you are confusing charging deep cycle batteries and auto batteries because what you are saying is largely true for auto batteries. I have assumed that Lewjack's question was relating to deep cycle batteries supplying considerable amounts of power – not minuscule amounts like auto batteries normally supply as he was talking of four stage chargers.

The link you gave to Bill Darden's site gives conflicting requirements for the necessary voltage for the absorption stage when charging AGM batteries. The graph shows 14.9V and the table 4.3V & 14.6V for spiral wound construction. As you said consult the manufacturers specifications.

You stated "YOU CAN FULLY CHARGE AN AGM BATTERY ON 13.8 VOLTS." Yes but how many weeks have you got to do it? Darden's site talks about 13.8V as a float voltage “The current is reduced to approximately 1% (C/100) or less.” At 1% charge rate the charge time is going to be a fair while when you subtract the self discharge current from this 1% - yes self discharge still takes place during charging and the float levels are designed to provide just a little more than this.

You make the claim that people are changing to AGM batteries because “they are so much less demanding on their charging needs.” Less demanding than what? If you want something more robust and you are likely to miss treat them then you go for flooded batteries that you can add water to. If you charge a battery too hard you boil water out of it – even AGM batteries. If you hit a flooded battery to hard then you just add water. The effects of over charging sealed batteries is terminal. There are still installers recommending flooded batteries as they are more robust.

The bit about AGM batteries needing specific requirements if left connected to a float charger applies to all batteries. AGM batteries need less voltage than flooded ones and gel cells less again.

Lets not confuse deep cycle batteries and auto ones. Cars and trucks both have internal combustion engines but we do not get confused with those do we!!!

PeterD

Keep in mind that 13V8 (or whatever) is only a float voltage when the battery is fully charged, until then it's a charging voltage as the AGM sitting on my bench being charged from my lab PSU at 13V8 is currently demonstrating. The charging current curve will, of course, be exponential but I doubt we're talking weeks.

Mike Harding

According to the Absorbed Power AGM Datasheet, it will take 4.5 hours to achieve 100% charge from 0%, using an initial current limit of 0.25C (25 amps for a 100 amphour battery).

Testing on my Remco AGM showed that it takes 10 hours to get up to 80% charge with a voltage of 13.8 volts.

13.8 is called a float voltage because you can leave a battery connected to it forever. It's not the fastest way to charge a battery - that's what the 3-stage algorithm is for. But for AGM's it will still get you to full charge - slowly.

I have yet to do teh test for wetcells.

Mike

I know this is off the topic of chargers you techos but I post another at Site Link and ask you to consider the alternator ratings of some modern vehicles if you have come across them. I have had email from a C/T manufacturer who now has to check the output voltage, to the C/T to provide the guarantee is sound, on dleivery.