To Robin re charging

Pepper2 , its pretty easy to verify your battery state by having one of those aftermarket battery monitors permanently attached to your car - Jaycar have some.

I get into habit of looking at mine each morning before starting the car , you get a good reading then as battery has stabilized.

I would expect you will see 12.5v most of the time which is about 80% charged, above 12.6 is 100% (assuming average temp overnight is roughly 10- 25c).

If you do lots of short trips , you may see 12.4 more often in the morning

Robin Miller

Yeh its funny how the facts can be twisted around on the internet !!! A lot easier to type furphys than to say something face to face !!

Same story about people saying you CAN'T mix wet cell and AGM batts ?

While not ideal, I have 4 x 35ah AGM's in the CT which charge ( thru a 40A relay ,which lets me decide when and if I want to charge them) directly off the main battery. It will take 4+ hrs of driving if they are down to approx 12V, but they will still get there. But even if they only get to 12.6 instead of 12.8 , its good enough for the next nights power use... and I fully charge them at home with the 3 stage charger ( or at a powered site if there is one )

If you have a look at the early websites which started promoting "alternators don't charge batteries fully" , these were webistes selling improved Alternator Regulators that would charge your battery fully. Hmmmm????

Why would you spend hundreds of dollars on an improved regulator, unless you were convinced your standard regulator was inadequate.

I'm not suggesting that a standard regulator provides for excellent charging, but in most circumstances it will charge your battery close to 100%.

Its that simple Pepper2

Robin Miller

The answer may have been simple, but it’s simply not correct.

Even if you have 2 large batteries and as you put it pepper2, “ all other things being equal “, the 2 batteries will charge at about the same time that it takes to charge one battery and is the MAJOR advantage of systems that parallel batteries once the vehicle’s motor is running.

In the vast majority of situations, they will allow for charging of 2 and 3 batteries straight off your alternator in a much quicker time than most of the so called “NEEDED” devices can.

Cheers.

Hi Drivesafe

Y'a gotta love these battery discussions.

My answer to Pepper2 statement that 2 batteries take longer to charge is correct.

Also in special cases - e.g. as Eric describes below, your statement is also correct.

I guess I was trying to keep this simple, but it reality the charge process is more complex and runs thru various conditions which add up to the correctness of the statement that "two batteries take longer".

Note- that I was careful not to say something like "It takes twice as long".

Central to the discussion is that an alternator is not always a constant voltage source, it is only a constant voltage source while it can deliver its load (70amp using Eric figures).
Beyond this it becomes a constant current source (70 amps again).

When using eric example of C/10 charge or 7 amps then it matters not much wether there are 1 or 2 batteries connected and they can charge at the same rate, this we agree on.

This is a reasonable but simplistic statement, which happens during part of the charge cycle.

If you go to a battery that has been used overnight on a fridge etc and is significantly discharged then things change.

The current taken by the battery is limited initially by its internal impedance typically 0.005 ohm and also the resistance of the connecting cables etc maybe 0.01 all up.

(I know you guys know this below, but I'm putting in all the steps for those that may not)

Now lets do the little calc. Assuming that the alternator is 14v and battery is 12 v

V=I*R --> (14-12)= I * 0.01 --> 2= I * 0.01 ---> I = 200 amps

I.E. The battery has the peak capacity to draw over 200 amps which of course the alternator cannot supply.

In real life however the battery is also chemically limited to what it can accept and peak current (capacity) is rarely more than 100 amps for long.


The key point is that either figure significantly exceeds that which an alternator can supply and hence the available current is split between the batteries.


It follows directly from the above that 2 batteries (given that they need charging)
must take longer to charge than 1.

The charge time is therefore not the same and it is also not twice as long but lies
somewhere in between.

As a practical example of the above, its good to check out how long it takes a
dual battery system to recover after a short winching session.

Robin Miller

I don’t know where you get those figures from but even the best quality AGM, like an Optima, of around 100 A/H capacity is unlikely to draw more that 30 to 40 amp while being charge from your average vehicle alternator and it’s not the alternator’s maximum capacity that is the limiting factor it’s the charging voltage that has the greatest effect on how much a battery is going to pull.

Folks, if you have access to a tong meter or clamp meter and you have a fairly discharged battery or two.

Start the vehicle and go for a 10 minute drive to let every thing warm up and then get someone to keep the revs up and measure just how much, or more to the point, how little current is going into each battery.

The total, even with two auxiliary batteries is not going to be anywhere near the maximum output of your average alternator and there won’t be any drop in voltage either.

Most vehicles run at around 14 to 14.2 volts and this is the ruling factor on how much current a battery is going to pull and this won’t be anywhere near 100 amps per battery even if you only had one battery to charge and your alternator was one of the newer 120 amp type. A 100 A/H AGM will only be pulling around 40 amps max and a wet cell would be pushing 30 but more likely around 25 max.

If your vehicle runs at 14.7 volts then you would be able to put more into the battery but it’s still not going to be 100 amps.

The theory is nice but has little to do with reality.

Hi Drivesafe

To answer your question of where figures came from, I have only used one non-derived figure above and that is for the internal resistance of the battery.
I used a figure of 5 milli-ohms which is conservative - hear is a link direct to my better type 48ah battery which states that its is actually lower at < 3 milliohms .

http://www.exide.com/pdf/orbital_gold_specs_and_features.pdf

Robin Miller

P.S. Theres probably no point in going over things we have before but I agree on your typical charge figures though, I wasn't happy with Eric 7amps.

Hi Brett, have you noted the charge slope for the battery from when you first started your motor to when ever you turned the motor off.

Cheers

I haven't actually noted it at set intervals but generally after a day or so running the fridge it's down around 12V, maybe a bit less under load. I have seen it go to 40 amps generally 30 something though, then it drops from there, comes down to under 20A within 30 minutes, after a 3 hour drive it is almost down to zero only 1 or 2A max. This is with a Delkor 70A deep cycle and a 125A alternator that varies from 13.9 to 14.2V , depending on temperature

The Delkor under the bonnet is only for short stays, my full battery setup should not work according to some theories but it does. With my camper attached I have the 70A Delkor under the bonnet, then 4AG cable to the back of the car where there is a 100A AGM sitting under the fridge then from there it's 8AG back to the camper via an Anderson plug where I have 2 X 6V 125A AGM's in series. All these are charged through a GSL 100A isolator. After a 3 hour drive the camper batteries are at 12.7V which is pretty close to full.

Hi Brett

The reason you get 40a or so is because the alternator is current limited over that part of its range as internal resistance easily allows for > 100amp.

If you look at the specs of typical alternator - say Patrol 100amp one then it puts out 35-40amp at 1300rpm, rising to >70amp at 2500rpm.

This is a secondary reason why it takes longer to charge multiple batteries.
As while your revs are below 1500-2000 and even when fast idling there is just not enough available current for the batteries to take.

The electronic regulator does indeed attempt to always voltage regulate , but until the alternator gets up to quite a high speed it can't generate enough current to achieve this, unless the battery is already well charged.

As a simple check of this , run your engine at 3000 rpm next time and your max current will go to > 60amp.

Mine regularly is in 70's and over but soons drops to the more common 25amp.
(But I use a high performance type battery).

Robin Miller

Again, I’m not sure where you get you info from but most alternators reach maximum current output at between 1500 to 2000 RPM so that they are producing full current outputs at normal highway cruising speeds.

What would be the use of a high current alternator if you had to rev the guts out of the engine to be able to get the high current advantage, wouldn’t be much of a real advantage would it?

And again, the reason Brett’s Delkor was taking in no more that 40 amps is because of the charge VOLTAGE being applied.

Brett’s vehicle could have a 1000 amp alternator but unless the charge VOLTAGE is higher the current being draw by the battery is not going to increase.

Thanks for the info Brett and it sounds like your set-up does what you need it to.

Cheers.

While it's true you don't get the maximum rated output of the alternator at idle I wouldn't call this current limiting, the alternator is not that smart, it's only producing a lower current as it's not spinning fast enough. My 125A alternator is rated max output at around 2000 engine rpm like most alternators, so as long as your doing over 2000rpm you will have full current available, so when my battery is drawing 40A at 2000rpm it's the battery limiting this not the alternator, at idle it is less, around 20A as the alternator is not producing enough current at that speed.

Hi Guys

Whatever you like to call it its not really an argueable point - one only has to look at alternator specs. (Patrol workshop manual page EL56 )

For the common Patrol one I gave - their is no escape -> at anywhere from around below 1500 rpm you just cannot get any more than about 40 amps out of the alternator, hence even at a fast idle you can't supply what a couple of batterys might like to pull initially.

P.S. This is a deliberate design feature so battery its not stuffed by huge currents

Robin Miller