Thread: 12v charger and dual battery

Originally Posted by THE BOOGER

Thats why he,s a mechanic not a sparky

Hi BOOGER and Oh brother ain’t that the gospel.

A week or so back, I had a diesel mechanic on the caravaners forum inform me that he know more about batteries than anybody who works with batteries because he IS a mechanic.

And the moderators reckon he is correct, go figure.

Originally Posted by Mundy

Excellent explanation, Drivesafe.

The only advantages that I thought I knew of DC/DC chargers are that you can select your battery type and if you have a calcium type which has a need for 14.7V/15V input, they will provide it (possibly only eventually given your explanation). Secondly, for maintenance charging, I thought they pulsed the charge to stop sulphonation. Is this correct?

It would seem logical to me from all this that you need to be careful in choosing two battery types which require a significantly different charging voltages.

Mundy

Hi Muddy and while the different maximum voltages that different batteries can tolerate can be a problem, this is really only a problem if certain batteries are located under the bonnet of most vehicles.

Batteries like Gel Cells and STANDBY type AGMs, like Fullriver, have a maximum charging voltage of 14.4v and while few older vehicles have operating voltages of much over 14.0v, there is an additional problem or number of problems.

The 14.4v is the maximum charge voltage of these batteries AT ROOM TEMPERATURE, which is usually 25 degrees C.

Now add to this the fact that STANDBY type AGMs actually have a current limit, both while charging and when being discharged, something most people are unaware of, so you have more potential to damage these batteries than you do with good old fashion flooded wet cell batteries or AUTOMOTIVE GRADE AGMs, like Optima.

A STANDBY type AGM fitted under the bonnet will be hit with high temperatures which means the charging voltage should be much lower that 14.4v and the higher voltage also means high charge currents being applied to these batteries, which all add up to much shorter operating life spans for these batteries.

Now back the DC/DC devices and while they will control the charge current, they still apply too much current to some batteries when those batteries are under the bonnet because very few DC/DC devices have temperature compensation and simply end up eventually cooking the very batteries they are supposed to be protecting.

Originally Posted by drivesafe

Battery chargers, DC/DC devices and solar systems CAN NOT do this because they are all constant CURRENT chargers

Originally Posted by drivesafe

Now back the DC/DC devices and while they will control the charge current, they still apply too much current to some batteries.

Huh?

Originally Posted by wozzlegummich

Huh?

And your point is?

If a battery is having too high a voltage applied to it, limited charge current is not going to stop it cooking.

So again, what is your point?

Originally Posted by drivesafe

And your point is?

I'm just confused. (Easily done these days)

In one post it was mentioned that all 12v/12v chargers are constant current yet in another it says they control the current.

In my experience nearly all 12v/12v chargers control both the current and the voltage.

That is why they are so good to maintain the correct charge in an auxillary battery. They allow you to mount the battery in the rear away from the heat of the engine bay, without voltage drop problems associated with a long cable run whilst isolating it from the vehicles charging system and battery. The better ones also have a battery temp sensor, something no O/E alternator based system has.


These forums are great - spirited discussion and a wealth of info.

Originally Posted by wozzlegummich

I'm just confused. (Easily done these days)

In one post it was mentioned that all 12v/12v chargers are constant current yet in another it says they control the current.

In my experience nearly all 12v/12v chargers control both the current and the voltage.

That is why they are so good to maintain the correct charge in an auxillary battery. They allow you to mount the battery in the rear away from the heat of the engine bay, without voltage drop problems associated with a long cable run whilst isolating it from the vehicles charging system and battery. The better ones also have a battery temp sensor, something no O/E alternator based system has.


These forums are great - spirited discussion and a wealth of info.

Wozzy,
I think what drivesafe is saying is that, an alternator applies a constant voltage across a battery and the difference between that voltage and the voltage of the battery being charged determines the current which varies as the battery is charged up. So when the voltage difference is large (ie 2.5v) the current is high but when the difference is small (say 0.1v) the current is small.

However, I assume he means, a DC/DC charger varies the voltage to maintain a constant current. So that it doesn't matter how low the SoC is, the charger pushes through a relatively low current which doesn't change much. This if the SoC is low, it will take longer to reach a substantial charge than with an alternator which will push more current through. But when the battery is nearing full SoC the DC/DC charger may well take less time as it increases the voltage to keep pushing the current through.

Finally, I agree these forums are great, especially this one. However, the issue of batteries and charging is the one which generates the most misinformation and where it is hardest to work out who actually knows. Drivesafe seems to.

Further to my last post and in the interest of better understanding, I have pondered Drivesafe's view that a solar panel is a constant current device. I wonder if its not a constant power device in that it varies both voltage and current to enable the energy it's producing to be pushed through the load, recognising solar panels have both a maximum limit current capability (shorted current) and voltage capability (open circuit voltage). Thus for a 60W panel, if the battery SoC is 12.5v the current would be 4.8A, and if SoC was 13.5V, current would be 4.4A.

Does anyone know if this is correct?

Originally Posted by Mundy

Further to my last post and in the interest of better understanding, I have pondered Drivesafe's view that a solar panel is a constant current device. I wonder if its not a constant power device in that it varies both voltage and current to enable the energy it's producing to be pushed through the load, recognising solar panels have both a maximum limit current capability (shorted current) and voltage capability (open circuit voltage). Thus for a 60W panel, if the battery SoC is 12.5v the current would be 4.8A, and if SoC was 13.5V, current would be 4.4A.

Does anyone know if this is correct?

Not really, constant current is a better description at an output voltage well below the panel's maximum power output voltage. Most "12V" panels produce their rated output power around 17V (at 25C panel temp) with open circuit voltage typically around 20V. Around 12-14V their current output is close to constant. Someone may have a solar panel output curve they can post up. The output of a panel is best fed via a good quality regulator where the output is programmable to the specific battery type. Nowadays MPPT (maximum power point tracking) regulators can extract more power from a typical panel than simple analogue regulators.