DC-DC charger- which one?

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Originally Posted by Redback

I was under the assumption that all lead acid batteries were the same and that an alternater will charge them all without a problem :(

Baz.

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No baz, not really. Some lead-acid batteries are designed specifically for 'house' duties and are termed deep cycle batteries. That means they can be deeply discharged without huge damage. They are not necessarily designed so much for very high current discharge but for heavy total discharge. That is, low drain for long periods.

Other lead-acid batteries are designed for cranking the engine which requires a very high current but only for short duration. These batteries can be damaged by even infrequent discharging.

There are of course also many different types of deep cycle and cranking batteries, all with their own ideal charging regimes.

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Originally Posted by Silenceisgolden

No baz, not really. Some lead-acid batteries are designed specifically for 'house' duties and are termed deep cycle batteries. That means they can be deeply discharged without huge damage. They are not necessarily designed so much for very high current discharge but for heavy total discharge. That is, low drain for long periods.

Other lead-acid batteries are designed for cranking the engine which requires a very high current but only for short duration. These batteries can be damaged by even infrequent discharging.

There are of course also many different types of deep cycle and cranking batteries, all with their own ideal charging regimes.

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Yep, and they'll all charge up no dramas just connected to an alternator, regardless of what mix you run.

so if alternators are so bad, why have car manufactures been using them for the last 60+ years instead of dc-dc chargers?

A couple of questions:

Do DC-DC chargers get a a battery up to a higher state of charge. Most alternators get a battery up to ~85%?

The five step charge process on a C-tek (for example), is it a gimmick or is there value in desulphation, float and pulse modes?

Its clear that an alternator charges faster, and that most do not need a DC/DC. But if charge time was not an issue, and the battery is mostly above 75% is a DC-DC charge going to keep a battery in better health?

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Originally Posted by manic

Most alternators get a battery up to ~85%?

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Myth.....

Most batteries run lower state of charge because of "urban cycling" - that is never run long enough to charge fully...

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Originally Posted by manic

A couple of questions:

Do DC-DC chargers get a a battery up to a higher state of charge. Most alternators get a battery up to ~85%?

The five step charge process on a C-tek (for example), is it a gimmick or is there value in desulphation, float and pulse modes?

Its clear that an alternator charges faster, and that most do not need a DC/DC. But if charge time was not an issue, and the battery is mostly above 75% is a DC-DC charge going to keep a battery in better health?

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Yes, DC-DC charger gets batteries close to fully charged, but it is only a small amount of benefit. The de-sulphation (pulse) mode is more for rejuvenating cranking style batteries that have been deeply discharged and is not totally relevant to deep cycle batteries.
The big benefit of DC-DC chargers is prolonging the life of the house batteries, because when the battery is fully charged the charger drops down to float mode instead of continuing to charge like an alternator will do. Over charging is not as damaging as over-discharging, but it will shorten battery life.
The alternator does not necessarily charge faster. Provided the DC-DC charger is chosen to suit the battery capacity it should be faster than the direct alternator connection and there is not the risk of the alternator charging at too rapid a rate for the battery.

Manic - the process of charging a battery is simple.

Apply Voltage above battery voltage (think of this as force or pressure).
Now apply amperage available for the battery to absorb this energy.

A battery charger, DC-DC device, or alternator produces this output... And if the battery is in a state to absorb it it will.

A battery in a lower SOC will draw more current initially and this reduces as the battery gets to higher SOC.

To answer your question; a good battery charger will have some functions to prevent sulphation however this is more suited to batteries in storage.

As for DC-DC charger Vs Alternator and time...
Either will charge a battery to full charge given the time...

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Originally Posted by Silenceisgolden

Yes, DC-DC charger gets batteries close to fully charged, but it is only a small amount of benefit. The de-sulphation (pulse) mode is more for rejuvenating cranking style batteries that have been deeply discharged and is not totally relevant to deep cycle batteries.
The big benefit of DC-DC chargers is prolonging the life of the house batteries, because when the battery is fully charged the charger drops down to float mode instead of continuing to charge like an alternator will do. Over charging is not as damaging as over-discharging, but it will shorten battery life.
The alternator does not necessarily charge faster. Provided the DC-DC charger is chosen to suit the battery capacity it should be faster than the direct alternator connection and there is not the risk of the alternator charging at too rapid a rate for the battery.

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Not sure how many times it's been mentioned, but an alternator WILL NOT overcharge a battery, it simply can't. PLEASE stop pedalling this myth. If alternators overcharged batteries, we would all have dead cars all the time. I don't care if it's a lead acid, AGM or pink with purple dots, as the batteries charge increases, the current draw drops off. The alternator will only provide what the battery needs, it CANNOT provide current to a battery beyond this.

Do the physics, chemistry and math - those laws are well proven.

Why do you not believe the people on here saying this? The 4 main protagonists against your argument are some of the most respected and highly qualified people in their fields. All have vast electrical, electronic and automotive experience and together would have over 100 years of experience in this sort of stuff. Thier not technophobes, they just intimately understand the subject.

Sorry, but you're continual denial of their explanations is just making you look silly. I'd cut my losses and run, even if you won't accept their advice.

Sure I get it. I have a voltage meter on the dash and I can see the alternator is often knocked down to ~13v so I figured a DC-DC at nearer 15v might be quicker at topping up batteries and the alternator quicker at restoring bulk.

I'm considering the following set up.

Starter Battery
Deep Cycle Battery (AUX)

All Auxilary sockets, fridge, lights etc run off a 100AMP relay that gets power from the starter/alternator when the car is running and switches to the Aux battery when starter voltage drops below ~12.4v

A DC-DC charger is the only device connected to the AUX bat when the engine is running.

This set up will see all devices powered by the alternator when engine running, and the starter battery will get charged quickly (priority) from the alternators bulk amperage. The AUX will get a regulated DC-DC charge without having to provide any power.

On short stops the starter battery is likely to provide the required power, on longer stops the AUX takes over with solar support.

On a 90 with a small fridge and LED lighting I think this set up might see the AUX battery kept at a high/better state of charge most of the time. Especially when the vehicle is not being used for camping.