backchannel: DC-DC charging myths

Outbackjoe’s info is excellent.  Here I will address some of his points about what he calls DC-DC charging myths.

 The most commonly reported myth is that your alternator cannot fully charge a battery.

It can, given enough voltage and enough time, as on a powered cruiser (boat), that spends many hours a day running the engine.   To some degree, time can be substituted for voltage (ie absorption takes longer at lower voltages).

Having said that, in actual usage alternator charging alone is usually insufficient to charge deeply-cycled lead-chemistry batteries to manufacturer specs in vehicles that aren’t running many hours a day.

The figure of 80% sounds like an arbitrary number that some marketing guy made up.

80%  is likely a reference to the percentage of amps replaced when absorption voltage (Vabs) has been reached and current acceptance starts dropping off.   It is not a reference to 80% state of charge by voltage.

When charging AGM at the max recommended rate (C/2.5), MainSail finds that 63.3% of amps had been replaced when Absorption commenced.  A gentler charging rate of C/5 showed 77.4% of amps were replaced.

If the alternator is able to hold Vabs for the duration the battery mfg specifies then all is well.  In the real world, it’s likely not hitting Vabs at all (hence DC-DC chargers), and not holding it anywhere long enough (a different problem).

Nothing special happens to a battery’s chemistry when it reaches 80% state of charge

Right, but something special happens to its behavior:  current acceptance starts falling until it hits endAmps.

In reality any voltage above the voltage dictated by the battery’s chemistry will charge the battery to 100%

True, tautologically speaking.  The questions here are:

  • can the battery be fully charged at barely-above-nominal charging (say, 12.8v) in the time available; and

  • will the battery remain healthy with such a regimen?

My answer:  No, not in an off-grid vehicle.   Maybe if fully charged first then sitting on shore power 100% of the time aftweward,  and even then Vfloat specs exist for a reason.

An elevated voltage will charge the final few % faster

Agreed, but for non-obvious reasons:

  1. higher Vabs indirectly extends bulk stage; and

  2. directly reduces the amount of time required to meet endAmps specs

and help reduce sulfation.


On average, under cycling conditions, an alternator will provide a greater state of charge.

Well, it will provide a greater rate of charge until Vabs than DC-DC, which is typically rate-limited.

After an overnight discharge, an alternator will charge faster than a DC-DC converter during the bulk charge stage.


This accounts for most of the battery’s capacity

80%, would you say?  :-)

It is only once the battery is almost fully charged that a DC-DC converter will charge faster than the alternator.

Once Vabs is reached, yes.  But speed isn’t the point of DC-DC charging;  fully charging the batteries to manufacturer specs is.

On average though, for a typical cycle of the battery, you’ll have more capacity charging from the alternator.

More capacity, yes.  Better charging, no.

I’ve seen other myths reported about DC-DC converters. For example that they are “better for your loads”, or provide “better isolation”

Yeah, I don’t know what that means.  Marketing speak, I suppose.

or that they help your starter battery charge to a higher capacity or improve longevity in your starter battery.

Some of the  DC-DC chargers will maintain the starter batt, which could conceivable have those effects.  I am, generally speaking, not that worried about my starter batt.

Some claim that if cost is not an issue, a DC-DC converter is universally the best solution for a dual battery design.

I do not claim that.

Any [design is a compromise](

absolutely agreed

If your auxiliary battery is able to accept a high charge rate from the alternator and will see regular charging from adequate solar and / or an intelligent mains charger then I prefer the advantages of a dual VSR setup.

100% agreed.  Solar and mains smart charging bring proper Vabs to the table, which makes DC-DC largely irrelevant.  My expanded thoughts on that matter.

If you have a fixed voltage alternator and its output is lower than the specified float voltage of your battery then you probably need a DC-DC converter.

I don’t think Vfloat charging is sufficient for deeply-cycled, offgrid batteries.   There simply aren’t enough hours of sun or driving in the day.

Or if you never have a solar or mains system giving regular high voltage top ups to give the battery 100% charge and reduce sulfation then again you should consider a DC-DC converter.

It’s almost like you’re saying that “your alternator cannot fully charge a battery”.  Glad to see we are reconciled at last.  :-)

And even with DC-DC, it still takes hours of Absorption.


elsewhere in the same article:

If you have a load that regularly draws current (like a fridge) then the DC-DC converter or solar regulator will interpret this as the battery requiring a charge and will crank up the voltage accordingly

No, it won’t.  It will increase power harvest from the panel up to the panel’s limit.  IF the load is heavy enough that

  1. the panel can’t keep up; and

  2. system voltage drops below a given setpoint for a given amount of time

then some controllers will restart Absorption.

This is a problem. The fully charged battery will be experiencing overcharge

No, it won’t.

Overvoltage-induced positive grid corrosion is associated more with calcium-doped non-deepcycle batteries than deep cycle batteries.   According to EastPenn (manufacturer) when it happens to deep-cycles (antimony-doped) it’s a function of very deep discharges.  Other research ties it to electrolyte concentration and duration of the deep cycle events (ie, failure to fully recharge in a timely manner), and to elevated temperatures.  Batteries should always have a temp sensor attached to prevent overheating.

There is some evidence  that deeply-cycled offgrid banks may last longer and perform better without the drop to Vfloat at all.

This is a serious problem for designs utilising DC-DC converters.

No, it’s not.

> The problem is less for solar regulators since this will impact the battery only when the solar panels are deployed and typically after an overnight cycle down where the battery is in a discharged condition.

There is no difference between the effect on DC-DC and solar controllers.

And I’d be surprised if most folks are running the engine in their vehicles longer than the sun shines each day.