Alternators, battery sensing.

[Bro]

Recent interweb trawl for an alternator seemed to indicate Battery sensing jobbies are becoming more readily available at sensible prices.  As an alternative to additional systems for faster battery charging (such as Adverc, good as they are) do they now make sense on our narrow boats?

[Tony Brooks]

If you have a poorly specified and installed charging circuit then a battery sensed alternator or am Adverc will cover it up and hide its shortcomings. In my view its probably more cost effective to get the charging system right and make sure the alternator regulates at around 14.4 to 14.6 PROVIDING that does not exceed the battery's maximum charging voltage.

Such devices are vital if you insist on using a conventional diode charge splitter but nowadays that's poor design and specification.

[Bro]

Hi Tony.

I may well have a poorly specified charging circuit, I don't know.  Having two alternators, one for engine start battery and other for aux/ domestic battery bank It looks conventional to me.  Boat instruments indicate both alternators charge at your 14.4 to 14.6V.

I do know that after a short time,  though the voltage remains high the amps output falls away dramatically and consequently it takes longer to charge the batteries.  I had assumed alternators with battery sensing might maintain high output for longer.

[Tony Brooks]

27 minutes ago, Bro said:

Hi Tony.

I may well have a poorly specified charging circuit, I don't know.  Having two alternators, one for engine start battery and other for aux/ domestic battery bank It looks conventional to me.  Boat instruments indicate both alternators charge at your 14.4 to 14.6V.

I do know that after a short time,  though the voltage remains high the amps output falls away dramatically and consequently it takes longer to charge the batteries.  I had assumed alternators with battery sensing might maintain high output for longer.

That is all perfectly normal. You are not discharging your batteries very much. If you did get them well discharged the voltage would be lower, gradually creeping up and the amps would be higher for longer.

If you are reading 14.4 to 14.6 volts on the voltmeter WITHOUT any solar input then in my view the circuit is fine.

I will be controversial now, the average boater has no idea how battery charging works and is therefore a sucker just waiting to be targeted by tye makers of so called advanced regulators.Your description seems absolutely normal to me  and perfectly acceptable. Your conclusion that the falling charge means it takes longer than it needs to recharge your batteries is false.

All boaters need to grasp that at any given charging voltage it is the BATTERIES that determine how much current they will accept. The more charged they are the less current they will accept. Now, if you boosted the charging voltage you would push more amps into the battery (that's what the Adverc does) but if you overdo it you damage batteries so the possible voltage boost is pretty limited on a decent system as yours seems to be. Your charging voltages are about 0.1 to 0.2 volts lower than the safe maximum for modern batteries. The time saved by using any form of advanced controller (Adverc) is somewhat limited for the reasons I set out below.

1. When first started the alternator will be delivering maximum current because that's what the battery wants but in ding so the charging voltage is depressed to a degree. As the batteries charge the current falls and the voltage rises. As the voltage is depressed any advanced regulator or a battery sensed alternator can have no effect on the charging voltage so its just junk at the moment.

This is the first stage of charging roughly analogous to the Bulk phase referred to in battery charger's graphs.

2. Before long, depending upon how discharged the batteries are, the voltage would rise to the point the regulator starts to control the voltage and limit it. At this stage the advanced regulator can short out the alternator's own regulator and provide a higher voltage that in turn results in a  higher charging current BUT how much of that current is charging the batteries, how much is heating the batteries and how much is making them gas is open to question. In any case all the modern systems pulse the high voltage at rough 15 minute intervals so you only get a charging boost for half the running time. If the controller is fitted with battery and/or alternator temperature sensing the amount of time spent at the higher voltage will be reduced. The controller is doing something now but how much good it is is questionable with your regulated voltage.

This stage of charging is roughly analogous to the acceptance phase of a chargers charging graph and it is likely to only last an hour or two.

3. Eventually the batteries will only accept a few amps BUT to get to 100% fully charged they will need that for many hours. Modern controllers are likely to simply switch them selves off for this stage so its just the alternator's regulator controlling the voltage and the batteries deciding how much current they need. A Sterling A to B or B to B charger may well reduce the voltage for this stage but other makes will just revert to the setting of the alternator's regulator.

This stage is a little different to a chargers float stage because after a few hours the engine will be turned off so charging stops.

You can see form the above that an advanced regulator can only shorten the charging time during stage 2 but probably by not as much as you have been lead to believe.

The easiest way to shorten the engine charging run time is to install some battery monitoring and only engine charge to about 80% of fully charged early in the day. This may take between 2 and four hours. Then fit solar to spend the rest of the day supplying those few amps required to fully charge the batteries. I think that you ca get 100 watt of solar and a PWM controller for a similar cost of an advanced controller. An MPPT controller is preferable.

A battery sensed alternator will make no difference to the charging time on your setup unless you start putting diodes in the sense lead to fool the alternator into boosting the voltage but on your figures I think the result would be damaged batteries.

 

 

 

 

 

 

 

[cuthound]

3 hours ago, Tony Brooks said:

I will be controversial now, the average boater has no idea how battery charging works and is therefore a sucker just waiting to be targeted by tye makers of so called advanced regulators.Your description seems absolutely normal to me  and perfectly acceptable. 

I have a term for this. It call it "marketing bollux".

At the beginning of my career I did electrical design and engineering to HNC level.

Mid career I went to university to undertake an MBA. The marketing lectures were interesting, the lecturer looked down on engineers for "wanting to gold plate everything", and was always banging on about marketeers being the most important people in an organisation because they increase sales so much.

I asked him to name one thing the he could sell that didn't grow or wasn't invented by an engineer. He was stumped. 

[Bro]

Thank you Tony for the reasoned explanation and thanks also to Cuthound for confirming Tony's view on usefulness of exotic contraptions.  I'll stick with the old adage and "keep it simple".

[Tony Brooks]

At those voltages very wise.

[DHutch]

Battery sensing, to my understanding, is only really useful if the alternator is a significant distance from the batteries. Where the two or physically cost, using sufficiently chunky cable should render it unnecessary.

Daniel

[Tony Brooks]

or where a diode splitter is involved. Also has been used on arctic spec vehicles where a  battery temperature sensor modifies the feedback to the alternator.

[Robbo]

Top marks for Tony for an excellent explanation, i think this topic should be pinned, etc.