Solar power confusion

[Top cat]

I'm a bit puzzled by the interaction of our alternator and solar charge controller. I think I have figured out whats going on but I thought I'd run it past you lot to sanity check my theory.

 

We have an Adverc controller on the alternator and a Victron 15/75 controller on our 190w solar panel.

At the end of a typical cruising day the 440 Ah battery is at 14.4V and is taking about 6-8 A.

I interpret that as being not quite fully charged. Is that correct?

I had expected that when we shut the engine down and moored with the panel in full sun that the panel would finish off charging the battery.

BUT that doesn't happen. Instead the batteries stay at 13.2V discharging at about 1.5A. This does not change if the sun goes behind a cloud. Eventually the current drops but by then so has the sun.

My theory is that the solarontroller sees the 14.4V and assumes the batteries are fully charged and it goes into float mode and hols them at 13.2V.

Does that sound reasonable?

 

As and experiment I put the hairdryer on during the 13.2v plateau and pulled the voltage down to 12.8v or so for a minute or so. After that the solar controller started to charge the batteries and continued to do soup above 14V.

 

So the big question is

 

Are my batteries getting full charge?

 

If not what to do about it?

 

Thanks

 

Top Cat

[bottle]

Top Cat you have described exactly how my system works and I regularly put a load (boiling a 1 kw kettle for coffee) on the batteries to drag the voltage down to make the solar 'kick in' after cruising because as you say it sees the high voltage and believes the batteries are charged.

 

It then carries on charging at absorption voltage.

 

Lots of discussion on here about when batteries are 'fully' charged, the figure banded about is 1% of battery capacity but as we all know that varies with age and usage.

[Paul C]

Sounds about right, solar controllers are notoriously conservative with the way they charge batteries. This isn't necessarily a bad thing in itself, but it does mean in marginal situations the batteries may be charged less than perfectly. Is it better to undercharge batteries than overcharge them?

[Cas446]

Try setting the charge voltage to very slightly higher than the alternator voltage with a short absorption time, I use 2 hours. After a full days cruising the solar stays in for the two hours but then does go to float.

[Top cat]

Thanks for the suggestions, I'm not sure if the Victron allows adjustment, but I'll look into it. It maybe easiest for us to put the kettle on when we stop.

 

I should have mentioned that my batteries are 3 1/2 yr old cheap sealed leisure type which cope with our demands easily, but they are showing their age by going up to max voltage early in the recharge process.

 

Top Cat

[WotEver]

Is it better to undercharge batteries than overcharge them?

No. Undercharging results in Sulphation. Overcharging results in electrolyte loss and shedding. It is much better to charge them correctly.

[Paul C]

No. Undercharging results in Sulphation. Overcharging results in electrolyte loss and shedding. It is much better to charge them correctly.

 

Yes of course, one would charge them correctly, but given the choice of under/over....... Basically it comes down to the non-sophisticated way the Victron solar controller monitors and determines battery state. Does there exist a more sophisticated solar controller, or are they all variations of this simple model (resulting in conservative charging) or simply have user settings/manual control ie you can alter the voltages possibly arbitarily and put more "management" onus on the user themselves?

 

Let's face it, they're all pretty dumb - a better system could easily be envisaged, where over time the battery monitor "learns" what your typical daily profile is, of battery use. Then it seeks to charge the batteries, just right, in the cheapest way possible. For example, if you're running the engine but its a sunny day and going to remain sunny the rest of the day, it would back off the alternator and let the solar do its thing, then once you've finished cruising for the day the solar would keep on charging a while longer. And if the weather is looking a bit iffy it would take more from the alternator. My central heating does it, why can't a boat?

[WotEver]

My central heating does it, why can't a boat?

A very good question to which I have no answer. It would seem to me to be a pretty simple algorithm for a manufacturer to include (which they could charge extra for) so why haven't they done it? No idea.

[Top cat]

Well if all the solar charge controller is seeing is the battery voltage how is it to distinguish between hitting 14.4v from the alternator vs getting there via the solar panel? You would need some means of determining the state of charge , not trivial with two inputs.

If the solar charge controller also measured current at say 14.4v and the user could set a threshold , ( in my case 4A) then we might get somewhere.

Any one got any better ideas?

 

Top Cat