Excess solar when there's a power demand - what happens at the leisure battery?

[Hagstrom]

In a lay by in Spain last week, I found myself thinking about, what was going on in the leisure battery as I used the 230v espresso machine on the inverter.

In strong sunshine, the services panel indicated that there were six to seven amps coming down from the solar panels. On the demand side, there may have been, say, 1 amp being consumed by the van in its ambient state. The 1200 watt espresso machine would be adding, say, 5 amps to the load, so that the solar supply still exceeded the demand. The numbers on the display panel indicated this.

In this situation, I wondered, what was happening in the leisure battery? Was the demand being met directly by the excess solar "bypassing" the battery processes or by separate but simultaneous battery charging and discharging processes?

When these musings consume me for miles down the Autovia, I find myself thinking " where is Grommit when you need him?"

 

[andy63]

On the demand side, there may have been, say, 1 amp being consumed by the van in its ambient state. The 1200 watt espresso machine would be adding, say, 5 amps to the load, so that the solar supply still exceeded the demand.

Hi..
You are getting your 230v and 12v sides mixed up.. The coffee machine will draw 5amps or more at 230v but that will place a 100 or more amp load on the leisure batteries... The solar will supply what it can but your leisure batteries will make up the majority of the loadd while you run the inverter..

Andy..

 

[Hagstrom]

Yes, you're right, of course but my question isn't answered.

OK, switching off the inverter and using a totally 12v load, from non-inverter supplied components like 12v extractor fan plus12v lights, to a point where it amounted to less than the supply from solar at whilst the battery capacity is shown as full. (My display panel shows solar supply in amps and, simultaneously, the battery drain in amps, and measures battery ins and outs, so I can see this is happening.)

I want to know what is happening at the battery when the solar supply exceeds the load and the batteries are fully charged. Does the feed to the load "bypass" the battery or go through the charge/discharge processes to produce it?

 

[DBK]

Power doesn't get lost. If the batteries are fully charged then they might take a float charge of say around one amp or less. This mostly dissipates as heat in the battery if they are genuinely fully charged.

Beyond that power consumed by the 12 volt loads will, disregarding heat losses in the wires, exactly equal the surplus power output from the solar over and above what it is putting into the batteries. Assuming a steady state.

If you have an MPPT controller you can't directly compare the amps coming from the solar to what is measured at the battery as they will be at different voltages.

But you either have loads you don't know about or your instrument readings are wrong. As I said at the beginning, power doesn't get lost.

 

[andy63]

Look at it like this...
Your solar will have the capacity to supply x amount of power on a certain day and under certain conditions.. Depending on sunshine..up to its maximum capacity...
You connect in loads which it will supply up to it's max capacity at the time..the batteries are just another load which may draw a current if required depending on their state of charge.. If the demand on the load side is zero then no current will flow..but its there as potential energy..
If your solar can't meet the full demand of the load the batteries will supply the difference..
Andy.

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[HandyAndy]

Think of it as water in pipes, much easier to get your head around..
If there is 6 litres flowing down from the roof and your battery tanks have 4000litres capacity and you open your espresso tap then you will use the 6L flowing from the roof and the other 94Litres will flow from the battery tanks.
Your inverter will have a bit of a leak as well so you'll need a bit more than a 100 litres to power the espresso machine

 

[DBK]

But you either have loads you don't know about...

That doesn't read very well! I meant electrical loads, not the OP's knowledge.

 

[autorouter]

I want to know what is happening at the battery when the solar supply exceeds the load and the batteries are fully charged. Does the feed to the load "bypass" the battery or go through the charge/discharge processes to produce it?

Yes, it bypasses the battery and goes straight to the load. You could run the load just off the solar power at that point, it's free energy. Until the sun goes in of course.

 

[Hagstrom]

Thanks to all, especially Auto router who has answered my question directly.

 

[Duckato]

It helps to understand why a battery charges and how an MPPT tracker behaves
A battery draws current at an ever deceasing amount as the internal voltage and impedance rises.
To the point where it's internal impedance and hence voltage matches that of the charging source then virtually little more than a trickle of current is flowing and the controller drops the voltage aka a float charge occurs.

An MMPT tracker does two things , it matches impedance presented to the solar panels to get maximum current at the required output.

The output is a result of a DC to DC conversion process within the Tracker voltage is reduced and current, pro rata, increased e.g assuming 100% efficiency 24v in at 1A = 12v out at 2A.

The DC to DC conversion output Voltage is e.g. 14.6v for bulk and absorption stages and 13.6 for float depending on battery chemistry and the controller will try and provide as much current as it can whilst maintaining that bulk voltage, the battery itself begins to determine the absorption current, eventually it will be (should be on a sunny day) less than available current from the controller.

Hence as the batteries reach full charge the MPPT adjusts the impedance it presents to the solar panels to the point that the Solar panels rise to their VCC and very little current flows because the panel and mppt impedance are equal.

Now lets assume your batteries are now 100% charged and at float

If you switch on your monstrous 1200W load that does two things, it very rapidly draws lots of current and begins to pull down the battery Voltage.

Mppt controller sees the batteries require charging and simply delivers max current it can until batteries are charged during and after you turn said load off.

Obviously in the current weather conditions it would take a month of Sundays to replace what your 1200W beasty can draw in a few minutes of regular daily use.

I hope you have decent B2B as well!

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[Duckato]

This is a really good in depth example of how batteries charge and why it takes so long to get to a true 100%
https://marinehowto.com/how-fast-can-an-agm-battery-be-charged/