Is it good to keep solar charging fully charged batteries?

[escapism]

At this time of year the solar panels (40W) and controller are keeping the batteries at a nominal 14.1V. With the boat currently out of use, there are no loads on the system except the memory of the audio unit and self discharge of the (newish) batteries. Is excess energy being generated by the panels, and if so, where is it being dissipated, presumably as heat? The solar panels are hot, but they're out in the sun and I'd expect them to be. The charge controller has a small heatsink at the back but its temperature is ambient. I'd be a bit worried if the batteries were being overcharged and heated, but again, their temperature seems ambient.

So: are the panels still generating? Where is the energy going? Am I worrying unnecessarily?

 

[Boathook]

I've got the same and the controller, a victron one with blue tooth floats the batteries at around 14 volts. The solar panels only work when the circuit is made? so I assume that the controller is switching on and off in a controlled way to charge and maintain the batteries.
However it works, and I haven't had to use a battery charger since installing solar panels.

 

[Rappey]

? so I assume that the controller is switching on and off in a controlled way

Basically yes .

 

[Graham376]

I've got the same and the controller, a victron one with blue tooth floats the batteries at around 14 volts.

Is 14 volts correct float voltage for your batteries? My fla Trojans specify 13.5v.

 

[noelex]

For longer term storage if there are no loads on the systems, you should lower the float voltage (around 0.3v is a good starting point) and decrease the absorption time (reduce this to only a few minutes).

if your solar controller is keeping the batteries at 14.1v it probably only has single stage charging (instead of bulk/absorption and float) so an upgrade is worth considering.

 

[MADRIGAL]

I've got the same and the controller, a victron one with blue tooth floats the batteries at around 14 volts. The solar panels only work when the circuit is made? so I assume that the controller is switching on and off in a controlled way to charge and maintain the batteries.
However it works, and I haven't had to use a battery charger since installing solar panels.

I have an even more basic type of charge controller and get the same result - I haven't used a battery charger since Madrigal went back into commission in April. It's been the same every since I got the battery three years ago. I put it on a charger a couple of times over the winter, but the solar looks after it on the boat from April to November. Mind you, it only powers LED nav lights and VHF, so there is not much drain.

 

[William_H]

Essentially if the solar PV panel does not have a load it just generates a voltage with no current so now power dissipation of any sort. Panel may produce internal heat at full current drain but other wise heat is from being in sun. ol'will

 

[geem]

For longer term storage if there are no loads on the systems, you should lower the float voltage (around 0.3v is a good starting point) and decrease the absorption time (reduce this to only a few minutes).

if your solar controller is keeping the batteries at 14.1v it probably only has single stage charging (instead of bulk/absorption and float) so an upgrade is worth considering.

If you use Victron Smart solar regulators and Adaptive charging, wouldn't a reduced absorption time happen automatically?
Why reduce factory float voltage? Float voltage stops sulphation doesn't it?

 

[Alan S]

Essentially if the solar PV panel does not have a load it just generates a voltage with no current so now power dissipation of any sort. Panel may produce internal heat at full current drain but other wise heat is from being in sun. ol'will

Not really relevant to the OP's question, sorry escapism, but something puzzles me...

About 20% of the energy from the sun that hits a solar panel connected to a load gets converted to electrical energy, part of the remaining 80% is reflected and part of it is absorbed as heat.

If the panel is not connected to a load, presumubly 100% of the energy must now be either reflected or absorbed as heat. I can't see how more can be reflected so the panel must absorb more and get hotter.

I'd love to know if this is true or wnere the fault lies in my reasoning.

 

[vyv_cox]

A lot of users in Greece have wrecked their batteries by leaving solar panels connected over winter with the boat unoccupied. One or two have narrowly escaped fires. Our own domestic bank of three batteries, only two years old, was destroyed. We find we need to top up wet cell batteries fairly frequently during summer use and we now disconnect totally in winter.

 

[geem]

A lot of users in Greece have wrecked their batteries by leaving solar panels connected over winter with the boat unoccupied. One or two have narrowly escaped fires. Our own domestic bank of three batteries, only two years old, was destroyed. We find we need to top up wet cell batteries fairly frequently during summer use and we now disconnect totally in winter.

I have this quandary. We lift out on Monday for 3 months of hot Tropical summer. Boat temperature internally will be circa 45degC during the day. Batteries get hammered. We have Victron smart solar regulators with temperature compensation. In theory bulk and absorption voltages and run times are automatically regulated using the adaptive settings.
Leaving the batteries disconnected will likely see me coming back to partially charged batteries and sulphated.
My plan is to leave them connected and let the Victron kit do its stuff, albeit with only half the solar connected
Edit
Trojan website suggests storing them in a cool environment. Not going to happen.
Also they say that the self discharge increases with temperature so they need some charge

 

[vyv_cox]

I have this quandary. We lift out on Monday for 3 months of hot Tropical summer. Boat temperature internally will be circa 45degC during the day. Batteries get hammered. We have Victron smart solar regulators with temperature compensation. In theory bulk and absorption voltages and run times are automatically regulated using the adaptive settings.
Leaving the batteries disconnected will likely see me co Inc back to partially charged batteries and sulphated.
My plan is to leave them connected and let the Victron kit do its stuff, albeit with only half the solar connected

The advice we were given is that batteries will easily cope without charging for six months. Disconnecting them from everything and each other works well for many.

 

[geem]

The advice we were given is that batteries will easily cope without charging for six months. Disconnecting them from everything and each other works well for many.

Not with Trojan T105s. They have a high self discharge rate.

 

[noelex]

Good questions.

If you use Victron Smart solar regulators and Adaptive charging, wouldn't a reduced absorption time happen automatically?

The solar regulators will start a new cycle of bulk/absorption/float each solar day. This is fine if the batteries have been discharged overnight, but is not ideal if the batteries are already 100%.

The adaptive absorption time incorporated into the Victron solar regulators algorithm is a help, but it does not reduce the absorption time sufficiently. Many authorities would suggest that batteries that are 100% charged should not have any bulk or absorption time at all. Personally, I think a very short absorption time (just a couple minutes) is helpful in preventing stratification of the electrolyte and helping to equalise the cells.

The Victron adaptive absorption time if kept at its default setting of 2 hours will increase the absorption time to 6 hours if the starting battery voltage is below 11.9v (for 12v system) and decrease the absorption to 1 hour if the starting voltage is above 12.6v. So in storage mode with a 100% charged battery the default will still apply a 1 hour absorption time and this is too long.

The Victron solar controllers have one other automatic mechanism to reduce the absorption time called the “tail current setting”. This can be a nuisance for conventional charging on a boat. This feature is better disabled on most installations, (unless you have a system where the current into the battery is conveyed to the controller) but if you leave the boat without any loads it does help reduce the absorption time further in a storage situation. Sometimes this can be effective, but the effect is a little erratic so reducing the absorption time manually is better.

Why reduce factory float voltage? Float voltage stops sulphation doesn't it?

If you look up battery manufacturers’ recommendations they state a float voltage which is for cyclic use and a lower storage voltage. Some of the Victron battery chargers will do this automatically, dropping to the lower storage voltage (rather than the normal float voltage) after an extended period without a bulk cycle. Unfortunately, none of the solar regulators incorporate this feature. It easy to manually lower the float voltage to the more correct storage voltage. It is best to look up your specific battery manufacturer’s recommendations, but around 0.3v lower than the float voltage is usually about right.

 

[RJJ]

I have this quandary. We lift out on Monday for 3 months of hot Tropical summer. Boat temperature internally will be circa 45degC during the day. Batteries get hammered. We have Victron smart solar regulators with temperature compensation. In theory bulk and absorption voltages and run times are automatically regulated using the adaptive settings.
Leaving the batteries disconnected will likely see me co Inc back to partially charged batteries and sulphated.
My plan is to leave them connected and let the Victron kit do its stuff, albeit with only half the solar connected

Why not leave a small load on, like a couple of LED lights on a timer to switch on by night?

Or, reduce the supply by partially shading the remaining panel.

Or, and if we're really talking about damaging your several hundred pounds of batteries and fire risk, why not invest a few pounds in a tiny 10w panel and keep it plugged in?

 

[Beneteau381]

I have this quandary. We lift out on Monday for 3 months of hot Tropical summer. Boat temperature internally will be circa 45degC during the day. Batteries get hammered. We have Victron smart solar regulators with temperature compensation. In theory bulk and absorption voltages and run times are automatically regulated using the adaptive settings.
Leaving the batteries disconnected will likely see me co Inc back to partially charged batteries and sulphated.
My plan is to leave them connected and let the Victron kit do its stuff, albeit with only half the solar connected

Since following your advice and fitting a Victron remote programmable controller my batteries are fine. The panels are left hanging down on the guard rails so less efficient.

 

[geem]

The advice we were given is that batteries will easily cope without charging for six months. Disconnecting them from everything and each other works well for many.

I plan to top up the water to the maximum. The batteries will be fully charged so bulk and absorption times will be shorter than normal. Water loss should be low. Even with the boat in use with fridges running, etc we only top up about every three months. The engine batteries have been set up with their own Smart solar reg and 40w panel for the last few years and we have had no problems. They use no water as they don't get aggressively charged and discharged.

 

[geem]

Why not leave a small load on, like a couple of LED lights on a timer to switch on by night?

Or, reduce the supply by partially shading the remaining panel.

Or, and if we're really talking about damaging your several hundred pounds of batteries and fire risk, why not invest a few pounds in a tiny 10w panel and keep it plugged in?

I am not sure what that achieves. How can I damage the batteries by leaving them to charge to the correct voltages as recommended by the manufacturer?
I am not being arsy. I just don't see a technical arguement for an ad-hoc arrangement for charging them.
I don't need to reduce the supply. The Victron kit can do that. I can set up the voltages and absorption time to what ever I want. They are fully programmable

 

[escapism]

Many thanks for all the contributions. My conclusion is that continuous solar charging CAN damage batteries:

A lot of users in Greece have wrecked their batteries by leaving solar panels connected over winter with the boat unoccupied. One or two have narrowly escaped fires. Our own domestic bank of three batteries, only two years old, was destroyed. We find we need to top up wet cell batteries fairly frequently during summer use and we now disconnect totally in winter.

and that the issue may be particularly relevant to people whose programmable controllers may force a daily absorption phase if so configured.

On the other hand:

Essentially if the solar PV panel does not have a load it just generates a voltage with no current so now power dissipation of any sort. Panel may produce internal heat at full current drain but other wise heat is from being in sun.

So my relatively primitive controller, which is constantly switching/sensing the batteries is not passing a current to the fully charged batteries, so VI=P=0.

 

[Daverw]

By using victron VRM we can remotely manage charging options if needed, makes it very easy from anywhere for the cost of wifi.