Another charging question

[pessimist]

Having just fitted a new battery bank (lead carbon) we find that we are unable to use the boat for 6-7 weeks We have 420ah of batteries and 350 watts of solar. There will be a constant discharge of +- 0.5a. Should I be "turning down" the solar controller? It's currently set to a 2 hour absorbtion time should I be reducing this?
Thanks for any aaadaavice.

 

[PaulRainbow]

Having just fitted a new battery bank (lead carbon) we find that we are unable to use the boat for 6-7 weeks We have 420ah of batteries and 350 watts of solar. There will be a constant discharge of +- 0.5a. Should I be "turning down" the solar controller? It's currently set to a 2 hour absorbtion time should I be reducing this?
Thanks for any aaadaavice.

6-7 weeks with lead carbon, you could turn the solar off.

EDIT, with a 0.5a discharge the above is incorrect, i missed that. Better to reduce the panel output.

 

[alahol2]

My Ah and watts are very much less than yours, however, I asked this very question of Renogy. They assured me that "If your battery is fully charged, it will not enter the bulk/boost charge stage until the battery voltage drops to the boost voltage set point." This reassured me somewhat.

 

[VicS]

6-7 weeks with lead carbon, you could turn the solar off.

But a constant discharge of 0.5A will run the battery completely flat in 5 weeks

 

[PaulRainbow]

But a constant discharge of 0.5A will run the battery completely flat in 5 weeks

That's correct, i missed the .5a constant discharge.

 

[Stemar]

Jazzcat's 200w of solar feed my 120AH of domestic and 60AH starter batteries via an MPPT two battery controller. Rightly or wrongly, I just leave her to get on with it.

Quick thread hijack: I'm going to have to remove the battery box for several days to allow other work. Should I cover the panels or just disconnect them from the controller?

 

[pessimist]

Jazzcat's 200w of solar feed my 120AH of domestic and 60AH starter batteries via an MPPT two battery controller. Rightly or wrongly, I just leave her to get on with it.

Quick thread hijack: I'm going to have to remove the battery box for several days to allow other work. Should I cover the panels or just disconnect them from the controller?

FWIW whenever I've had to muck around with the batteries I've simply disconnected the panels from the controller. Doesn't seem to do any harm, but that's only for a few hours not days. Presumably someone who knows what they're talking about will be along soon.

 

[pessimist]

Thanks for all the replies so far. I'm still not sure whether I should be taking any action and really don't want to bugger the new battery bank.

 

[alahol2]

Ask the manufacturer of your controller if it will enter the absorption phase even if the battery is fully charged.

 

[noelex]

With a 0.5A discharge you will not do any great harm over 6-7 weeks leaving the solar controller with its normal voltage set points and this is the simplest solution.

If leaving the boat for longer, especially with no load, it is better to reduce the voltage set for bulk/absorption and float to a lower level and cut back the absorption time to only few minutes. The ideal voltage set points depend on the circumstances and battery type etc but around 0.3v lower than normal is a good starting point.

 

[noelex]

My Ah and watts are very much less than yours, however, I asked this very question of Renogy. They assured me that "If your battery is fully charged, it will not enter the bulk/boost charge stage until the battery voltage drops to the boost voltage set point." This reassured me somewhat.

This is true of some controllers (but not all) , but even when this is the case the battery voltage will typically drop below the re-bulk voltage set point by the morning.

For example, with the popular Victron controllers the re-bulk voltage set point is (by default) 0.2v below the float voltage. So if you have a float voltage of 13.6v the solar controller will initiate a new bulk cycle when the voltage drops below 13.4v. Even with no load, the surface charge will have largely dissipated by morning. The voltage will be sitting at below the re-bulk voltage, although like many controllers the Victron algorithm will initiate a new cycle each solar day even if this is not the case.

The bottom line is no matter what system your solar controller uses you are likely to have new bulk, absorption, float cycle each solar day. This, of course, is ideal if the battery has been discharging overnight,as is the case with normal cyclic use, but it will be overcharging the battery if there is no discharge and the battery is sitting at 100%.

Some solar controllers do have a mechanism to try and mitigate this overcharging. Victron use an adaptive absorption time and a tail current setting to try and terminate the charging earlier on a 100% charged battery, but these mechanisms reduce rather than eliminate the overcharging when the boat is in storage mode with no discharge and few controllers have an algorithm this sophisticated.

So if leaving the boat for long periods with no loads it is helpful to reduce the battery voltage set points and absorption time to numbers better for this storage mode. About 0.3v below the normal settings with a very short absorption time (say 10 minutes) is a good starting point. However, with a constant 0.5A load and only a short absence I think leaving the settings unchanged is reasonable in this case and removes the danger of adjusting something incorrectly.

 

[Refueler]

Jazzcat's 200w of solar feed my 120AH of domestic and 60AH starter batteries via an MPPT two battery controller. Rightly or wrongly, I just leave her to get on with it.

Quick thread hijack: I'm going to have to remove the battery box for several days to allow other work. Should I cover the panels or just disconnect them from the controller?

The single and two battery Solar MPPT controllers I have - both manuals say to connect batterys before connecting panels - to allow the controller to setup. Both manuals also say not to connect panels first ... so I assume that means disconnect panels before disconnect batterys.

 

[pessimist]

Thanks again. Doing nothing is always appealing! I'll keep an eye on what's going on and, if necessary, reduce the duration and/or voltage, of the absorption phase.

 

[noelex]

My Ah and watts are very much less than yours, however, I asked this very question of Renogy. They assured me that "If your battery is fully charged, it will not enter the bulk/boost charge stage until the battery voltage drops to the boost voltage set point." This reassured me somewhat.

Out of interest I looked up the specifications for the Renology Rover 60A for you.

It has a default boost return voltage of 13.2v for all battery types, although this is user adjustable.

Renology have chosen a slightly lower than typical boost return voltage, but wth no solar charging overnight the surface charge will have largely dissipated by morning and the battery is likely to be sitting below 13.2v so a new bulk/absorption/float cycle will be initiated even if the battery is at 100% (or very close to this level) state of charge.

 

[alahol2]

Renology have chosen a slightly lower than typical boost return voltage, but wth no solar charging overnight the surface charge will have largely dissipated by morning and the battery is likely to be sitting below 13.2v so a new bulk/absorption/float cycle will be initiated even if the battery is at 100% (or very close to this level) state of charge.

You may very well be correct. As I said my setup is very much smaller than the OP's. The controller is a Wanderer 10 with only a 30W panel but the boost return voltage is, as you say, 13.2V with a 2hr absorption phase.
My engine start battery, charged only by the alternator, will sit at 13.4/13.5 volts for days on end, indeed that is what the voltage normally reads each time I join the boat.
It may well be that the parasitic draw of the solar controller is sufficient to remove the surface charge and allow the house battery to drop below 13.2V.
As the batteries get older I expect the 'fully charged' voltage will drop somewhat.