Battery question

[greeny]

I'm going to fit a second battery to my Sun 2000. It will be solely used for supply to "secondary domestic loads" such as laptop and a couple of extra led lights to supplement the existing lighting onboard which is minimal. It will be completely separate from the main battery. No connection at all between them. It will be charged solely by solar panel. I have no other means of charging . It will be used in the evening mainly so will deplete overnight and be topped up in the day by the panel. Pretty much guaranteed sunshine in the summer months here in Portugal,
What is the forum wisdom on percentage discharge before the battery is "damaged" given it will be a standard lead acid or gel that I fit. Nothing fancy or expensive.
Then I can work out estimated loads and hence battery size and solar panel size. Some people around me seem to think 50% maximum discharge is ok but that seems optimistic to me.

 

[CapPugwash]

Ah, you are approaching this, as I did, expecting it to be a simple topic.

It turns out voltage isn't a great measurement of State of Charge. Here is a useful article that explains it. Under Load Battery Voltage vs SoC.

My advice would be to wire them as one bank, but to answer your question, my understanding is 50% ........ if you can get an empirical measurement

 

[VicS]

I'm going to fit a second battery to my Sun 2000. It will be solely used for supply to "secondary domestic loads" such as laptop and a couple of extra led lights to supplement the existing lighting onboard which is minimal. It will be completely separate from the main battery. No connection at all between them. It will be charged solely by solar panel. I have no other means of charging . It will be used in the evening mainly so will deplete overnight and be topped up in the day by the panel. Pretty much guaranteed sunshine in the summer months here in Portugal,
What is the forum wisdom on percentage discharge before the battery is "damaged" given it will be a standard lead acid or gel that I fit. Nothing fancy or expensive.
Then I can work out estimated loads and hence battery size and solar panel size. Some people around me seem to think 50% maximum discharge is ok but that seems optimistic to me.

50% is the commonly accepted figure but the smaller the depth of discharge the better. It's not a hard and fast rule.
A leisure battery will probably cope with your pattern of use better than an ordinary starter battery but the 50% guide still applies.
You could of course splash the cash on a genuine deep discharge battery or consider going for a lithium battery.

The open circuit voltage of a 50% discharged lead acid battery which has rested with no load or discharge for 24 hours will be about 12.2 .

 

[Refueler]

These figs are what I keep to and have proven good :

Wet flooded Lead Acid >
Standard car starter battery : use only 30 - 40% of rated capacity
Leisure battery - compromise version that can start engine / allow deeper discharge : use only 40 - 50% of rated capacity
True leisure battery that is not designed to start engine and rated as deep discharge : use only 50 - 55% of rated capacity

Lead acid batterys will recover when over discharged and not left in that state - but damage is accumulative.

For this reason - I advocate finding a local scrap car yard and buying second hand car battery ... scrappers usually have a load tester - so you know battery is good ...
I've had such batterys last many years on my boats.

 

[greeny]

Thanks guys. Seems like if I size everything to keep discharge to maximum of 30/40 % I should be in the ball park. The reason I'm not making the system any more complex by combining the batteries etc is that she is mainly a day sailor with an existing battery for simple electronics that is charged by a solar panel daily as required through a controller. This seems to work well with a 85 Ah battery and 50w panel. There is no engine start needed as it is a 4hp outboard. I don't want to unbalance this system by using laptop, occasional extra lighting and maybe a small freezer box and then find the load is causing too deep a daily discharge. A completely separate battery and panel won't be expensive and will give me security in that the old system remains untouched, and also the ability to link across the batteries using temporary leads should I ever need to supplement the main battery capacity.
Another quick observation. The existing 50w panel is charging the battery at approx a maximum (from what I've seen) of about 2 amps. 50W divided by 13 volts gives 4 amps so why the big difference. I know the panel has to be at more than 13 volts or so to push into a battery at 13 volt but it still seems to be a big difference between 2 amps and 4 amps so want to get the sizing right.
When I look at the specs of new panels for sale, they all seem to reflect this, in that 100 W panels are only stating 4/5 amp charge rates. So this means I will have to go for a new panel twice the size of my calculation on battery usage. i.e. if I need circa 4 amps maximum I will need a 100w panel. I've had solar on my boats for many years but always as a supplement to engine charging and marina mains charging, now I'm going to be totally dependent on it and need to get the balance right. Physical size and weight is also important as the sun 2000 is only 20 ft so the smaller the better.

 

[V1701]

If you haven't already I think I would see whether what you already have can cope with the additional load before anything else. If not add another battery & see if the existing panel can keep up, if not add another panel or swap existing panel for a larger one. I can see the appeal of having two separate systems but it's not really necessary IMHO...

FWIW when I had a boat in Greece one 80W panel kept 220AH worth of batts charged, switching the fridge off overnight was the only concession I had to make...

 

[KompetentKrew]

If you look at the data sheet for Trojan's T-105, they list the battery's estimated lifetime in cycles based on discharge to 80%, 50% etc.

I'm sure they do the same for some of their other battery models, although I expect them to be mostly similar.

I daresay you could put these numbers into Excel and calculate the cost per watt based on each depth of discharge.

 

[Birdseye]

Another quick observation. The existing 50w panel is charging the battery at approx a maximum (from what I've seen) of about 2 amps. 50W divided by 13 volts gives 4 amps so why the big difference. I know the panel has to be at more than 13 volts or so to push into a battery at 13 volt but it still seems ton it so want to get the sizing right.o be a big difference between 2 amps and 4 amps.

The current flow into the battery depends on the difference in voltage between the panel and the battery. It also depends on the circuit resistance. So as the battery becomes more charged and its voltage increases, the difference reduces and so therefore does the current. You have exactly the same effect with alternators - it might be rated at 60 amps but you will never ever get that much flowing into the battery and as the latter gets near full the current can drop to 5 amps.

The panel rating will be measured by connecting it to a known resistor which unlike a battery doesnt have its own voltage which is riding all the time the current is flowing. Also it will be measured under absolutely the best possible conditions.

 

[PetiteFleur]

Slightly off topic - but have considered a PWM "dual battery controller? " Photonic Universe supply them, you can set it to charge at various percentages to each battery and when one is charged all the charge goes to the second battery.
Be careful as some "dual battery controllers" only give a trickle charge to the second battery.

 

[greeny]

Good info and good ideas. All food for thought. Thanks guys.

 

[William_H]

Regarding solar panel current. Solar panels are deigned to charge a battery (via PWM controller which is in effect a direct connection panel to battery. The panle must at all times provide a voltage above that of the battery. ie 14v To ensure more than 14v at lower light levels they add more solar panel cells to give around 18 to 20v no load.
This means that typically with lots of sun the difference 14v to 20v is wasted in the panel itself. But the max current remains the same. The wattage rating of the panel is calculated at max current by max voltage. Hence OP disappointment is valid but normal.
In reality his choice of size of solar panel if he goes an additional panel will be totally dependent on where he can mount it. (how much room he has) I think he will be pretty limited for a reasonable boat layout. But in any case buy as big a panel as manageable. The panel size will then dictate the total current he can use from this system over 24 hrs to relate to power he can get in day time from panel(s).
The battery should be deep cycle type (SLA) with a capacity at least twice the daily expected demand.
But in reality trim the demand to suit the supply.
In the end probably just use the existing battery and panel. ol'will

 

[greeny]

Thanks for that William. The boat is on a mooring and normally just day sailed. The panel is only deployed when moored not when sailing. In my situation the existing battery will supply and the panel recharge it as the boat has "days off" during the week which allows it to get fully topped up. I'm thinking the extra backup is needed for the occasions I am away sailing and onboard for a few days when the panel can't be used all day and will only get a bit of use each day after anchoring. Maybe 2 /3 days sailing then a couple of days on anchor with the panels deployed all day will top the batteries right up again. I just felt that I needed a bit more buffer in battery capacity. You are correct in that panel size is one of the main limiting factors in this hence my disappointment in the panel specifications of charge.

 

[simonfraser]

with a victron shunt you can see how much goes in / out
with a victron solar controller you can see how much the panel has generated and wasted, e.g. how much spare charge it had
in my case i can put it all back into the 2 AGMs in two days, when the sun is shining

 

[Refueler]

If you look at the data sheet for Trojan's T-105, they list the battery's estimated lifetime in cycles based on discharge to 80%, 50% etc.

I'm sure they do the same for some of their other battery models, although I expect them to be mostly similar.

I daresay you could put these numbers into Excel and calculate the cost per watt based on each depth of discharge.

Take care ..... the % quote has to be stated as to what it actually is ... capacity remaining - or capacity used ...............

Problem is too many people AND documentation fail to specify which !

As example - if you discharge repeatedly a Lead Acid by USING 80% of its capacity - meaning only 20% remaining ........... then you are not going to have long life out of that battery ! Wont be long before a plate buckles.

 

[Refueler]

]quote]
Slightly off topic - but have considered a PWM "dual battery controller? " Photonic Universe supply them, you can set it to charge at various percentages to each battery and when one is charged all the charge goes to the second battery.
Be careful as some "dual battery controllers" only give a trickle charge to the second battery. [/'quyote]

There are plenty of dual solid state charge controllers out there for small money - that are literally fit and forget. The RV - caravan market is the creator ...

I have a small controller that I got from Maplins years ago ... it automatically senses each battery and sends charge as required ... it has no voltage drop like diodes. It cost about a tenner .... Any steady charge source of up to 10A into it ... so a 50W or more Solar is fine.

 

[Refueler]

The battery should be deep cycle type (SLA) ...

SLA are not all deep cycle ... in fact apart from the case design and terminals - are in fact usually same limitations as any other Lead Acid - but sealed.

Deep discharge a typical SLA a few times and you will be replacing it !! Take the example of House Alarms - they usually have an SLA in them ... on constant charge.

 

[KompetentKrew]

Take care ..... the % quote has to be stated as to what it actually is ... capacity remaining - or capacity used ...............

Problem is too many people AND documentation fail to specify which !

As example - if you discharge repeatedly a Lead Acid by USING 80% of its capacity - meaning only 20% remaining ........... then you are not going to have long life out of that battery ! Wont be long before a plate buckles.

It's pretty obvious from Trojan's documentation:

This is from their T-105RE model, not the standard "Motive" T-105. The 105RE is marketed as "Smart Carbon for improved performance - 8 years battery life based on IEC 61427" but Plymouth Battery Centre are showing it as cheaper than the standard 105, so I don't know what's up with that. It looks like they're no longer publishing this information for the standard 105's - I'm sure they used to.

 

[Stemar]

My only indication of the state of charge on Jissel was a voltmeter. I worked on the basis that with no load, 12.7v was fully charged. With a load, this would drop, but if the voltage went below 12.0, it was time to reduce the load. I tried to avoid going below 12.2V with no load. I won't pretend it was perfect, but it worked and my domestic battery seemed to last as well as I could expect.

 

[Refueler]

It's pretty obvious from Trojan's documentation:

This is from their T-105RE model, not the standard "Motive" T-105. The 105RE is marketed as "Smart Carbon for improved performance - 8 years battery life based on IEC 61427" but Plymouth Battery Centre are showing it as cheaper than the standard 105, so I don't know what's up with that. It looks like they're no longer publishing this information for the standard 105's - I'm sure they used to.

Well ... all I can say to that ..... Not for me ............ OK ? If you want to trust the graph of 100% discharge and ~750 charge cycles ............ then I wish all the luck ...

What I do find interesting is the 40% level which indicates I may not be so wrong in my earlier statement ..............

And anyway - the physics of Lead Acid Battery's cannot be played with ... basically avoid approaching 50% discharged ............... regardless of whether its starter or leisure ..............