sizing solar for battery charging

[retsparkeedave]

what is the recommended solar panel size to keep a 12v 18 ah battery charged to support 2 10watt led lamps for 12 hours? Would the 12v 18 ah battery be large enough or should 2 12v 18 ah batteries in parallel be used?

 

[ggunn]

what is the recommended solar panel size to keep a 12v 18 ah battery charged to support 2 10watt led lamps for 12 hours? Would the 12v 18 ah battery be large enough or should 2 12v 18 ah batteries in parallel be used?

What kind of battery? Assuming your lights are 12V, your need is for 20Ah of available battery capacity, but if it's lead acid you'll need twice that since you do not want to draw the battery down more than 50%. This sounds like it might be a dusk to dawn outdoor sign lighting project.

 

[retsparkeedave]

Yes, it's for dusk to dawn outdoor lighting.
Thank you!

 

[ggunn]

Yes, it's for dusk to dawn outdoor lighting.
Thank you!

Then you definitely will not want to use lead acid batteries. Are you designing this from scratch or are you going to take advantage of any of the off-the-shelf options? Amazon and Morningstar have some.

 

[jaggedben]

The solar part really depends how critical the application is.
Like, how many days in December are you willing for the lights to not stay lit all the way until dawn?

Also it depends on the location, azimuth, angle, shading, and soiling of the solar panel.

You are saying you need ~240 watt hours per day, i.e 7.44 kWh for 31 days in December. I would use PV Watts to find the solar panel size to get that bare minimum as an average for December. For perspective, in my area with ideal exposure that would be about a 100W panel. But I would then multiply both panels and battery capacity by some factors to account for how reliable you want this thing to be. Say, double the panels and do enough battery for 5 days (accounting for acceptable depth-of-discharge as ggunn is pointing out), if that's what you need. There will be days without enough sun.

A single 12V 18Ah battery is nominally only 214 watt hours so that definitely isn't enough for your application even if it's a chemistry with a better depth-of-discharge than lead acid. Then if you want multi-day reliability you should be multiplying your battery capacity as mentioned above.

Also regardless of battery chemistry, if you don't want to damage the batteries you need some kind of electronics to shut the load off when battery charge drops too low. Hence using an off-the shelf solution might be better.

 

[jaggedben]

My gut feeling is that two 100W solar panels will do you fine but that you want a lot more battery capacity. Unless it's just fine if it doesn't work very long after a cloudy day.

 

[ggunn]

My gut feeling is that two 100W solar panels will do you fine but that you want a lot more battery capacity. Unless it's just fine if it doesn't work very long after a cloudy day.

Design for more than the minimum. Hope for the best, expect the worst. Extra capacity on the front end is cheaper than fixing it later.

 

[topgone]

The solar part really depends how critical the application is.
Like, how many days in December are you willing for the lights to not stay lit all the way until dawn?

Also it depends on the location, azimuth, angle, shading, and soiling of the solar panel.

You are saying you need ~240 watt hours per day, i.e 7.44 kWh for 31 days in December. I would use PV Watts to find the solar panel size to get that bare minimum as an average for December. For perspective, in my area with ideal exposure that would be about a 100W panel. But I would then multiply both panels and battery capacity by some factors to account for how reliable you want this thing to be. Say, double the panels and do enough battery for 5 days (accounting for acceptable depth-of-discharge as ggunn is pointing out), if that's what you need. There will be days without enough sun.

A single 12V 18Ah battery is nominally only 214 watt hours so that definitely isn't enough for your application even if it's a chemistry with a better depth-of-discharge than lead acid. Then if you want multi-day reliability you should be multiplying your battery capacity as mentioned above.

Also regardless of battery chemistry, if you don't want to damage the batteries you need some kind of electronics to shut the load off when battery charge drops too low. Hence using an off-the shelf solution might be better.

There is a specific limit as to how much one can squeeze out of a certain type of battery.
As a rule of thumb, the depth of discharge allowed for lead-acid batteries is 50% (e.g. for a 200AH battery, limit energy draw to just 100AH). LiFePO4 batteries can be discharged up to 80 to 90% of its capacity.

 

[Flicker Index]

what is the recommended solar panel size to keep a 12v 18 ah battery charged to support 2 10watt led lamps for 12 hours? Would the 12v 18 ah battery be large enough or should 2 12v 18 ah batteries in parallel be used?

There is a specific limit as to how much one can squeeze out of a certain type of battery.
As a rule of thumb, the depth of discharge allowed for lead-acid batteries is 50% (e.g. for a 200AH battery, limit energy draw to just 100AH). LiFePO4 batteries can be discharged up to 80 to 90% of its capacity.

The relevance of this depends on battery's capacity vs the solar output but it's worth noting that charging slows down considerably in the top 20-25% of battery capacity. Sizing the battery array such that they don't charge beyond 80% during ordinary use will maximize energy capture. That's the case with phones and EVs too. Once you hit 75-80%, the miles of range or minutes of use you gain per minute of charging starts falling down. This characteristic is shared between lead acid and lithium.

Sizing the battery such that it's always ready to absorb whatever the panel is putting out maximizes panel utilization. Normally, the pack is charged in constant-current mode until a set voltage is reached, then the power input sent to the battery is reduced to maintain the same voltage. The best panel utilization is achieved when MPPT is used to push power into battery by MPCV (maximum wattage, then constant voltage) charging. MP means always send whatever power the panel is capable of producing at every opportunity. If the battery hits constant-voltage stage, then the input power has to be cut back.

So, if the battery is sized such that the panel will never push the battery bank to constant voltage phase, it maximizes the power captured. If the panels can make the battery reach CV phase on a regular basis, the panels are not being utilized to their full potential. This is something more feasible with lithium chemistry as constantly operating between 20-80% is just fine while lead acid needs to hit maximum state of charge to maintain good life.

 

[ggunn]

There is a specific limit as to how much one can squeeze out of a certain type of battery.
As a rule of thumb, the depth of discharge allowed for lead-acid batteries is 50% (e.g. for a 200AH battery, limit energy draw to just 100AH). LiFePO4 batteries can be discharged up to 80 to 90% of its capacity.

Another factor in the depth of discharge tolerance is the number/frequency of discharge cycles; the more and deeper discharges a battery experiences the more quickly it becomes non viable. In this case the battery will be significantly discharged every night.

 

[topgone]

The relevance of this depends on battery's capacity vs the solar output but it's worth noting that charging slows down considerably in the top 20-25% of battery capacity. Sizing the battery array such that they don't charge beyond 80% during ordinary use will maximize energy capture. That's the case with phones and EVs too. Once you hit 75-80%, the miles of range or minutes of use you gain per minute of charging starts falling down. This characteristic is shared between lead acid and lithium.

Sizing the battery such that it's always ready to absorb whatever the panel is putting out maximizes panel utilization. Normally, the pack is charged in constant-current mode until a set voltage is reached, then the power input sent to the battery is reduced to maintain the same voltage. The best panel utilization is achieved when MPPT is used to push power into battery by MPCV (maximum wattage, then constant voltage) charging. MP means always send whatever power the panel is capable of producing at every opportunity. If the battery hits constant-voltage stage, then the input power has to be cut back.

So, if the battery is sized such that the panel will never push the battery bank to constant voltage phase, it maximizes the power captured. If the panels can make the battery reach CV phase on a regular basis, the panels are not being utilized to their full potential. This is something more feasible with lithium chemistry as constantly operating between 20-80% is just fine while lead acid needs to hit maximum state of charge to maintain good life.

Slow charging during the upper 20% of the battery capacity is the battery's charging characteristics. Because the internal voltage of the battery has risen to a higher level, the charging current tapers when the applied voltage is the same. The result is a longer charging time as he charging amp is reduced. Charging amps = (charger voltage - internal battery voltage)/battery internal resistance.
In modern chargers, the charging mode starts with "constant-current mode" (charging amps set equal to the rated AH divided by the rated discharge hours, 8 or 10 hours). The charging mode shifts to "trickle-charge" or "float charge" once the internal voltage approaches the rated voltage --> meaning, the charger monitors the battery voltage instead of forcing a set charging current. This is done to do away with battery cells getting hot or lead-acid cells gassing up/ losing water in the process or worse, shorten the battery life.

 

[topgone]

Another factor in the depth of discharge tolerance is the number/frequency of discharge cycles; the more and deeper discharges a battery experiences the more quickly it becomes non viable. In this case the battery will be significantly discharged every night.

Yep. Batteries have definite charging cycles available in them. Even if you have batteries that were never abused, they will conk out in time because it's beyond its charging cycle times. Stationary batteries can last 20 years as the battery maintenance procedures are followed to the letter. Lead acid batteries can last about 1000 charging cycles (3 years+, @1 cycle per day) while a LI-ion battery lives until about 3000 cycles (8 years).

 

[jaggedben]

...while a LI-ion battery lives until about 3000 cycles (8 years).

Depends on the lithium technology. We have companies warrantying LFP for 15 years or 6000 cycles, sometimes more cycles (but that's pretty meaningless in a solar application with typically less than 1 cycle per day). Of course they have their pretty advanced BMS on it.

LFP is longer lasting than NMC (all else being more or less equal). At least that seems to be what many are banking on.

 

[ggunn]

Depends on the lithium technology. We have companies warrantying LFP for 15 years or 6000 cycles, sometimes more cycles (but that's pretty meaningless in a solar application with typically less than 1 cycle per day).

In the OP's application the batteries will have a deep discharge every night.

 

[jaggedben]

In the OP's application the batteries will have a deep discharge every night.

Depends how much extra capacity he provides.

 

[ggunn]

Depends how much extra capacity he provides.

Well, yes, of course; if you look at the OP, you will see that initially, anyway, he was planning on a 100% discharge every night, but I think that he has walked back from that a bit.