Battery systems for stationary applications

[Landshark]

I have an off-grid cabin that I am just now upgrading to have 3.1 kW solar power and have 20 kWh of battery storage. My total cost was about $6000 CAD.

I will be charging my new Solterra (off-road capable) at 1.9 kW there this summer.

What type of batteries are you using?

Are you in an area that might be subject to wildfire smoke? I’ve seen our solar production reduced by 70% or more on some smoky days, compared to a clear day at the same time of year. Being grid-tied, that normally isn’t an issue. However, in 2020, a fire caused an 8 day power outage and by the 3rd day the smoke prevented the panels from maintaining a charge on the batteries. Had to run a
portable generator during the day to take some loads off the batteries so they could charge. Now we have a 48V charger and a backup generator on the to do list. Something to keep in mind for an off-grid system. Of course, any well designed off-grid set up will have a back up generator.

 

[R P]

What type of batteries are you using?

Are you in an area that might be subject to wildfire smoke? I’ve seen our solar production reduced by 70% or more on some smoky days, compared to a clear day at the same time of year. Being grid-tied, that normally isn’t an issue. However, in 2020, a fire caused an 8 day power outage and by the 3rd day the smoke prevented the panels from maintaining a charge on the batteries. Had to run a
portable generator during the day to take some loads off the batteries so they could charge. Now we have a 48V charger and a backup generator on the to do list. Something to keep in mind for an off-grid system. Of course, any well designed off-grid set up will have a back up generator.

I have 16 GC2 size 6V flooded lead acid batteries (48V system). Can get them here very cheap. Over priced Li Ion batteries are waste of money when weight and space are not an issue. I have never depleted my batteries below 50% and most of the time, not below 80%. Will have to watch it though, with my Solterra charging.

Rarely get smoke here on the coast, so not a factor. Interior of BC gets it sometimes in the summer. Unlike winter (more rain and clouds), summers here get a lot of sunshine. The only time I have used a generator in the past is for some power tools and my log splitter, because of my past undersized inverter. But with my new inverter (4000W peak, 2000 continuous) that is no longer necessary.

And remember this is a cabin, so we don't live there. Although in summer it gets a lot of use with family and friends. It's a great location. I have a floatplane, so fly up most of the time, but sometimes have to take a car, and of course others have to drive. But 1/2 hour flight sure beats a 2 hour drive, incl last part over a very rough FSR. The lake is near sea level at 33 ft.

 

[Jim Matthews]

Solar panels don’t reduce power demands, they have the potential to increase capacity in order to meet demands.

Semantics.

Home user, connected to grid who consumes less power in a year than their array produces equates to a reduction of supply from the power line. This is the bookkeeping behind net metering.

https://news.energysage.com/how-solar-helps-the-grid/

 

[Landshark]

I have 16 GC2 size 6V flooded lead acid batteries (48V system). Can get them here very cheap. Over priced Li Ion batteries are waste of money when weight and space are not an issue. I have never depleted my batteries below 50% and most of the time, not below 80%. Will have to watch it though, with my Solterra charging.

Depending on the application, Lithium batteries may provide electricity at a lower cost per kWh over their lifetime than other battery chemistries. Wet cells are delicate flowers and they are frequently murdered prematurely by their owners.

We’ve had Rolls Surrette wet cell 6V L16’s at the house since 2012 and they appear to still have at least 90% of their original capacity. They aren’t discharged frequently. When the time comes to replace them, the likely choice will be FLA’s, since we aren’t cycling them daily. The need to inspect for corrosion, check water levels and perform occasional equalizations will continue.

When your GC2’s need to be replaced, Rolls L16’s might be something to consider.

 

[Landshark]

Semantics.

Home user, connected to grid who consumes less power in a year than their array produces equates to a reduction of supply from the power line. This is the bookkeeping behind net metering.

https://news.energysage.com/how-solar-helps-the-grid/

Words matter.

Solar panels are another source of generation. Solar production peaks between noon and 2:00pm. Grid demand peaks between 4:00pm and 9:00pm. Solar panels create a surplus of electricity at a time when it isn’t necessary to have an increased supply.

Net metering is a look at energy production and consumption over time, perhaps a month for the typical billing cycle. It isn’t particularly relevant to a discussion about supply and demand at a specific point in time.

 

[R P]

Depending on the application, Lithium batteries may provide electricity at a lower cost per kWh over their lifetime than other battery chemistries. Wet cells are delicate flowers and they are frequently murdered prematurely by their owners.

We’ve had Rolls Surrette wet cell 6V L16’s at the house since 2012 and they appear to still have at least 90% of their original capacity. They aren’t discharged frequently. When the time comes to replace them, the likely choice will be FLA’s, since we aren’t cycling them daily. The need to inspect for corrosion, check water levels and perform occasional equalizations will continue.

When your GC2’s need to be replaced, Rolls L16’s might be something to consider.

That's very true. We know several people at the lake, incl one of our neighbours who went through a couple sets of L16s in 5 years because they severely mistreated them. They would run them dead periodically, and also overcharge them. Their systems had older charge controllers (but expensive) that were professionally installed (also very expensive). They were told to periodically boil the batteries (equalizations). However they did it too long and at high current. I looked at their batteries, and they were bulging at the sides, totally ruined.

My controller is cheap, but works great. It does a 1 hour auto equalization once every 30 days, at higher voltage but very low current.

I don't expect to have to replace my GC2s for a long time yet. And they are way, way cheaper than equivalent capacity L16s. There is a local battery guy here that picks up surplus stale inventory from stores and distributors (they write them off after a certain time) and sells them as used. But each one is tested and only sold if they meet the new battery spec. The price is about 1/4 of a new one.

 

[Landshark]

We know several people at the lake, incl one of our neighbours who went through a couple sets of L16s in 5 years because they severely mistreated them.

Not all batteries are the same. Entegra makes some high end motorhomes, yet they equipped many of them with Discovery L16’s which had a knack for failing at about 2 years of age, despite owners keeping the coaches plugged in and rarely using the batteries.

My controller is cheap, but works great. It does a 1 hour auto equalization once every 30 days, at higher voltage but very low current.

It’s odd that your neighbor’s equalization protocol was not set up appropriately. Perhaps they performed the procedure with low water levels? In your case, it may not be necessary to equalize every 30 days. The process is a necessary evil. Necessary to maintain the battery, while at the same time eating away at the plates. If the batteries aren’t going through charge cycles for several months, there’s almost no reason to put them through an equalization on a monthly basis.

I don't expect to have to replace my GC2s for a long time yet. And they are way, way cheaper than equivalent capacity L16s. There is a local battery guy here that picks up surplus stale inventory from stores and distributors (they write them off after a certain time) and sells them as used. But each one is tested and only sold if they meet the new battery spec. The price is about 1/4 of a new one.

That price is tough to beat. Even if they are not the best quality battery, it’s worth the risk of having to replace them more frequently. It’s just as easy to kill an expensive FLA as it is to kill a cheap one.

 

[R P]

Not all batteries are the same. Entegra makes some high end motorhomes, yet they equipped many of them with Discovery L16’s which had a knack for failing at about 2 years of age, despite owners keeping the coaches plugged in and rarely using the batteries.



It’s odd that your neighbor’s equalization protocol was not set up appropriately. Perhaps they performed the procedure with low water levels? In your case, it may not be necessary to equalize every 30 days. The process is a necessary evil. Necessary to maintain the battery, while at the same time eating away at the plates. If the batteries aren’t going through charge cycles for several months, there’s almost no reason to put them through an equalization on a monthly basis.



That price is tough to beat. Even if they are not the best quality battery, it’s worth the risk of having to replace them more frequently. It’s just as easy to kill an expensive FLA as it is to kill a cheap one.

The default is 30 days, and from reading the forums, that is pretty normal. Some go 2 hours, but mine is only 1 hour. What is most important is very low current (amps/watts) at the higher voltage. I have been doing that for several years with this charge controller, and batteries are all still good. And yes, need to keep an eye on water levels.

The neighbours were old (finally sold their cabin last year) and had a hard time comprehending how to look after the batteries. I gave up trying to help them. The equalization process was not automated with their controller. And their solar guy was no help with his instructions and advice. I think he probably figured if they wreck their batteries, well, more business for him. They did have a back-up generator, and had to use it a lot.

 

[Landshark]

The default is 30 days, and from reading the forums, that is pretty normal.

Normal may not be the best choice for every application. If normal is cycling the batteries on a daily basis, equalizing every 30 days might be an appropriate schedule.

As an example, our batteries most sit fully charged, I equalize once or twice a year. After 11 years, the plates look like new and the capacity is still near what is was when they were new.

 

[R P]

Normal may not be the best choice for every application. If normal is cycling the batteries on a daily basis, equalizing every 30 days might be an appropriate schedule.

As an example, our batteries most sit fully charged, I equalize once or twice a year. After 11 years, the plates look like new and the capacity is still near what is was when they were new.

Not sure what you mean by cycling on a daily basis (what %). We or someone is usually using the cabin about twice a week for a day or two each time. In summer it is used almost continually by someone. In winter we go up there maybe once a month. Most of the time the batteries don't go below 90% (as per first thing in the morning). Often not below 95%. Occasionally, we might have darker clouds for a few days (white clouds still charge up). and it could drop to 70, but not very often. If we used it in winter, would be a different matter.

Every time we go up our batteries are sitting at 55.4V (float charge for 48V system), before we turn on the inverter. The controller does keep history, so I should check when and how often the batteries were equalized.

So based on that type of usage and cycling, what do you recommend? The setting are easy enough to change, so could have different intervals depending on time of year and usage.

 

[Landshark]

Not sure what you mean by cycling on a daily basis (what %).

A daily discharge and charge cycle. There isn’t necessarily a specified amount of discharge to qualify for a cycle. This is why I mentioned that “normal” doesn’t work for every situation. Keep in mind that fully charging a FLA, even from 90% can take many hours.

Every time we go up our batteries are sitting at 55.4V (float charge for 48V system), before we turn on the inverter.

Are you using a temperature sensor on the batteries? 55.4V seems a bit high unless the batteries are near freezing. Float voltage should change with temperature.

So based on that type of usage and cycling, what do you recommend? The setting are easy enough to change, so could have different intervals depending on time of year and usage.

I’m hesitant to give a recommendation, other than to follow the battery manufacturers recommendation. That said, your shallow discharge cycles might allow you to equalize less frequently in the summer, perhaps every 2-3 months, and possibly every 4-6 months over the winter.

 

[R P]

A daily discharge and charge cycle. There isn’t necessarily a specified amount of discharge to qualify for a cycle. This is why I mentioned that “normal” doesn’t work for every situation. Keep in mind that fully charging a FLA, even from 90% can take many hours.



Are you using a temperature sensor on the batteries? 55.4V seems a bit high unless the batteries are near freezing. Float voltage should change with temperature.



I’m hesitant to give a recommendation, other than to follow the battery manufacturers recommendation. That said, your shallow discharge cycles might allow you to equalize less frequently in the summer, perhaps every 2-3 months, and possibly every 4-6 months over the winter.

Yes, have a temp sensor on the batteries, well just between two of them. Can't do it on all 16 of them. 55.4V is the equivalent of 13.8V which is a normal float charge for 12v batteries.
Below is from the manual for FLA:
Constant voltage Floating voltage
14.6V 13.8V


a) Constant Current Charging_CC(Bulk Charging)
In this stage, the battery voltage has not yet reached constant voltage (Constant or Boost Voltage), the controller operates in constant current mode, delivering its maximum current to the batteries (MPPT Charging).
b) Constant Voltage Charging_CV( Constant and Boost Charging)
When the battery voltage reaches the constant voltage set point, the controller will start to operate in constant voltage charging mode, this process the charging current will drop gradually. The constant charge voltage will increase 0.2V on the basis of constant voltage at 1st of each month, charge time is 60 mins. ( The data of boost charge voltage can be set via PC software and APP)
c) Floating Charging_CF
After the constant voltage stage, the controller will reduce charging current to maintaining the battery voltage on the Floating Voltage set point. Charging the battery with a smaller current and voltage on Floating Voltage stage, while maintaining full battery storage capacity.
In Floating charging stage, loads are able to obtain almost all power from solar panel.
If loads exceed the power, the controller will no longer be able to maintain battery
voltage in Floating charging stage. If the battery voltage remains below the Recharge
Voltage, the system will leave Floating charging stage and return to Bulk charging
stage.

 

[Landshark]

Yes, have a temp sensor on the batteries, well just between two of them. Can't do it on all 16 of them. 55.4V is the equivalent of 13.8V which is a normal float charge for 12v batteries.
Below is from the manual for FLA:
Constant voltage Floating voltage
14.6V 13.8V

One temperature sensor is sufficient.

From what FLA manual did you obtain the float voltage figures quoted above?

Here are 2 screenshots, 1 from Trojan, the other from Rolls. Note the standard temperature of 25C/77F. Both manufacturers show a float voltage 13.5V at that temperature. That temperature is not “normal” unless the batteries are stored in a climate controlled enclosure.

I’d encourage you to examine the charging protocol literature provided by the manufacturer of the batteries that you have installed. I’d also be interested in seeing those documents.

 

[R P]

The charge numbers are from my charge controller manual. And I do know that some battery specs say 13.5 for a float charge. When I see my charger voltage, when there is zero load, it sits at 13.8 (12v mode), but when I shut off the PV panels it immediately drops to 13.6 or lower, and holds at 13.2.

At home in the past, I have charged up some 6Vs and here is what I found. I have 2 chargers. One had a float charge of 13.8 while the other was 13.5 (6Vs were in series for charging). The 13.8 one would fully charge the battery to 6.36v (after surface charge removed) and hold there. The 13.5 would finish early and the holding charge was only 6.30 or 6.31. So at least for these batteries (Interstate), they liked the higher float charge one better.

While I could lower my float charge setting slightly, not sure if that is necessary. I do want a fully charged battery. And have not seen any excessive water boiled off. I add a bit of water about every 2 years.

Here is an excerpt from a deep cycle battery charging guide. They say that 13.8 is OK, although an upper limit. I could change mine to 13.7 or 13.6, but not sure if that is necessary as have been doing it this way for years, and batteries are still good. And I don't want undercharged batteries.
https://deepcyclebatterystore.com/the-battery-blog/how-to-charge-12-volt-6-volt-batteries/
Sustained charging, where the batteries are “floated” at a constant charge (as in the RV converter or with an automatic portable charger) should not be done at more than 13.8 volts (and 13.65 makes batteries last longer).

Here is another table from a charger manual, that my local solar store sells. It's specs are pretty well identical to mine, also with 13.8v for FLA. And I think pretty well all the charge controllers I have seen are this way.

 

[Landshark]

Here is another table from a charger manual, that my local solar store sells. Its specs are pretty well identical to mine, also with 13.8v for FLA. And I think pretty well all the charge controllers I have seen are this way.

My preference is to follow the guidance from the battery manufacturer rather than use the default settings of a charger. It eliminates the guesswork and increases the likelihood that best practices are being implemented.

 

[R P]

My preference is to follow the guidance from the battery manufacturer rather than use the default settings of a charger. It eliminates the guesswork and increases the likelihood that best practices are being implemented.

What does your charger manual specify for a float charge? And what do you see on your display panel when in float mode?

 

[Landshark]

What does your charger manual specify for a float charge? And what do you see on your display panel when in float mode?

I doubt that the charger manual “specifies” a float voltage. It likely has a “default” setting. As far as I’m concerned, the default setting is irrelevant unless it happens to be what the battery manufacturer specifies. My solar installer set the parameters. At present, it is showing 52.5V-52.6V.

 

[R P]

That's about what mine shows when there is a load, ie inverter is turned on and usage is happening in the cabin and with Starlink. When everything is off, but PVs on, it show 55.4. Be interesting to see what yours show when all load is turned off (ie inverter off) and PVs still on, but with a full battery.

 

[Landshark]

Be interesting to see what yours show when all load is turned off (ie inverter off) and PVs still on, but with a full battery.

Being a grid-tied system with battery back up, there is no load on the battery unless there is a loss of grid power. The inverter has AC input from the grid and DC input from the PV array. It energizes the essential loads sub-panel via, solar, grid or a combination of the two. With fully charged batteries, the battery shunt typically shows a very low current from the panels of ~0.2-2.0A and a voltage of 52.5-52.6V. This would indicate a Float charging voltage of 13.1-13.2V per 12V cell.

 

[R P]

Being a grid-tied system with battery back up, there is no load on the battery unless there is a loss of grid power. The inverter has AC input from the grid and DC input from the PV array. It energizes the essential loads sub-panel via, solar, grid or a combination of the two. With fully charged batteries, the battery shunt typically shows a very low current from the panels of ~0.2-2.0A and a voltage of 52.5-52.6V. This would indicate a Float charging voltage of 13.1-13.2V per 12V cell.

Can you not shut down your grid tie-in and shut off your inverter leaving only the PVs on charging the battery?