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diy solar

Can anyone assist with a calculation?

medic5678

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How much can I expect from a bank of six 6 volt lead acid golf cart batteries if they are fully charged? (expressed in terms of watts and hours). They're configured for 12 volts. How far down can I draw them? I have these batteries and am trying to use them for backing up my property wide security system. How far down can I draw them?

I have decided to look for more stand alone uses for my solar panels, without batteries. My goals with solar were grossly unrealistic, so I'm making the best of it. Thanks to all who reply :).
 
How much can I expect from a bank of six 6 volt lead acid golf cart batteries if they are fully charged? (expressed in terms of watts and hours). They're configured for 12 volts. How far down can I draw them? I have these batteries and am trying to use them for backing up my property wide security system. How far down can I draw them?

I have decided to look for more stand alone uses for my solar panels, without batteries. My goals with solar were grossly unrealistic, so I'm making the best of it. Thanks to all who reply :).
Generally, LA batteries should not be discharged below 50%, so whatever the Ah rating written on the batteries is, multiply by 3 to get Ah and multiply Ah by 6V to get Wh.

For example, if you have 230Ah cells like these: https://www.batteriesplus.com/produ...MI1tGZ7dOR9AIVXhitBh3YpQY5EAQYAyABEgJ9H_D_BwE

That would translate to ~690Ah or 4140Wh of usable capacity…
 
Guys, are we talking 4347 watt hours, so that if I were drawing 100 watts continuously, then the batteries would last about 43.47 hours (theoretically).?
 
Guys, are we talking 4347 watt hours, so that if I were drawing 100 watts continuously, then the batteries would last about 43.47 hours (theoretically).?
Theoretically, yes. Practically though it's going to depend on the specifications of the actual batteries (above was just an example of the calculation), their condition and what state of charge they have to begin with.
 
Guys, are we talking 4347 watt hours, so that if I were drawing 100 watts continuously, then the batteries would last about 43.47 hours (theoretically).?
You probably need to knock of at least 10% of that for the ~90% efficiency of your inverter (4347Wh in = ~3812Wh out)…
 
Would I need an inverter if I used 12 volts dc for all items? We are talking very low wattage on any one item. Also wondered what sort of wire I'd have to use. Low voltage DC, would imagine you'd have to use pretty thick wire to avoid voltage drop. I am sure I'll have to knock off a considerable % in efficiency due to, among other things, my general ineptness. But even if I could get 60 watts for 24 hours, that would be *HUGE* for my goals. .
 
You probably need to knock of at least 10% of that for the ~90% efficiency of your inverter (4347Wh in = ~3812Wh out)…
Plus puekert Exponent...

Which may increase the available power of lead if less than rated current or decrease it (effectively) if higher.
 
Battery capacity depends on temperature the colder it gets below 20c the less ah's are available, also in colder conditions you will have less charging from the solar array.
UPS systems general operate via grid charging to maintain battery bank float charge so it has a fully charged battery bank when required.
Solar UPS systems need to maintain 20 to 30 percent SOD (state of discharge) or battery bank life span will be reduced dramatically.
Over sizing the system is recommended because solar systems are only efficient during the summer months in the UK.
If you calculate power required and battery bank capacity you will need to times that by 5 because solar charging in the winter months will be no more than 20 percent that in the summer months.
 
would imagine you'd have to use pretty thick wire to avoid voltage drop. I am sure I'll have to knock off a considerable % in efficiency due to, among other things, my general ineptness. But even if I could get 60 watts for 24 hours, that would be *HUGE* for my goals. .
60w/12v = 5 amps

current carrying capacity of wire is rated based on its size (cross sectional area) any wire size increase required for lenght of run is based on allowable voltage drop over that distance. The lenght thing usually only becomes significant for distances measured in multipul tens of feet. (like 50' and up) so short runs (stuff under several feet) its not considered important. only current carrying capacity.
 
Battery capacity depends on temperature the colder it gets below 20c the less ah's are available, also in colder conditions you will have less charging from the solar array.

Do you have any source of data on that? Winter temps in my basement where the LiFePO4 battery are are generally 15-18C and I’m interested in any data to show how much I should derate capacity for the cooler temps.

Also, does cycling a battery at 20C result in shorter cycle life or longer cycle life than cycling at 25C?

UPS systems general operate via grid charging to maintain battery bank float charge so it has a fully charged battery bank when required.
Solar UPS systems need to maintain 20 to 30 percent SOD (state of discharge) or battery bank life span will be reduced dramatically.
Over sizing the system is recommended because solar systems are only efficient during the summer months in the UK.
If you calculate power required and battery bank capacity you will need to times that by 5 because solar charging in the winter months will be no more than 20 percent that in the summer months.
Uhh, perhaps in the UK, but not in California. My winter production in December and January is more than 42% of my Summer production in June and July…
 
Found this article including some data on LiFePO4 capacity change with temperature: https://www.sciencedirect.com/science/article/pii/S1002007118307536

ED5E3989-2843-472D-B180-465C67F52B7C.jpeg

Between 25C and 35C there is a capacity increase of ~9% and from 35C to 45C there is a further capacity increase of ~10%, so it’s probably realistic to assume 8-9% capacity decrease between 25C where the cells are rated and 15C (59F).

So a fully-to-spec 280Ah battery probably only delivers closer to 255Ah if you cycle it ‘cool’ (60F rather than 77F).

That same data also suggests that the cycle degradation at lower temps is less than at higher temps (but that may be largely explained by the fact that they are cycling less charge at lower temps because of the reduced capacity).
 
Do you have any source of data on that? Winter temps in my basement where the LiFePO4 battery are are generally 15-18C and I’m interested in any data to show how much I should derate capacity for the cooler temps.

Also, does cycling a battery at 20C result in shorter cycle life or longer cycle life than cycling at 25C?


Uhh, perhaps in the UK, but not in California. My winter production in December and January is more than 42% of my Summer production in June and July…
OP has lead batteries.

Lithium is much less.
 
Regarding my batteries, I'm unsure as to the real condition. They were hardly used but fully discharged batteries I kept in my shed. Ultimately, they are able to be charged, though I did boil them with 50 amps to help desulfate. At this point, they seem to be functioning perfectly. They are all holding a full charge. I'm going to be draining them down and recharging them to see if they are really worth using This were my batteries in my initial attempt to do a solar powered travel trailer when I was working as a traveler. So I've got them, they're holding a charge and they are essentially free to me because I don't have to pay anything. Certainly not like a bank of new LifePO4's. But as long as they can hold up my security system for a day, they are good enough for the task at hand. They will end their lives as a big UPS battery backup.

Hoping the next few years some amazing battery research trickles down to production. I'm in an optimal place for solar in Florida and we are going to be building a new house in the next few years. Going fully solar with special appliances, etc is just too much for me. But I would like to move my home's lighting, television and satellite internet on a separate solar system not tied into the grid so they will work 24/7 whether the power is on or not (a problem here).

Have decided to use my solar panels to power greenhouse fans, sans battery.
 
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