Battery Charging

[zander1976]

Hello,

There seems to be a disconnect, or at least I haven't figured it out yet and I can't really find a formula that says a battery will take so long to charge. Lets say I have a typical homemade 50 watt 18volt solar panel. Batteries are measured in amp hours so lets say i get a 12v 100a/h battery right. My panel produces around 2.5 amps give or take so lets say 3 amp hours. Now lets say I have 5 hours of sunlight a day. So I produce about 15 amps a day so it will take me like 7 days to charge the battery? Does that sound right. Did I do the math right.

So now lets say I get a bunch of batteries. I use 26 kwatts a day in my house, so lets say I got 2,000 amp hours in batteries ( lets say, not practical I know ) * 12 volts would be around 26 kilowatts right. How much power would I need to produce to charge that a day.

Thanks, Ben

[zander1976]

Its kind of funny,

I understand the physics of turbines and read all the papers I could find on it and solar cells I understand the different crystals and even the nano technology behind thin film and read the papers on that but a battery confuses me. It must be that I simply have no experience at all with electricity so batteries, cables and all that just seem to go over my head.

Ben

[southpaw]

Hello,

There seems to be a disconnect, or at least I haven't figured it out yet and I can't really find a formula that says a battery will take so long to charge. Lets say I have a typical homemade 50 watt 18volt solar panel. Batteries are measured in amp hours so lets say i get a 12v 100a/h battery right. My panel produces around 2.5 amps give or take so lets say 3 amp hours. Now lets say I have 5 hours of sunlight a day. So I produce about 15 amps a day so it will take me like 7 days to charge the battery? Does that sound right. Did I do the math right.

So now lets say I get a bunch of batteries. I use 26 kwatts a day in my house, so lets say I got 2,000 amp hours in batteries ( lets say, not practical I know ) * 12 volts would be around 26 kilowatts right. How much power would I need to produce to charge that a day.

Thanks, Ben

Hi Ben
The formula for battery charging is usually C/10 or C/20 or something similar depending on the type of battery.
This means that you would in the case of C/20 charge your battery at a rate of CAPACITY/20 or in your example of 100a/h divided by 20 meaning a rate of 5 amps for 20 hours. In your case you have a charging current available of 2.5 amps it would take twice as long or 40 hours. (by the way you cant interchange amps and amp hours they are different measurements) [" My panel produces around 2.5 amps give or take so lets say 3 amp hours."] If you get 5 hours of sun per day you will get 5hrs.X2.5 amps or 12.5a/h of power per day and it will take 100 a/h (battery capacity)  divided by 12.5 a/h per day or 8 days to charge the battery.

If you use 26kwh of power in your home (note 26kw is an instantaneous measurement of power and is meaningless in this case without the time h) you need to produce 26 kwh to charge your batteries back up. This is simplified not taking into account inefficiencies in the charging system, power loss in the conductors , inverter efficiencies, self discharge of batteries etc.
This would mean you would need 26000w/h divide by 5hrs of sun per day or 5200watts of panels or 104 of your 50 watt panels.

To store this power you would need 26000w/h divided by volts (12) or 2167ah of storage  but you don't want to run your batteries flat so to keep them above 80% you need 5X2167ah or  10,835 ah of storage batteries. And if you want some cushion in case the sun doesn't shine for 4 or 5 days you need to multiply this by 4 or 5. You really need to reduce your power consumption to make this feasible.


Southpaw
 

[madlabs]

Ben,

If you completely discharged a 100aH battery, it would in theory take 6.6 days to recharge the battery. In reality it would take longer than that, as charging isn't 100% effecient. It's usually between 20% and 30% more. All of this assumes things like that your wires are all large enough to prevent losses and so on.

So, to replace 26kwA a day, with your 5 hour sun window, you would need 6500 watts of panels. This is assuming 25% charging losses.

2000 aH battery bank is quite practical. I'm sure lots of folks here have that and more. I have 1200 aH of batteries and will be buying more. The thing is you really don't want to discharge batteries below 80% charge. You can discharge them more, but you start to damage the batteries. You need to use 2000 aH a day, so for decent battery life you would actually need around 8,000 to 10,000 aH bank. That is getting pretty big and expensive.

I don'y know if you are running a buisness that uses all that power or if that is your home consumption. If it is just your home, I would look at seriously reducing your power usage.

I'm just a RE nub, so take this with a grain of salt.

Jonathan

[madlabs]

South,

Guess we posted at the same time. Glad to see that my figgers agree with yours!

Jonathan

[zander1976]

Thank you both, that made it much clearer. Nah, I wasn't planning on actually putting in that many batteries but more just trying to figure out the math of it all. I think I will be installing a few batteries to drive things when the power went out or maybe just to put a few things on battery instead of the grid. So I am trying to get a grip on how much of my house I could actually power based on the money I have to buy batteries. Once I buy a bunch of batteries then I need to charge them and I couldn't find any math that said you have 10 batteries so how many panels would I need to recharge them on a daily bases. There is lots of information on calculating battery stack size but the direct relation to that is charging that stack.

Thanks again.

[zander1976]

So based on that if I have a 100amp/h c20 battery it would produce 5amps per hour. In a 12 volt system that would end up being 60 watts per hour. So I could drive a 60 watt light bulb for 10 hours to hit 50% battery capacity. Am I figuring that correctly.

Thanks

[ghurd]

You may be confusing yourself with mixing up, and making up, terminology.

"5 amps per hour"
is like saying 'My car has a 17 gallon per hour fuel tank'?  Sort of confusing.

"My panel produces around... 3 amp hours. Now lets say I have 5 hours of sunlight a day. So I produce about 15 amps a day"
The panel produces 3 amps.  Plain old 3 amps.
In 5 hours, it produces 15 amp-hours.
3A X 5H = 15AH

Something people rarely talk about is how fast a battery is able to be recharged.
Can only recharge them so fast.  Going with huge panels related to the battery capacity is mostly a waste of panel.
Example- 
If a 100AH battery is 50% DoD  (half full?), then it needs 50AH to be recharged.  Plus the inefficiencies which I will ignore for this.
If you have a battery charger that will force 25A into the battery, then it should be full charged in 2 hours.
However, forcing that kind of amps into a battery that size makes the voltage increase very fast.
When the voltage increases to some value, the controller acts on the incoming amps to slow them down,
otherwise the high voltage causes lots of gassing, water loss, acid spitting around the area, etc, and possibly battery damage from excessive voltage.
Meaning if the panel is large compared to the battery, the controller will hold back what the panel can make, so much of the panel's power is not used anyway.

Even a 50W panel in good sun, charging a 50% DoD 100AH battery will soon have the battery up to regulation voltage, when the controller will not allow the full available charging current into the battery.
Just because the panel can make the power, and the battery can store it in theory, does not mean it happens that way in real life.
Need to get a balance between power usage, battery size, and panel size.
G-