Cariboocoot wrote: »
The charge rate is expressed as a percentage of the battery bank's Amp hours (at the 20 hour rate).
For systems that have no loads on during charging (such as one meant to supply nighttime lighting only) you can use the manufacturer recommended minimum of 5%:
220 Amp hour battery * 5% = 11 Amps peak charging current.
For systems that are in daily use (that is loads drawing while charging at the same time) you target 10%:
220 Amp hour battery * 10% = 22 Amps peak charging current.
Other circumstances such as heavy loads or poor insolation conditions may warrant a higher charge rate. 13% has traditionally been the practical maximum because above that you spend a lot of money on PV that doesn't do much most of the time; it simply isn't needed. With PV prices as low as they are now that could be 15%, but then you get into the area of low charge acceptance for flooded batteries. AGM's can take more current.
Keep in mind this is a peak charge current, not a constant one, and may not necessarily be seen because of the many factors involved in current demand. It is important to recognize also that the actual charge rate will be the controller output minus the load demand: if you have 22 Amps from the controller and loads take 5 Amps then the net amount going to the batteries is (22-5) 17 Amps.
Another caveat: the 'tall case' batteries such as L16's require that 10% net rate due to the need for fairly long Absorb to mix the electrolyte and avoid stratification; the faster you can get them through Bulk the more time there is for Absorb before the sun goes down.
javadz wrote: »
thank you for your precious information
if i understand ,
that measn if i have a 250Ah of battery and a charge controler give me 40A at a moment ,then the 40A will not go to the battery ,only (10% of 250AH =25A)?
Cariboocoot wrote: »
The current rating of the charge controller is the maximum it can handle: 60 Amps for a 60 Amp controller.
If there is enough PV and light to output 40 Amps then 40 Amps is what is going to the battery. If there are load detracting from that output then what is going to the battery is the controller's output minus the current going to the loads.
The 10% is the 'target' for peak charge current:
You have 250 Amp hours so you want to be able to supply 25 Amps for charging.
On a 12 Volt system with an MPPT charge controller this requires (25 Amps * 12 Volts / 0.77 efficiency) 389 Watts of solar panels (and good insolation).
It would be okay to go as high as 37.5 Amps (15%) to the batteries (584 Watts of panel to supply it).
Minimal charge rate of 5% would be 12.5 Amps and would require 194 Watts of panel.
These are not constants. Actually current varies with the State Of Charge of the battery, any loads on, and available sun. There is a range that will work, and usually if you try for that 10% peak current all will be well. Adjustments need to be made for unusually heavy loads (enough that will pull the current to the battery below 5%) or poor sun conditions.
As a rule it is better to err on the side of caution: round up battery bank capacity requirements to the nearest available. Round up solar panel Watts to the nearest available. As in the first example you probably won't find panels that add up to exactly 389 Watts, but you can buy two 220 Watt panels and have 440 Watts with a resulting peak current of 28 Amps which the batteries can take without issue.
Cariboocoot wrote: »
When you design a system you use the 10% rule-of-thumb to make sure you get enough charging for the battery. You do not have to limit it to that, but there is a problem with going too high in current to the battery.
If there's enough panel to supply 70 Amps to your 250 Amp hour battery that would be a 28% charge rate. For flooded cells that is very high and will result in a lot of heating of the battery during charging. This will shorten the lifespan and could even cause sudden failure. AGM's can take higher currents; check the makers specifications for those.
Do not confuse Amp hours with Amps: Amp hours (at a given Voltage) is a capacity, Amps is a rate. The battery charging rate (Amps) is based on a percentage of the capacity (Amp hours).
To get 70 Amps peak current on a 12 Volt system you'd need about 1090 Watts of PV and a charge controller capable of handling 80 Amps. Do not confuse the controller's maximum output capacity with the actual current that will flow: if there's not enough PV and/or sun to provide the current or not enough demand for it you will not see the maximum current. In other words if you have 400 Watts of PV on an 80 Amp charge controller you will never see 80 Amps from it because there is not enough power available from the solar panels to generate that much current.
1000 Watts of panel over 4 hours of equivalent good sun will actually generate about 3 kW hours DC. Less when converted to AC and even less if it all has to go in and out of batteries (end-to-end efficiency is then down around 52%).
1000 Watts of panel on a 24 Volt system can generate about 32 Amps peak charging current, which would normally be enough for a 320 Amp hour battery bank. Said bank at 25% DOD would supply (320 Amp hours * 0.25 / 24 Volts) 1920 Watt hours DC. At 50% DOD it would be double that: 3840 Watt hours.
Again these are 'ballpark' calculations used for the basic design; quite a few factors may make your system more or less efficient. For example I am at higher elevation so my panels receive more than the average amount of light and actually run at 82% efficiency on average rather than 77%.