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I have a 100w solar panel connected to a charge controller that is connected to a 35ah battery that is connected to a 400w inverter. From what I read a solar battery can be discharged down to 50%. However, when the battery gets below 11 volts (its full charge is around 13.96v) the power shuts off. How can I get around this to get more out of my battery? Is the issue with the inverter? Any help is greatly appreciated. Thanks
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Hello and welcome to Solar Panel Talk
1st. Most deep cell batteries do not like to go down to 50% daily. That will shorten it's life if you discharge more than 25%.
2nd. A 12volt battery is pretty much at 50% SOC when it gets to 12.06volts. At 11volts it is below 10% SOC and is pretty much dead.
At 100% SOC it will be ~ 12.7volts. That 13.96v you measured is before the battery has rested after charging and not the true voltage level.
The safe voltage range to discharge is between 12.3 and 12.4volts which is about 25% DOD.
My guess is that 400watt inverter is over discharging your 35Ah battery or your 100 watt panel (which is really a 67watt with a PWM Charge controller) is not putting back into the battery what you take out.
Sunking is correct. While my data is based on a slightly different chart for all purposes a battery with 11volts is toast. -
Your numbers are all wrong. A Pb battery at 100% SOC is 12.6 volts. At 50% is is 12.1 volts. 11 volts is throw the battery away because you destroyed it with ignorance.
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Your inverter is doing its job by shutting off. It is called Low Voltage Disconnect or LVD. Default of 11 volts is way too low and should be 12 volts.Your second problem is your Inverter is way too large for your battery. Your whole system is out of whack.
If we work backwards from your Inverter, the minimum size battery at 12 volts is 250 AH. To properly charge and maintain a 12 volt 250 AH battery requires a 30 amp Charge Controller using a 400 watt panel.
If we work forward with a 100 watt panel using a PWM controller you can only support up to a 55 AH battery and a 100 watt Inverter.
Nothing you have is made to work with the other pieces. Couple that with your lack of knowledge, and you have a RECIPE FOR DISASTER.Everything you think you know about batteries and solar is rubbish.MSEE, PEComment
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Thank you both very much for the info. I appreciate the help and knowledge... I am here because of my ignorance (check definition
) but thank you for your post and assistance.
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The SOC voltages do vary a bit. That is because of the battery plate alloy type and percentage. There is Lead Calcium, Lead Antimony, Lead Carbon, and Pure Lead.
Me I am a simple man and use KISS tables. For me anything below 12.3 volts is trouble.
To the OP The higher voltages you observed are voltage under charge, not at rest or under load. You need to understand that before you can catch on.
Good Luck and ask questions.MSEE, PEComment
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Do not depend on the inverter shutdown. It is there to protect the inverter more than the battery.
As Sunking said, when you get to the inverter cutoff voltage your battery is probably at about 10% SOC, an abusively low level and one which would require 10 or more hours of full noon sun in a single day to recharge. And that is not going to happen.
You can either watch the battery voltage or count the number of amp hours used to decide when to stop using power or you can get a separate voltage controlled switch to turn off the inverter at some point above 12.0 volts. (Lower if under a high current load at the time.)SunnyBoy 3000 US, 18 BP Solar 175B panels.Comment
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Here is some additional detail that might be easier to remember. For one thing, you are using an AGM battery that has slightly different voltages when measured *at rest* than does a flooded.
While not supremely accurate, especially in the case of a badly abused agm battery, this rest (no charge, no load) of at least 2 hours or more will get you in the ballpark.
12.9 - 13.2v 100% <--- this is what a *healthy* agm looks like at rest for at least 2 hours after a full charge, preferably more.
12.5 - 75%
12.2 - 50% < --- the canonical point which one should not go, again measured while at rest.
12.0 - 25% <--- avoid this or below as cycle life is really cut short.
With your small 35ah agm, don't expect your 400 watt inverter to be able to sustain more than just drilling some quick holes with a small drill motor.
For continuous draw, we really need to lighten that load up.
Napkin calc of your power budget:
1) 35ah batt * 12v = 420 w
2) Since only half that is usable for a 50% DOD, your total power reserve is really 210 watthours.
To avoid a quick voltage drop during discharge, draw no more than 10% of the wattage of step #1 from your inverter with your *continuous* load. This is the "peukert" effect which you can check out - basically the faster you charge or discharge, the more inefficient the process becomes.
If your load is this large (say 40w incandescent bulb and an inverter), then you'll be able to run that for about 5 hours before reaching the 50% DOD mark. After rest, your battery should be in the 12.2v range.Comment
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No problem - the biggest moral here is that an inverter low-voltage shutoff is a method of last resort to protect a battery from total discharge, and is not to be used as the normal signal under day to day use as your stopping point.
Here comes the fun part. While a bigger battery will allow you to power more stuff, or the same stuff just a lot longer, the BIG question now is do you have enough solar-insolation to actually recharge that battery from 50% DOD? If you are cycling daily, this becomes really important, because now, you have to plan your panel size, CC current specs of course, and choice of battery. For instance, many small agm's in the 35ah range like you are using can only withstand a max constant current charge of C/4 , maybe C/3.
That is, for your 35ah conventional agm, then you are looking at a recharge current of anywhere from about 8-11A, so size your panel accordingly - 120 to maybe 200 watts. There are other agm's out there that don't have a recharge current limit (within reason), like Enersys/Odyssey/Optima, Lifeline, Concord and so forth. Most of these are under the "pure-lead" category. Whether that's important to you for your application is up to you to decide.
Solar insolation depends on where you are located and running geographically, and the smart ones choose the "winter" hours from the online charts that detail this. These hours differ from mere sunrise-to-sunset when used for your calculations.
So don't just throw money at a bigger battery UNLESS you can be sure to recharge it in time.Comment
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