basic question about inverter configuration

samitrix · March 2011

hi,

konnichiwa from Japan! im a newbie here and this is my first post! i haven't seen forums on RE so im very pleased to be here.

i have a big interest in off grid solar projects but i lack knowledge so i would be very happy if you experts can point me in the right direction

my first question is regarding inverter configuration in a very basic solar home system. precisely, why the standard configuration is the connect inverter directly to the battery?? we tried connecting inverter to charge controller which links the solar panel and battery and its working fine. is this a big NO NO design?
Our system component has: 1 x 125W solar panel + 100Ah deep cycle battery + 10A charge controller. system voltage 12V.

i would really appreciate your input.
sam

dwh · March 2011

Re: basic question about inverter configuration

Konnichiwa,

Hope you are well there in Japan.

Yes, you need the battery in case the inverter needs to draw more than the charge controller can supply.

BB. · March 2011

Re: basic question about inverter configuration

All our prayers to you, your family, and country.

Solar panels have a fairly wide output voltage range.

A typical "12 volt" solar panel can range from 10.5 volts (typical AC inverter cutoff voltage) to ~21 volts or a bit higher... Many inverters will turn off around 15-16 volts on their DC input to prevent damage.

In the end, most AC loads are constant power devices (computers, TV's, tools, pumps, etc.) and the Sun is a variable energy source. And having the AC power turn on and off (due to sunlight/clouds/etc.) is not usually a good thing for those devices.

You either have to have an Extra Large solar PV panel / array to run a given load. Or, you use a battery to store the variable power from the solar array, and feed the energy (as needed) to the AC loads.

There are, however, some very useful applications where solar PV arrays are directly attached to the load (no batteries). That is for water pumping.

Use DC pumps (or special controllers that take DC power in and output variable frequency AC power to pumps). Basically, the pumps turn slower with less PC Voltage / Current, and faster with lots of voltage / current.

The controllers are a form of Variable Frequency Drives.

And here are some very nice pumps that can take DC solar power, DC battery power, or even AC power... Very flexible (and not cheap).

Grundfos Solar Submersible Well Pumps (link now working)

-Bill

stephendv · March 2011

Re: basic question about inverter configuration

Hi Sam,

I assume that your charge controller has a DC output plug, and that's where you've connected the inverter? This DC output point is really designed for low current 12V DC appliances that you may want to connect.

An inverter will work fine connected to this point as long as it doesn't draw more current than the charge controller can handle. That's the main reason why the standard configuration is to connect the inverter directly to the battery- so that the charge controller doesn't limit the current draw.

samitrix · March 2011

Re: basic question about inverter configuration

hi dwh and Bill,

thanks for your reply and concern about Japan. i can still vividly recall the evens of March 11. im based in yokohama japan so my family and i were fortunate to escape the devastating tsunami. however, the quake was terrifying. our 22-storey high rise swung almost 1M right and left...we ran for our lives taking stairs. it was very difficult as we couldnt run straight. but anyway, we are alive and that's a gift...thanks once again for your thoughts and prayers.

now back to the topic, i understand both of your reasoning. however, in the case of directly connecting the inverter to the battery, it seems a high risk config in terms of battery cycle life as the inverter will heavily drain the battery. one solution will be to use inverter with build in charge controller functions but its way more expensive than using a separate charge controller and inverter.

Bill, the way we wired the inverter to the charge controller will not lead to any fluctuation in voltage as solar panel V fluctuates. on any charge controller there is there is plus (+) and minus (-) terminals for load connection. what we have done is connected the load + terminal to the + terminal of a 300W DC/AC inverter. and the - terminal as grounding. this way the charge controller will only be supplying constant 20A current to the charge controller. this will create bottleneck as the inverter will not reach its max capability but the big advantage will be battery protection.

ok a little lengthy but that was my explanation. please dont tell me im crazy

lol

cheers,
sam

samitrix · March 2011

Re: basic question about inverter configuration

stephendv wrote: »

An inverter will work fine connected to this point as long as it doesn't draw more current than the charge controller can handle. That's the main reason why the standard configuration is to connect the inverter directly to the battery- so that the charge controller doesn't limit the current draw.

hi stephendy,

thanks for pointing that out. in my crazy mind however, it seems justified to limit inverter so that it doesnt abuse the battery. after all, battery is the 2nd most expensive component in an SHS and pretty much the heart of the system.

what i cant understand is why the standard configuration of connecting inverter directly to the battery is OK when it carries a huge risk of battery abuse?? any thoughts...

cheers,
sam

stephendv · March 2011

Re: basic question about inverter configuration

The inverters I'm familiar with also have a low voltage disconnect, so they will turn off if the battery voltage drops below a certain level.

If the charger controller has a 20A limit, at 12V that's 240W, so you will not be able to draw the full 300W from your inverter if it's connected to the controller.

samitrix · March 2011

Re: basic question about inverter configuration

stephendv wrote: »

The inverters I'm familiar with also have a low voltage disconnect, so they will turn off if the battery voltage drops below a certain level.

If the charger controller has a 20A limit, at 12V that's 240W, so you will not be able to draw the full 300W from your inverter if it's connected to the controller.

yup, you are right. inverters do have an LVD but its more to protect the inverter than the battery. the LVD cut off voltage of the current inverter i have at hand is 10.5V and for a 12V battery, this is like a death point-almost 100% DoD. so this is what i mean by abuse of battery. or am i looking at wrong kind of inverters?????

stephendv · March 2011

Re: basic question about inverter configuration

The LVD is to protect the battery not the inverter. Yes 10.5 is quite low, but surely you can adjust this value to something more suitable for your battery?

If inverters limit current and have an LVD then they can perform the same "protection" for the battery as is offered by the charge controller. Which getting back to your original question, is why you can safely connect them directly to the battery.

BB. · March 2011

Re: basic question about inverter configuration

Sam,

I am from just south of San Francisco California... So I have been in my share of earthquakes--but none that large (remember the one in 1989 that shut down the world series?).

samitrix wrote: »

now back to the topic, i understand both of your reasoning. however, in the case of directly connecting the inverter to the battery, it seems a high risk config in terms of battery cycle life as the inverter will heavily drain the battery. one solution will be to use inverter with build in charge controller functions but its way more expensive than using a separate charge controller and inverter.

Battery cycling is always a problem... For smaller systems, a 3-5 year battery life is doing good... For larger systems (i.e., golf cart to fork lift sized batteries), upwards of 8-15 years. And with industrial batteries, 20 years is possible.

There are the various Lithium battery chemistries which are possibly the next lighter weight, high power density, better cycle life batteries...

Lithium Iron Phosphate Batteries; LFP; LiFePO4 discussions

There are people now packaging these batteries for light weight/portable Solar PV systems.

Bill, the way we wired the inverter to the charge controller will not lead to any fluctuation in voltage as solar panel V fluctuates. on any charge controller there is there is plus (+) and minus (-) terminals for load connection. what we have done is connected the load + terminal to the + terminal of a 300W DC/AC inverter. and the - terminal as grounding. this way the charge controller will only be supplying constant 20A current to the charge controller. this will create bottleneck as the inverter will not reach its max capability but the big advantage will be battery protection.

Interesting. There are several major charge controller / voltage controller topologies that could be used. Each with its own advantages and disadvantages:

Linear (pass transistor), cheap, quiet, wastes lots of power, low amount of power transfer
PWM (Pulse Width Modulation), cheap, need filter caps, efficient
MPPT (Maximum Power Point Tracking) Digital Switching; expensive, needs filtering, optimum I*V power matching from Solar Panel output to inveter's DC input stage.

What type are you experimenting with?

ok a little lengthy but that was my explanation. please dont tell me im crazy lol

cheers,
sam

When you have a single 15,000 character reply that needs to be broken into three posts--Then I will call you crazy. Right now, I have that title locked up.

;):p

-Bill

samitrix · March 2011

Re: basic question about inverter configuration

stephendv wrote: »

The LVD is to protect the battery not the inverter. Yes 10.5 is quite low, but surely you can adjust this value to something more suitable for your battery?

If inverters limit current and have an LVD then they can perform the same "protection" for the battery as is offered by the charge controller. Which getting back to your original question, is why you can safely connect them directly to the battery.

i guess im not looking at the good charge controllers. the one i have right now can not be adjusted so that's a bummer. but i bet the ones you can adjust might be quite expensive no? what do you recommend for a charge controller?

samitrix · March 2011

Re: basic question about inverter configuration

BB. wrote: »

Sam,

I am from just south of San Francisco California... So I have been in my share of earthquakes--but none that large (remember the one in 1989 that shut down the world series?).

Yeah i am aware of that. we live in a dangerous world i guess :roll:

BB. wrote: »

Battery cycling is always a problem... For smaller systems, a 3-5 year battery life is doing good... For larger systems (i.e., golf cart to fork lift sized batteries), upwards of 8-15 years. And with industrial batteries, 20 years is possible.

well with the kind of users i have in my project, you would be lucky if the batteries last 1 year lol. these users just rent the system so they dont really give a damn about good battery management or any management for that matter

BB. wrote: »

There are the various Lithium battery chemistries which are possibly the next lighter weight, high power density, better cycle life batteries...
There are people now packaging these batteries for light weight/portable Solar PV systems.

interesting read. its a new product so i rather wait till it matures in the market.

BB. wrote: »

Interesting. There are several major charge controller / voltage controller topologies that could be used. Each with its own advantages and disadvantages:
Linear (pass transistor), cheap, quiet, wastes lots of power, low amount of power transfer

PWM (Pulse Width Modulation), cheap, need filter caps, efficient

MPPT (Maximum Power Point Tracking) Digital Switching; expensive, needs filtering, optimum I*V power matching from Solar Panel output to inveter's DC input stage.

What type are you experimenting with?

Well we are using PHOCOS CML 10A charge controller. its definitely not an MPPT so im assuming its a PWM

BB. wrote: »

When you have a single 15,000 character reply that needs to be broken into three posts--Then I will call you crazy. Right now, I have that title locked up.
-Bill

damn! lolzzzz. so that's the record to beat aye?? my first day started quite well here so one day records shall be smashed :D8)

stephendv · March 2011

Re: basic question about inverter configuration

There's nothing wrong with the Phocos controller, it just looks like that specific model you have is quite basic in operation and only has pre-programmed settings that can't be changed.

Their CX model does have a programmable LVD, but that's a 40A controller and will likely cost a bit more.

techntrek · March 2011

Re: basic question about inverter configuration

Bill mentioned Lithium, I'll mention flooded NiCad batteries. More expensive than flooded lead-acid but not nearly as expensive as Lithuim. With your rental system it has several important advantages. First, unlike lead-acid NiCad can be completely discharged w/o damage (and can even be stored long-term discharged). So a 10.5 volt cutoff on your inverter is ok. Second is they have long life compared to lead-acid which mostly offsets the additional cost. Third they don't need to be watered as often as lead-acid. Fourth, they have a higher energy density so they are lighter. Finally, they can be charged using standard lead-acid charge controllers.

BB. · March 2011

Re: basic question about inverter configuration

I agree with techntrek that Ni-Cad are actually a pretty nice and rugged battery chemistry (don't worry about "memory effect"--that is a specialized "issue" due to precision charge/discharge cycles--Satellite use--or from constant float charging).

However, the Cadmum content of cells can make them very difficult to import/use/recycle in many countries. Depends on your local regulations

-Bill

mike95490 · March 2011

Re: basic question about inverter configuration

techntrek wrote: »

Bill mentioned Lithium, I'll mention flooded NiCad batteries. More expensive than flooded lead-acid but not nearly as expensive as Lithuim.....

Any sources, or just surplus as they appear ?

techntrek · March 2011

Re: basic question about inverter configuration

http://www.sbsbattery.com/subpage_index.php?_subp_=142

From what I've read online, they will sell direct but they may require specs and line drawings of your application, before they'll cover them under warranty.

If you dig down to the spec PDFs on their site, they have a wealth of info such as charts showing available power for constant current vs. constant voltage charge sources (very little difference from what I saw). http://www.sbsbattery.com/PDFs/sbsNCPP%20SINGLE%20TECH.pdf

samitrix · March 2011

Re: basic question about inverter configuration

techntrek wrote: »

http://www.sbsbattery.com/subpage_index.php?_subp_=142

From what I've read online, they will sell direct but they may require specs and line drawings of your application, before they'll cover them under warranty.

If you dig down to the spec PDFs on their site, they have a wealth of info such as charts showing available power for constant current vs. constant voltage charge sources (very little difference from what I saw). http://www.sbsbattery.com/PDFs/sbsNCPP%20SINGLE%20TECH.pdf

thanks techntrek for the suggestion. its the first time i was advised to use NiCad batteries for solar home systems. i have already started reading on it. seems like very few companies make these kinda batteries. i couldnt find a rough price. do you have any idea on the pricing?? i will be emailing SBS shortly after this and RFQ.
do you or anyone you know here in this forum has experience with flooded NiCad batteries??

basic question about inverter configuration

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