# Off grid system basic principles

Posts: 34Registered Users ✭✭

Hi!

I have some fundamental questions around the interactions between the PV array, charge controller and battery.

When a battery and charge controller is involved, does the electricity from the PV panels (charge controller) always go via the battery before entering the inverter and load? In other words, does the battery always feed the load? In that case, I have som questions:

1) Say the load is 1000 watt and the PV panels produces 1000 watt. Will the electricity just go through the batteries without charging them and straight to the load? Or will the electricity delivered to the load come from the existing energy stored in the batteries, and electricity from PV panel will always just charge the batteries?

2) Must the battery always be able to deliver an instantaneous power that matches the load? Say the PV array can deliver 10 kW and the load is at 12 kW. Will the battery need to be able to deliver an instantanous power of 12 kW? How does it work when the PV power and battery have to deliver power at the same time?

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> @schmek said:
> Hi!
>
> I have some fundamental questions around the interactions between the PV array, charge controller and battery.
>
>
> When a battery and charge controller is involved, does the electricity from the PV panels (charge controller) always go via the battery before entering the inverter and load? In other words, does the battery always feed the load? In that case, I have som questions:
>
>
>
>
>
>
>
> 1) Say the load is 1000 watt and the PV panels produces 1000 watt. Will the electricity just go through the batteries without charging them and straight to the load? Or will the electricity delivered to the load come from the existing energy stored in the batteries, and electricity from PV panel will always just charge the batteries?
>

The batteries ARE a load, to the extent they draw power to charge. If the batteries are fully charged, pv will supply current directly (via charge controller) up to the available pv current. Say you have pv capable of producing 4kw. With fully charged batteries, the pv will produce 1kw to supply your 1kw load (ignoring system/wire losses for simplicity). If the load increases to 2kw, pv will supply 2kw for the load. If the load increases to 6kw, 4kw will come from the pv, and 2kw from batteries. When the other load drops off, pv will provide current to recharge batteries as required according to charge controller program.

> 2) Must the battery always be able to deliver an instantaneous power that matches the load? Say the PV array can deliver 10 kW and the load is at 12 kW. Will the battery need to be able to deliver an instantanous power of 12 kW? How does it work when the PV power and battery have to deliver power at the same time?
>
>

In this case, 10kw would come from pv, and 2kw from batteries. It's a bit more complicated (mppt sweeps, etc), but that's the basic concept.

It's much more efficient to run loads with pv directly than to charge and discharge batteries.
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Off-grid.
Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter
Welcome to the forum Schmek,

How detailed do you want the answer to be--And remember there are different conditions at different times (day/night, battery full/empty, zero loads/full loads/surge loads, etc.).

Are you familiar with automotive (car, boat, etc.) electrical systems? At a basic level, Off Grid solar operates in a very similar manner.

Basically, think of the battery as the "center" of your off grid power system. Its job is to keep the DC Battery Bus at a (reasonably) constant level. roughly, if the bus voltage falls below ~12.7 volts, the battery will output current. If the battery bus voltage is >~13.0 volts, it will start to sink current (recharging). The DC bus voltage is "stabilized" by the battery bank.

The job of the charge controller (electrical current from Alternator, solar panels/etc.) is to keep battery bus voltage (on average) between 13.8 and 14.75 volts (depending on if the battery needs charging, higher voltage for Absorb charging mode. Or if the battery is charged, the lower voltage--Float charging mode).

When you have a load, if there is no sun (engine not running), the battery has to supply all energy needed to run the loads/electrical system (between 12.7 and 10.5 volts typical).

While people like to think that they can change the system design (such as large solar array and very small battery bank), if you need power rain or sun (alternator running or not), generally you have to design for worst case operation (1/2 charged battery, maximum load current plus support for surge current--such as starting a pump).

When the sun is up (motor is spinning alternator), the battery bus voltage rises to ~13.8-14.8 volts VDC. If there are no loads, all of the energy goes into recharging the battery. The charge controller (alternator voltage regulator) will adjust its output current to keep the battery voltage at ~13.8 volts (float charging) or ~14.8 volt charging (flooded cell lead acid storage battery at less than ~80-90% State of Charge). Note that (in general) solar charge controllers and alternator charge controllers (on cars/vehicles) are not designed to output stable DC Voltage by themselves... If, you tried to run the system with the battery disconnected, the DC Bus voltage can vary from zero volts to >100 VDC (car alternator/wind turbine alternator) with no battery connected. The role of the battery is to buffer the DC Bus Voltage.

If, for example you have both loads and sun--The battery will either take near zero current (it is fully charged), and the loads (AC inverter, etc.) will draw (on average) 100% of the current from the solar panels (solar charge controller, car alternator, etc.). And the battery will buffer slight voltage/current swings (i.e., load drops, battery bus voltage a bit, and battery sinks/take a bit of extra current for charging) (if the load suddenly increases, the battery bus voltage will drop well below 12.7 volts, and the battery will supply "extra current" to the load--Until the charge controller can increase solar/alternator current--or until the load drops back to normal).

As the sun goes down, you have less available power (watts, current) from the solar panels, and the DC bus voltage slowly drops to 12.7 volts and below, and the solar panels will supply whatever current is available from the sun, plus the battery will make up the difference. When the battery bus voltage/output voltage falls to ~11.5 to ~10.5 volts, the AC inverter will typically shut down the AC output and will keep it off until battery bus voltage rises to >~12.7 volts or more (i.e., the battery is getting recharged somewhat).

Answer to 1) is that current does not flow from the solar panels through the battery to support the loads. The battery is "off to the side" and averaging the current flow (and spikes) to between ~11.5 and 14.8 volts. With the battery either discharging, charging, or not much depending on actual conditions (available solar energy, state of charge of battery, current going to loads).

Answer to 2) specific question, 10 kWatt solar array (full sun, size of array, maximum output through charge controller) to 12 kWatt load, the battery will have to supply 2 kWatts (12 kW load - 10 kW solar available power) until conditions change (more or less sun, more or less loads, or battery goes dead, etc.).

Sometimes people call Solar PV panels "solar batteries". Technically, a "Battery" is a constant voltage device. 12.8 volt battery will output current if DC voltage falls below 12.8 volts and will take current if voltage goes over 12.8 volts.

A solar panel is a "constant current" device. More or less, the avialable output current is some percentage of available sunlight. Say you have a solar panel rated at:
Vmp~18 volts
Imp~10 amps
Pmp=Vmp*Imp= 18 volts * 10 amps = 180 Watt "12 volt" solar panel (just nice round numbers).

Vmp is roughly ~18 volts under any sunlight condition -- It is the solution to Pmp=Vmp*Imp... Since Imp (current) is more or less fixed (100% sun = 100% * Imp-rated), and the Vpanel voltage can range all over the place... A dead short Vpanel=zero volts
• P=V*I= 0 volts * 10 amps = 0 Watts

And if you have the panel disconnected (no loads), its Voc (voltage open circuit) is ~21.0 volts with zero current

• P=V*I= 21 volts * 0 amps = 0 Watts

And there is the optimum voltage Vmp (voltage maximum power):

• P=V*I= 18 volts * 10 amps = 180 Watts

And if connected directly to the battery bank needing charging (for example)

• Pbatt = 14.5 volts charging * 10 amps = 145 Watts into the battery bank
Note that the battery bus can range from 10.5 to 14.8 volts (or higher) on the battery bus--So the Watts available from the solar array is "sort of" variable (note this is describing a PWM -- Pulse Width Modulated -- charge controller).

There are other types of charge controllers like MPPT (maximum power point tracking) that work a little but differently (and are more expensive than PWM controllers), but that is the next discussion (if you wish).

Please ask questions--I went through this very fast and my English is not always clear (my only language- ).

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Posts: 34Registered Users ✭✭
Thank you for your detailed answer. Actually a MPPT is the most relevant charge controller for me at the moment, and I know the basics of it.

Regarding this:
Answer to 1) is that current does not flow from the solar panels through the battery to support the loads. The battery is "off to the side" and averaging the current flow (and spikes) to between ~11.5 and 14.8 volts. With the battery either discharging, charging, or not much depending on actual conditions (available solar energy, state of charge of battery, current going to loads).

In pure practical terms, when you say "off to the side": where does the electrons go? If we again look at a situation where the PV power is just sufficient to serve the load and the batteries neither charge or discharge. PV electricity comming from the charge controller, do the electrons that enter the inverter go through a seperate circuit? Or does the electrons go via the batteries?

-Sindre

• Posts: 4,764Solar Expert ✭✭✭✭
Let me add another take on this, and perhaps having a bunch of views might make this easier to understand.

The energy from the sun flows through the solar panels and the charge controller to provide energy to the system. There is a unique feature to solar panels. They don't 'have to' provide any current. They can simply be turned off. (This isn't true of generators, wind chargers, and water mills.)

The charge controller's responsibility is to turn the solar energy on and off to properly charge the batteries. It mainly does this by 'watching' the system voltage. Over night it sees the voltage drop below 12.7 volts and it starts the day allowing energy to feely flow into the system. The battery will accept current as long as the voltage is higher than the resting voltage but likes/needs a voltage about 10-15% higher to get it's maximum flow to the batteries. Batteries have a maximum charging current that they can accept.  Once a flooded lead acid battery reaches about 80% full, it is no longer capable of accepting it's maximum current and the voltage will 'run away'. The charge controller will maintain the system voltage high enough for the batteries to accept current without them having to dissipate the extra energy as heat.

When additional loads are applied to the system, the energy at first is provided by the solar array. If the loads become greater than the solar array can provide, the battery will become an energy source. You can watch your system voltage to see what is happening as loads are applied. Once the voltage reaches about 12.7 for a 12 volt system, the battery has become an energy source and the current is flowing away from it as it is being discharged.

Often when inductive loads are applied, the energy required to start them is greater than the solar can provide, but is nearly instantaneous, so you might not see a voltage drop, but the solar would be unable to provide the energy to start them without the battery.
Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Prosine 1800 and Exeltech 1100, 660 ah 24v ForkLift battery. Off grid for @16 of last 17 years. Assorted other systems, and to many panels in the closet to not do more...lol
• Posts: 34Registered Users ✭✭
edited April 13 #6
Thanks, I appreciate your detailed answer! I think I understand the basics. But i believe my question may be simpler than what you think. I understand that energy going to the load is first provided by the solar array. When the battery is fully charged and PV power feeds the load: Does the electrons flow in a seperate wire/circuit? Because in many figures i see, it looks like the electricity flow always goes into the battery, before traveling further on. (Like in this figure: https://pureenergycentre.com/wp-content/uploads/2013/09/Solar-off-grid-Pure-Energy-Centre.jpg)

I am now wondering about the physical connection/wireing between the charge controller, battery and load.

-Sindre
• Posts: 4,764Solar Expert ✭✭✭✭
edited April 13 #7
No, There's no reason to try to go to the battery. If the load is requiring more energy than the array can provide the system voltage will drop and there is no pushing the electron 'up stream'. In most home/large systems, there is a single set of cables going to the battery bank attached to a main breaker in a breaker box. as the system voltage drops the battery is sending electrons into the system to provide energy for the loads.
Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Prosine 1800 and Exeltech 1100, 660 ah 24v ForkLift battery. Off grid for @16 of last 17 years. Assorted other systems, and to many panels in the closet to not do more...lol
• Posts: 2,391Solar Expert ✭✭✭✭✭
The charging input and the battery are in parallel if the battery is fully charged there would be no ( actually there is always a little due to losses ) current, or electron flow, if you like. The panels would be waiting for the opportunity to work, the inverter, which is also be in parallel with the battery is now turned on with a load almost equal to the capacity of the array, the electrons will flow to the point of least resistance. Since the battery if full, therefore not requiring any additional electrons, the flow would be directed to the inverter. When the load exceeds the array capacity, the additional electrons will be made up by the battery and the voltage will drop. When the load is removed the electrons will be directed to the battery to restore the voltage setpoint, when satisfied the flow will cease. There is no separate circuit as everything is in parallel, it is however important to allow the batteries to be charged, before applying such a load.

Another point is if a cloud passes for a period of time the current required for the load would be taken from the battery, which acts as a buffer or voltage regulator of sorts, this needs to be replaced, it a balancing act which takes time to learn. Naturally having an array capacity greater than the load will make the ballance easier to achieve.
1500W, 6× Schutten 250W Poly panels , Schneider MPPT 60 150 CC, Schneider SW 2524 inverter, 400Ah LFP 24V nominal battery bank
900W  3 × 300W No name brand Poly panels, Morningstar TS 60 PWM controller, no name 2000W inverter 400Ah FLA 24V nominal as a backup system.
5Kw Yanmar clone single cylinder air cooled diesel generator for rare emergencies and welding.
• Posts: 34Registered Users ✭✭
Thanks a lot guys! It is clear to me now

-Sindre
• Posts: 303Registered Users ✭✭✭
edited April 14 #10
This discussion brings to mind something that I have wondered about.

If in using an off grid system with just Solar Array, CC, Batteries and Inverter,   When the batteries are old and near end of live,  but probably not with bad cells... as long as those batteries are connected,  will the system still provide for the loads in best case conditions?   I realized the batteries have to be at least in decent enough shape to provide the very small amount of power for the Inverter and CC  (usually about 1 amp DC x 24v = 24 Watts),   but the way I am reading this discussion ... do even the inverter and CC  (which need battery power to power on and remain on at night) get their power from the Array thru the day?

In other words,  in 5-10 years when the batteries have reached near end of life as far as energy storage is concerned for loads, Would the system still function thru the day on the Array power alone?   And at that point,  couldn't the system work - in the day - for a long time past effective battery life as long as cells were not bad and the battery could provide 1-2 amps DC to power on the CC and Inverter?

What happens if a person does not replace their batteries when the batteries stop holding charge under heavier than 1-2 amp DC load?

REC TwinPeak 2 285W 3S-3P 2.6kW-STC / 1.9kW-NMOT Array / MN Solar Classic 150 / 2017 Conext SW 4024 Inverter latest firmware / OB PSX-240 Autotransfomer for load balancing / Trojan L16H-AC 435Ah bank 4S connected to Inverter with 7' of 4/0 cable / 24 volt system / Grid-Assist or Backup Solar Generator System Powering 3200Whs Daily / System went Online Oct 2018 / System, Pics and Discussion
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Among other issues, a too small apparent bank capacity would be a problem when initially presenting a suddenly large load to the system. The charge controller "sweeps" the array at intervals to determine an optimal voltage/current combination. Although the pv may be able to handle the load, it may take some time (likely just seconds, but not instantaneous) for the controller to get the new mppt point, during which time the bank supplies the load.

Assuming the load is AC, there may also be problems with the inverter, as it may require the battery to properly smooth DC->AC between AC cycles.
Off-grid.
Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter
• Posts: 2,391Solar Expert ✭✭✭✭✭
The purpose of the batteries is to store energy to be used at a later time, nights for example, if the capacity is lost due to age, the ability to power loads is lost. The plates within the battery are sacrificial so material will drop into the well below the plates possibly causing short circuits which will consume what the array is producing, to the point of actually being dangerous. This analogy is for flooded batteries. If capacity is lost, the first signs is water consumption without any actual storage capacity, so basically they would be a reference voltage for the controller to follow whilst powering loads during the day. This is fine but what happens when a few clouds pass by, the battery voltage collapses and the inverter shuts down, so the question is, is it worth flogging a dead horse, naturally the answer is no. When the service life of the batteries is passed, it's time for replacement, no point in comprising the entire system.. Opinions and thoughts FWIW.
1500W, 6× Schutten 250W Poly panels , Schneider MPPT 60 150 CC, Schneider SW 2524 inverter, 400Ah LFP 24V nominal battery bank
900W  3 × 300W No name brand Poly panels, Morningstar TS 60 PWM controller, no name 2000W inverter 400Ah FLA 24V nominal as a backup system.
5Kw Yanmar clone single cylinder air cooled diesel generator for rare emergencies and welding.
• Posts: 8,000Solar Expert ✭✭✭✭
MrM1 said:
This discussion brings to mind something that I have wondered about.

If in using an off grid system with just Solar Array, CC, Batteries and Inverter,   When the batteries are old and near end of live,  but probably not with bad cells... as long as those batteries are connected,  will the system still provide for the loads in best case conditions?   I realized the batteries have to be at least in decent enough shape to provide the very small amount of power for the Inverter and CC  (usually about 1 amp DC x 24v = 24 Watts),   but the way I am reading this discussion ... do even the inverter and CC  (which need battery power to power on and remain on at night) get their power from the Array thru the day?

In other words,  in 5-10 years when the batteries have reached near end of life as far as energy storage is concerned for loads, Would the system still function thru the day on the Array power alone?   And at that point,  couldn't the system work - in the day - for a long time past effective battery life as long as cells were not bad and the battery could provide 1-2 amps DC to power on the CC and Inverter?

What happens if a person does not replace their batteries when the batteries stop holding charge under heavier than 1-2 amp DC load?

A failing battery, at some point will drop below the voltage required by the inverter, and the inverter will shut off.   Eventually, the charge controller will not have enough voltage (overnight) and it too will shut down, and may not restart when the sun comes up
Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
|| Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
|| VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

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• Posts: 4,764Solar Expert ✭✭✭✭
The dial in (voltage settable) PWM charge controllers would likely continue to operate until the battery fails due to internal shorting. I would suspect a battery would present the correct voltage until this point as well for morning startup of MPPT type controllers as they require very minimal current, just a reference voltage.

I left a charge controller and inverter on with a small system and a pair of GC batteries, when I left for the winter. I had pulled them out of the sun and only left a single panel connected set on top of the box housing the batteries. It got covered with leaves. Leaving the inverter on was not intentional. After 4 months I returned to a dead system, but did have some power during the day. Battery capacity was nothing. Don't recall even putting a meter on them. This was an old system before MPPT type charge controllers. happened maybe circ 2004. Charge controller was a Specialty Concepts Mark IV /30 amp from the 1990's (I think from my faulty mind, still have it somewhere)
Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Prosine 1800 and Exeltech 1100, 660 ah 24v ForkLift battery. Off grid for @16 of last 17 years. Assorted other systems, and to many panels in the closet to not do more...lol