SMA Sunny Island battery questions.

1) LIFePO4 is Lithium Iron Phosphate battery chemistry (most likely the Lithium Iron battery they were talking). It tends to be the "safest" of the Li Ion family (least likely to catch fire if "something goes wrong"). LiFePO4 batteries tend to have a lower charging voltage (around 3.6 volts or so vs 4.2 volts for other "common" Li Ion rechargeable chemistry types). Also LiFePO4 tend to be have a bit less energy per kg than other Li Ion types.... Overall, a very common battery type for homes and RVs (where a slightly larger/heavier battery is OK).

With solar panels, the "controllers/inverters/etc.) can draw as much or as little energy (aka current) from the solar panels as needed by the loads/battery charging/etc. without damage to the panels.

Batteries, in general, will eventually be destroyed by over charging (or under discharging). Lead Acid batteries tend to be more forgiving... Ii Ion batteries much less so.

GT / Hybrid inverters can be setup to send charging current to the battery bank and the loads. If there is "excess" solar power available, they can feed current back to the main AC panel--For use by other AC loads and/or feeding back to the utility and "turning the utility meter backwards" (details matter here--But this is the basic idea).

In many cases, feeding energy back to the utility is allowed and supported by "most" utilities. There are variations in billing/credits for power, and in many locations, it can be "illegal" to feed back energy to the utility (their power lines, their rules).

Limiting power back to the grid from a Hybrid (GT/Off Grid capable AC inverter+battery bank) is a commonly requested function, but you have to find Hybrid inverters that will perform this task--Many/most(?) do not support zero backfeed rules.

Solar energy (from solar electric panels) is a "use it or lose it" proposition. If you do not use it to power loads, charge battery bank, or feed back to grid (and get credit on your bill)--Any unused electricity is "lost".

Getting this all this stuff to play well together can be a pain. And if the hardware does not support what you need--Won't work. If the vendor does not provide detailed support for what you need or does not answer your questions--Can be a big issue too.

There are different types of charge controllers... The "series" type that (basically) turns on and off current to battery bank to set charging voltage... And their are parallel or dump type charge controllers... These (typically) connect to the battery bank and simiply turn on current to a resistor bank and get rid of "excess charging" current.

Series type controllers are typically used with solar panels (they can be "turned on and off" just fine).

Parallel type "dump/bypass" controllers are used (typically) with wind turbines and hydroelectric turbines. Typically these must have a constant load (such as a battery bank or resistor bank)--They cannot be "turned on and off" electrically--If turned off, the wind/water turbine can overspeed and self destruct.

Feeding energy from a Hybrid Controller is like a "dump type" charge controller--Excess solar power is passed to the grid to reduce your electric bill/receive billing credits. But since most are solar array types--They can also "modulate" the array current to run DC/AC loads and keep the battery bank properly charged just fine.

Note: Many/Most Li Ion battery banks have a BMS -- Battery Management System. Yes, in many cases you can install a "standalone" Li Ion system just like a Lead Acid bank (following the "rules and specifications")... But with BMS--They are "smart devices" and there are advantages to integrating your BMS with the Hybrid Inverter-Charger. Can make monitoring and managing your system much easier.

But, that BMS+Inverter is another level of complexity--And generally a Hybrid Inverter mfg. will pick and choose which BMS/Li Ion batteries (brand & model) they will support... This is not a trivial task.

2) Generally, you should work from your loads (Watt*Hours or kWatt*Hours per day and peak power requirements Watts). What is needed battery bank AH capacity wise to "run" the inverter-charger is usually smaller than the AH capacity needed to run your daily loads...

Yes, you can run a small 150 AH (at 48 VDC battery bus?)--But that is like placing a small fuel tank on a large tractor trailer rig... It will start the engine and get the truck to the next city--But it will be out of fuel when it gets there. There are other issues with "size of battery bank" (peak starting load support for motors and such). But a good "rule of thumb" for Lead Acid batteries is around 100 AH @ 48 Volts per 1,000 Watt (1 kWatt) AC inverter power (and solar array charging power). A 4.4 kWatt AC inverter would work "nicely" with a minimum of 440 AH @ 48 VDC battery bank... You probably could get away with a 1/2 size (AH) Li Ion battery bank for Discharge and Charging current--But it would still be a "small" battery for typical home off grid power usage (i.e., power your home for 1 day without sun, vs the 2-3 Days of stored energy suggested for full time off grid solar power systems).

3) GT AC inverters feed 100% of the available solar array energy to the AC mains box in the home. That energy is "transparently" used to run your home's AC loads, and either "slow the billing meter down" if there are more load current than solar panel energy... Or even "turn the billing meter backwards" if there is more solar power than AC loads in the home).

Can this unit do that? Can you provide a link to the SMA unit you are thinking of using?

Are you planning on doing the install yourself? Most mfg. do not support Do It Yourself installs (DIY homeowner installs). They typically want installers trained by the manufacture to do the system design and install--And provide Mfg support to those companies/installers.

If you DIY install... You can be in a difficult situation. Spent the time and money to purchase and installs the product--But it does not do what you need. Is that your fault, not enough configuration information in homeowner manual, or there is a problem with the hardware itself (need firmware update, turns out the hardware does not do what you want (even if sales literature says yes, it can) Mfg. may not provide technical support to answer those questions, and may be difficult to return the equipment for cash back).

I would ask for references from the retailer for local (to you) customers that have installed systems like yours, and that do what you want/need the system to do--Successfully.

3a) No--A simple mains disconnect will not "solve" the energy backfeed problem. Feeding back to the grid will not 'hurt" your battery bank. Typically, the solar will recharge the battery bank (with AC mains/AC backup genset as "backup" charging sources). Usually, it your electric utility that has rules on what GT/Hybrid AC inverters can be connected to their AC Grid "legally" (legal vs functional--Always an interesting question--One which the utility will "win" if you ask them for answers).

3b) What/how you wire your hybrid AC power system to your home's wiring is up to your needs and what is legal in your region/utility service area (which may not always make engineering sense).

Your two main options... You setup your off grid/hybrid system to only run "emergency" loads (lights, refrigerator, washing machine, well pump, etc.) during power outages and when it is sunny. This is setting up a "protected" sub panel (breaker panel) which takes energy from the hybrid inverter and supplies the minimum number of circuits to your home.

Another option is to size your off grid/hybrid power system to power all of your home's loads--Lights, fridge, pumps, washer, dryer, entertainment system, garage shop, etc.... Typically design for running 9+ months directly from Solar+battery bank, and the "dark months of winter", use utility power and/or backup genset as needed (during bad/cloudy weather).

Note that GT Solar Inverter system (soalr panels => GT inverter => House mains) is the cheapest and most reliable/cost effective solar power system. And can save you money (if you have enough sun, and your utility allows GT solar with "customer friendly rate plans"). This system only provides power when the utility power is present (car or storm takes down power lines, your solar power system is "dead weight"--There are a few options for a bit of daytime only emergency power for some brands/models of GT solar).

Hybrid solar power systems (solar panels => charge controller => battery bank => AC inverter Off grid or Hybrid). This type of system will not (in general) save you money (batteries, hybrid inverter, replacement batteries every 5-15 years, etc.) generally cost more than your utility power bill would (and you are paying most of the costs "up front" to install and get the system working vs 20 years of payments to your utility).

You really need to decide what you want--Save money / Emergency Backup power / "Go Green" / etc... And do a bunch of paper designs and costs first--Before spending the money.

If this is your first time--Having a local designer/installer with local experience and references can save you a lot of headaches.

The second system--After you have gone through the first design and install--Maybe you can tackle that one yourself.

-Bill