re-commissioning a system, new batteries

[ralph day]

Also posted with OUtback and WindSun forums. 

Hi all
A friend of mine has had bad luck (experiences) with his off grid system.  The original installer made a mess of things leaving the owner with a badly executed system.  Undercharged batteries for the first year, no way to find out why (no generator either at first).  The first set of L16's experienced a runaway charge event and many of the 16 batteries were toast.  He ran a little longer on some scavenged/salvaged L16's.

He then installed a 10kw system like mine, straight feed to the grid, no net metering possible (or use of any of the power).  At the same time an electrical service was installed for the house and the off grid system was de-commissioned.  Batteries sold for scrap value.

My system (with batteries still) allows me to use the grid when I want, charge or just have the utility carry the loads and let renewables charge and or charge and charry all the loads.  I think I've convinced my friend to get some batteries so he can still use the valuable MX60 and double Outback inverters etc, and use the solar power available to him. 

Here's the question:  How much battery capacity is required to really just have the system act like a big UPS?   Cost is a factor.  Enough battery to be off grid for 3 days...not needed, the propane generator is always available still.  1.8kw of pv, 2x 3600watt OB inverters and one MX60.  I'm thinking a minimum of 400amp hours (48 volt system).  AGM would be better than FLA's, time and inclination to work with FLA's is lacking on his part.

Contributions and suggestions are appreciated.

Ralph

[mike90045]

The size of your emergency loads, and the expected duration, determines the size of the battery. 

Often in areas with generally reliable grid, a backup generator is a better choice than a battery bank that is aging and dieing the day you install it.

[ralph day]

Hi Mike
The generator is usable for backup.  The site has a pre-existing off grid infrastructure (MX60, panels, inverters, just no batteries).  We're trying to figure out the minimum amount of batteries needed to resume use of the off grid infrastructure with the grid  available too (night, cloudy etc).  The grid tied panels and inverters are strictly feeding the grid, no net use possible (80.2 cents per kwhr paid out)

Not like a Schnieder 6048 grid tied, but with a battery backup.  The backup is not really necessary, just the ability for the inverters to make use of the solar power available (maybe HBX mode on the mate).


Ralph

[mike90045]

Are you asking for battery backup for a Grid Tie system with a string inverter ?  Those generally run 300- 500VDC, and a battery is not safe for consumers at those voltages.

[CDN-VT]

So cost we'll deal with first .
Many golf courses that rent out carts will sell old GC 110Ah  6V  ones , the 8Vdc are a pain but some of the prices are two good to pass up. Many are just not charged or looked after ,suffer bad connections etc.

When I helped a few , I collected many old starting batterys as cores the golf course maintenance shop could return a 12Vdc unit & keep the old 6Vdc GC battery that I could buy for cheap 10--20 each and I would find one out of ten  that was toasted. These were 2014 dated battery we picked up in feb 2015.

They don't really know why the cart wont make the full course , so they replace all it seems. 

Here you need charge them and remove electrolyte & keep till the water boils off , and add back in (as that what we found).
Enough to get 16 for a 220Ah 48Vdc &  mate & hub it to grid tie back feed , & cheap training for the friend to learn .

Hydro  needed

VT