A few questions on battery sizing, maintenance and charging.

philreed52

Hi all, new member here from the UK.

My parents have been living off-grid now for the past 3 years and so far relatively successful. Starting out with a single Iconica 48v 5kw hybrid inverter with 2 strings of 8 panels and a fair sized bank of heavy duty 12v starter batteries. As imagined, the batteries didnt last forever and we then got our hands on a used set of 2v 960ah forklift batteries. We have since added a second inverter with 16 more panels (some of which are installed on the western side of the building for the time being as roof space is limited) We also have an 18kva perkins generator as back up.

Anyways, unfortunately our 'used' batteries have a fair few dead cells now that are over sulphated and swelling the cases so the whole lot are being swapped out for a replacement set of brand new batteries in the next couple of weeks. We know the batteries were second hand but feel poor maintenance on our behalf hasnt helped - we have never eq charged them. We do keep on top of the distilled water and inspect them regularly.

My own family are shortly building a bungalow on the same plot of land with the view of adding at least one more inverter depending on the amount of roof space and the power rating of the panels i can get hold of/afford. I will also look to purchase a second bank of batteries (they will be identical to my parents set). The plan being we all share the same grid with a15-20kw pv setup, 2x 24no. 2v 960ah flooded batteries. The generator and hopefully a wind turbine if is financially viable.

I have a few questions regarding the batteries.

Can our old set of batteries be revived by EQ charging? we dont have the data sheet unfortunately and dont know the brand to be able tofimnd out although I can ask the dealer we got them through as he supplies them new to forklift engineers,

What is the recommended way of sizing a battery bank to the inverters? Im of the opinion its better to be oversized to reduce the DOD on the bank BUT we do need to ensure they are getting adeqately recharged. (The generator is going to be wired to the remote start, on a timer so it doesnt fire up in the middle of the night)

Frequency of maintenance, how often are you all checking the SG?

Is there a rule of thumb for how often an EQ is performed? I'll have the data sheet when we get the new set.

Can we rely on the inverters telling us what is going on with the banks or should we add a monitor?

Sorry if its a long post LOL

Regards

Phil

Photowhit

Lots of general questions, Forklift batteries require equalizing once a month and checking electrolyte levels and topping of once a month for there early years and more often as they age. 

Used batteries are always a "roll of the dice". Typically, someone is selling because they are under performing. 

I'd suggest reading about equalizing in Rolls/Surrette manual as they split up maintenance and corrective equalizing. I'd hunt a link but it's too late now.

BB.

Welcome to the forum Phil,

Can you tell us a bit more about your system and power requirements...

What is the daily usage of the home (Watt*Hours per day, Amp*Hours @ xx volt battery bank, etc.)? Winter vs Summer?

What was the AH and voltage (48 vdc?) of the existing/old battery bank? Did the bank support your needs OK before it "failed"? Ideally looking at 25% discharge per day for 2 days of storage (50% max planned discharge for longer battery life).

The solar array--Somewhere around London or Exeter UK? Presumably, you do not have that much sun in your area. Or somewhere near a city on this list:

http://www.solarelectricityhandbook.com/solar-irradiance.html

Exeter
Average Solar Insolation figures

Measured in kWh/m2/day onto a solar panel set at a 39° angle:
(For best year-round performance)

Jan	Feb	Mar	Apr	May	Jun
1.34	2.25	3.00	4.07	4.70	4.64
Jul	Aug	Sep	Oct	Nov	Dec
4.68	4.34	3.60	2.45	1.65	1.13

And how many Watts of solar panels do you have in the array?

As a rough starting point, suggest for flooded cell battery bank a minimum of 100 AH @ 48 volts per 1,000 Watt of AC inverter (i.e., a 5 kW inverter would need a minimum of 500 AH @ 48 volt Lead Acid battery bank--And would be "nice" to have 2x that size or needed (1,000 AH @ 48 volt FLA battery for a 5 kW inverter if you are using a lot of energy per day).

For solar charging, suggest a minimum of 10% rate of charge for off grid solar power system... But for a in a "poor sun" area (call it less than 3 hours of sun per day which is probably October through February for your location)--A larger array is usually needed (13% to 20% at a guess) will usually keep the battery much happier (batteries are expensive and keeping them well charged and avoiding "deep cycling" prolongs life--And solar panels are relativity "cheap" these days).

We can go through the math here... But need a bit more information. Can design a system to support your needs. Or take your setup and "predict" its useful output.

-Bill

philreed52

Thank for the reply Bill, we have 960ah @48vdc battery bank (made up of 2v 960ah cells) but what we have found today is a complete array on one of the inverters has gone down and not been charging the batteries properly. Hopefully its not too late for them, the generator is running as we speak to charge them fully and ill measure the SG again. We live not far from exeter. I'll do the math on the panels later cant remember the spec,im sure they are 260watts 2x8 on one inverter, string of 8 and 6 on the second inverter

We have the whole set on the second inverter measuring 7v individually. something has killed the panels and we cant work out what yet. The original inverters panels are running at 30v each

thanks

phil

philreed52

https://forum.solar-electric.com/discussion/357740/low-panel-voltage-on-one-inverter-normal-on-the-other-could-they-have-all-failed

BB.

Lets do the math first... Using our general rules of thumbs for a full time off grid system:

960 AH of battery bank @ 24 volts. 10-13% is generally a minimum suggested rate of charge... probably 13%+ for forklift/traction batteries (higher self discharge rate):

• 960 AH * 59.0 volts charging * 1/0.77 panel+controller deratings * 0.10 rate of charge = 7,356 Watt array minimum off grid
• 960 AH * 59.0 volts charging * 1/0.77 panel+controller deratings * 0.13 rate of charge = 9,563 Watt array suggested min "Forklift" battery bank

If you have 260 Watt panels:

• 7,356 Watt array @ 10% / 260 Watt panels = 28 solar panels suggested minimum
• 9,563 Watt array @ 13% / 260 Watt panels = 37 solar panels possible minimum for "forklift" bank

And so some calculations based on your average loads and amount of sun you receive. A good optimum usage is to use 25% of battery bank capacity per (dark) day, for 2 days to 50% of capacity (you can go down further if needed, but try to avoid):

• 960 AH * 48 volt * 0.85 AC inverter eff * 1/2 days usage * 0.50 max planned discharge = 9,792 WH = 9.8 kWH per day load

And amount of sun you get per day over 12 months for Exeter UK, fixed array facing south:
http://www.solarelectricityhandbook.com/solar-irradiance.html

Exeter
Average Solar Insolation figures

Measured in kWh/m2/day onto a solar panel set at a 39° angle from vertical:
(For best year-round performance)

Jan	Feb	Mar	Apr	May	Jun
1.34	2.25	3.00	4.07	4.70	4.64
Jul	Aug	Sep	Oct	Nov	Dec
4.68	4.34	3.60	2.45	1.65	1.13

Toss the bottom three months (assume genset and/or cutting way back on energy usage during poor weather):

• 9,792 Watt*Hours per day * 1/0.52 off grid AC system eff * 1/2.25 hours of Sun February average = 8,369 Watt array "February break even"
• 8,369 Watt array / 260 Watt per panel = 32 panels Feb Break even array

You can add a larger array to reduce genset fuel usage... For example, suggest you only use 50% to 65% of "planned" average harvest... If you wanted "avoid" genset usage in February, then for example:

• 8,369 Watt array * 1/0.65 genset weather fudge factor = 12,875 Watt array for "low genset usage" in February...

You could do the math down to December at 1.13 Hours of sun per day... But that would be yet a 2x larger array. May not be economical for your needs (just not much sun in deep winter for you).

In general, over paneling your off grid solar system is a good idea... But not always practicable in low sun regions.

The above are not hard and fast rules--But usually are "close enough" to guide you to a working/cost effective system. If you have other or special needs for power (running fans for drying crops, arc welding, different power needs for winter vs summer, etc. Can modify the results.

I will go over to your other thread about possible loss of array power...

-Bill