One battery bank or several smaller in parallel?

rabbit_39 · March 2012

So assuming the calculations are done correctly, one of the systems that we came up with requires 2000Ah batteries x 48VDC. Of course we would need several charge controllers and pv arrays to charge that.

The question now is, would it be better to have several smaller battery banks wired in parallel for output (but fed by their own individual pv arrays and charge controller) or a single large battery bank fed by the same as above individual pv arrays and charge controllers wired in parallel into the single battery bank?

Thank you very much. We're getting conflicting answers from the battery guys and from the solar charge controller guys (Outback and Schneider).

Andre S

westbranch · March 2012

Re: One battery bank or several smaller in parallel?

what are the conflicting recommendations?

I guess it all depends.....on what they want to sell you,.... but more specifically how it works for you.

A few more details would help.

HTH

rabbit_39 · March 2012

Re: One battery bank or several smaller in parallel?

The system is for a communication network device that runs 24 hours/day. But the load depends on the amount of "chatter" on the network. Max power is 1500Watts and the expected usage breakdown is:
1500W x 12hours = 18kwh
1200W x 6hours = 7.2kwh
900W x 3 hours = 2.7kwh
400W x 3 hours = 1.2kwh
TOTAL = 29.1kwh/day

So about 30kWh/day.

The arrays we're planning are 3500Wp PV panels x 3 (in an area that gets average insolation of 5hours according to the NASA data).

The battery companies are saying we should use several banks of smaller capacity batteries.

The solar charge controller companies are saying we can use a single bank (charge controller output in parallel) or several banks (charge controller output into their own respective banks and the output of the battery banks are wired in parallel to serve the loads).

What are the pros and cons of each system? I would think there would be less losses if I use a single battery bank.

Thank you,

westbranch · March 2012

Re: One battery bank or several smaller in parallel?

IMHO think Redundancy, if one fails there would be at least 1 , or more backups to keep things running.

big banks are harder to manhandle etc.
small banks have more (duplicated) issues with connections, wiring and more spares
multiple banks can discharge a differing rates.

Is that NASA data for December?

BB and others are far more knowledgeable than I so they will probably be on tomorrow.

HTH

Can you be a bit more detailed on the 30Kwh, those numbers don't quite add up

rabbit_39 · March 2012

Re: One battery bank or several smaller in parallel?

Yes, that NASA data is for worst month of the year (which is in January for this location in Indonesia). I assume that's what you're getting at?

I added the total kwh/day to be 29.1kwh/day that I rounded up to 30kWh. Am I missing something?

Thanks for the input

westbranch · March 2012

Re: One battery bank or several smaller in parallel?

yes you want to use the worst month, at least as a starting point.

Hmmm Indonesia, you will have warm temps so there will be a temp derating for the PV's due to temps above 25 C. this in addition to general system derating, PV to battery.

I was just thinking out loud after adding to 29 too. rounding... yup
the other point I see is the 24 hour demand mapped (graphed) out to visualize the max and min usage, then seeing which configuration(s) will work for you, demand vs 1 or more Batt banks.
HTH

rabbit_39 · March 2012

Re: One battery bank or several smaller in parallel?

The demand will be higher during the day, night usage is about 40-50% of that 30kwh. Perhaps I'd do better with a 1500Ah and increasing the PV panels. Hmmm I'll have to see if I have the space for it. The space is 15m x 10m with a 5mx5m tower and I'll have to use up all that room I think. Which means I'll have to make sure the sides that will get shaded by the tower be their own array. Nuts! grrrrrr!!! Oh well, will have to see what I can figure out.

Yes, average PV temp here is quite high, all year round. But it does get made up by having sunshine everyday (just about). It's a rare day that we get dark and cloudy throughout the day. Just looking at rough data, I get twice the daily energy here than in Portland, OR.

westbranch · March 2012

Re: One battery bank or several smaller in parallel?

I found that taking site pics with the sun at your back, at 15 minute intervals, was an eye opener regarding shadows. best if you use a tripod, load them into power point and just watch the show, amazing what you will 'see'...

you haven't mentioned battery voltage, any preferences? the load/capacity will have a determining influence.

rabbit_39 · March 2012

Re: One battery bank or several smaller in parallel?

It will be 48VDC for something this size. Plus some of the equipment are 48VDC input already.

The loads will really be mostly the communication gear (1400W max) and the rest are small things like:
- tower aircraft lights (I may use a solar powered unit with built in battery)
- outside lights (about 30W LED lights total)
- exhaust fans (about 50W)

niel · March 2012

Re: One battery bank or several smaller in parallel?

don't forget about those pesky cloudy/rainy days i'm sure you get. you must account for the worst case you can expect to continuously need to run (days of autonomy) without the sun and a generator would be a good backup to this too even though gasoline is high priced. continuity is crucial for you.

this same line of thinking goes for the pvs as well for you may need to up the pv capacity to bring the batteries up faster due to weather or other unforeseen event interrupting power production with controllers, inverters, and even battery failures are possible. the efficiencies also need to be compensated for.

this is not a small undertaking.

rabbit_39 · March 2012

Re: One battery bank or several smaller in parallel?

Yes, it does get cloudy/rainy once in a while. I was hoping after a one year run, if needed we can add another array of 3500Wp. I suppose this is another plus for smaller battery banks. I can add more capacity later without having to double a 2000Ah battery bank. I like to keep battery banks that are being paralleled for output to be the same capacity.

There will be an 8kw diesel generator hooked up to the inverter/charger (Outback).

All this talk makes me think that for 30kWh/day usage I might want to use 4 arrays x 3000Wp from the start. After accounting for the losses I should get 40-45kWh/day of it. (12000Wp x 5 hours x 0.7)

I should still be good with 96kWh of battery storage (48VDC x 2000Ah or whatever configuration I'll end up using).

Any issues with my calculations so far?

If the numbers are right, I'd like to also hear more about the single battery bank vs multiple (ie 2 battery banks each fed with 2 of the arrays/CC OR 4 battery banks each fed with 1 of the arrays/CC).

Thank you all.

vtmaps · March 2012

Re: One battery bank or several smaller in parallel?

rabbit_39 wrote: »

The battery companies are saying we should use several banks of smaller capacity batteries.

The solar charge controller companies are saying we can use a single bank (charge controller output in parallel) or several banks (charge controller output into their own respective banks and the output of the battery banks are wired in parallel to serve the loads).

What are the pros and cons of each system? I would think there would be less losses if I use a single battery bank.

Your instincts (single bank) are good. Read: http://forum.solar-electric.com/showthread.php?14674
--vtMaps

rabbit_39 · March 2012

Re: One battery bank or several smaller in parallel?

The issues in that thread seems to be if there is only one charging source? Would some of those issues be irrelevant since each battery bank is being charged independently?

Oh wait, but the output is being paralleled. So the same issues will still happen just on the other end. Heh, silly me.

stephendv · March 2012

Re: One battery bank or several smaller in parallel?

I'd go for 1 big bank, with individual 2V cells. You can then replace individual cells if/when they fail without losing the whole system (check whether the outback inverters can handle this, but I know other inverters can be programmed to work with a 46V or 44V nominal battery bank).

You could also consider partial AC coupling using a high efficiency transformerless inverter since the majority of the loads are during the day. E.g. 1500W on a grid tie inverter, and the rest of the PV on DC charge controller. If you're dead set on using the outback inverters and AC coupling then you'll either have to:
a) make sure that the peak PV power will always be consumed by the loads, e.g. if you connect 1500W in AC, then you must always use at least 1500W from the loads.
b) put a dump load controller across the battery to absorb excess power if the AC connected PV produces more power than the loads can consume.

rabbit_39 · March 2012

Re: One battery bank or several smaller in parallel?

stephen: If I understand you correctly I will need 2 inverters. A grid tie and an off grid. This will increase cost and complexity, right?

With the outback inverters and CC, the way I understand it, I won't need a dump load controller. If the batteries are full and I am using less than what the PV produce, the PV simply just won't produce more. If I had wind, then yes I would need a dump load controller. Same with hydro. Am I understanding this correctly?

So the way I will have this is:
- several arrays going into their own CC
- output of CC will be paralleled into a single big bank
- batteries will power a 48VDC offgrid inverter/charger (if I go with outback it'll be their 2300W or 3000W)
- batteries will power some loads directly with 48VDC
- generator will be hooked up to the inverter/charger

anything I am missing?

stephendv · March 2012

Re: One battery bank or several smaller in parallel?

rabbit_39 wrote: »

stephen: If I understand you correctly I will need 2 inverters. A grid tie and an off grid. This will increase cost and complexity, right?

Yes and yes

What you'll gain is maybe a 2-5% increase in efficiency for the power produced during the day.

rabbit_39 wrote: »

With the outback inverters and CC, the way I understand it, I won't need a dump load controller. If the batteries are full and I am using less than what the PV produce, the PV simply just won't produce more. If I had wind, then yes I would need a dump load controller. Same with hydro. Am I understanding this correctly?

Yes, correct again. You'd also need the dump load if you connect a grid tied inverter to the outbacks.

rabbit_39 wrote: »

So the way I will have this is:
- several arrays going into their own CC
- output of CC will be paralleled into a single big bank
- batteries will power a 48VDC offgrid inverter/charger (if I go with outback it'll be their 2300W or 3000W)
- batteries will power some loads directly with 48VDC
- generator will be hooked up to the inverter/charger

anything I am missing?

Seems fine. You'll also need the additional outback bits and bobs like the mate, hub etc. And maybe an FNDC to better control the whole system. Might be worthwhile checking the prices for aSi (thin film) solar panels since they're slightly better at higher temperatures than poly. But the recent price drops in poly might not make them economically viable.

BB. · March 2012

Re: One battery bank or several smaller in parallel?

One other specification to check for... Some of Outback's Inverter/Chargers are designed for grid use only. They have a very tight input frequency specification (something like +/- 1% or ~0.5-0.6 Hz) which is fine for utility power--But not (usually) good enough for generator power (+/- 10% at least).

You don't want your AC backup charging system to go down just because a generator governor is slightly out of RPM specifications.

-Bill

westbranch · March 2012

Re: One battery bank or several smaller in parallel?

did a quick check on Victron Inverter and their 1200 to 5000VA models will take a wide range of input voltages especially the '48v' model 38v - 66v
http://www.victronenergy.com/upload/documents/Datasheet%20-%20Phoenix%20Inverter%201200VA%20-%205000VA%20-%20rev%2005%20-%20EN.pdf

65DegN · March 2012

Re: One battery bank or several smaller in parallel?

I'm voting for the single battery bank, it would be a pain to have to maintain all those connections and monitor the state of charge of multiple banks and balancing the loads with the charge sources.

One battery bank or several smaller in parallel?

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