Minimum Battery size?

[kingofbanff]

If the focus was not on worrying about how many Kwh you need to power your critical loads but instead the smallest battery string that wouldn't be fried by lots of power coursing through it on the way to the loads or the grid, how would one determine that?

Specifically I will have 13kw array feeding through Outback 8048A and 4048A which then goes to battery charging/loads/sell to grid.

I don't quite grasp how the flow of electricity works. Let's say I have a nice sunny day and 12kw being produced. The Outbacks are maxxed. The battery string is charged. Does any electricity even pass through the battery at that point? Or do the Outbacks sense resistance in them and say" nope, those guys don't need power lets send it to the loads and grid"?

If this is the case could, in an extreme example, you have one lonely battery?

thanks in advance.

[Eleceng1979]

Short answer...no, you need more than a basic, small battery. Why buy a battery inverter if you have no plans to buy the required batteries?

Imagine putting your mouth on a fire hose...this would be a 13kw array on the smallest battery. It would look like a cheech and chong movie...up in smoke

Outback specifies a minimum battery size per kw of solar/inverter. Without wasting my time... I would guess the minimum size to be above 1300ah @48v or in other words $$$...like $7000 for a used forklift battery as the cheapest solution. Usually something like 100ah per kw, but I'm playing yard darts here...

Power flow is simple...you have a hose (pv) and a bucket (battery) and a hole (load).

If the hose overflows the bucket due to a small hole, overflow goes to grid. The radian can be programmed in different modes btw... but I'm being simple...

If the hole is big, bucket is never full, grid fills in

If hose is kinked and off, bucket empty, grid charge , grid power only

If your hose is itching or burning consult a doctor

You need an adequately sized bucket to your hose...ever pissed in a Dixie cup? Or a swimming pool? One overflows and one never knew you was there...batteries are identical. The charging/discharging rates have limitations and guidelines, hence the sizing.

No matter what, a battery inverter needs a properly sized battery 100% of the time. Hence the minimum requirements. It can ac/dc couple, bypass grid to loads, get charged from a Gen or grid, have wind, hydro, pv, or magical elves as power inputs but will always need a proper battery.

[Mike90250]

I recall that for every 1Kw of inverter load, you need 100ah of battery (at 48V, this will change to 200ah @ 24V) to forestall "ripple effect" of the AC loads (local or grid) disturbing the DC bus.
This requirement has nothing to do with battery depth of discharge - which is another subject, just inverter loading vs battery size

[Wy_White_Wolf]

If you wish to ignore the load than minimum battery would be determined by the maximum charge/discharge rate. So if you battery manufacturer claimed a maximum charge rate of C/5 an all of your 13KW array is programmed to the batteries that would be 270A @ 48V. So 270A * 5 = 1354AH battery bank. You would also need to run the number for discharge as that rate may be different from the charge rate.

WWW

[Sunking]

Generically speaking the maximum charge and discharge current for a Flooded lead Acid Battery is C/8, and for AGM C/4. Where C is the battery Amp Hour rating. So if you have a 100 AH battery it can only be discharged at a maximum of 12.5 amps for FLA or 25 amps for AGM. You might be inclined to think AGM would be less expensive, but it is NOT.

So lets say you have a 48 volt battery and a 13,000 watt load and you want to use a FLA battery. At 13,000 watts / 48 volts = 271 amps. It would require 8 x 271 amps = 2168 AH battery. Just call it 2200 AH. You are looking at a 6300 pound battery costing you $25,0000 ever few years in replacement cost.

Bottom line is you have to consult the battery manufacture as max charge rates for FLA range from C/10 to C/6 with C/8 being typical.

That ought top put an end to your adventure. If you use 48 volt battery with 13,000 wat panel will require 3 very expensive charge controllers that cost $600 each.

[ButchDeal]

I agree FLA is generally the way to go for off grid but for on grid backup I would generally push AGM and particular long shelf life AGMs. There are several reasons, the homeowners do not want a bunch of maintenance on these systems, you can go with smaller battery (we size for the load and owners have correct expectations), The long shelf life versions are important.
With the long shelf life versions they lose power more slowly, meaning that there are less days the charge controller goes into charge mode. This means more power is produced AND less cycling of the batteries.

Given that this topic is about minimum battery size AGM is the way to get that.

[Sunking]

Butch I have always said consistently AGM has it place in Solar applications. You just have to justify the expense of using them. I will not go into all the reasons as I have said it a thousand times. But one reason is high charge/discharge rates in excess of C/8.

I would agree with you on shelf life if you are talking about storage. AGM does have less self discharge, thus can stay in storage longer without be refreshed as often. I will give you that. But on a system that Floats every day is a moot point. In the morning when the sun rises, rather the battery is AGM or FLA, the controller goes through the 3 cycles in a few seconds and into Float. We are talking milli-watt hour difference. About one small beer fart.

[ButchDeal]

Butch I have always said consistently AGM has it place in Solar applications. You just have to justify the expense of using them. I will not go into all the reasons as I have said it a thousand times. But one reason is high charge/discharge rates in excess of C/8.

I would agree with you on shelf life if you are talking about storage. AGM does have less self discharge, thus can stay in storage longer without be refreshed as often. I will give you that. But on a system that Floats every day is a moot point. In the morning when the sun rises, rather the battery is AGM or FLA, the controller goes through the 3 cycles in a few seconds and into Float. We are talking milli-watt hour difference. About one small beer fart.

I think we are pretty much in agreement here. BTW, mine are cheaper AGMs without the long shelf life (slightly better than FLAs) and my CC gives them about 200wh every morning. But on the 14th they got 3kwh boost. This happens about every other week. The longer shelf life ones wouldn't need that boosting less often. But then I got my batteries from Battery mart (on sale

[Sunking]

I think we are pretty much in agreement here.

I think so, just a different spin on it. Am curious how you burn off 200 wh over night? Something is pulling that power.

[ButchDeal]

that is just what outback reports to opticsRE that is going into the batteries during the morning float charging. I should have qualified where that info comes from. As you mentioned normally it spends little time in the float charge.