Battery loss grid-tie

[veggieburner]

I have a 1.5 kw wind generator and 1.44 kw of solar, both are connected to a 48 volt battery bank (4 12 volt UPS battery's the size of a car battery). Then I have an outback 3648 grid-tie inverter selling back to the house/grid. My question is, would 4 small 12 volt motorcycle type batteries be less of a loss than the 4 larger ups batteries.

[dnix71]

If you have a reliable grid, you don't need batteries. If you don't have a reliable grid then you need enough batteries to carry you over.

4 UPS batteries isn't much backup. Motorcycle batteries would be almost useless, as well as not being suitable anyway. Motorcycle start batteries are not made to be deep discharged. It will ruin them. The UPS batteries are made to do that and recover when recharged.

[Ungrounded Lightning Rod]

He's not using the batteries as a backup.  He's using them as a filter.  Like an enormous capacitor.  They fill in the gaps in the alternator ripple and slow-ramp the voltage when the wind shifts or a cloud shades the sun, giving the grid-tie alternator (which understands battery bank levels) a chance to react properly.

[Ungrounded Lightning Rod]

My question is, would 4 small 12 volt motorcycle type batteries be less of a loss than the 4 larger ups batteries.

The loss from leakage would probably be lower.  The loss from battery resistance would be higher.

You want a battery capable of handling the full current provided by your panels without taking damage, overheating, or going very resistive due to polarization (gas bubble formation causing a significant part of the plates to lose contact with the electrolyte.)  This is in the ballpark of 125 amps - perhaps much higher during wind gusts on a sunny day.  Your batteries have to handle the difference between the charge and load currents at all times - and if the line trips out your load is just your dump load, which will probably be either off or full-on.

Imagine the currents seen by the batteries if there's a storm (frantically spinning the mill) which knocks out grid power...  Seems to me your car-sized deep-cycles are at the low end of what you'd want for this much generation.

Meanwhile, losses from leakage are pretty low if the batteries aren't at end-of-life.

So I'd stick with the (moderately) big batteries.

[Ungrounded Lightning Rod]

OOPS!  Divided by 24 rather than 48.  So that should read:

[Your charging current] is in the ballpark of 63 amps - perhaps much higher during wind gusts on a sunny day.

That's still pushing a "car battery sized" deep cycles darned hard.  Their capacity is probably a bit under 100 amphours, right?  So when they have to swallow the whole output of your generation farm you're feeding them about C/1.5 or more.  That's getting pretty close to cranking the starter continuously, then feeding that much current back to fast-charge the batteries.  B-(

So IMHO you're still at the low end of the batteries appropriate for this service.

[electronbaby]

On a system with an inverter that is capable of stand alone, OR grid-tie operation, size the battery bank for what the inverter is capable of doing, supplying power off grid. Especially if you have wind. It is better to have a decent size bank because of the large current inrushes you may see in stormy weather. I understand you want to increase efficiency, BUT  if you really wanted to increase efficiency, you wouldnt have bought the battery based inverter anyway. The manufacturers have guidelines for this sort of thing. It should be in the outback manuals.

Having wind hooked up will make your batteries live a longer life (if the bank is sized appropriately) based on the constantly fluctuating current coming in. The larger currents help to better mix the electrolyte and stand to "equalize" better than the PV can on a normal basis.

I would check the manuals.