Question on battery replacement

[mblevins]

Hello,

Found this forum and have been lurking for a while. I'm looking for a sanity check:

I have a hybrid solar system that's about six years old.

- It has 48 150W panels for around 7kw output
- 3 Xantrex (Schneider) MMPT60-150 MPPT controllers
- 2 Xantrex (Schneider) XW4548 Inverters master/slave (4500W each)
- Originally 16 6 volt batteries in two series of 48V each
- "Deka Solar" batteries, I think 370AH @ 20 hour rate.

After doing zero maintenance on the batteries for five years, the system completely failed last year. I removed some obviously failed batteries and was able to make one string of 8 not-completely-dead batteries. This brought the inverter back up, but it doesn't seem to be running at full capacity, and I have about a minute and a half of battery backup.

I need the backup less than I did when we bought the system -- the power grid is better, we put our water tanks for our well up a hill for gravity feed water, and we have a wood stove for heat/cooking. So we really just need to run lights/fridge/tv for convenience. I have a potable generator for extended outages for refreshing the water tanks or cooling down the fridge Honestly if I were to do this over again, I would have gone grid-tie/generator.

My question is if I can get new batteries significantly smaller than my original installation. My main concern is making sure the inverters are running efficiently in grid-tie mode, some amount of battery backup is a plus. I'd like to use 8 6 volt Trojan T-105RE's (225AH @ 20 hour rate). There's a Trojan dealer nearby where I can get the batteries.

Will this work, is there anything I'm missing? This gives me about a 1/3 of the backup capacity of my original system, but I'm good with that if it gets the inverters working reliably again in grid-tie mode. The manual for the inverter just says at least "100 AH" for the batteries.

[Sunking]

Where is your generator?

Batteries have minimum and maximum charge prameters and max discharge parameters.

On the charge side of FLA batteries like the T-106 you do not want to exceed a C/8 charge rate. On the T-105 you can go as high as C/6 or 37 amps. At 48 volt battery is around 1800 watts maximum. That means throw 2 charge controllers away, and reconfigure your panels so one charge controller has about 1800 watts on it.

On th edischarge side a 48 volt 225 AH T-105 batery can only handle a 2000 watt load for just 2-3 hours max.

[mblevins]

Thanks Sunking,

Where is your generator?

Generator is portable, it isn't tried to my system. I can plug it into a dedicated plug in the house or to a mechanical transfer switch near my water pump. Is just there for multi day outages, which we seem to have every few years. I'm good with that part of it.

Batteries have minimum and maximum charge prameters and max discharge parameters.

On the charge side of FLA batteries like the T-106 you do not want to exceed a C/8 charge rate. On the T-105 you can go as high as C/6 or 37 amps. At 48 volt battery is around 1800 watts maximum. That means throw 2 charge controllers away, and reconfigure your panels so one charge controller has about 1800 watts on it.

I see, I completely missed that (and looking at the wiring diagram, I can see the controllers connected to the batteries now). So if I want my controllers feeding the grid, I still need to size for max battery charge, so I need
7000 = c/6 *48, c = 7000*6 / 48, c = 875 AH

That's actually over my original installation. Is there any other way of preventing over charge for the batteries while still keeping my controller output for grid-tie? Should I contact Schneider?

On th edischarge side a 48 volt 225 AH T-105 batery can only handle a 2000 watt load for just 2-3 hours max.

That part is fine for me, although might be moot w/ my fumble w/ charge rate.

[mblevins]

Replying to my own post. Thanks to Sunking for his excellent stickies in the battery/off-grid forums, and guiding me which questions to ask.

Here's the reply from my question to Schneider:

When you are selling back to the grid with the XW/XW+ inverters, we recommend that you have a capacity of at least 440Ah at the C20 rating.

At lower capacity, we have seen the battery voltage waver or ripple. This is cause by the inverters actually digging into the batteries capacity rather than just the surface charge like it normally would. If the batteries are very small (capacity-wise) or sulfated, then the ripples can drop the battery voltage enough to start unnecessary charge cycles.

Overcharging should not be an issue with the charge controllers. They can regulate their output while charging to make sure your batteries are not getting overcharged. The inverters are a different story, you will want to limit the 'Max Charge Rate' on those to make sure they do not put too much current into the batteries. Each one is able to put in 85A into the batteries if allowed to.