What batteries should I buy?

[TRW]

If I were to go to an automotive battery supplier, could I buy 12 volt batteries to use with my 10 KW solar system? If so what kind of battery should I buy? Is this a reasonable alternative?

 

[gnubie]

Anything is possible but you'd have to make sure they don't try to sell you a cranking battery. You want a storage battery. Are you aiming for series connection of the batteries, say 48V? 10kW at 12V is up around 850A.

 

[TRW]

Interesting, any idea how many I would need? I understand hooking 4 in series would give me 48 volts, but that is all I know.

 

[Hedges]

Here's the kind I bought:

Sun Xtender Solar Batteries - Photovoltaic Battery

The Sun Xtender Battery line consists of specialty lead-acid solar, photovoltaic batteries, and renewable energy storage batteries manufactured by Concorde Battery Corporation.

sunxtender.com

AGM, so no maintenance.
For lower cost, there are wet cell, golf cart type and others.


"10 kW solar system"
Is that 10 kW peak inverter output? Or 10 kW peak PV output?

Batteries have various requirements, including maximum (and sometimes minimum) charging rates.
The type of system you have may determine whether the battery has to be sized to take maximum PV current, or of current can be limited to allow a small battery.

Beyond that, how many Ah or Wh depends on what you want to operate, and for how long, before you see the sun again.

 

[TRW]

In an idea world I would like to power a ground water heat pump that is about 30,000 BTU. The 10 KW is what the PV array can put out under full sun. Thanks for the info!

 

[gnubie]

The trick with DC coupled systems is that unless the charger (standalone or integrated in the inverter) can separate battery charge current from load current, the recommended charge rate for the battery bank becomes a limiting factor in the design.

You can have 10kW of solar panels, but unless a 12V battery bank can safely accept about 850A (insert picture of volcano here) of charge current you can't actually allow your charger to produce 10kW. Even at 48V this still means your battery bank must be capable of accepting 200A without damage. Battery voltage is about more than just coping with supplying the load.

 

[Hedges]

In an idea world I would like to power a ground water heat pump that is about 30,000 BTU. The 10 KW is what the PV array can put out under full sun. Thanks for the info!

That will vary during the day, and with weather.
How about a 3-phase heat pump, and operate it with a vfd? You could implement a servo to adjust the frequency/RPM such that battery stays full. Or maybe feed PV directly to DC rail of vfd (if you determine Voc and Vmp are amenable to vfd specs) and do a system with no battery.

 

[Hedges]

"my 10 KW solar system"

Do you have something already? What inverter?
I use AC coupling (grid-tie inverter) and get away with a very small battery bank. My PV array is oversize.
You can do that to handle heat pump turning on and off. Use a state of charge signal to enable the heat pump. This will cycle the battery (if load is more than PV), so you have to select/size battery for the lifespan you want.
For summer A/C I think it is cheap to just put in more PV. Winter, you may never have enough.

 

[TRW]

Thanks for the input and info. I currently have no system at all, I am wondering if I can do some PV without any permitting and that is why I ask these questions. I have a remote shop area that has no power other than a gas power generator.

To answer your questions, I have no invertor yet either. I do understand your point on the three phase, is that something I should consider for a heat pump? If I had a grid tie system I assume that would not be a reasonable option, is that correct.

 

[Hedges]

Thanks for the input and info. I currently have no system at all, I am wondering if I can do some PV without any permitting and that is why I ask these questions. I have a remote shop area that has no power other than a gas power generator.

To answer your questions, I have no invertor yet either. I do understand your point on the three phase, is that something I should consider for a heat pump? If I had a grid tie system I assume that would not be a reasonable option, is that correct.

With grid-tie you wouldn't worry about when it draws how much power, nor about starting surge. So only cost, lifespan, efficiency, noise level would matter. No need to spend extra to match power draw to variable source. Grid-tie PV on a net-metering plan, no batteries, is the most economical and efficient.

Mass-marketed consumer products will be the lowest cost, and 3-phase commercial units would likely be more expensive. Searching a little I haven't spotted a suitable 3-phase heat pump yet, but I did find variable speed. Trane had several models, and the variable speed of course was the most expensive. That would be 3-phase motor and VFD built in.

Split-compact units are often variable speed, and those are low cost. Not sure their lifespan - Trane indicated 15 to 20 years for their large units.

Since you have no power in the shop, you could set up an off-grid PV system to run it. Maybe have battery state of charge feed a control loop that runs the heat-pump temperature setting up & down to just consume surplus energy and not run battery down. Heat (or cool) a tank of water as thermal storage and use that to heat and cool the building. You might need a separate dehumidifier or window A/C to control humidity if that's a problem in the summer.

Batteries - Several choices are maintenance free AGM lead-acid (70% discharge 500 cycles, 15% discharge 5000 cycles), wet-cell golf-cart or forklift (lower cost, more cycles), Lithium (deep discharges 5000 cycles)

Lithium typically has similar cost to lead-acid per kWh of cycle, but costs more up front for more cycles. Some of they guys here are setting up DIY lithium for similar capital cost to commercial AGM batteries but with 10x longer life. I use AGM for lower cost no maintenance, occasional cycle during power failures. If your application doesn't drain batteries after dark, might last 10 years for you.

What I have is a large PV array and undersized battery bank (about 1/4 the size vendor recommends.) The PV is AC coupled (grid tie inverters) and Sunny Island battery inverters charge at appropriate current - when off grid PV production is curtailed to match load. If you have an electric start or manual generator, that can be used to support load and recharge battery.

With a generator, you can do combined heat and power - since you will often have insufficient sun in the winter, capture heat from generator's cooling system and exhaust pipe to heat the water tank, while also using the electricity. If air-cooled, could run the cold water return from system past a radiator in the airflow first to capture some of its heat.

 

[TRW]

Wow, thanks for all the great info, I will reread all of this tonight and likely respond more then.

 

[iceledoar]

Interesting, any idea how many I would need? I understand hooking 4 in series would give me 48 volts, but that is all I know.

The amp hours will remain the same with the batteries in series, but add in parallel. Calculate how many amp hours you will need and scale out the batteries to meet your needs.

 

[TRW]

Here's the kind I bought:

Sun Xtender Solar Batteries - Photovoltaic Battery

The Sun Xtender Battery line consists of specialty lead-acid solar, photovoltaic batteries, and renewable energy storage batteries manufactured by Concorde Battery Corporation.

sunxtender.com

AGM, so no maintenance.
For lower cost, there are wet cell, golf cart type and others.


"10 kW solar system"
Is that 10 kW peak inverter output? Or 10 kW peak PV output?

Batteries have various requirements, including maximum (and sometimes minimum) charging rates.
The type of system you have may determine whether the battery has to be sized to take maximum PV current, or of current can be limited to allow a small battery.

Beyond that, how many Ah or Wh depends on what you want to operate, and for how long, before you see the sun again.

Here is my question: How much do these cost? I assume they would be hard for me to get them to Wisconsin, so can I go with something from our local auto store or Walmart?

Lots of people have little slogans, I think mine is going to be " I don't know what I don't know and that is a lot!"

 

[Hedges]

Price varies with capacity. I bought eight, 6V 405 Ah, cost about $5000. Retailer had a location in the same state, and I went to freight terminal to get the pallet.

Just Google part number and "price" to find vendors.

 

[Hedges]

Wisconsin - isn't that where the small town "OSHA" is located?

 

[snoobler]

Here is my question: How much do these cost? I assume they would be hard for me to get them to Wisconsin, so can I go with something from our local auto store or Walmart?

Lots of people have little slogans, I think mine is going to be " I don't know what I don't know and that is a lot!"

One does not purchase quality power system batteries from Autozone or Walmart. They are not for the unwashed masses.

I'm going to answer the original question:

NONE.

It sounds to me like you don't have a plan at all. Maybe I'm wrong. Sorry if I'm making assumptions.

You need to use link #5 in my signature to determine your available solar hours for your location, panel orientation and tilt.
Then you need to use link #1 to download the energy audit spreadsheet and determine what you want to run, it's usage and for how long you want to run.

THEN you can begin to evaluate a system. Maybe grid-tie is the better option. If off-grid is the way to go, now you know how much battery and solar you need.

 

[Trukinbear]

No to Wal-mart 'deep cycle' batteries, they are nothing of the sort, been there and regretted it.
We graduated (quite quickly they died) from the Wal-Mart batteries to 16, then 32 6V T-105a Trojan golf cart batteries - maintaining a battery bank like that was a pain. No also to the AGM 'mainitinence free' batteries as they won't last. We finally ended up with a 986 Ah 48V 'forklift' industrial battery - $5000 and 2200 pounds but she's guaranteed for 10 years!

 

[snoobler]

No to Wal-mart 'deep cycle' batteries, they are nothing of the sort, been there and regretted it.
We graduated (quite quickly they died) from the Wal-Mart batteries to 16, then 32 6V T-105a Trojan golf cart batteries - maintaining a battery bank like that was a pain. No also to the AGM 'mainitinence free' batteries as they won't last. We finally ended up with a 986 Ah 48V 'forklift' industrial battery - $5000 and 2200 pounds but she's guaranteed for 10 years!

and there she is in your avatar... byoot.

 

[TRW]

Wisconsin - isn't that where the small town "OSHA" is located?

Yes, OSHA is in Wisconsin!

 

[TRW]

One does not purchase quality power system batteries from Autozone or Walmart. They are not for the unwashed masses.

I'm going to answer the original question:

NONE.

It sounds to me like you don't have a plan at all. Maybe I'm wrong. Sorry if I'm making assumptions.

You need to use link #5 in my signature to determine your available solar hours for your location, panel orientation and tilt.
Then you need to use link #1 to download the energy audit spreadsheet and determine what you want to run, it's usage and for how long you want to run.

THEN you can begin to evaluate a system. Maybe grid-tie is the better option. If off-grid is the way to go, now you know how much battery and solar you need.

Yes you are right I don't have a plan yet, I like this entire concept, but have so much to learn, I hesitate to move forward yet.