# Right Battery For rooftops solar

#### daljeet795

Joined Jul 2, 2018
295
What kind of battery's are you using For rooftops solar ? sealed lead acid batteries?, lead-acid ? lithium-ion battery? I see In most cases, lithium ion batteries are the best option for a solar panel system,

#### andrewmm

Joined Feb 25, 2011
1,751
An off the shelf system or a home custom job ?

Lead acid are better for home jobs, they are resilient of abuse, and easy to replace, just make certain you get a deep discharge one.
( I have friend who got bunch of Lead acid open cells from an old telephone exchange, each about 200 A at 2 V I think, he just has to keep the acid toped up )

Off shelf systems,
you need to look at the warranty / guarantee for number of cycles and what a cycle means ,
some you can only charge / discharge from 80 to 30 percent, so you get half the KWh advertised.

#### Ian0

Joined Aug 7, 2020
3,186
Good old-fashioned tubular plate batteries. That’s what we install for off-grid customers. 48V 840Ah 1.7 tonnes. Good for 2500 charge-discharge cycles.

#### andrewmm

Joined Feb 25, 2011
1,751
@Ian0
Out of interest, what depth do you charge / discharge to ?
How is the cycle calculated ? I was told by one company that was per day, another told us that was when it stopped charging and started discharging, which with the clouds we have, could be many times per day ,

#### Ian0

Joined Aug 7, 2020
3,186
@Ian0
Out of interest, what depth do you charge / discharge to ?
How is the cycle calculated ? I was told by one company that was per day, another told us that was when it stopped charging and started discharging, which with the clouds we have, could be many times per day ,
we try to keep it above 50%, but the 2500 cycles is quoted as discharge to 25% (I.e. using 75% of capacity)

#### andrewmm

Joined Feb 25, 2011
1,751
we try to keep it above 50%, but the 2500 cycles is quoted as discharge to 25% (I.e. using 75% of capacity)
Thank you,
I'm looking myself at the battery idea,
last time I looked it did not add up, but we will see this time.

#### Just Another Sparky

Joined Dec 8, 2019
216
I've seen examples of people with nothing better to do salvaging hundreds or thousands of old 18650 cells and assembling them into massive storage batteries. I would only look into this option if you have loads of time to kill and can be extremely responsible and diligent in researching and understanding lithium cell chemistry, welding & protection techniques. 18650 is merely a form factor - the actual chemistry (and hazards involved, as well as charge and discharge characteristics) will vary with each individual part number.

A better option might be new lithium iron phosphate cells as these are inherently safer, offer long cyclic lives (>2000) and are maintenance free. Not sure on the exact depth of discharge on them but the rule of thumb with lithium cells is 10-90% for maximum service life. Meaning discharge to no less than 10% of the rated voltage range, charge to no more than 90%. Bear in mind that lithium cells drop off rapidly below right around 3.00~3.30 volts depending on load and chemistry, so the bottom 30-45% of your voltage range is basically useless.

Example of a typical lithium cell discharge curve:

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#### Ian0

Joined Aug 7, 2020
3,186
Most of the lithium batteries sold for domestic energy storage are second hand, having first seen service in electric vehicles, thus saving the electric vehicle companies a serious problem in disposal. Otherwise they would end up in landfill where they will corrode until the contents leaks out and catches fire, maybe igniting the methane from the landfill.

#### Janis59

Joined Aug 21, 2017
1,346
Just Li-ion have two stoppers: 1) them are about 10x expensiver as lead; 2) them are tended to self-ignite causing a serious fires, as more capacity, as more several ill.

#### Ian0

Joined Aug 7, 2020
3,186
There are two main sorts of lithium batteries: those that only blow up if they are overcharged, and those that can blow if they are overcharged or overdischarged.
The laptop and electric car type are lithium cobaltate, which are used because of a higher energy density, mainly due to half a volt more terminal voltage. The safer (relatively speaking) type is lithium ferrophosphate. There's also lithium titanate, which are ridiculously expensive, have no more terminal voltage than lead-acid, but have an immensely long life.
If domestic batteries are second hand car batteries, then they are the rather more blowy-uppy type and as they are being used towards the end of their lives then they are probably even more blowy-uppy.
In terms of volume, there is not much space-saving - the only real advantage over lead-acid is weight, and for a house supply that's only a concern to the bloke that has to install them, and that's his job - he spends all day assembling lead-acid cells into tanks, mostly into fork-lift trucks.

#### bassbindevil

Joined Jan 23, 2014
171
Nickel-iron "Edison" batteries have a lifespan measured in decades rather than years, but they are about as expensive as lithium. Maintenance is simpler than lead-acid; no equalizing charges needed or checking specific gravity, just add distilled water to maintain the correct level. They may also have the useful property of turning surplus energy into hydrogen, but that's still in the research stages.
If I was putting together a small system for a van or trailer, LiFePO4 is ideal but expensive (roughly a buck per Wh); the voltage range is a close match to lead-acid. So I'd settle for a conventional deep cycle RV/marine battery at a fraction of the price, and plan to buy/build some kind of lithium battery in the future as prices drop or opportunity arises.
For a house, I'd be tempted to try a whole bunch of recycled car batteries. With enough of them, depth of discharge or high internal resistance shouldn't be a problem. If I could get enough used lithium ion cells, I'd go the DIY "power wall" route. Electronics for monitoring and balancing battery banks are cheap now, so there's less need for perfectly matched batteries.

#### Ian0

Joined Aug 7, 2020
3,186
For a house, I'd be tempted to try a whole bunch of recycled car batteries.
I wouldn't - they'd be dead in no time - SLI batteries are good for peak current, but dreadful for cyclic use.

#### Ian0

Joined Aug 7, 2020
3,186
Nickel-iron "Edison" batteries have a lifespan measured in decades rather than years, but they are about as expensive as lithium.
Why are they so dear? They're not exactly complicated and the design in a century old. The cynic in me suspects that it is something to do with never being able to sell replacements!

#### Just Another Sparky

Joined Dec 8, 2019
216
Nickel-iron "Edison" batteries [...] may also have the useful property of turning surplus energy into hydrogen, but that's still in the research stages.
Lead-acid batteries do that too. That 'useful' property is the reason why smoking is strictly prohibited in battery vaults and why non-sparking brass tools are generally required inside of them. Work on battery vault ventilation equipment is done with a live work permit because shutting down the exhaust fans creates a greater danger.

https://www.mcmaster.com/Nonsparking-Tools/

Look into Rolls-Surette if going the lead-acid route. They make large stationary storage batteries.

Also, consider the difference in cycle life when evaluating the cost of lead-acid vs lithium. 800~1000 80% DoD cycles vs 2000 (or more) as a rule of thumb until you start getting into the really massive 2-6 volt stuff.

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#### Ian0

Joined Aug 7, 2020
3,186
Rolls are good if you want to buy in 12V blocks that you can possibly move on your own!
If that's not important, look at the PzB and PzS series made by several manufacturers, which generally have a slightly longer life, and can have individual 2V cells replaced if they fail.
In either case, ventilation is important although hydrogen is only evolved towards the end of the charge cycle.
What would be handy is a way of storing all that hydrogen which you could produce in the summer and would be nice to burn in the winter. . .

#### Just Another Sparky

Joined Dec 8, 2019
216
What would be handy is a way of storing all that hydrogen which you could produce in the summer and would be nice to burn in the winter. . .
How much energy would be wasted in compressing said gas into an 1800+ PSI high pressure cylinder, and how much would you net when it comes time to burn it? Compressors and their discharge piping tend to get mighty hot. Despite burning hot, hydrogen is a very low energy-density fuel compared to hydrocarbons. How would you separate the hydrogen from any oxygen contamination so your storage cylinder doesn't become a bomb?

Nevermind the investment cost for such a system. The expense for the volume of gas produced would be unrealistic. If the batteries have been properly maintained, we're talking about a mouse-fart's worth of gas. It's primarily under adverse conditions (overcharging, lack of ventilation, lack of maintenance) that dangerous gas concentrations can be present. Consider how much water you add to your batteries on a regular basis - that is an indication of how much gas they are generating. Compare that to how much energy is in a 20lb propane cylinder which can be recharged for $30. How long would 20lbs of propane run your furnace in the depths of winter? A day maybe? Last edited: #### Ian0 Joined Aug 7, 2020 3,186 How much energy would be wasted in compressing said gas into an 1800+ PSI high pressure cylinder, and how much would you net when it comes time to burn it? Compressors and their discharge piping tend to get mighty hot... Nevermind the investment cost for such a system. The expense for the volume of gas produced would be unrealistic. There's plenty of people working on it: https://www.h2gopower.com/ for instance. If the hydrogen is produced by surplus solar energy (e.g. off grid location with solar panels sized for winter electricity requirements) then the energy to compress it would not be "wasted". There's a lot of energy in hydrogen 140MJ/kg compared to 50MJ/kg for propane However, I totally agree about the cost. #### Just Another Sparky Joined Dec 8, 2019 216 If we're talking surplus, sure. But at the same time... how much does an extra battery cost, versus gas collection equipment for every battery, a gas separation system, a high pressure compressor, high pressure plumbing, a high pressure gas cylinder, a regulator, a dual-fuel furnace and a robust control scheme for all of the above? Not to mention the room all of this is happening in will become a Class 1, Div 2 hazardous location and will require explosionproof wiring and seal-offs, forced-air ventilation, et. al. Consider too that ~80% of the energy which is sunk into compressing the gas and not immediately lost as heat *will* later be lost when it is brought back down in pressure via a regulator - and the cylinder & piping will at that time tend to *absorb* heat whilst you are trying to generate it. And propane can be easily stored as a liquid. So can gasoline, kerosene, diesel, bunker 'c', biodiesel, etc. Hydrogen is always a high pressure gas unless refrigerated. Much less energy per unit volume, as well as for any given investment cost.$50 for a new full propane tank - \$425 for an empty 300CF (140lb dry-weight, 141.54lbs full) cylinder, not including pressure regulator.

Without claiming to be an expert, my caveman-level math tells me you'd need 13 such cylinders to equal the stored mass within one LPG tank.

...For one day of heat in the dead of winter if your battery bank is really that severely undersized.

Hydrogen power is an exercise in futility unless it is burned as a byproduct at the time it is produced. Just buy an extra battery, lol.

Or better yet, more PV modules so you can heat up your house hotter in the daytime and demand much less from your batteries at night, prolonging their useful life and maximizing your system's efficiency whilst minimizing your environmental footprint. Batteries have to be manufactured and then eventually disposed of - and recycling requires logistics and capital investment, consumes energy & generates waste just like any other industrial process. Using your solar energy immediately on the other hand costs nothing and generates no waste until some component in the system inevitably fails and needs to be replaced. 30 year life for modules which can be landfilled, vs 5-8 year life for batteries which become hazardous waste.

It is clear that minimizing one's need for storage infrastructure is imperative for any sort of ecologically-driven solar installation. Put your system to work in the daytime and let it rest at night. Otherwise it would probably be more responsible to buy from the grid.

TL;DR; K.I.S.S.

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