Backyard Aquaponics
A place to discuss aquaponics

I''ve started this thread to try and encourage any battery discussions to end up here so that its easier to find.

I'm sure I'll be one of the people that goes off topic but it would handy if we could keep the discussion on batteries and related issues.

I came accross this:

http://redflow.com/wp-content/uploads/2015/03/Redflow-Understanding-the-Redflow-Battery.pdf

and wanted to discuss it and any better solutions.

The two things that immediately jump out at me are the efficiency (80%) and the charging rate (3kW).

Gordon has previously said that he is getting greater than 98% efficiency from his Li batteries which is pretty impressive but the problem I have with is that while Li batteries have an efficient high rate of discharge they have a low rate of efficient charge.

I'm looking at a system where I can have a generator, PV panels and batteries. The idea is that the panels provide primary power with the batteries supplying secondary power. When the batteries get low (15% discharge for lead acids, ~60% discharge for Li) the generator turns on, runs at peak efficiency (~80%) and provides power as well as charging the batteries and then turning itself off. A crucial requirement of this system is the ability of the batteries to deliver charge to multiple kW motors (either 3 phase or DC) and except the charge from the generator.

In lead acids this means that the battery bank must be pretty massive in order to deliver the current when required. Li batteries have no problem delivering the current when needed but that are not so good at excepting charge which means that the battery bank still has to be pretty big.

Could these ZBM batteries be the solution I need? A battery that can be quickly charged?

Check out this youtube clip of Tesla's new batteries

Before I watch a 17min video on my limited download limit....


The Tesla solution is based on Li Ion batteries and they have limited ability to be charged quickly. They are pretty cool but I don't think they suit my application.

With a power system for AP one of the challenges is that you need power 24hrs per day. Power usage overnight can be reduced but you still need a minimum amount of aeration or flow or both.

Lets say i want to power a pump drawing 1kW. To do that I need 24kWH. Assuming the PV panels provide energy during the day I still need to provide the load over night. The Tesla systems only deliver 7 or 10kWH.

Tesla's powerwall 7kW battery will only deliver 2kW continuously, which isn't particularly high for a Lithium battery.

Stuart- Lithium batteries can be charged a lot faster than Lead-acid. 80% efficiency for the Redflow is even worse than Lead-acid.
I've done some changes with my charging voltages and get pretty close to 99% efficiency now, and I charge at 3kW+ ATM, but that will soon be increased to about 5kW. I don't expect my efficiency will drop off too much. LiFePO4 can be charged at 0.5-1C without any problems- so for my domestic 21kWh battery, that's 10-20kW.
How big is your generator?

The Tesla batteries can be stacked, but that's not the way I'd go. They are still more expensive than my battery, which I installed in 2012.

they stack up to 9 times stuart.... so up to 90kwh per day (if you can fully recharge each day).

The 10kWh battery can't be used for daily cycling- it is designed for only about 1 cycle per week, ie it is purely a backup battery.

Ok so not so good on the efficiency side of things.


It doesn't exist yet.


At a rate of 0.5-1C a 1kW generator could charge your battery in 21 to 42 hours?

1kW is only ~0.05C. 1C is 21kW = ~1 hour + a bit charge time (have to slow down at the top end to keep within voltage limits. 1kW is a pretty small generator, and you'd never take a Lithium battery to 0% state of charge. I've been down to 75% discharge a few times and 80% once, and my 4.19kW of panels can recharge it and supply household needs on a sunny day.

thats damn good to find out thanks.

hey, how would the 48v 1200ah battery in EM's 12kw/day off grid system stack up against the teslas? how many kwh roughly would that get you?

So for your battery a 20kW generator could charge it to close to 100% in about an hour?

But you don't know at what efficiency? Bit less than 99%? A lot less?

What sustained load can they deliver?

Is that comparison just based on battery capacity or does it include all the smarts as well?

The Tesla battery is only for grid-connected, you cant use it for any existing off-grid systems, so its not really possible to do any direct comparisons. The Powerwall does have inbuilt charging and balancing functions, but it operates at 300-450V, and only works with certain grid-connected inverters, of which Fronius will be one. My battery and BMS and some other mods I did cost a bit under $12000 for ~17kWh usable capapcity, the 7kW Tesla is likely to be AU$5-6000 retail, or maybe a bit more I suspect, for 7kWh usable. However, the cost for what I have has probably gone up a bit due to the exchage rate changes since 2012. There are other similarly priced commercial Lithium batteries around, but not yet in Oz.

Yes you could charge most of it's capacity in a bit over an hour, the last little bit is slower, but is best done with PV panels rather than a whacking great generator running at a fraction of its capacity. I would think efficiency would be in the mid 90% somewhere, certainly still way better than a Lead-acid battery that could not be charged at anywhere near that rate. For a long life you probably want to limit discharge to 5-10kW, but it could do 20kW for many minutes at a time and up to 50kW for brief peaks. I'm sure that would knock efficiency down a fair bit, but that's the sort of treatment they get in electric vehicles.

The EM 12kWh/day Lead-acid battery (57.6kWh) would ideally only be discharged by about 30% or 20kWh with any sort of regularity if you wanted it to last a long time. I haven't looked at the price, but that's about my average energy use with a 21kWh LiFePO4 battery, and I havent used a generator since installing it. I do limit energy use to necessities in extended cloudy periods, but when I add some more panels that will be a bit easier to do, and I have to make sure I have enough energy for the 2nd big AP system!

Is that what we would pay? I mean were you able to get it at that price because of the work you did your self?


The idea is that when the system is sized the generator is matched to the charge capacity of the battery bank. You run the generator to maximize its efficiency which is generally at about 80% of rated capacity.

When you say "best done with PV panels" is this because sunlight is free or some other reason?

The reason for the generator is primarily as back up but also to reduce the required size of the battery bank and hence reduce the capital cost of the system.

The system I'm working on at the moment needs about 1.5kW 24hrs per day but I want to design it so it can be scaled up to 5kW.



that is still pretty good.


Yeah lead acid won't cut it. I've already worked that out.


Most of the loads I have to deal with at this stage are more constant in nature rather than peak. This is part of the problem I'm having since most systems are designed to provide power during the day and for peak periods of usage. Mostly in AP systems the duties are relatively constant. That may change later as more control systems vary pumping rates and turn blowers up and down.

The other challenge is the voltage to run the system at.

Couple of things Stuart.

I take it Toshiba weren't interested or were too expensive for the titanate batteries?

Slightly off track (but not too much) what's the overnight winds like where you are? At one stage I had no serious interest in wind generators (as part of a hybrid system) however I am beginning to take a little more interest. I live at the base of the Perth hills (south); for much of the year our overnight winds are adequate to provide at least a reasonable power generation. One of the land developers near me is using what looks like a small cheap wind generator and a couple of solar panels to hold up ?security and possibly led floodlights at night.

Also I managed to get a hold of some power optimiser modules from a solar panel array (I thought they were micro inverters initially but no such luck) haven't had a chance to play with them yet but the theory of them seems reasonable.