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Powerwall using 50 AH cells
#1
I have put together a system without the tedium of using 18650 cells which for me was a hell of a lot of work.

I have used  8  LFP or LiFePo4 cells which are in alloy cases in a prismatic form and are readily available for under $1 an amphour plus postage or freight to you from china.

The system has been running well now for 18 months.

The nominal voltage for these is 3.2V and cutoffs are high 3.55V and low of 2V. The BMS is sealed and waterproof.

8 cells gives me 28.4V max and this runs a 24 V  Inverter of 500 watts nicely.

I run my 55 inch TV and a light and my security lights at this time to prove the effectiveness and longevity .This approach is rapidly being installed by Boaters and RVers.

With everything turned on there is less than 7 amps at say 27V flowing into the inverter so I am using a quarter of its potential
I am charging it with two normal house panels of 260 watts and can run the TV 24 / 7 if the sun permits . The battery feeds the system for about 10 hours without charging.
Korishan and Redpacket like this post
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#2
Sounds like a "just right" sized system, good job.
What are you doing for fusing, etc?
Running off solar, DIY & electronics fan :-)
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#3
Fusing? Nothing .
The Inverter is fused for its rating ,the solar controller is fused and protected for its own rating too.
If I put it in a boat I would fuse and battery switch on the negative line with something suitable if I felt a need but I don't.
The battery is capable of outputting a huge current by short circuit but the BMS's maxs protect the battery again on the negative line.
Any more would be overkill and money wasted.
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#4
I went 18650 but I have been wondering about LifePO4 / fusing / do same principles apply. For example, if you built a 300ah LiFePO4 battery using 50ah(s) in parallel it would be 6 of them for each 'pack'. Do LifePo4(s) have same general issues as 18650 - e.g. you should fuse each of the 50ah(s) to the busbar for the pack?

>without the tedium of using 18650 cells which for me was a hell of a lot of work.
I must confess that after 5,034 18650 cells for my 1st 780ah @ 48v battery bank - I'm thinking of doing the next one with LifePo4 prismatics as soon as I can find a reliable source.
kitty tan likes this post
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#5
No that type of cell fusing was not necessary for mine. The BMS connects between each cell and if one cell discharges more than the set current the BMS disconnects the entire pack.  If the temp is too high or the voltage too high or low on any cell the BMS just turns the whole pack off .

Systems can be built as you wish , so if you put say 2 off 50AH in parallel, as many do, I would start to think about fusing but perhaps one BMS would still protect each group of parallel cells.

If you put 8 cells in parallel it would not be the way I would go because you can't protect every cell properly.

I would make 8 battery packs of 8 cells each in series and give each one its own BMS. (They are $30 each for 8S),
then I would parallel the 8 packs knowing if one cell goes out then one pack will go out and 7 will remain in service.

Less capacity but full voltage.
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#6
(06-17-2019, 01:15 AM)tytower Wrote: Fusing? Nothing .
The Inverter is fused for its rating ,the solar controller is fused and protected for its own rating too.
If I put it in a boat I would fuse and battery switch on the negative line with something suitable if I felt a need but I don't.
The battery is capable of outputting a  huge current by short circuit but the BMS's maxs protect the battery again on the negative line.
Any more would be overkill and money wasted.

At least a fuse or breaker in series with the main battery would be a good idea.
The BMS might not operate as expected if there is a large short current, it's pretty stressful for the MOSFETs...
Running off solar, DIY & electronics fan :-)
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#7
BMS without fuse is not good enough. A fuse or breaker should be used inline with the system . In this case it should be after the BMS so the BMS isnt disconnected if something trips.
The Ultimate DIY Solar and build place
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Current: 10kW Mpp Hybrid | 4kW PIP4048 | 2x PCM60x | 83kWh LiFePo4 | 10kWh 14s 18650 |  66*260W Poly
Upcoming: 14S 18650~30kWh | Automatic trip breakers, and alot more
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#8
(06-17-2019, 08:07 AM)Redpacket Wrote: At least a fuse or breaker in series with the main battery would be a good idea.
The BMS might not operate as expected if there is a large short current, it's pretty stressful for the MOSFETs...

As you wish .
That's why I bought the BMS, so how many systems are you going to duplicate for paranoid safety to be good enough ?.
Inverter is fused too and that's all I am using atm.


Quote:Daromer - BMS without fuse is not good enough. A fuse or breaker should be used inline with the system . In this case it should be after the BMS so the BMS isnt disconnected if something trips.


Same for you . Just your personal opinion and I do not agree. The fuse would be too slow and needs 10 times the current for more than one tenth of a second to blow and cut the line .
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#9
One of the things about this forum is we share knowledge, give feedback & learn from each other's experiences.
Collectively this adds up to a lot of experience.
We're suggesting accepted basics on the stuff we've all built.
Fuse disconnects/breakers are one of those accepted things - even if just to be able to isolate things to work on your gear! :-)
We get it, your system isn't as complex as some here (not having a shot just saying).
The trouble is the short circuit current of your batteries is many hundreds of amps (at least) & that can easily make for spectacular outcomes with just a slipped tool.
Running off solar, DIY & electronics fan :-)
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#10
A fuse is not about current limitation. That should be done by the tools connected. Its not My personal opinion. A fuse should Always be used between electronics. You say you have One then you have One and thats fine. But a bms is not a fuse or dead short protection. If you get a dead short a bms potentially can fail in open state. Thats what the fuse are for.

And what do you mean with same for you? I have fuses for each and every device rated att their spec but also have a Main breaker as needed by legal where i live.

I have yet not seen any commercial device without fuse.
The Ultimate DIY Solar and build place
YouTube / Forum system setup / My webpage  Diy Tech & Repairs

Current: 10kW Mpp Hybrid | 4kW PIP4048 | 2x PCM60x | 83kWh LiFePo4 | 10kWh 14s 18650 |  66*260W Poly
Upcoming: 14S 18650~30kWh | Automatic trip breakers, and alot more
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