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I bought these LifePo4 batteries....

Mark_B

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Nov 28, 2020
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Hi All,
So I bought 36 of these batteries from Bathookup.
The Navitas Prismatic cells. I know I should have asked this prior, but I was pretty sure they weren't going to last long, and that turned out to be true. The price was amazing.
I have a couple of questions if anyone might know.

1. Can anyone recommend a good BMS for 2 4P4S 12 volt batteries built with these cells? I plan to build two 12v batteries (4P4S) and then series connect them in my mini-power wall as a 24v battery bank. I want to be able to grab one battery or both when I need to for taking camping. But otherwise they will power some select items in my house.
2. In the advert it talks about "Calendar life" which I haven't seen often for batteries. I am wondering why they are calling that out specifically? It says 5 years. Since they were manufactured in 2019, does this mean they have 4 useful years left? I know that things like temperature and SOC/discharge can affect life. Are these going to be useless in 4 more years?
 
You would be far better off applying KISS and make either 4x 24V/8S or 2x 24V/2p8s battery packs.
These are New Old Stock. If they have been stored at proper storage voltage (3.200V +/-0.100) then they should be just fine.
Calendar Aging is not much of an issue IF properly stored.
Lifecycles are relative to usage which is on the datasheet panel. Indicacted at 2000 cycles if used at no more than 1C-rate.
3.2V X 25AH = 80kWh per cell. kWh is always calculated at 3.2V which is nominal cell voltage. Powercurve IS 3.000-3.400 for LFP.
12.8Vx25AH = 320kWh.
25.6Vx25AH = 640hWh.

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Thank you Steve.
It isn't clear to me off the top why the 24v would be better than 2x12v.
Is it because it's just easier and/or cheaper to build a single 24v cell?
If it isn't an increased performance issue, I would be willing to trade off ease of build and even a little cost for the convenience of being able to grab one 12v battery to take camping.
 
I'm considering these cells when they come back in stock.
how are you tying them together?
Battery bars? What's your source?
 
I'm considering these cells when they come back in stock.
how are you tying them together?
Battery bars? What's your source?
Well, initially I am using aluminum bus bars I am making myself. I may go to copper (crushed pipe) bars.
If all else fails, then I will buy copper bars of some type.
Batteryhookup is where I got mine.
 
With LFP Batteries most BMS' will not handle 12V+12V Series connections to make 24V. If One BMS kicks off for any reason the other battery pack will try to deliver 24V and fail, if charging and all of a sudden one battery shuts down the other may shutdown or get fried. Most BMS' do not support Series. Parallel is not usually a problem, so it is more rational & safe to build 24V Battery Packs with an 8S BMS and set each battery pack into Parallel which increases the Amp Hours storage available.

The cost of pure 110 Copper can be ridiculous BUT you only need 4mm thick and as wide as the cell terminal faces (usually 10-15mm) depending on brand & cell capacity. Crushed copper pipe is a false economy when you consider the potential issues and folks have had plenty of issues trying that bodge. Aluminium Flat Bar Stock is also cheap, also and 6mmX30mm will carry a lot of juice without issue.
 
I like the idea of crushed pipe. Might go with solid copper though. Bus bars are neater.

The cables will give me more flexibility though.
I might go with battery cables with copper connectors. I could make my own eailsy.

I'm definitely waiting until I have the batteries in hand.
Well, initially I am using aluminum bus bars I am making myself. I may go to copper (crushed pipe) bars.
If all else fails, then I will buy copper bars of some type.
Batteryhookup is where I got mine.
Found these at batteryhookup.
"Headway 38120 100a Copper Busbars – Battery Hookup" https://batteryhookup.com/products/headway-38120-100a-tinned-copper-busbars
 
With LFP Batteries most BMS' will not handle 12V+12V Series connections to make 24V. If One BMS kicks off for any reason the other battery pack will try to deliver 24V and fail, if charging and all of a sudden one battery shuts down the other may shutdown or get fried. Most BMS' do not support Series. Parallel is not usually a problem, so it is more rational & safe to build 24V Battery Packs with an 8S BMS and set each battery pack into Parallel which increases the Amp Hours storage available.

The cost of pure 110 Copper can be ridiculous BUT you only need 4mm thick and as wide as the cell terminal faces (usually 10-15mm) depending on brand & cell capacity. Crushed copper pipe is a false economy when you consider the potential issues and folks have had plenty of issues trying that bodge. Aluminium Flat Bar Stock is also cheap, also and 6mmX30mm will carry a lot of juice without issue.
Thanks for the heads up. I checked with my BMS maker and they say up to 24v series is ok.
I am using the aluminum stock for now and will check the heat on it in use. I don't expect to be pulling many amps I don't think.
 
I would be absolutely surprised, if the headway dimensions are remotely close to the Navitas dimensions
Navitas are about 1.07 inches center to center stacked side by side, so shorter than headway. Those busbars should work, but you'd have to drill a hole.
 
Hi. You wrote: "If One BMS kicks off for any reason the other battery pack will try to deliver 24V and fail, [or] if charging and all of a sudden one battery shuts down the other may shutdown or get fried. " I've heard this before and believe you, but don't understand yet.

If the batteries are in series, controlled by separate BMSs, if one shuts-down (causing a Contactor to open?), how can this cause a problem? I say this knowing my thinking is wrong. I have a sneaking suspicion it has to do with varying reference voltages for the Contactor, but you would think some kind of opto-isolator could fix that.

I'll be very grateful if you can explain this.
 
You put the answer in your comment. 24V going to a Series String of batteries, One BMS shutdown be it FET / Contactor based for one batt in teh string, then the balance of voltage & amperage get's dumped to remaining batteries in the string. Same in reverse for Load/Draw, one kicks off the 2nd Batt has to try and deliver it which it won't. A 4S Battery will never be capable of delivering more than the 12V spec it is designed for.

Like most everything else in the world, whatever you build or use, it is only as strong/reliable as the sum of its components, the weakest item within the assemblage is the limiter. A BMS is a critical component, yet many opt to "Cheap Out" and go with a lowest priced option and often regret doing so but will Never Ever admit to it. Instead of thinking that it's a valuable lesson to learn, they let their emotional embarrassment stop them from sharing "Lessons Learned". The IRONY IS, Everyone who has DIY'ed ANYTHING has messed up something, Learned a Lesson or Two and 50/50 share that knowledge. Some of us here do share that knowledge.
 
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