Advice on 48v to 12v LifeP04 reconfigure & bms

camperman1000

100 µW
Joined
Apr 30, 2019
Messages
7
Hi everyone,

I have a 30Ah 48v LiFeP04

https://goldenmotor.bike/product/48v30ah-lifepo4-battery-in-a-stainless-steel-case/

It is currently set up as 3s1p 10Ah packs and I want to reconfigure it as 4s for use with a 1500w inverter for use in my truck camper. It will be used for LED lights, a small fan and running a laptop and a small stereo.. stuff like that.. nothing heavy duty.

In my limited reading I understand 4s to be better in terms of proper voltage for 12v system and also easier on batteries as a 4s wont need to be charged to a cell's max voltage and a 4s setup will more readily stay above 12v.

If this is all correct... I am now at a loss for what BMS setup I should get.. I am not really up on my BMS info and what I should do.

Guidance welcomed!
 
There is no leeway, 4S is the **only** way to get a nominal 12V pack from LFP.

Opinions vary widely on BMS, for that sort of House bank use, many do without, or rather DIY their own OTS gear for monitoring, cutoffs if automation is needed and occasional balancing.

But the "smart" bluetooth thread has lits of reco's, Many use Daly, Chargery is solid, lot depends on what you want to spend, some think $100 is expensive. . .
 
I think I wasnt looking at it correct... you are right it's 4s but they have it as 3 10Ah cells in parallel to get 30Ah and have 4 of those in series to get the 48v so ya, it's a 4s... just the way they built it tricked my eyes into into thinking it was a 3s.

So this would mean I can keep this setup close... essentially 4 of these 3s4p in parallel to make it a 12v 120Ah... is that correct?

Im ok spending $100 or more on a BMS as long as I make sure it's all 100% safe and there is over draw and over charge protection.

Other concerns are this batt will be charged by my truck while driving and by a 100watt solar panel while not.

Also, my inverter will connect to the battery terminals or through the BMS? I assume the BMS if it is to prevent over draw. If this is the case I will need a BMS with large AMP rating?

I found this... but it seems too inexpensive https://www.amazon.ca/LiFePO4-Lithi...words=4+12v+lifep04+bms&qid=1604020457&sr=8-2
 
The amazon bms only has a max of 10 amp charging, thats too low for your alternator. I been using the chargery bms8 on my 220ah 4s lifepo4 for the past 2 years 24/7, its been rock solid. The only disadvantage is you have use your own contactors (2 of them) one for charge and one for discharge. I been using a 4 dollar 30amp automotive relays as a contactor, it works good. You can find 200a solenoids/relays for 20 dollars. The amps the chargery can handle depends on the size of the contactors you use.

The chargery works for some people and other users have run into problems, most of the problems I seen are with the chargery bms16. It's not cheap in the 110 range. And this bms works extremely well with solar, just install the charging contacter between the panel and controller. If the bms activates it will disconnect the solar panel and stop charging. The cheap mosfet bms might cause voltage surges if your controller is not design to handle the bms activating. Unless your solar controller states in the manual that it won't cause voltage surges I wouldnt get a mosfet bms, I learned the hard way the damage voltage surges cause.
 
camperman1000 said:
4 of these 3s4p in parallel to make it a 12v 120Ah... is that correct?
It is better to just use the cell size - Ah capacity - you want for the pack as a whole, just one 4S string.

A Group of **cells** (means in parallel) in effect works as a single cell long as they are healthy, ideally well matched in actual capacity

best to put 4*4=16 cells in parallel for one group

then string them in series to get 4S

By paralleling 4 strings you are likely to get bigger balancing problems, uneven wear, the current won't be flowing evenly between them

____
Nothing is "100% safe"

Yes if your BMS is providing OCP rather than a fuse, it needs to be sized 15-20% higher than the max amps you will be pulling.
 
Hi John,

The 4x4=16 confused me a little. Do you mean put 16 of them in a row and connect them all in parallel to get 120 Ah 3.2v then connect 4 of these in series to get 120Ah 12v ?

Thank you

Edit... I get it now... I screwed up when I wrote 3s4p I think I meant 4s3p but even that doesnt make sent as that's 9v.

I better go look at the cells and see how many I have.
 
ok.. I figured out why I was confused. I assumed that my 48v 30Ah would be split into 4 giving me 12v 120Ah but the pack isnt 4s it's 3s packs to get their "12v"

So.. I guess this means if I configure it to mimic a Pb battery I will have to solder it up as packs of 4s and then I will only have 12v 90Ah and not the 120Ah I was thinking.

I'm thinking this 4s 12v 100amp BMS https://www.amazon.ca/4Series-Prote...jbGlja1JlZGlyZWN0JmRvTm90TG9nQ2xpY2s9dHJ1ZQ== just to keep an eye on each bank of 16 if it's even necessary (?) and this solar charge controller https://www.amazon.ca/gp/product/B01MU0WMGT/ref=ox_sc_saved_title_1?smid=A18M12GR6V2Z8F&psc=1 and perhaps at the beginning of each season I can open it and check each cell individually and balance if needed. I'll hook up all the 12v trailer stuff to the BMS and the 1500w inverter straight to the battery and probably a low voltage beeper so that I'm not caught off guard by a depleted battery because I'm assuming those BMS's just kill the power when the batt is low.

Please correct anything I'm thinking wrongly about or offer other ideas please.

Thanks everyone.
 
Now I'm confused.

But, bottom line, best layout is all the paralleled cells connected first at 1S to get your Ah capacity.

Then those connected into series to get your voltage, here 4S gives 12V nominal.

If you want to parallel two 4S strings at the 12V level, say for mission-critical redundancy, then you need 2x BMS.

I would not go past three strings though.


camperman1000 said:
the pack isnt 4s it's 3s packs to get their "12v"
Not possible with LFP, at 3.2Vpc nominal, and if one of the other LI chemistries at 3.6-3.7Vnom still way too low!

would need DC-DC converters for both charge an discharge using 12Vnom gear

Test the cells at 1S, try to find out what specific cell model you have.



 
So Sorry... im not sure why I am struggling with this cell set up. I think it's because the had them in stacks of 3 and i cant shake that number outta my brain when trying to think through the math and just mixing myself up.

ok.. So I ripped apart the pack into individual 10Ah cells and there are 48 of them. Currently they are all sitting at 3.3 volts but I seem to remember setting up the charger last year and setting it to 'storage' for LifePO4. I am putting them parallel into 12s to get 3.2v not the 16 we were talking about earlier.

They are 10Ah cells.... does that mean each row of 12 will be 3.2v 120Ah and then put these packs in series to get 12v 120Ah ?

TY
 
Why the heck are we talking 12S now????

so confused. . .

______
Before proceeding be 100% sure they are LFP.

If you bring one up to say 3.6V and hold until the amps rate falls to nothing

then let it sit a few hours

put a light load on it for a short time, if you have a wattmeter draw off under 1% of SoC

what is the resulting voltage?

If you only have 12V range gear then do this at 4S, all four at the same starting voltage.

And I'm not saying use 3.6Vpc normally, too high, but for an occasional test NP.

_____
Forget "row". In parallel at 1S is a "group"

so yes, a group of 12 will be 120Ah at 3.2V nominal, that is the 12P part.

Connect these in series, 12P4S gives 12Vnom 120Ah.

Bigger than a Battle Born costing $800 plus. Even better if these cells are new, and you learn how to not murder them, maybe put some protective circuitry in place.
 
Haha I'm so sorry... I didnt understand as much as I thought I did when I came in here at first and want providing correct information. I'm not sure how we got on 4x4 = 16 because I always assumed splitting a 48v 30Ah into four = 12v 120Ah.

But I have to thank you for putting up with my confusing contributions to the thread!

I finally figure it out... and now I have a 12s4p I think it would be called now.... I hope lol. I have 48 3.3v 10Ah packs so they've been turned into four 3.3v 120Ah (12 cells) packs then I wired the four of them together in series like suggested. Now I measure 13v just like I wanted.

They are defo LifePO4 chemistry Ive had them for a few years from a big E-bike pack I bought and never used. Thankfully this $1000 (at the time) lump of batteries will serve good life in my truck camper now. I will order 2 x 100w solar panels and find a decent charge controller and a decent 1500w inverter although I have no real intentions of running anything that will pull that much.. some LED lights, a radio and a couple laptops for my partner and I. MAYBE I will trying running a smallish A/C unit on it if I find it unbearable at night or something.... it should be able to power that here and there with no issues.

Im on the fence about BMS at 4s 12v to keep each bank in check. Im just going to run balance leads and use that to keep an eye voltages and just connect my B6 if I ever need to balance it. Sound sane?

Thanks again :)
 
You need 4s or 4 in series for 12v. You can do 1 p parallel or as many as you have 2p or 10p still 4s @ 12v. 3 10ah is 30ah or capacity capacity is volume.
 
camperman1000 said:
I finally figure it out... and now I have a 12s4p
Nope, backwards

12P gives you your pack's Ah capacity

4S gives you nominal 12V

Connecting the groups in parallel first, then stringing those 4 groups in series is called

12P4S


 
camperman1000 said:
2 x 100w solar panels
Might get 50-60Ah on a perfect day ~700Wh

Aircon??!! for even a little while, need 8-10 times that,

really you need a genset running for that kind of power

> LED lights, a radio and a couple laptops

no inverter needed for any of that, all native DC loads, just wasting money and energy converting in both directions like that

> just connect my B6 if I ever need to balance it.

that a hobby charger?
 
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