4s diy 12v battery BMS issue

[Jamer76]

There is only 8 volts coming out of the C- of my BMS, yhe chat bot says connect that to the forth cells negitive so I did. Hope it's correct to set up this way. See the photo and any help would be great.

 

[Two on Tour]

There is only 8 volts coming out of the C- of my BMS, yhe chat bot says connect that to the forth cells negitive so I did. Hope it's correct to set up this way. See the photo and any help would be great.View attachment 1056896

Chat bot is talking out of its backside.
Your "C" cable does not connect to any of your cells, as it is the output negative.
I'm not sure that I would be too happy about pull much power through the posts thingies on your positive and "B" negative.

 

[Jamer76]

Thanks I knew that, but newded reconfirmation, but still I listened to chat bot. it was late and the chat bot seemed so real, and the bms and site are chinnese so I did it any way. I disconnected though when charging was finished.

 

[Jamer76]

Looks like I brought a dud anyway, or something is very wrong, I've got 14v from the terminals which is great. But the cells are so unbalanced. 3.52, 3.46, 3.55, 3.41

And as said. The output from the BMS the C- cable should go to (in my case) a negitive bus bar, but that's outputting 8.6volts (so unless. Don't know what to do, need the power set up till waiting for answers. So going to hook it up, connect it to fan and lights (low amp drain) and just buy another BMS ( cheap but easy) daly 100amp.

Thanks I knew that, but newded reconfirmation, but still I listened to chat bot. it was late and the chat bot seemed so real, and the bms and site are chinnese so I did it any way. I disconnected though when charging was finished.

 

[Jamer76]

All seems fine, as c- outputs 14volts now. Blue light is off on bms. Must still be doing something as I I unplug the balancing lead volts drop to 12v. If blue light is on volts from c- is 8.7. So yeah I'm going with it.

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[Carpmart]

Jamer76 - what individual cell numbers did you get when you top balanced the cells before final assembly? Actually, did you do this?

 

[herald_400rl]

Jamer76 - what individual cell numbers did you get when you top balanced the cells before final assembly? Actually, did you do this?

Would suggest not... BMS probably fried anyway connecting the positive to the negative out, think I read or watched somewhere (might well have been Roger) saying that when they get requests to look at DIY batteries typically this is quite common and it kills them instantly.

Although, looks like C- and B- were both connected to negative posts so maybe no current flowed. Guessing off the picture and terminal colours being correct, though.

 

[Jamer76]

Jamer76 - what individual cell numbers did you get when you top balanced the cells before final assembly? Actually, did you do this?

Nope, I relied on the bms, which wasn't working to make it all happen. Now two cells are 13.5 and two cells are 13.4. Am I up the river?

 

[Raul]

Nope, I relied on the bms, which wasn't working to make it all happen. Now two cells are 13.5 and two cells are 13.4. Am I up the river?

Well that is 100mA of difference, quite a bit for a passive balancer. I would throw that bms in the bin, put all cells in parallel and let them stay that way till your new bms arrives. Get a jk bms with active balancer and an app where you can actually set and configure all the parameters to your like.

 

[Carpmart]

Nope, I relied on the bms, which wasn't working to make it all happen. Now two cells are 13.5 and two cells are 13.4. Am I up the river?

Have you seen the the diagram above from Two on Tour? Is your pack wired as this?

How can two cells be 13v+? Two cells in series would be around 7v assuming nearly fully charged.. why on earth are you measuring voltage on pairs of cells? Actually, how are you measuring pairs of cells voltages?

You need to change what your doing completely… Each cell is 3.5xV when normally fully charged and You need to connect all four cells in series to make a useable battery for a Motorhome application.

Are you aware of the process one follows to build a battery? Have you watched some YouTube videos and followed these instructional steps?

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[Two on Tour]

All seems fine, as c- outputs 14volts now. Blue light is off on bms. Must still be doing something as I I unplug the balancing lead volts drop to 12v. If blue light is on volts from c- is 8.7. So yeah I'm going with it.

There is normally a procedure for starting up a BMS for the first time, which may be the lack of output from your BMS, did you not get information with your BMS ?

The most common method is to connect a battery charger to the battery + terminal and the negative output of your BMS, and this will usually wake up the BMS.
Even a brief connection (a few seconds) might be enough to turn the BMS on.

 

[Jamer76]

Yes. Thanks. I persumed that the blue light means error. Worried that after a day the cells are not balanced. Two cells at 13.5v and two cellss at 13.4v are a big difference when it comes to lifepo4. Can't work out all the voltages from the C- cable. 8.7v is useless, thats with the blue light on, now that I'm thinking right I want 12v from the C-. Except I only get that without the balancing leads connected. With the leads connected and without the blue light on the bms it's 13.9v now?, the same as from the terminals, so i guess its not even working, it's a mystery to me. Why only 8.7volts?, I'm thinking it's only senses 3 cells not four, I will use my multimeter to check the connections again.

 

[MisterB]

Or start again as suggested, by top balancing the cells ...

Ps, don't worry if you've made mistakes or errors, but please don't use chat bots to advise you, there are plenty of you tube videos. I can recommend that you watch off grid power solutions videos from a few years ago, where they go through it from start to finish.

 

[herald_400rl]

Yes. Thanks. I persumed that the blue light means error. Worried that after a day the cells are not balanced. Two cells at 13.5v and two cellss at 13.4v are a big difference when it comes to lifepo4. Can't work out all the voltages from the C- cable. 8.7v is useless, thats with the blue light on, now that I'm thinking right I want 12v from the C-. Except I only get that without the balancing leads connected. With the leads connected and without the blue light on the bms it's 13.9v now?, the same as from the terminals, so i guess its not even working, it's a mystery to me. Why only 8.7volts?, I'm thinking it's only senses 3 cells not four, I will use my multimeter to check the connections again.

13.9v is morenaccurate if you've got Cells at 3.5 and 3.4.
Someone with knowledge will maybe swoop by to explain why we call them 12v batteries when they're 12v at 20% SOC and ~14v fully charged.

 

[Raul]

Someone with knowledge will maybe swoop by to explain why we call them 12v batteries when they're 12v at 20% SOC and ~14v fully charged.

We call them 12.8v nominal, the cells nominal voltage is 3.2v.

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[Raul]

Yes. Thanks. I persumed that the blue light means error. Worried that after a day the cells are not balanced. Two cells at 13.5v and two cellss at 13.4v are a big difference when it comes to lifepo4. Can't work out all the voltages from the C- cable. 8.7v is useless, thats with the blue light on, now that I'm thinking right I want 12v from the C-. Except I only get that without the balancing leads connected. With the leads connected and without the blue light on the bms it's 13.9v now?, the same as from the terminals, so i guess its not even working, it's a mystery to me. Why only 8.7volts?, I'm thinking it's only senses 3 cells not four, I will use my multimeter to check the connections again.

The bms has shut down discharge hence you see 8v on the output. That because needs turning on by placing a charger on the battery , or simply because is a large imbalance. Without knowing the settings, you don't know for sure. The 3.5 and 3.4v difference can be corrected to some extent, by placing all the cells in parallel and let them sit a day. Then re assemble the bms and battery pack as 4S.
If the bms does not turn on with a charger aplied to the battery, you may need to parallel piggyback another fully charged battery, to force the charger into charging.
Another way to start abms is using a multimeter set on diode, you have to put the meter + lead on b- of the bms, and the meter - lead on tbe C- of the bms.
Search youtube procedure how to turn on bms using a multimeter.
What bms is you got there? There may be documentation online by other users.

This is an easy task if you understand the relation and role of the bms and cells.

 

[Dargish]

You cannot rely on the BMS to balance your cells when you first build your battery. You need to top balance first, there are plenty of guides about how to do this. This is not an optional step.

You then need to follow instructions on building and starting the battery, this is not something you can just "vibe". If that cable on the right of your first photo connected C to the cell above then I'd be very impressed with the BMS if you didn't fry it. Follow diagrams, if you are unsure then ask for help before doing anything.

Building a DIY battery is a good learning experience but it needs to be done carefully and safely. You can easily burn down your house or campervan if done wrong.

 

[Jamer76]

Jamer76 - what individual cell numbers did you get when you top balanced the cells before final assembly? Actually, did you do this?

Top balancing, sorry missed that stage. My issue was, does my lifepo4 charger charge cells in parallel as that only 3.2 volts? So I didn't try it. I will remove the bms and put the cells in parallel for a day though.

 

[Raul]

I will remove the bms and put the cells in parallel for a day though.

Without any charge, unless you have a adjustable power supply that can be set at 3.6V.

 

[Raul]

You cannot rely on the BMS to balance your cells when you first build your battery. You need to top balance first, there are plenty of guides about how to do this. This is not an optional step.

That used to be the case long time ago.
You can build a battery without prior top balancing, providing you understand the process, and the bms is capable of active balancing.
The initial top balancing is needed more where a passive balancing bms is used, (50-100mA ) if it has active balancing a 2A will top balance within the first charge.

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[herald_400rl]

That used to be the case long time ago.
You can build a battery without prior top balancing, providing you understand the process, and the bms is capable of active balancing.
The initial top balancing is needed more where a passive balancing bms is used, (50-100mA ) if it has active balancing a 2A will top balance within the first charge.

If you connect 2 (or more) cells with a voltage difference won't they transfer massive current between each other when you attach the bus bar?

 

[Raul]

If you connect 2 (or more) cells with a voltage difference won't they transfer massive current between each other when you attach the bus bar?

Not massive enough to worry about, and is only at the beginning, then tappers of as the voltages equalises.
I have coupled partially discharged batteries to full ones with no problems. The inrush is only valid where you have a empty completely empty coupled with full ones, but it has to be a large battery to worry about the amps.
On partially charged ones, the current transfer is not that big.

 

[Growly bear]

Does your BMS have a software switch to enable and disable charging and discharging? Mine is a JBD which does. When I turn off both charge and discharge the voltage drops like yours. The voltage is there but when any load is put on it, the voltage falls away.

 

[autorouter]

My issue was, does my lifepo4 charger charge cells in parallel as that only 3.2 volts?

A standard LiFePO4 charger will charge the cells in series, not parallel. Exactly the same amps goes through each cell. So over a period of time, exactly the same amp-hours of charge goes into each cell. If they start out a bit unbalanced then they will always be a bit unbalanced.

That's why you need a BMS with a balancing function. It measures the voltage of each individual cell, and can put a little more into the cells with lower voltage, and less into the cells with higher voltage. So eventually they end up balanced, ie with exactly equal voltages.

Of course it's a good idea to start out with the voltages equal before you build the battery, but a good BMS can correct any inequalities and make the cells balance perfectly.

 

[oldiesontour]

All 8 of my cells arrived within a MV or 2 from china ,just assembled and left the BMS to it.

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[Jamer76]

Have you seen the the diagram above from Two on Tour? Is your pack wired as this?

How can two cells be 13v+? Two cells in series would be around 7v assuming nearly fully charged.. why on earth are you measuring voltage on pairs of cells? Actually, how are you measuring pairs of cells voltages?

You need to change what your doing completely… Each cell is 3.5xV when normally fully charged and You need to connect all four cells in series to make a useable battery for a Motorhome application.

Are you aware of the process one follows to build a battery? Have you watched some YouTube videos and followed these instructional steps?

Sorry bit grammar on my part, I meant out off four cells two fall are around 3.4v whilst the other two are somewhere in the 3.5v range..I messed up. Somehow. I've ordered a new bms. The cells I will put in parallel to balance out....yet I'm short two busbars. Got to order, living of a power station atm.

 

[herald_400rl]

Sorry bit grammar on my part, I meant out off four cells two fall are around 3.4v whilst the other two are somewhere in the 3.5v range..I messed up. Somehow. I've ordered a new bms. The cells I will put in parallel to balance out....yet I'm short two busbars. Got to order, living of a power station atm.

Jk-b2a8s20p is the one most seem to use though it lacks heating if that's something you'll need.

 

[Dargish]

Sorry bit grammar on my part, I meant out off four cells two fall are around 3.4v whilst the other two are somewhere in the 3.5v range..I messed up. Somehow. I've ordered a new bms. The cells I will put in parallel to balance out....yet I'm short two busbars. Got to order, living of a power station atm.

You can use cables to attach the cells too, there shouldn't be a lot of current, short cables would be best to minimise resistance.

Fwiw you don't need active balancing so long as you top balance first, my 24v 280ah battery with a jbd passive balancing bms stays within tolerance. Been running for a couple of years now.

 

[Carpmart]

You can use cables to attach the cells too, there shouldn't be a lot of current, short cables would be best to minimise resistance.

Fwiw you don't need active balancing so long as you top balance first, my 24v 280ah battery with a jbd passive balancing bms stays within tolerance. Been running for a couple of years now.

You’re 100% not needed, but for the small additional outlay on the BMS, with something you’re likely to have in service for many years, why not!

 

[Dargish]

You’re 100% not needed, but for the small additional outlay on the BMS, with something you’re likely to have in service for many years, why not!

Oh sure, I'd get a bms with it now that I know more but I wouldn't bother binning a bms without it just to get one with active.

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