Battery charged - Voltage readings dancing.

[KJU]

At the end of charging, solar or AC-DC, voltage readings on (either of) our two 24V LiFePO4 130AHr ready-made batteries, dance. V skips above and below 28V, a few skips/second.

My amateur guess, and hope, is that the BMS is working hard balancing the cells.
Can anyone confirm or enlighten?

BMS is JBD-SP08S004 V1.2 (Bluetooth module on order.)
Constant Voltage is set 28.8 or lower
Float 27.6 or disabled
LowV Disconnect should be 22.4V
Seller claims there's '3A Active Balancing' but I wonder...

I'm trying to 'cycle them into balance,' on the advice of the seller, hoping they'll stop cutting out at about 25V in a capacity test. (Well, 50V because they're used in series as a 48V bank, but being charged & discharged separately for now.) I've only been able to draw 105AHrs or so before BMS cutoff. A bit of a hazard, as they're meant to power our yacht's propeller. In practice, I'd only want to run down to 24V + a bit.

Batteries are 'Platinum LiFePO4' drop-ins from MicroMall in Auckland NZ. Solidly put together, stainless steel case, and I was happy to see a JBD BMS inside. I can't comment on the quality or matchedness of the cells. Yet.
Anyone in NZ or anywhere with experience of these batteries?

Thanks,
KJU

 

[svetz]

How far above and below 28V does the voltage vary? That it goes above sounds more like a bad contact in the measuring apparatus.

 

[KJU]

How far above and below 28V does the voltage vary? That it goes above sounds more like a bad contact in the measuring apparatus.

Thanks.
I see 27s and 29s and the odd 26; the decimals flicker by too fast.
The voltmeter contacts are solid, and the dance is shown in each on-battery meter, the solar mppt controller display and a separate battery monitor.
It also occurs with the two 12V LiFePO4 batteries of the house bank.
I'd be surprised if this is unique, whatever is happening. Unexpected.
KJU

 

[svetz]

Does it eventually settle? That it happens on another system points more to the DMM. I'd try another DMM just to be sure it's not something flakey in it.

 

[KJU]

Does it eventually settle? That it happens on another system points more to the DMM. I'd try another DMM just to be sure it's not something flakey in it.

I'm afraid the dance has already appeared on 9 different voltage reporting devices.
I don't see the dance as a problem; it's just behaviour I don't understand.

 

[KJU]

It settles when charge is taken off.

 

[svetz]

Since you can rule out the DMM and it happens during charging, my "guess" is there's something fast switching (e.g., PWM) either charging the capacitors that provide the charge output or in the active balancers shifting voltage around.

You said it happens on the other battery... is there anything common to both systems?

 

[KJU]

Well, the 24V batteries are said to have '3A Active Balancing' but no such claim for the 12V ones.

The behaviour is shown with several different charging devices (solar MPPT controllers, switching-mode AC-DC chargers - 8 in all) suggesting the batteries contain the puzzle. That's why I thought it might be a normal BMS thing, and I hoped it meant, 'Leave me alone. I'm balancing your cells.'

But nobody's said so...

 

[svetz]

On the pack I built I measured before and after the BMS while charging and it was steady. The BMS just turned the power off when the voltage got high (but it was a standard BMS, not active-balancers). I also have a "dumb" charger (not an MPPT or PWM) so it wasn't trying to do anything fancy other than keep voltage/current steady. MPPTs and PWMs typically do have battery charging controllers that need to be set for the battery chemistry. They alter the voltage to control the amount of current flowing into the battery to prevent it either from charging to fast or to much and it does change over time...but AFAIK it shouldn't change much and should be rock solid once the battery gets to constant-voltage charging mode (near the end of the charge cycle).

The voltage you see is the voltage of the charger in circuit with the voltage of the battery. So, if the BMS is connecting/disconnecting the charger at the end of they cycle I could see the voltage bouncing around. My pack charged very slowly, so when the BMS cut it off, it was essentially done (it would actually reconnect / disconnect after that, but there would be several minutes between occurrences, so more like on/off rather than dancing).

 

[KJU]

Hmm...

 

[roly1]

Hi,
Curious as to your final appraisal on Micromall LiFePO4 batteries? I guess it will apply generally to MM stuff.. I want to convert my house bank to Lithium. Being a dropin is probably definitive enough!
I have a regular ICE for propulsion so only need 12V. I think I would be happier getting grade A from China and fitting my own BMS & control
circuits. They just looked appealing mainly because of availability. Currently have 4x6v 232Ahr FLA's. I'm tired of PSOC problems, smell, etc.
I have no problem with alternators, regulators,BM's, etc. A few permutations & combinations to consider.

 

[mikefitz]

My guess is that the BMS is seeing cell overvolts and shutting down the charge path.
I suggest lowering the charge volts to 28 volts or lower if the condition persists, (27.5).
This may give time for the balance process to function and pull down the high cell.
Cutting out at 25 volts suggests a cell dropping below minimum cell volts.
Once you have access to cell volts it will be easier to determine what's happening.

As a customer it should not be you that has to rectify issues. Get the seller to replace the batteries. There may be mechanical issues, bad joints, or a weak cell in the battery .

Having to 'cycle into balance' advice from the seller is bad news. A well built battery should not need this.

If its not too late, building your own battery from quality cells , as a traction battery, would be an ideal solution. A yacht propulsion system needs to be reliable and able to deliver power in a worst case situation. You dont want a BMS shutting down at a critical point.


Mike

 

[KJU]

To roly1 & mikefitz,

I can't recommend MicroMall's batteries or customer service, although they may have many satisfied customers who are: lucky; or undemanding; or in ignorance of how LiFePO4 batteries should work.

There is no sign of the advertised '3A Active Balancing' inside. Cell uniformity questionable. The batteries appear well-designed physically, and the cases are nice.
Best of all, the lids can be opened! Bluetooth dongle and CarpLounge app working now. (Password-avoidance comments in another thread.)

My pragmatic solution will be to remove the 24V BMSs and install an ANT 16s BMS across all 16 cells, to make one 48V battery.

to roly1 - In NZ, SunnyTech has drop-in batteries, some with Bluetooth BMS communication. The 12V ones we have (no Bluetooth) are serving us well, and tested above nominal capacity. (I have no business interest in SunnyTech. I'm just a customer.)

Thanks to mikefitz for very sensible comments. I reduced top charge voltages on all our batteries some time ago, after some study.
And yes, with hindsight I wish I'd succeeded in sourcing good LiFEPO4 cells in the first place.

Cheers,
KJU

 

[treenz]

To roly1 & mikefitz,

I can't recommend MicroMall's batteries or customer service, although they may have many satisfied customers who are: lucky; or undemanding; or in ignorance of how LiFePO4 batteries should work.

There is no sign of the advertised '3A Active Balancing' inside. Cell uniformity questionable. The batteries appear well-designed physically, and the cases are nice.
Best of all, the lids can be opened! Bluetooth dongle and CarpLounge app working now. (Password-avoidance comments in another thread.)

My pragmatic solution will be to remove the 24V BMSs and install an ANT 16s BMS across all 16 cells, to make one 48V battery.

to roly1 - In NZ, SunnyTech has drop-in batteries, some with Bluetooth BMS communication. The 12V ones we have (no Bluetooth) are serving us well, and tested above nominal capacity. (I have no business interest in SunnyTech. I'm just a customer.)

Thanks to mikefitz for very sensible comments. I reduced top charge voltages on all our batteries some time ago, after some study.
And yes, with hindsight I wish I'd succeeded in sourcing good LiFEPO4 cells in the first place.

Cheers,
KJU

When you say 'with hindsight I wish I'd succeeded in sourcing good LiFEPO4 cells in the first place.' Do you mean that the SunnyTech battery cells are not good?

 

[KJU]

Hi Treenz,
I can't say that anybody's cells are not good in general. Cell uniformity and quality, and the ability of a BMS to keep (maybe even imperfectly-matched?) cells in balance are basic.
Reading here about successes of people who chose their cells well (and with luck) even without using a BMS, makes me wish I had had access to great sets of cells instead of the ready-made batteries that I've struggled with. I'd still have used BMSs.
On top of old puzzles, as in this thread, I've had new trouble with a SunnyTech battery. No doubt other ones have been trouble-free. Hope so.
KJU