Fogstar Drift shenanigans

[nicknorman]

OK not on my boat where I have a "proper" lithium battery system, but in my caravan. One 105Ah LiFePO4 Fogstar Drift battery charged by solar in summer and a genny in winter. I use a BMV712 to stop the solar charging at 90% SoC as I didn't want the battery to be continually rammed full every day by the solar when I wasn't there. It's been in for a couple of years now I think. I fully charge it on occasion to sync the BMV712, every 3 months or so (not that fussed about the accuracy of the BMV). It seemed OK when I got it but one cell (cell 4) was always a bit odd. But I haven't paid much attention until recently when the SoC got quite low due to excessive electric blanket use! Cells 1,2 and 3 are always within a few mV and they started going down the knee. Meanwhile cell 4 was way up. Hmmm, big imbalance.

So I fully charged it but cell 4 topped out very early. I left it connected to a power supply for a few hours whilst the balancing thing was going on. But it was not touching the sides. The balance current is only about 50 or 100mA I think.

So in the end I decided to investigate further and opened up the case. Firstly, the BMS is reading the cell voltage correctly. So I then disconnected the cell connections and connected a resistor across cell 4 to give around 0.8A. This has now probably been on for at least 10 hours and probably nearer 15, but Cell 4 still overtakes the others as full charge is approached. I am discharging it more as we speak.

So somehow, cell 4 has managed to accumulate at least 10Ah (out of 105Ah) more charge than cells 1-3. This is a crazy amount of imbalance and I am wondering how it happened.

I notice that the BMS which is a JBDSP04S034 can be used with either 3 or 4 cells in series. So I am wondering if it is so crappily designed that it actually takes the power to run the BMS and the Bluetooth, just from cells 1-3 and cell 4 has no parasitic drain. Thus over 2 years this 10 or more Ah imbalance has built up. I can't think of another explanation and I certainly can't see how 3 cells can be perfect and one cell has far too much charge. I could understand it if one cell was low - knackered cell. But when cells become knackered they don't gain charge!

confused.com

[David Mack]

12 minutes ago, nicknorman said:

Cells 1,2 and 3 are always within a few mV and they started going down the knee. Meanwhile cell 4 was way up.

Is that why they use the name 'Drift'?

[GUMPY]

I will have a look at mine next time I'm there to see if it's similar at the same age. Big difference is that I just leave the MPPT charger to do it's thing, none of the only charge to 80% stuff. If there's sun and I'm not there it gets to 100% every day drops back to 98% overnight back to 100% the next day. Spends a lot of time on float at 13.5v  or about 99%.

[MtB]

Would you not get the same symptoms if the capacity of cell 4 has dropped with use more than the other three? 

They are after all, the cheapest LFPs money can buy, and it seems unlikely they are selling top notch quality products that cheap. 

 

 

[nicknorman]

12 minutes ago, MtB said:

Would you not get the same symptoms if the capacity of cell 4 has dropped with use more than the other three? 

They are after all, the cheapest LFPs money can buy, and it seems unlikely they are selling top notch quality products that cheap. 

 

If the cell just topped out early during charging, loss of capacity would be an explanation. But I first noticed the problem when the other 3 cells were nearly flat, whilst the rogue one still had loads of charge. So I don't think the capacity has been lost, it is just that more charge has been put in or less charge taken out of cell 4, than it has from cells 1-3. Which bearing in mind they are in series, is weird!

[MtB]

16 minutes ago, nicknorman said:

 

If the cell just topped out early during charging, loss of capacity would be an explanation. But I first noticed the problem when the other 3 cells were nearly flat, whilst the rogue one still had loads of charge. So I don't think the capacity has been lost, it is just that more charge has been put in or less charge taken out of cell 4, than it has from cells 1-3. Which bearing in mind they are in series, is weird!

 

 

Ah I missed that bit in your OP. So yes your hypothesis is the only explanation I can think of too. 

[PeterF]

The fourth cell has a much lower self discharge rate than the other 3 cells which are similar. However, I doubt that this is the case and your explanation closer to the truth.

[nicknorman]

3 minutes ago, PeterF said:

The fourth cell has a much lower self discharge rate than the other 3 cells which are similar. However, I doubt that this is the case and your explanation closer to the truth.

Something I hadn't really thought about is the self-discharge rate of the actual cells, vs the self discharge rate of the battery including BMS. The JBD BMS quotes a running current of 25mA and a sleep current of 0.3mA. Sleep mode is entered when the there is no current or other stuff going on. So 0.3mA sounds quite small but is still 2.6Ah a year or 2.5% of my battery. But the bluetooth module is an "extra" which is probably not included in that 0.3mA and it has to remain vaguely active so it can connect with the phone app at any time. So I wouldn't be too surprised if the actual sleep consumption including the bluetooth was not more like 3mA or 26Ah (25%) a year.

Whereas I think the self discharge of the cells themselves is much lower. I was careful with my own BMS design to minimise sleep current - all the peripherals (canbus transmit, microprocessor clock etc) is shut down when it's sleeping and it just wakes up every 20 seconds or so to sample cell voltages (which takes a few milliseconds). That way the sleep current is under 100 microamps. But of course if I want to interact with it, I have to wake it up with a button, not with my phone!

[Jen-in-Wellies]

3 minutes ago, nicknorman said:

But of course if I want to interact with it, I have to wake it up with a button, not with my phone!

The horror! What is this, the 20th century? 😀

[Col_T]

23 hours ago, nicknorman said:

So I am wondering if it is so crappily designed that it actually takes the power to run the BMS and the Bluetooth, just from cells 1-3 and cell 4 has no parasitic drain.

Curious as to what Fogstar say about this?

[nicknorman]

59 minutes ago, Col_T said:

Curious as to what Fogstar say about this?

I didn’t asked them as they didn’t design the BMS, they just followed the chinglish instructions. I might yet turn out to be something up with the cell, so next step (now it is properly top balanced) is to discharge it a fair bit and see if it remains balanced.

[Col_T]

Fair enough, but it seems so obvious to run the BMS from all the cells within the battery that I wondered if it was a manufacturing error, which Fogstar may have an interest in?

[IanD]

19 minutes ago, Col_T said:

Fair enough, but it seems so obvious to run the BMS from all the cells within the battery that I wondered if it was a manufacturing error, which Fogstar may have an interest in?

It seems unlikely anyway, the BMS in a 4 cell battery won't work properly if it only connects to/is powered by 3 cells, it has to connect to the ends of the stack to function.

[nicknorman]

26 minutes ago, Col_T said:

Fair enough, but it seems so obvious to run the BMS from all the cells within the battery that I wondered if it was a manufacturing error, which Fogstar may have an interest in?

The BMS is wired as per JBD’s instructions. I don’t think I have enough firm data to go to Fogstar with a categoric issue. Anyway, it doesn’t particularly matter as I have (hopefully) restored the battery to normality. I will keep a closer eye on it to see if the problem recurs, but even if it does after another 2 years, I’ll know what to do.

In the end I think I must have taken 15Ah out of cell 4.

Edited October 23, 2025 by nicknorman

[nicknorman]

I’ve discharged to 75% SoC. Cells are all within 1mV which is pretty good! So I don’t think there is anything badly wrong with a cell.

[GUMPY]

Could it have happened because you don't charge to 100% and cell balance that often?

[Gybe Ho]

2 hours ago, nicknorman said:

I’ve discharged to 75% SoC. Cells are all within 1mV which is pretty good! So I don’t think there is anything badly wrong with a cell.

 

Put a 0.5c load on the battery with the lid off, get your thermal camera out and see if the cell-4 terminals look hotter. Could you get a clamp around the inter cell connections or it it busbar based?

 

Alternatively switch to hot water bottles then in 3 months time you can start a new thread about removing black mould above the gas hob.

 

BTW Mr. Fogstar is pleasant to deal with and I think the company would be interested in a coherent report on top-balance mal function.

 

BTW2 The BMS is powered off the cell voltage sense wires and so a man of your electronics calibre should be able to test your theory about which cells are powering the BMS.  

[IanD]

6 hours ago, nicknorman said:

I’ve discharged to 75% SoC. Cells are all within 1mV which is pretty good! So I don’t think there is anything badly wrong with a cell.

Unfortunately -- as I believe you yourself have pointed out! -- the cell voltage difference at 75% SoC tells you almost nothing about the actual SoC of each cell, since the LFP SoC/voltage slope is so flat.

 

For example, the last time I did a charge to 100% SoC (57.3V) there was one cell well out of balance with the rest (more than 100mV), but the voltage difference beforehand (e.g. at 75%) was close to zero -- and that cell wasn't even the lowest one...

 

3 hours ago, Gybe Ho said:

 

Put a 0.5c load on the battery with the lid off, get your thermal camera out and see if the cell-4 terminals look hotter. Could you get a clamp around the inter cell connections or it it busbar based?

 

Alternatively switch to hot water bottles then in 3 months time you can start a new thread about removing black mould above the gas hob.

 

BTW Mr. Fogstar is pleasant to deal with and I think the company would be interested in a coherent report on top-balance mal function.

 

BTW2 The BMS is powered off the cell voltage sense wires and so a man of your electronics calibre should be able to test your theory about which cells are powering the BMS.  

Series resistance (e.g. at a terminal) can't affect SoC since current into the terminal (e.g. cells 1-3) and out (e.g. cell 4) are identical, all you get is a voltage drop so the terminal gets hot (bad!) and the cell voltage under load looks different -- which is not the problem here.

 

IIRC the Fogstar BMS circuits are powered from the top and bottom of the cell string (Vbatt and GND), the terminals in between cells are used for voltage measurement (no current) and cell balancing (should be no current when not balancing).

 

So the only real possibilities are a leaky cell (but this would leave it at lower SoC and it appears to be higher), or maybe a problem in the balancing circuits -- perhaps the one across that cell (if it's active, don't know if this is the case or not) is faulty and is pushing a small balancing current in all the time?

 

 

Edited October 26, 2025 by IanD

[Gybe Ho]

36 minutes ago, IanD said:

 

IIRC the Fogstar BMS circuits are powered from the top and bottom of the cell string (Vbatt and GND), the terminals in between cells are used for voltage measurement (no current) and cell balancing (should be no current when not balancing).

 

 

A JBD BMS will boot up and an attached bluetooth port will support configuration of the BMS before the principal 12v output of the battery is connected to the BMS. This should not be a surprise because silicon chips typically do not need a 12v power source.

 

43 minutes ago, IanD said:

 

Series resistance (e.g. at a terminal) can't affect SoC since current into the terminal (e.g. cells 1-3) and out (e.g. cell 4) are identical, all you get is a voltage drop so the terminal gets hot (bad!) and the cell voltage under load looks different -- which is not the problem here.

 

 

Not convinced cells are equally affected by the voltage drop at a hot connection, does it not depend on whether the bad connection is between say cell-4 and the main +ve terminal or between cell-4 and cell-3? Yes the current will be the same through all cells but the power extracted by a cell undercharge will be proportional to the voltage seen by the cell.

 

Anyhow putting that open question aside, a poor connection could throw out balancing as applied to individual cells.

 

Whatever, given the mysterious behaviour of @nicknorman's lithium battery, a routine check for a worrying resistance somewhere within the battery internals would make sense. I am sure someone with @nicknorman's back ground will intuitively feel a 1N or 5N or 10N torque at connection.

[IanD]

3 hours ago, Gybe Ho said:

 

A JBD BMS will boot up and an attached bluetooth port will support configuration of the BMS before the principal 12v output of the battery is connected to the BMS. This should not be a surprise because silicon chips typically do not need a 12v power source.

 

 

Not convinced cells are equally affected by the voltage drop at a hot connection, does it not depend on whether the bad connection is between say cell-4 and the main +ve terminal or between cell-4 and cell-3? Yes the current will be the same through all cells but the power extracted by a cell undercharge will be proportional to the voltage seen by the cell.

 

Anyhow putting that open question aside, a poor connection could throw out balancing as applied to individual cells.

 

Whatever, given the mysterious behaviour of @nicknorman's lithium battery, a routine check for a worrying resistance somewhere within the battery internals would make sense. I am sure someone with @nicknorman's back ground will intuitively feel a 1N or 5N or 10N torque at connection.

You're misunderstanding how cells in series work -- they should all have the same current regardless of any connection resistances, and it's current (into the voltage of that cell) that charges the cells.

 

How do you think the BMS is powered up, if not from the series stack of cells (or the input terminals) to get the Bluetooth going in the first place? The "silicon chips" need a power source, which will come via a low-power internal DC-DC converter from 12V.

[Gybe Ho]

1 hour ago, IanD said:

You're misunderstanding how cells in series work -- they should all have the same current regardless of any connection resistances

 

 

Did you miss the section of my previous response where I said...

 

5 hours ago, Gybe Ho said:

Yes the current will be the same through all cells but the power extracted by a cell undercharge will be proportional to the voltage seen by the cell.

 

 

1 hour ago, IanD said:

 and it's current (into the voltage of that cell) that charges the cells.

 

 

Not so. For a battery to be charged work has to be done to convert electrical power to stored chemical energy. The amount of work done is the voltage multiplied by current observed while a battery is being charged. The designed nominal voltage of a battery is something else. The amount of power put into a battery is proportional to the excess charge voltage applied above the battery's nominal voltage up to a safe maximum.

 

Voltage drives battery charging not current, current is just a consequence of voltage and resistance.

 

1 hour ago, IanD said:

 

How do you think the BMS is powered up

 

 

I explained earlier in the thread. It is entirely logical that the BMS designers chose to power the BMS circuit from the cell sense wires because 3.2v is closer to the power requirement of the chips than the main 12v supply. 12v input DC/DC conversion losses would be greater.

[IanD]

11 hours ago, Gybe Ho said:

 

Did you miss the section of my previous response where I said...

 

 

 

Not so. For a battery to be charged work has to be done to convert electrical power to stored chemical energy. The amount of work done is the voltage multiplied by current observed while a battery is being charged. The designed nominal voltage of a battery is something else. The amount of power put into a battery is proportional to the excess charge voltage applied above the battery's nominal voltage up to a safe maximum.

 

Voltage drives battery charging not current, current is just a consequence of voltage and resistance.

 

 

I explained earlier in the thread. It is entirely logical that the BMS designers chose to power the BMS circuit from the cell sense wires because 3.2v is closer to the power requirement of the chips than the main 12v supply. 12v input DC/DC conversion losses would be greater.

So why the comment suggesting that poor connections can have anything to do with cell imbalance?

 

With LFP batteries the cell SoC pretty much defines the voltage, it's very hard to force this to change because of the low internal resistance, so it's current that determines charging.

 

If you think the bit at the end is true in practice, you don't understand anything about modern microelectronics... 😉

[Gybe Ho]

46 minutes ago, IanD said:

So why the comment suggesting that poor connections can have anything to do with cell imbalance?

 

 

We are going around in circles but I am pleased to note you now accept I stated that the same charge current flows through all cells even when a dodgy intercell connection exists.

 

Let me try to define my open question more concisely. For work to be done (namely battery charging) power is expended. Electrical power is calculated as volts x amps. Given that we have finally achieved consensus that charge amps are uniform through the battery cells we now have to verify that voltage is the same across all cells, for your opinion to be correct.

 

Given a 4-cell 12v lithium battery that is part charged and the cells are initially in soc balance. Now introduce a poor connection between cell-3 an cell-4 that generates 60 watts of heat when a charge current of 100 amps is flowing. If the charge voltage is 14.4 volts we would expect to see 3.6v potential difference across each cell under charge if the internal wiring is healthy. However at 100 amps that poor connection is generating 60w of heat and by definition 0.6 volts of the 14.4v is lost generating heat at the poor connection.

 

My open question is how is the remaining 14.4 - 0.6 = 13.8v of charge voltage distributed across the 4 cells in the presence of a 0.6v voltage drop between cells 3 and 4? If the voltage is not even then we have a battery where cell imbalance can accumulate.

 

[nicknorman]

21 hours ago, IanD said:

Unfortunately -- as I believe you yourself have pointed out! -- the cell voltage difference at 75% SoC tells you almost nothing about the actual SoC of each cell, since the LFP SoC/voltage slope is so flat.

 

 

Yes I know, my point was merely that there probably wasn't anything catastrophically wrong with the cell. After all, it was about 15Ah (out of 105Ah) out of balance, which is HUGE!

21 hours ago, IanD said:

IIRC the Fogstar BMS circuits are powered from the top and bottom of the cell string (Vbatt and GND), the terminals in between cells are used for voltage measurement (no current) and cell balancing (should be no current when not balancing).

 

So the only real possibilities are a leaky cell (but this would leave it at lower SoC and it appears to be higher), or maybe a problem in the balancing circuits -- perhaps the one across that cell (if it's active, don't know if this is the case or not) is faulty and is pushing a small balancing current in all the time?

 

 

The BMS is a 3 or 4 cell BMS, but the wiring for 3 cells puts the 4th cell + input connection on the 3rd cell +, the 3rd cell + input is unused. So clearly the BMS does take power from the top cell. The balancing is only passive (resistors) so unless 3 of them are active when they shouldn't be, this doesn't explain it. All very odd. I will update in another couple of years to see if the problem has recurred!

[IanD]

1 minute ago, nicknorman said:

 

Yes I know, my point was merely that there probably wasn't anything catastrophically wrong with the cell. After all, it was about 15Ah (out of 105Ah) out of balance, which is HUGE!

 

That is indeed huge, and suggests something is not right -- but it's difficult to see what, most faults I can think of (e.g. a faulty balancing circuit for that cell) would lead to it ending up with lower SoC than the rest, not higher.

 

Unless the BS uses active balancing which can pump up the charge of one cell in the stack, which seems unlikely given the low cost...