2 x 12v fogstar out of balance

[whibble]

As per the description. I have 2 x fogstar pro 280ah lifpo4 wired in series to give 24v for my inverter.   They work fine, but have a niggle. 

The one connected to the positive end discharges faster than the other one. As a result, when charging it doesn't reach full capacity before the charger cuts out. The second battery can be at 100% but the first can be mid 80s %.

I have done a full top balance ie removing and charging each one to 100% then left them for a few hours before reconnecting them. They still go out of balance. 

Can these be connected using the network sockets on the top to then act as a single battery ? They came with a cat5 cable each. They also came with a balancer each. 

 

[BEngo]

Give fogstar a ring or drop them an e-mail.  They are very helpful on tech support and it goes all the way to Ben the boss if needed.

 

[jonathanA]

1 hour ago, whibble said:

As per the description. I have 2 x fogstar pro 280ah lifpo4 wired in series to give 24v for my inverter.   They work fine, but have a niggle. 

The one connected to the positive end discharges faster than the other one. As a result, when charging it doesn't reach full capacity before the charger cuts out. The second battery can be at 100% but the first can be mid 80s %.

I have done a full top balance ie removing and charging each one to 100% then left them for a few hours before reconnecting them. They still go out of balance. 

Can these be connected using the network sockets on the top to then act as a single battery ? They came with a cat5 cable each. They also came with a balancer each. 

 

Are you sure they are drift pros ? If Gen2 then you should have specified you needed series support when you ordered them. I can't see a 280ah drift Pro available on the frogspawn website. So just check what you actually have.

[whibble]

Definitely drift pro. I have had them just under a year. 

There is a rs485, and nc, and a  can socket on them. Plus of course the balance round socket.

[MtB]

1 hour ago, whibble said:

The one connected to the positive end discharges faster than the other one.

 

Point of Order M'Lud,

 

It is not possible with two batteries in series. The same current is by definition flowing through two batteries in series so they MUST be discharging at the same rate if you measure the AHs drawn from each battery.

 

I imagine you might be looking at the percentage SoC of the batteries and you see one battery falls faster than the other. If so, then this is telling us the capacity of one battery is lower than the other. 

 

How are you measuring the rate of discharge?

 

Also, did you top-balance the two batteries to 100% before connecting them together in the first place? 

 

Edited Wednesday at 18:41 by MtB

[GUMPY]

Surprised that in series it runs with 2 separate BMS.

My home system has a master battery and up to 2 slaves all in series, battery numbers set by the BMS.

I end up having to top balance them about every 12 months as the total capacity drops over time from 17.4kWh to somewhere about 14.5kWh.

 

[MtB]

1 hour ago, GUMPY said:

Surprised that in series it runs with 2 separate BMS.

 

Why would it not? 

 

The BMS monitors cell voltages and temperatures. Anything else? What does it do that would stop a pair of batteries in series running? 

[GUMPY]

My batteries are in series and only the master battery has a full BMS this monitors the master and up to two series slave batteries. Why have the excess hardware of a full BMS for each battery when it's not needed. The slaves will not function without a master and you can't use two masters in series. 

Yes two batteries each with a full BMS should work in series but each BMS will handle things slightly differently.

One BMS might shut down due to high/low voltage where the other is still on, similar for temperature and current.

 

 

 

Edited Wednesday at 20:47 by GUMPY
Adda word

[MtB]

5 minutes ago, GUMPY said:

One BMS might shut down due to high/low voltage where the other is still on, similar for temperature and current.

 

Which seems to be exactly what the OP is complaining about, and strikes me as correct operation. Once any single cell dips too low, the BMS on that battery should turn the battery off to prevent cell damage. 

 

I notice you ducked my question! 

 

 

[GUMPY]

9 minutes ago, MtB said:

 

Which seems to be exactly what the OP is complaining about, and strikes me as correct operation. Once any single cell dips too low, the BMS on that battery should turn the battery off to prevent cell damage. 

 

I notice you ducked my question! 

 

 

The BMS should turn the whole bank off not one half of it as it does with two unlinked batteries in series.

[wakey_wake]

1 hour ago, GUMPY said:

The BMS should turn the whole bank off not one half of it as it does with two unlinked batteries in series.

Unless there is a centre tap (12V), it can't turn half the bank off.

 

They're in series, and if either BMS goes open circuit then the switch is now off and the entire bank's voltage will be across those FETs.

This will be the origin of the limit on how many integral-BMS batteries (often 4x 12V) may be strung in series.

 

@whibble there do exist 24V (2x 12V) balancing devices. The simplest ones slowly burn off excess charge in the higher, and the better ones transfer it to the other(s).

Sorry, I know nothing about Fogstar-specific products, or whether they offer / need / can use / already should include such a device.

[PeterF]

Always best to have a singe 24V battery with a single BMS managing all 8 cells and keeping them in balance.

 

Can you use an external balancer such as the Victron one for lithium, these are normally used for lead acid.

 

https://www.victronenergy.com/batteries/battery-balancer#downloads-manuals

[wakey_wake]

44 minutes ago, PeterF said:

Always best to have a singe 24V battery with a single BMS managing all 8 cells and keeping them in balance.

Unless you want a centre tap at 12V, and are prepared to deal with the issues of using it.

 

Sorry... I am a pathological finder of counter examples. 🤓

[Gybe Ho]

13 hours ago, whibble said:

I have done a full top balance ie removing and charging each one to 100% then left them for a few hours before reconnecting them. They still go out of balance. 

 

 

Just curious, I assume you used a 12v charger for this independent charging?

 

I wonder if your methodology is wrong. Before independently bringing up each 12v battery to 100% from a 12v charging source you should first plug in the external Fogstar active balancer to level the internal 3.2v cells, having completed the active balance then bring the 12v battery up to 100%

 

Just doing external cell balancing using the active balance port on each battery *OR* charging each battery up to 100% at 12 volts won't resolve the problem.

 

Having said that its all a bit of a palaver and leaves open the question as to why you are seeing such drift in capacity between your two 12v batteries. What sort of time period or over how many 80% cycles does this drift occur? 

 

[IanD]

The missing info here is cell/battery voltage -- knowing Fogstar's propensity (at least, in the past) for SoC drift as reported by the internal BMS, it's possible that this is the problem not battery/cell imbalance. For example, if both batteries are at 14V (3.5V/cell) and one says 100% SoC but the other says 80%, they are actually balanced (assuming the cells inside each battery match) even though the SoC says they're not.

To reset the SoC counter normally needs the batteries to be held at the absorption/balancing voltage (e.g. 14.4V) for a period (at least 30mins) long enough to reset the SoC counters to 100%.

 

(these voltage levels are examples, exact numbers depend on Fogstar)

[Gybe Ho]

2 hours ago, IanD said:

The missing info here is cell/battery voltage -- knowing Fogstar's propensity (at least, in the past) for SoC drift as reported by the internal BMS, it's possible that this is the problem not battery/cell imbalance. For example, if both batteries are at 14V (3.5V/cell) and one says 100% SoC but the other says 80%, they are actually balanced (assuming the cells inside each battery match) even though the SoC says they're not.

 

 

This is a reasonable theory, the OP mentions RS485 and CAN ports on his batteries but does not clarify if these are connected to the charger. At this point we do not know what event causes charging to finish, is it:

1. The internal BMS dropping to charge portcullis at the FETs due to high cell voltage? or
2. The charge source respecting the 100% SOC on the lower capacity battery?
3. In a sophisticated distributed BMS setup that the OP might have it is more usual for the charge source to respect the advanced warning message from the BMS that high cell voltage is about to be reached.

Edited Thursday at 11:05 by Gybe Ho

[Gybe Ho]

12 hours ago, GUMPY said:

My batteries are in series and only the master battery has a full BMS this monitors the master and up to two series slave batteries. Why have the excess hardware of a full BMS for each battery when it's not needed.

 

 

In the case of parallel 12v batteries with parallel master BMSs I can think of multiple reasons:

 

Halve the current through each BMS. A modern boat electrical setup should be designed to a minimum spec of 300 amps and as a precaution we would not want run a BMS at 100% of its rated capacity for an extended duration. Commodity BMS boards above 300 amps are not common and/or cheap therefore it makes sense to design a system around multiple 300 amp BMSs.

 

Liveaboard Redundancy. A single BMS failure in a system with two battery banks keeps the lights on assuming the boat owner can exercise some self discipline with the immersion switch, electric kettle and air fryer.

 

Where a Victron Cerbo hub/brain is plumbed in there is free software available that can logically merge the data feeds from two physical BMSs and present both battery banks as a single entity to other components onboard such as a 240v charger or MPPT. 

[GUMPY]

1 hour ago, Gybe Ho said:

In the case of parallel 12v batteries with parallel master BMSs I can think of multiple reasons:

 

We were discussing batteries connected in series.

Parallel is another kettle of fish.

Edited Thursday at 10:41 by GUMPY

[jonathanA]

16 hours ago, whibble said:

Definitely drift pro. I have had them just under a year. 

There is a rs485, and nc, and a  can socket on them. Plus of course the balance round socket.

don't frog spawn say you should connect the active balancer to the battery every 3 to 6 months and leave it for a couple of cycles ? (but  not permanently connected). Have you done that  ?  if so and that hasn't sorted it i'd be giving them a ring and talking warranty claim... 

 

I think the RJ45 sockets are for communication with Victron GX but i might be thinking of the Gen 2 batteries, don't think you have to do anything if you series the original version. (max 4 x 12V batts) 

[whibble]

Firstly, thank you for the input. 

The sockets are currently (can etc) not connected to anything.

The top balancing was done by removing each battery then charging using a lipo charger. Both were charged until the charger cut off. They were then left to settle before connecting back up as my 24v bank.

They tend to start to drift after about a week.

I heard tell that each battery can be connected to my victron inverter charger using the data cable but after that I can't see how that would help. The Bluetooth dongle I got with the inverter doesn't give a lot of functionality I'm thinking I should have got the USB one. 

 

[jonathanA]

if you haven't used the external active balancer on each battery, thats the first thing Fog Star will say to you... 

[IanD]

5 hours ago, whibble said:

Firstly, thank you for the input. 

The sockets are currently (can etc) not connected to anything.

The top balancing was done by removing each battery then charging using a lipo charger. Both were charged until the charger cut off. They were then left to settle before connecting back up as my 24v bank.

They tend to start to drift after about a week.

I heard tell that each battery can be connected to my victron inverter charger using the data cable but after that I can't see how that would help. The Bluetooth dongle I got with the inverter doesn't give a lot of functionality I'm thinking I should have got the USB one. 

 

When you say "they start to drift", do you mean the reported SoC per battery (calculated by the BMS) or the voltage per battery (actual measurements)?

 

You need to work out if the SoC readings are accurate -- which they often aren't after some time...

Edited Thursday at 17:07 by IanD

[whibble]

Back at the boat.today, the fogstar app showed battery 1 at 79% and battery 2 at 96%.

I have disconnected both and plugged in the external balance box and will leave it until tomorrow.  Then charge each up to full capacity and leave overnight tomorrow. 

This should give both batteries 100% soc.

It will be interesting to see if they drift again. On the app, both battery individual cells read within 0.5v, equal on both cells. 

My bmv712 just shows 100%soc

[IanD]

27 minutes ago, whibble said:

Back at the boat.today, the fogstar app showed battery 1 at 79% and battery 2 at 96%.

I have disconnected both and plugged in the external balance box and will leave it until tomorrow.  Then charge each up to full capacity and leave overnight tomorrow. 

This should give both batteries 100% soc.

It will be interesting to see if they drift again. On the app, both battery individual cells read within 0.5v, equal on both cells. 

My bmv712 just shows 100%soc

 

I suspect your units are wrong -- even 0.5V between two batteries is a massive imbalance...

 

Instead of just reporting SoC (which is known to be inaccurate sometimes with Fogstars) please give the actual battery/cell voltages -- without these it's impossible to tell if the SoC imbalance is real or a figment of BMS imagination... 😉 

Edited 15 hours ago by IanD

[MtB]

2 hours ago, IanD said:

I suspect your units are wrong -- even 0.5V between two batteries is a massive imbalance...

 

Perhaps not if both are actually close to 100%?

 

One might be at 99.8% SoC and the other at 99.9%....