LiFePO4 issue?

[GeoffL]

Hmmm, don't think that'll happen Geoff.
They suggest charge voltages of 14.6V so the BMS over voltage threshold MUST be higher and is probably considerably higher than this (3.65v / cell) and therefore alternator charging at 14.4 volts won't trigger cut off unless one cell is massively out of whack with the others.
Good luck with the Festival BTW, it looks great

According to the 'net, cell voltage at 100% SOC is 3.65V while charging and drops to 3.40V after resting. So, for a 4-cell battery (which is what I have), that's 14.6V and 13.6V respectively. Thus, unless the cells are perfectly balanced, an over-voltage condition in one cell seems possible. For example, if three cells were 0.1V lower than the fourth, 14.6V would be split 3.725V for the highest voltage cell and 3.625 for the other three. As you can see from the screenshots I uploaded, the differential is over 0.1V at 100% SOC although it's less than 0.005V at 80% SOC. Hence the cells don't appear to need to be massively out of whack. A 0.1V differential would be enough to trigger an over-voltage condition at just 14.3V...

Thanks re. the festival: I got press-ganged into a shanty band a couple of months ago and I'm still busking for a lot of their repertoire; so that good luck might be needed :vbg:

 

[merl]

Yep, I get where you're coming from, I guess it'll ultimately depend on exactly what the BMSs cell overvoltage is set at. I'd anticipate this will be significantly higher than the upper limit of recommended charge voltage?
The other thing is that IF the BMS does trip off when does it re-connect again?? And what happens when it does? Without detailed info it's all a bit finger in the air Geoff but personally I think you'll be fine, LFP seems to be very tolerant and not fussy at all. Discharge below 2.3 v/ cell excepted.

 

[Chasn]

Geoff. Depending on which BMS was used in your battery the balance current that's drawn from the strongest cell can often be tiny, it therefore takes ages to drag that cell down to a similar level to the lowest cell in the pack, this problem is often exaserbated by the fact that a lot of BMSs only provide cell balancing while the battery is actually being charged or even worse being charged above a set current. (maybe this is the 10A figure that you mentioned?)
Take a look at the BMS settings and see if there's a 'balance' setting page where you can set the charge current when balancing commences and set this as low as possible, ideally there'll be an option to make balancing happen continuously or ALWAYS, so the cells will balance constantly.

Do not do this. Setting the balancing at below 3.45v/cell (13.8v battery voltage) will upset balancing. I have three 30kwh packs balanced with 2A and it works just fine after an initial top balance. The expert on all things LFP batteries is Andy in Australia (see the Off-grid Garage YouTube videos). With such a flat charge curve you only know which are the strong and the weak cells once you are higher than the plateau voltage.Balancing below 3.45v/cell will make this worse.

 

[merl]

Do not do this. Setting the balancing at below 3.45v/cell (13.8v battery voltage) will upset balancing. I have three 30kwh packs balanced with 2A and it works just fine after an initial top balance. The expert on all things LFP batteries is Andy in Australia (see the Off-grid Garage YouTube videos). With such a flat charge curve you only know which are the strong and the weak cells once you are higher than the plateau voltage.Balancing below 3.45v/cell will make this worse.

Hmm, can't agree there and neither does Andy. He himself recommends setting the balance to 'always'

 

[GeoffL]

Do not do this. Setting the balancing at below 3.45v/cell (13.8v battery voltage) will upset balancing. I have three 30kwh packs balanced with 2A and it works just fine after an initial top balance. The expert on all things LFP batteries is Andy in Australia (see the Off-grid Garage YouTube videos). With such a flat charge curve you only know which are the strong and the weak cells once you are higher than the plateau voltage.Balancing below 3.45v/cell will make this worse.

Thanks for the pointer to Andy's channel. However, in my case this is moot since the BT app supplied only lets me monitor what's going on and I have no way of adjusting any settings. As others have mentioned or at least implied, if I set my SCC up for fixed time absorption at 14.2V or more, my LFP battery should receive an adequate charge each day without needing me to continue to intervene!

 

[wildebus]

Thanks for the pointer to Andy's channel. However, in my case this is moot since the BT app supplied only lets me monitor what's going on and I have no way of adjusting any settings. As others have mentioned or at least implied, if I set my SCC up for fixed time absorption at 14.2V or more, my LFP battery should receive an adequate charge each day without needing me to continue to intervene!

The reason why the app won't let you adjust anything is quite simple really.... more people f*** up their self-built/white-box lithium batteries by messing with the settings then ever make them better! So the more professional/dedicated battery suppliers usually lock down the configuration options. When I install Lithiums, I MUCH prefer to use batteries that are not configurable as that way I know they will carry on working after installation and not get "optimized".
I have never once had the need to do ANY balancing of my Lithiums as they were built correctly in the factory and configured prior to shipment to the distributor.
I wonder how all those people with the 'dumb' Lithiums without Bluetooth BMSes every managed to use them successfully!


PS. Good luck with Andys channel I know people like to watch him but he does my head in Can't manage more than 5 minutes

 

[mariesnowgoose]

 

[GeoffL]

The reason why the app won't let you adjust anything is quite simple really.... more people f*** up their self-built/white-box lithium batteries by messing with the settings then ever make them better! So the more professional/dedicated battery suppliers usually lock down the configuration options. When I install Lithiums, I MUCH prefer to use batteries that are not configurable as that way I know they will carry on working after installation and not get "optimized".
I have never once had the need to do ANY balancing of my Lithiums as they were built correctly in the factory and configured prior to shipment to the distributor.
I wonder how all those people with the 'dumb' Lithiums without Bluetooth BMSes every managed to use them successfully!

I now know what you mean. If I'd gone with my first choice (Fogstar Drift), I'd have had a JBD/Overkill BMS and a BT app that would let me configure it (soz: f*** it up) trying to correct what turned out to be a simple misunderstanding. Nice to know that you never had the need to rebalance any of your batteries, particularly since two of them are the same model as the one I've fitted! I'll hold off buying that expensive Victron IP65/10A until (if ever) rebalancing is needed!

PS. Good luck with Andys channel I know people like to watch him but he does my head in Can't manage more than 5 minutes

if you can't manage more than 5 minutes, how am I supposed to cope :vbg: That said, I've still got to get my head around your words of wisdom re hybrid systems!