LiFePO4 soc 100% but volt 13.7

torfal

Hi, I have two 200 Ah in parallel and my Bluetooth BMS tells me I have 100% Soc on both batteries despite voltage of 13.6.  The solar panel still charges the battery and my charger is set to turn off charging at 14.1 volt.... can I just disregard the soc and rely on the voltage to determine the “real” soc? Any help appreciated! 

mcgivor

The information is too vague, you need to state the topography, such as x parrallel x series, also are the batteries a drop in replacement type or a DIY bank, the assumption would be the latter. What is the charging source? Solar or grid? 

Based on the settings of 14.1V for charging that seems reasonable for a XP 4S bank, as there is very little added capacity above this region, so perhaps the BMS is recording 13.6 V as a 100% SOC as part of a conservative indication. What is the application of the battery, off grid, standby or ??? Lots of guessing on my part but essentially 13.6 V is in the 90% + safe region  preferred for off grid use, what is the low voltage disconnect?  That would be the next question, hopefully it's greater than 12V.

Additionally were the cells ballanced before application and if so we're they top or bottom ballanced? If applicable of course.

torfal

Thank you so much for your input😉. I Will try to clarify; the batteries are Topband with BMS and they are linked in parallel for a 12 v 400 Ah system all together.  Both batteries where initially at 0% Soc due to discharge and then charging has been built up through one solar panel and wind power (when windy). So they were bottom balanced. I have set my Coleman air C440 HVAD on bulk set point 14.1, absorb 120 minutes, Float 13.8 and rebulk 12.6. Please see photos. All is off-grid. So what do you think? Any concern for overcharging my batteries? Thanks a lot for help🙏

Raj174

With an internal BMS the batteries need to be charged according to battery mfg specs. When full, it will reset SOC to match capacity. They are top balanced and do this during the end of the charge.

torfal

So you mean that I continue to charge until I reach 14.6 volt?

Raj174

Most LFP 12v batteries with internal BMS absorb to around 14.4v. What are the recommended charging specs for your battery?

mike95490

torfal said:

Thank you so much for your input😉. I Will try to clarify; the batteries are Topband with BMS and they are linked in parallel for a 12 v 400 Ah system all together.  Both batteries where initially at 0% Soc due to discharge and then charging has been built up through one solar panel and wind power (when windy). So they were bottom balanced.

Having Li batteries at a 0% state of charge, is going to be a problem.  That's the initiation for a battery fire at some point later on.

At  least, that's what I understand from the interwebs.

BB.

Have you kept any history on how many Amp*Hours (WH, etc.) your wind turbine is producing per day?

From the photos, it looks like the turbine tower is too short (lots of buildings and trees as tall, or taller than the turbine).

Also, what is the wattage of your solar panel... Assuming ~250 Watts:

• 250 Watts * 0.77 panel+controller deratings * 1/14.0 volts charging = 13.75 Amps peak charging current (best case, cool/clear days, solar noon)

Assuming 250 Watt panel/array, fixed tilt, facing south, Stockholm:

Stockholm
Average Solar Insolation figures

Measured in kWh/m2/day onto a solar panel set at a 46° angle:
(Optimal summer settings)

Jan	Feb	Mar	Apr	May	Jun
0.84	1.86	3.39	4.76	5.67	5.57
Jul	Aug	Sep	Oct	Nov	Dec
5.46	4.76	3.64	2.09	1.16	0.64

Using June, assuming the solar panel/array is doing most of the charging:

• 400 AH / 13.75 Amps charging (est.)= ~32 hours of sun
• 32 hours of sun / 5.57 Hours of sun per day (June ave) = 5.75 Days of average sun (June) to fully charge

-Bill

torfal

Thanks for helpful comments! Indeed the wind tower much to low and badly placed, but as a matter of fact, the solar panel is doing the job, and deliver 9 AH when optimal, so summer time I have plenty of energy. The wind is more effective and needed during low sun October to March and strong winds bring in 10-15 AH for short periods. Since this is my summer house it is sufficient. And when batteries are ocassionally drained I run my gas generator to load the batteries at 40 AH. Just got a 100 AH charger that I will try this fall😉. The specs for my batteries are 

Bulk: 14.1V±0.2V

Absorption time: 180 min.

Re-Bulk: 14.1V±0.2V

Float: 13.8±0.2V

Charge current: 100 A

So I take it that my BMS will cut off charging when these parameters are achieved and that my soc values are faulty. In any event my HVAD charger will cut off charging also since the same parameters are entered. Hence I take it that I have a safe system with two independent monitoring systems. 

mcgivor

The BMS should cut the load when the voltage drops to somewhere in the neighborhood of 12 V, perhaps slightly higher, this actually wouldn't be fully discharged, although the monitoring may say 0%,SOC , it's a safeguard to prevent cell damage. Likewise on the top, it may say 100% SOC when in fact it's in the mid 90% SOC range, given the 14.1 V recomendations.

Yes the built in BMS makes the battery intrinsically safe (safe in and of itself, by design ) One thing to avoid is frequent deep discharge, if you're regularly nearing 0% indicated SOC,  it would be prudent to limit such events to extend cycle life expectancy, in other words increase the charging capacity to compensate and or increase battery capacity, see @BB. #8 post 

torfal

mcgivor thanx, makes sense to me  - I am waiting for my charge controller to kick in and divert the charging to the solenoid controlled dump load when parameters are meet; before my inbuilt battery BMS stops it; so that the latter serves as added security. I don’t want my system to catch fire due to overcharging.