Issues and questions to solar system in camping mobile

[Dietmar Miller]

Hello all,

I have built up a solar system for my Camper Fiat Ducato 5 years ago. It worked until now with only one issue.

I just come back from a travel and my system is defect. The LiFePO4 battery has 2 cells which have only 1.6 V ea.

The Electrodacus showed still remaining 40 % of charge. the load and the solar panel were switched off from the Victron BP100 and BP64.

However the inverter was still on for about 15 hours. I think the standby current of the inverter has then further discharged my battery.

The converter was not connected with the remote control to the BMS, so it was not switched off.

Therefore I have added the remote control in the schematic, but it is not yet implemented.

In general I have noticed that 2 cells have a little lower voltage than the 2 others.

I have also seen that the battery cells have been expanded to some extent. see picture attached.

I have some basic knowledge about electronics but I do not have much experience in this subject. 

My use model for the camping mobile is:

First use in April, last use in October/November. the battery stays in the mobile during winter with about 70% of initial charge. 

during travel it can happen that the battery is long time in between 50 and 80 % of displayed charging level when there is not much sun. When the charging level exceeds 90 %, I often switch it off and switch it on again when it is below 60%.

My questions are:

1)how can I ensure that the displayed charging level is correct? How can I calibrate the system after winter time?

I think the battery discharges in winter and it is not recognized by the system since the currents are too low.

2)do I need an additional balancer for the battery to ensure that all cells have the same voltage? In my use model the cells are very rarely charged fully. 

3) could you please review my schematic circuit for any issues? please check if the polarities of the control exits are correct.

4) to ensure that the inverter does not discharges the battery again, I have added the remote control of the converter. Does it make sense like this or is it better to connect the remote control to the BP 100 out pin?

thanks a lot for your support in advance

Dietmar

[Dacian Todea (electrodacus)]

Dietmar,

Unfortunately your battery is damaged and you will need to replace it. Discharging below 2.5V is what damaged the cells and the reason for cells being expanded.

1) You need to introduce the real capacity in the SBMS and you need to allow full charge to 100% at least once a week. Also the current shunt need to be setup correctly so it measures the current accurately.

Over the winter you need to fully disconnect the battery that includes the SBMS and leave the battery close to fully charged before winter storage.

2) Cells even in your photo are fully balanced since all cells are between 0% and max 5% SOC The 3.1V cells are also close to fully discharged.

3) The schematic seems fine except I do not think you can parallel the remote ON/OFF for the 500W inverter and BP100.  You can use EXT IO5 set as type 2 for the inverter.

4) You could connect the remote of the inverter to the Out of the BP100 just not sure exactly witch pin I think it will be the L pin that will connect there. But you need to make sure there are only Loads connected to BP100 Out so there is no voltage source there.

a) So the battery failed due to over discharge thus adding the inverter remote and making sure battery is fully disconnected over winter should prevent that for happening on the new battery.

b) Let the battery fully charge to 100% as that is not the reason your battery will degrade and it allows cell balancing and correction of the SOC. 

[Dietmar Miller]

Hello,

thanks for your detailed answer. 

Can I leave the batterie in the car over winter, also when it is very cold?

how did you conclude that the 4 cells are in good balance? 2 cells have about 1.6V and 2 about 3.1V. is a difference of 5% SOC still good enough?

I have an electrical fridge in my car which is the main load. it consumes about 3.5 A. When it switches on the displayed current on the  SBMS display is showing values in between 3 and 4A, very inconsistently. Is this because it is an inductive load? does this lead to inaccurate counting of the SOC?

[Dacian Todea (electrodacus)]

The difference of 5% is when cells are fully discharged. Likely the difference is much lower when cells are fully charged. There are differences in capacity and internal impedance between the cells from the factory.

Is the fridge a 12V DC fridge or is a 120V / 240V ac working trough an inverter ? Also if DC is the fridge compressor based or Peltier based ?

It is possible that there is that variation in current while the fridge is running.  It is more about a reading of say 3.3A when in reality real current is 3.4A that will result in SOC calculation error.

Also there could be multiple issues like multiple charge discharge cycles with no full charge that also increases the SOC error as it requires a full charge for the SOC calibration. To that it could also be that real battery capacity is lower than what you set in SOC menu. Say you set 200Ah that is the spec capacity when new but over time the capacity decreased to say 160A so 80% of initial capacity then just based on that alone the SOC at the end of discharge will show 20% instead of zero then the current reading error could add another 3% and then multiple cycles without full charge could multiply that 3% reading error to say 7x in a week and thus you get 3 x 7 = 21% + 20% = 41%

This is just an example of how you can end up with 41% at the full discharged battery but can be any combination of those factors.

[Dietmar Miller]

Hi,

the fridge has a special compressor system:

 " An Engel fridge uses a special system called the Sawafuji Swing Motor. Unlike normal fridges that use gears and rotating parts, it uses simple magnets and an electric field to move the piston back and forth. This means it has only one moving part. [1, 2] "

it runs with 12 V DC and the displayed current is jumping between 2.8 A and 4 A.

My failure was that I very rarely charged the cells completely. I always thought that 100 % charge has a negative impact to the lifetime of Li batteries. But this seems not to be the case for LiFePO4 batteries as you confirmed.

When the battery remains in the car in wintertime at cold temperatures (without any load), does this affect the lifetime?

[Dacian Todea (electrodacus)]

Likely that current variation is normal for those types of fridges.

The battery can stay outside without any degradation as long as your outdoor temperature is above -20C.   Below -20C will be problematic but is not chemical degradation is structural as the electrolyte can freeze and can damage plastic parts / seals around the connectors.  In therms of calendar aging the battery degrades less if stored at low temperature compared to storing at higher temperature due to reduced chemical reactions.