Low battery issue with Blue Nova

[LourensJB]

Hi all - first time post here

I have a 54 Volt Blue Nova battery approximately 10 months old now.  It is controlled by a Victron Multiplus 48/5000 unit - age of installation approx 5 years.  The available energy from the battery is constantly decreasing  up to a point where it now disconnects the Multiplus while even being at 40-50% State of Charge.  This is with an approximate continuous load of around 120watt overnight.  Originally when new it could go down to below 5% SOC with such a low load, only requiring a big load for it to go into protection  (Low Battery state) while still at say 20% SOC.  

I have a full island system being on a small holding and have a generator as backup but it does not auto-start. The island setup means that the battery is fully charged every day and drained to on average about 25% SOC overnight, infrequently down to 15% and lower.  It never stays at that  SOC for longer than a few hours - it is usually charged back up to at least 50% within 90 minutes - so chances of it getting damaged through prolonged discharge is slim. I have been advised by Blue Nova to allow for Equalisation charge which it does currently once a week (and has always been set like that) - we draw very low loads in the afternoons so the battery is fully charged daily.

Does anybody else have the same experience with Lithium batteries?  If so - how does one handle this? What is the safe Equalization repeat setting for Lithium batteries - will it do any damage to the batteries if set for "Daily"?

[Solarphile]

On 2019/08/05 at 7:07 AM, LourensJB said:

I have been advised by Blue Nova to allow for Equalisation charge

On a Lithium battery??!!!  There is something very wrong here! No lithium battery that I know of is EVER given an equalize charge!! 

On 2019/08/05 at 7:07 AM, LourensJB said:

drained to on average about 25% SOC overnight

Perfectly normal, lithium batteries are designed to do this.

On 2019/08/05 at 7:07 AM, LourensJB said:

will it do any damage to the batteries if set for "Daily"?

Most definitely!!!

 

My advice, disable the equalize charge immediately! It is destroying your battery. 

Oh, and welcome to the forum.....

 

[___]

In most cases, if you do actually try to equalise the battery, the voltage will rise too high and the BMS will activate its protection circuitry and disconnect the battery. So you may not sustain damage... but it's also not going to do any good.

There is something else to keep in mind though, and that is that LFP batteries usually balance (aka equalise) at the top. This is done using passive or active balancing. Passive balancing is when a small bit of charge is bled off a high cell (using a high power resistor) in order to let its lower brothers catch up: The current that flows through the bleeding resistor also flows through all the lower cells while keeping the high cell down a bit. Active balancing is done using a DC/DC converter setup, where charge is removed from a higher cell and released into a lower cell. Passive balancing is more common, cheaper to do, and usually works only at the top (in other words, only when the battery voltage is above some threshold). Active balancing may work at lower voltages.

For this reason you must take an LFP battery up to a balancing voltage occasionally. If it's a 16 cell battery (I believe the BNs are), then you want to take it up to 3.55V per cell, that is 56.8V. Of course, you should follow the battery-makers instructions if at all possible, this is just a general guideline. I can't imagine that they would tell you to do a lead-acid-type equalisation, so I must assume they meant you have to allow for balancing to take place.

[NigelL]

My post here https://powerforum.co.za/topic/2111-blue-nova/?do=findComment&comment=58949 may be useful.

[LourensJB]

Good morning all - thanks for all the feedback.

1 hour ago, plonkster said:

so I must assume they meant you have to allow for balancing to take place.

I must just maybe make a correction - to me as a noob asking a question without looking at the setup screens, is dangerous it seems - I used the word "Equalisation" while I should have used "Absorption".

The default setting on the Multiplus 48/5000 for Lithium batteries comes up as :
Repeated Absorption Time = 1Hr / Repeated Absorption Interval = 7 Days / Absorption Time = 1Hr 

Since getting the loss in performance over time, I have increased these settings now to:
Repeated Absorption Time = 0.5Hr (Shorter time lapse between peaks) / Repeated Absorption Interval = 1 Days (Every day instead of 7 days)/ Absorption Time = 1Hr 

Since I cannot find specific info on what is advised for these settings for the Lithium battery I need to find out what the pro's/con's are of increasing the Absorption process to the intensity I have done now to try and recover capacity as quick as possible.
 

There is already a huge improvement in individual cell voltages early morning compared to before making this change two days ago, so it seems to be a step in the right direction.

[___]

1 minute ago, LourensJB said:

I should have used "Absorption".

That makes a world of difference!

As I've said on another thread, Absorption as a term really only applies to lead acid batteries, which has to be held at a higher voltage for long periods to soak them all the way to the top. But LFP batteries have something a bit like this, in that their balancers only work at the top, which also means you have to do something very similar.

Batteries that are cycled daily already get an "absorption" charge once a day. For batteries that are not cycled daily, and that are held at a lower voltage for some of the time, I think you're right, you do need to do repeated absorption more often. It all depends on how low the "float" voltage is (again, float is really a lead acid thing). Most battery makers will specify a "float" voltage that is only slightly lower than the "absorb" voltage, and still high enough for the balancers to work.

That's the trick. You got to take it high enough often enough for balancing to happen.

[LourensJB]

1 minute ago, plonkster said:

That's the trick. You got to take it high enough often enough for balancing to happen.

Is there a top "safe" limit to keep in mind or would the internal protection processes both in the battery and the Mutliplus kick in? 

[___]

1 minute ago, LourensJB said:

"safe" limit to keep in mind

Keep it below 3.6V per cell. 3.55V per cell is technically full. Very little energy is stored above this. 3.55*16 = 56.8V. Stay below that and you should be good. Not sure what you're using now.

In my opinion 55.2V (3.45V per cell) should already be good enough to get a good balance. Again, it depends what you use now.

[LourensJB]

2 minutes ago, plonkster said:

In my opinion 55.2V (3.45V per cell) should already be good enough to get a good balance. Again, it depends what you use now.

In line with Blue Nova instructions, I am using Absorption Voltage of 56.2V currently.

 

[root]

What model/capacity BlueNova is it for interest sake? Also please update us if the BlueNova fails or gets replaced.

[LourensJB]

On 2019/08/07 at 11:17 AM, root said:

What model/capacity BlueNova is it for interest sake? Also please update us if the BlueNova fails or gets replaced.

Its is a 70 Ah 54 volt unit.  I will do so - currently the unit seems to behave a lot better since the change in settings.

 

[Rob Humphry]

I have a question regarding my Blue Nova Rac Power 52v battery. When it is fully charged the 4 SOC lights go out. Does this indicate that it has gone to sleep mode? As the battery begins to draw down the 4 lights come on and as the battery is continuously drawn down the SOC lights go out progressively.