Strange battery voltage readings

[gertc]

Hi all

My system is about 3 months old with the following specs:

- 3x 5 KVA Axpert inverters in parallel, 48v system
- 840 ah battery bank (Sonic Gel batteries)
- 7 KW solar panels

I use AICC to monitor and record the system together with a BMV 702 battery meter. I have set the inverters to go to grid on 44v and back to batteries to 46v, both the lowest possible settings, with the idea that AICC will control the system and kick over to grid at 55% SoC.

What worries me is that the inverters kicks over to grid at between 65-70% SoC.  Compared to what I remember just after installation, the battery voltage much faster from the 54v fully charged state.  Today, as an expample, I let the house run on grid and charged the batteries with solar energy to 100% (it went through both the bulk and absorption phases).  But right now the BMV shows 99.2%, the current load is 867w / 17.3 amps and the battery voltage already shows as 50v.

Should I be worried?

[KLEVA]

Gert

Are you using the midpoint of the BMV?

When it is near to 70% on the BMV, take a voltmetre and test each battery individually (whilst still on load). This will tell you if any single battery is at fault (They should all be within about a volt of each other, but 2 or more volts could indicate the start of a problem that needs more attention to isolate). If you do find one or more batteries at much lower voltage, then try feeling the sides and see if any one area of the battery is hotter that the others (there are 6 individual cells in each - like mini-batteries), a much warmer cell could indicate a faulty battery.

This is obviously not comprehensive, but is a quick starting point to try see if there is a fault. This is pretty much what I did last night to find my faults without any fancy kit on hand.

[gertc]

Thanks.  Will try this tomorrow morning when the reading is closer to 70%.  I do not have the midpoint reading installed, but are thinking of doing it now.

[Chris Hobson]

12 hours ago, KLEVA said:

When it is near to 70% on the BMV, take a voltmetre and test each battery individually (whilst still on load).

In my experience any differences happen during late absorb between 90 - 100% SOC. Nothing is apparent at a lower SOC. Before I had my battery balancers I would sometimes have more than 1V difference between batteries.

They start off being out of balance for about ½ hour and then this increases.

 

[gertc]

Chris, what is a battery balancer?

 

[KLEVA]

@Chris Hobson - The 70% of load pushes each battery to a lower level, and without a midpoint, the differences become obvious on even a cheap voltmeter at this level. And as I found out, there may be more than one battery out, which can throw out your mid-point (I had eleven perfect cells on each side of the midpoint, so no real differences picked up). Also, under load for a period of time, any dead cell will normally noticeably heat up.

@gertc - A simple explanation is that a battery balancer goes across 2 batteries and ensures that all the batteries sit with the same voltage/charge. For 1 battery no balance needed, for 2 batteries 1 balancer needed, for 2 x 2 batteries 2 balancers needed, for 4 batteries you need 3 balancers because of the way they connect and work together, for 2 x 4 batteries you need 2 x 3 balancers, etc. (one Set of balancers for each Bank of batteries). Hope that made sense.

[superdiy]

On 4/26/2016 at 7:08 PM, KLEVA said:

 

@gertc - A simple explanation is that a battery balancer goes across 2 batteries and ensures that all the batteries sit with the same voltage/charge. For 1 battery no balance needed, for 2 batteries 1 balancer needed, for 2 x 2 batteries 2 balancers needed, for 4 batteries you need 3 balancers because of the way they connect and work together, for 2 x 4 batteries you need 2 x 3 balancers, etc. (one Set of balancers for each Bank of batteries). Hope that made sense.

Depending on the type of balancer used...

The idea behind battery balancing is to ensure that all the cells/batteries in a bank are at equal SOC - in other words balanced. There are many different types of balancers available, from simple zener diode types to sophisticated types where you have a balancer on each cell which communicates with all the balancers on the other cells and transfers some of the charge from more charged cells to less charged cells etc. etc. So depending on your budget and the cost of your battery bank, you have many different balancing options to keep that investment in tip-top condition and to actively extend its life.