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kevgermany avatar image
kevgermany asked

Another Battery Balancer Query

I've searched, can't find a clear answer.


System is 4*106Ah lead carbon Wired in 2 pairs for 24V. Victron balancer, BMV712 monitor. Pretty new, kit is roughly 12 months old, batteries have never been discharged below 90%. Permanent charge ftom Easy solar.

One bank is always lower than the other, but doesn't alarm. Difference usually about 0.1-0.2 V. Have measured this on the batteries, at the balancer and with the monitor. All agree. Measured the current at the balancer with a clamp meter. It's draining the bank with the lower voltage.

I'm about to start using it more heavily, down to about 20%DOD.


So

Is this normal?

Is it going to lead to battery damage?

What to do?




Battery Balancer
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3 Answers
kevgermany avatar image
kevgermany answered ·

Update

I pulled the balancer out to test on the bench.

Balancer turns on at 27.3V Cuts at roughly 26.6, so within specs.

Can't get it to discharge to lower voltage


Following tests all with lower feed at about 14 V

Discharge of higher battery starts at 120mV one side, 70mV other side


Warning light of higher battery starts when voltage difference is 510mV (should be 100mV)

Alarm comes on at 700mV one side, 970mV other side.

Current never gets to 0.7A

So it's way out of spec. No wonder my batteries are always out of balance.


Anyone know how to get this fixed/changed? It's about a year old, bought through Amazon.


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Alexandra avatar image Alexandra ♦ commented ·
@kevgermany


If you are sure your wiring is a100% correct, current sharing is good and are unhappy then you should be able to go to the amazon supplier first as they should be backed by distributors.

if they do not respond then you can use the support form to track down the dealership it came from.

It may be time (or overdue) to rotate your batteries in your bank as well, ohms law and all that jazz being the pain that it is.

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kevgermany answered ·

Just in case anyone comes across this and is misled.

I repeated the testing. I was using small adjustable buck/boost converters to test the battery balancer. I'd previously checked voltage displayed at beginning of test to voltage measured at the yerninals of the balancer. It corresponded, with very slight differences. However I was worried that the current limits in the converter might be affecting the measurements. Removing the current limits on the converters resulted in no change to voltage displayed on their screens, but a big drop in voltage recorded at the balancer as it passed more current. I was also worried that there may be some effect on the balancer from unsmoothed output from these converters, so added smoothing capacitors and 5K drain down resistors across the capacitors.

I then repeated the tests.

So about all I was able to measure was turn on/off voltages and voltage at start of balance cycle. Turn on voltage is within specs. Turn off lower than spec, but at the official turn off current is so low it's not worth worrying about. Balance voltage not. Current starts flowing at a difference of 80mV on one side, 120mV on the other. This measured at the balancer. Should be 50mV. But given the observed oddities in the converters, I'm not sure how good these results are.

I've been monitoring the out of balance situation of the batteries with the balancer disconnected. It's worse. So the balancer is having a positive effect.

Not sure what to do now.

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Alexandra avatar image Alexandra ♦ commented ·
@kevgermany

If you are able to, charge each battery up individualy through its full charge cycle. Do you have any equalisation on your system at all? If you are planning deeper discharges this will also help keep their voltages in line.

Rotate the batteries in your bank and put the balancer back on.

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kevgermany answered ·
  1. Thanks. The batteries are victron lead carbon (4*106Ah), driven by an easysolar 1600W integrated unit. So there's an equalisation option, but I thought this was not for use on lead carbon batteries.

Individual charging/rotating the batteries isn't an option. They're very difficult to access.

What's an acceptable level of imbalance? I assume that as the balancing only starts at 50mV, this is the target/normal value.

You asked if wiring was correct. It's exactly according to Victron diagrams. Battery to busbars, isolator on positive before busbar. Balancer connections to busbars. Easy solar to busbars with smartshunt for BMV712 on neg side. Only battery connections other than this are the mid point connections for the shunt and balancer.

But as I'm measuring at the terminals of the balancer in a test setup and the BMV values show that overnight when no charging the banks come to low deviation (0-0.1%), I suspect the balancer.

I'm going to turn off the panels, reinstall the balancer and plug in the 240V cable. As there's been no mains power for a long time, the multiplus should go into absorption for a long time. Let's see.


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Alexandra avatar image Alexandra ♦ commented ·
Equlisation is really more for deeper discharges. Which you said you were looking doing.



To be honest I have never stressed about 0.1v to 0.2v (it is 50mv from mid point so potentially 0.1v between the two banks-thought that was good enough as it is the chemistry none of the batteries will ever be the same anyway) difference and had a set of lead calciums for over 5 years. Just made sure they got through their whole charge cycle each day without being dischrged part way. Did the bank rotation evey few months and had a separate 12v charger if i felt one of the babies was sick and needed more attention, this happened more as the bank got old.

Are you measuring them when they are fully charged or during discharge? Are all the legs off the balancer exactly the same length?

The batteries have a 0.3v voltage range for the charge cycle stages anyway. So if they are making it into those ranges then you are all good.


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kevgermany avatar image kevgermany ♦♦ Alexandra ♦ commented ·
Thanks

Mid point leg is longer, others are same length. Last time I was at the batteries, a few weeks back, I measured voltages, fully charged. Didn't disconnect.

The difference between top and bottom banks was identical to the difference on the BMV. Within the banks each pair was at the same voltage, something like two at 12.7 in the lower bank, 2 at 12.8 in the top bank.

Once since installing they've discharged to about 25% DOD, the rest of the time DOD has been max 10%, usually 5%.

Batteries are being charged to 100% daily, only once has the solar failed to go down to bulk due to weather.

I'm going to run them down to 50% DOD today, then start the mains charger. Maybe that will help.

But looking as if I have to live with it.




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kevgermany avatar image kevgermany ♦♦ Alexandra ♦ commented ·
Thanks, The info available from the system isn't really good enough for diagnosis. But the balancer is bringing the balance into the region you mentioned. And it can be either bank higher.

Even though the amber lights aren't working properly, I'm going to leave it for now.

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