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Mid point on Batteries on .70%


Gman007
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Hello I have 2 omni power 240ah batteries with a bmv 712 that measure the midpoint of the batteries. When my batteries discharge and they reach 79% the alarm for midpoint goes off and the midpoint reading sits on 0.70%. My batteries is permanently connected to utility power. What is wrong? and how can i test that the batteries is fine and that the metrics on the bmv is not wrong?

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Hi,

If you are using the midpoint measurement then I will assume you have the batteries in series? If so then I will take a stab and say that you will need a battery balancer (HA-01 at least) to balance your batteries and prevent the midpoint from deviating that much.

I am sure someone else will be along to confirm this.

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Balancers usually only work when the battery is charging. Well, this is one of those "it depends" things. The Victron balancers do their work when the battery is charging. The HA01/HA02 people use has a DC/DC converter, and it is unclear how exactly that works... it may work all the time. With Lithium batteries, I've been told that balancing during discharge can often be a waste of time, as there is a natural amount of back and forth that happens during discharge. I do not know if it is the same for lead acid, but I would expect that something similar could happen. More below*.

On discharge it is normal that there would be some midpoint drift, and this is actually perfectly okay (within reason). The point of balancing is to ensure that one battery is not overcharged while the other us undercharged (because of the voltage split).

So before you assume something is wrong... check the balance when fully charged. Maybe... just maybe... your batteries are simply entering middle age 🙂

The balancer remains a good idea though. If not necessary now, it may become necessary in future.

* So this is the issue with balancing during discharge. If the balancer notices (for example that the one battery is 0.2V lower than the other, it may begin to shunt some current past the other one (or it may transfer charge across using an isolated DC/DC stage). This may restore the balance, but because these are chemical beasts, it may well be that the battery that was low a second ago has a bit more low-down grunt... so within seconds, the 0.2V that was just eliminated shows up on the other side and has to be moved back. Each time charge is moved, there is an efficiency penalty.

Edited by plonkster
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2 hours ago, vlan_one said:

Hi,

If you are using the midpoint measurement then I will assume you have the batteries in series? If so then I will take a stab and say that you will need a battery balancer (HA-01 at least) to balance your batteries and prevent the midpoint from deviating that much.

I am sure someone else will be along to confirm this.

Thank you vlan_one 

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2 hours ago, plonkster said:

Balancers usually only work when the battery is charging. Well, this is one of those "it depends" things. The Victron balancers do their work when the battery is charging. The HA01/HA02 people use has a DC/DC converter, and it is unclear how exactly that works... it may work all the time. With Lithium batteries, I've been told that balancing during discharge can often be a waste of time, as there is a natural amount of back and forth that happens during discharge. I do not know if it is the same for lead acid, but I would expect that something similar could happen. More below*.

On discharge it is normal that there would be some midpoint drift, and this is actually perfectly okay (within reason). The point of balancing is to ensure that one battery is not overcharged while the other us undercharged (because of the voltage split).

So before you assume something is wrong... check the balance when fully charged. Maybe... just maybe... your batteries are simply entering middle age 🙂

The balancer remains a good idea though. If not necessary now, it may become necessary in future.

* So this is the issue with balancing during discharge. If the balancer notices (for example that the one battery is 0.2V lower than the other, it may begin to shunt some current past the other one (or it may transfer charge across using an isolated DC/DC stage). This may restore the balance, but because these are chemical beasts, it may well be that the battery that was low a second ago has a bit more low-down grunt... so within seconds, the 0.2V that was just eliminated shows up on the other side and has to be moved back. Each time charge is moved, there is an efficiency penalty.

Thank you plonkster, I was thinking that the one battery maybe not get enough charge? So is there no way to test something and see if it could get resolved?

My batteries is not even a year old and cant think that they have aged that much, and they sit inside my house so the temperatures is very good. I also use the BMV to cut off my house with the relay to contactor so the batteries dont discharge below 50%

Is a balancer something that i can try or is it just how it is? Whats the point of having the midpoint monitoring if there is no way to fix it?

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34 minutes ago, Gman007 said:

Thank you plonkster, I was thinking that the one battery maybe not get enough charge? So is there no way to test something and see if it could get resolved?

My point is that the deviance was measured at an SOC of around 80%, probably while being discharged. That it no way says anything about the charging of the batteries. Many batteries will indeed have a small deviation which slowly goes away as the battery goes to 100%. If it does... then your batteries are charged just fine.

If the deviation persists even after prolonged charging, then there might be a reason to worry. Then you want to install a balancer to help sort it out.

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23 minutes ago, Gman007 said:

Thank you plonkster, yes my batteries is permanent on charge at 100% so this is a concern for me

If it shows an 0.7% deviation when fully charged, then I would definitely look at balancers right now.

On my last lead acid batteries, I had a deviation during absorption charging (with the batteries at 14.5V). Then one battery would push higher to 14.8V while the other would lag behind at 14.2V. If I held the batteries at absorption for long enough, the voltage would always close over time until both batteries were fine. Of course this wasn't very healthy for the one that was held at 14.8V.

The moment the charger dropped to float, the deviation disappeared.

So just to be clear, if the deviation is during discharge, or at lower SOC... that's probably nothing. If the deviation is at absorption charge voltage, but disappears as the battery charges, then it might also not be that bad (yet). If the deviation persists, or is too large (more than 1%)... then it is time to panic 🙂

 

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Sounds like your batteries have reached end of life. Are you able to do a capacity test on both (separate)? Or at least measure the voltage when full and when this 0,7 deviation appears.

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Its impossible that batteries only can last 7 months. I have a axpert 3KVa. So i did a discharge to test if it will cut off at 50% but when i got back home the inverter is completely dead and my power off. But the bmv monitor shows that i still have 80%. What can be the issue?

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22 minutes ago, Gman007 said:

Its impossible that batteries only can last 7 months.

No it is not, if you want you can destroy them much quicker than that. So test or let your batteries be tested. Besides that measure the voltage of each battery after BMV says full and 50% dod. That will give you some clues.

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  • 1 month later...
On 2020/03/06 at 10:10 PM, Gman007 said:

Its impossible that batteries only can last 7 months. I have a axpert 3KVa. So i did a discharge to test if it will cut off at 50% but when i got back home the inverter is completely dead and my power off. But the bmv monitor shows that i still have 80%. What can be the issue?

The Axpert is lousy/useless at reporting SOC. I have checked this numerous times. The BMV will report a SOC of 98% and the Axpert will report 20-30% less depending on the load. 
You will see the best results when pairing the BMV and a control software with the Axpert. The control software will then use the BMV SOC reporting to control the Axpert. 

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On 2020/03/06 at 10:10 PM, Gman007 said:

Its impossible that batteries only can last 7 months

It depends on the battery and how it was used. A UPS-spec battery that was cycled daily could very well be starting to go at 7 months, in fact 7 months could be considered a respectable life in such a case. Remember that big companies replace UPS batteries every 2 years even if there were no power failures at all...

On 2020/03/06 at 6:43 PM, RikH said:

Sounds like your batteries have reached end of life

I disagree with this. They are not EOL yet, but they are beginning to tell you that they are no longer new. That is why I asked about when the imbalance shows up. An imbalance during discharge is not as big a problem as one during charging, and a small imbalance during charging that eventually goes away is again much less of a problem than a persistent imbalance that just won't go away. The important thing is to ensure that the batteries get a good charge, that is to say, if the imbalance eventually disappears as the battery is fully charged... you can get away with this for a while longer.

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Just now, Gman007 said:

Hello Plonkster how can i give the batteries a proper charge? Can I use my inverter?

They may already be getting a good charge. I asked earlier if the imbalance disappears as they reach 100%, and you indicated that they do. As long as this remains the case, you're fine.

The situation I'm talking about is where you have two batteries in a 24V system (as an example) with one at 14.8V and the other at 14V. The inverter is unaware of the problem, the terminal voltage is 28.8V which is absorption, so it merrily completes the "absorption phase" unaware that one battery was low while the other is being cooked. This is the situation where balancers helps to get the 14V battery up a bit faster.

But if the imbalance is mild, eg one battery is at 14.2V and the other is at 14.6V (again totalling 28.8V) but then narrows over time and ends at 14.4V each... then the imbalance is not as much of a concern as it would be otherwise.

In other words, what I am saying is there are levels of severity in this picture. An imbalance isn't ALWAYS an immediate reason to panic. But it is a sign that things are beginning to age.

Investing in battery balancers will be a good idea in the long run, because the next phase in battery wear is going to be a more persistent imbalance that won't want to go away.

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8 minutes ago, Gman007 said:

voltage drops really quickly and getting really high midpoint

OK, so the last phase went really quickly for you then. Again, this varies according to the quality of the battery. I had a set of Victron AGMs once that ran for MONTHS with that slight imbalance during absorption charging. I eventually sold them on to someone else (I asked only for the scrap value) and they continued their life somewhere on a farm in the Karoo (I don't know how long after that).

In your case, you definitely have a failed cell somewhere. What batteries are they, for interest sake?

Edit: Aaah, you said Omnipower. Yeah, those are supposed to be better than crappy UPS batteries, but you won't be the first person who had bad luck with them.

Edited by plonkster
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