What would you do?

[SteveW]

This must be shattering news!

[native pepper]

Terry, don't have any balancing issues at all with any of my 3 packs and never have beyond what is pretty normal and without controls. That's what I've been saying, but you keep saying I have problems, weird really. The set up I have now, has just made it so much better and there is no lose whatsoever anywhere.

The pack in the bus hasn't been used for awhile and it sits there with the equaliser on it at 13.4v, disconnected from any panels and charge controller, as the panels are half off, but the rain and other priorities always take over. It will get finished soon when we have a few days without rain, at least the gardens look alive and we have a fruit crop of massive proportions this year.

Got into this habit of always checking my packs when they have no controls on them but the balancers, don't trust all this electronics but they are all fine and the house is never without controls as energy is used 24/7, even when we are away for months.

[grizzzman]

My re-start charging begins when the pack drops to 13.6v, but that rarely happens as when the pack is full, the input is directed to the load. It's only low input days that the pack will get used when full, as the input doesn't correlate with the output and the pack compensates, then when it drops to 13.6v, on comes the charger.

That way at night, we almost always have a full pack and during the day when the pack is full, we use the direct input as much as we can. Others will be different and don't know if their charging system work that way, or take their energy always from the pack.

Do you know what the SOC as in percent at 13.6 volts?

Thanks

[grizzzman]

Unfortunately there is no way of knowing for sure all cells will reach 3.45v at the same time, so the pack voltage might still be 13.8v but one cell could be 4v while the others are still around the 3.3v and 3.2v mark. Less likely to happen at 13.8v granted, 14.4v is very likely as all the remaining cells could be over the 3.45v mark and one cell at 4v, any one of the 3 remaining cell running a little behind and the 4v cell would be well into its death spiral.
Cell balancing alone does no help here as the cell voltage run away only occurs once a cell reaches fully charged, it can't store any more current so the voltage climb is near vertical. This is why all charging must be controlled at some point by monitoring cell voltage and stopped if a cell reaches 3.6v and no charging recommencing before that cell has dropped below 3.6v.
The time lapse between charge off and charge back on really depends on the load on the battery pack as well as the rate of charge. This part will probably have your eyes rolling back in your head and the risk of you dropping into a coma, so maybe best to get a coffee before reading any further.

Ok, when a cell is below the 90% charged mark it can accept a very high current rate up to 3CA (3 x the advertised capacity read as amps or 300 amps per 100Ah advertised capacity), well beyond what would normally be seen on a house battery pack. After the 90% full mark the voltage will rise rapidly if the current acceptance rate is not as high as the supply rate. Each cell will be different to the next in just which point this max acceptance rate is reached, all to do with internal resistance and actual capacity and how well the cell was condition charged but high rate charging will see a cell voltage run away happen where it is less likely at a lower charge rate...... I can see the heads rolling back already so I won't go any deeper on that bit.
At a trickle charge rate the run away doesn't occur until the cell reaches 100% full or saturation charged, then even at a 5 amp charge rate the cell voltage will climb rapidly, just not quite as rapidly at it would at a high charge rate. One cell only needs to be 0.1% ahead of the others to reach a voltage run away point while the others are still charging, a set voltage of 13.8v that turns back on at 13.7v will belt 5 amps in until the total pack voltage reaches 13.8v.... so what?
This is what happens, cell 1 reaches 3.45v, cell 2 = 3.36v, cell 3 = 3.6v, cell 4 = 3.39v. Now we have 3.45 + 3.36 + 3.6 + 3.39 = 13.8v, charging stops. cell 1 is nearly full so it will probably drop to 3.4v, cell 2 hasn't reached the nearly full mark so the charge is still spreading evenly through the cell, it will probably drop to 3.2v, cell 3 is saturation charged and has started the first steps to voltage run away, it is unlikely to drop below 3.5v, cell 4 has not reached fully charged so it will probably drop to 3.3v. Now we have 3.4 + 3.2 + 3.5 + 3.3v = 13.4v. This means charging has recommenced at 5 amps until a total of 13.8v is reached and the charge stops again. Not really a problem for cell 1 just yet, cell 2 will continue to accept charge, cell 3 is already full so voltage run away will be in full swing and cell 4 will be accepting charge. Now the make up will see all the other cell voltages lower and cell 3 will have run away to a voltage that will be causing damage. Why? Cell 3 could not accept any more charge so the voltage climbed immediately, the other cell were accepting charge so their voltage didn't climb much above their settled voltage. Effectively each time charge resumes the lower cells get lower and the high cell gets higher to equal the same 13.8v.

Unless the cell voltages are monitored and charging stops if a cell's voltage starts to run away the above scenario is a real possibility. Equalising can not shift this amount of out of balance as it only occurs at the top end of the charging cycle. Simply shifting capacity from one cell to another could make the problem worse, if the problem cell has lost capacity or has a high resistance connection it will show a lower voltage while discharging and the balancer will try to move capacity from the other cells to this lower voltage cell. Now the actual capacity of the cells has been altered making the cell that showed the lower voltage slightly fuller than the other cells, it will see saturation charged before the other cells.
Stop the charging at 13.8v or even 14v OR if a cell reaches 3.6v and not recommence charging until either high voltage has been cleared and you won't have a problem. Start balancing at this point and stop balancing once charging resumes and the out of balance problem will slowly disappear and the primary control of 13.8v or 14v will take over and everything will return to normal.

T1 Terry

Dang Terry that's some good info there for sure! Now lets assume the cells are in balance. do you use a voltage or perhaps a SOC to bring charging back on?

Thanks

[grizzzman]

How do you decide when the DC to DC charger needs to be cut off? Or for that matter back on? Using voltage? Counting amp hours?

In my case my main DC DC Charger is connected via a relay from the ignition, so only comes into play while motor is running. I relay on the inbuilt charger algorithms to control the charge to the battery, but then I am working with AGM Batteries.

Thanks for you're method of charging. It is similar to how i charge my FLA house bank but limited amps/volts due to small charging wire and distance.

thanks

[grizzzman]

ok here's my quick sketch of charging from all points, hope it makes sense and maybe this is how everyone does it.

Are you running a 100 AH and a 200 AH bank together? ( I am assuming 100 AH cells?)

[grizzzman]

Also is it ok to equalize during discharging or does it need to be at the upper knee?'

[T1 Terry]

Dang Terry that's some good info there for sure! Now lets assume the cells are in balance. do you use a voltage or perhaps a SOC to bring charging back on?

Thanks

That is the whole point behind the long post, you can't assume they will be in balance at the top end of charging, SOC or battery voltage are wide open to creating a cell voltage run away so neither are suitable but battery voltage is a good primary control method. The battery protection control system must be cell voltage controlled, any other method is simply asking for trouble.

T1 Terry

[T1 Terry]

Also is it ok to equalize during discharging or does it need to be at the upper knee?'

Once you start actually using Li cells you will see that there is no out of balance below the upper knee and above the lower knee, if there is then that is indicating a problem and hiding it is being the ostrich with the head in the sand. If the philosophy is what you can't see can't hurt your battery then no point in monitoring any of the voltages or worrying about equalising eh We use Junsi 8S cell loggers to take a 7 day sample of cell voltages if we think there could be a problem, that tells us if there is a problem or just part of normal cell behaviour, it can also indicate memory charging/discharging effect within a cell group, but you do need to be skilled in reading the graph to see that. Normal cell behaviour will have a few milli volts difference between cells at different stages of charging and discharging, the problem indicator is if it is always the same cell group, normal behaviour has each cell group doing it at different times and sometimes the lead cell group will go from highest the middle to lowest during the same charging period, the next day it will hang in the middle, just part of their normal behaviour. If you add an equaliser to correct something that would happen normally the pattern is hidden and possibly the normal operation of the cell has been altered. A bit like drinking a high caffeine drink all the time, stop and tiredness and headaches hit, so was the caffeine stopping the tiredness and headaches or making the body reliant on the caffeine and can't operate normally without it?

T1 Terry

[T1 Terry]

Terry, don't have any balancing issues at all with any of my 3 packs and never have beyond what is pretty normal and without controls. That's what I've been saying, but you keep saying I have problems, weird really. The set up I have now, has just made it so much better and there is no lose whatsoever anywhere.

The pack in the bus hasn't been used for awhile and it sits there with the equaliser on it at 13.4v, disconnected from any panels and charge controller, as the panels are half off, but the rain and other priorities always take over. It will get finished soon when we have a few days without rain, at least the gardens look alive and we have a fruit crop of massive proportions this year.

Got into this habit of always checking my packs when they have no controls on them but the balancers, don't trust all this electronics but they are all fine and the house is never without controls as energy is used 24/7, even when we are away for months.

Not what you said here

Done that Terry, the only time the cell lines get out of balance is when they are at 3.45v and the difference between them all is less than .3v.

300mV is a long way out of balance, we generally ignore anything below 150mV at the top end of charge but 300mV indicates a problem that needs attention.

T1 Terry