In-line manual rebalancing

[s/v Jedi]

So I worked on coming to a decision on a manual balancing method that works well on boats.

During my initial top balancing I noticed how great my little cheapish 0-30V 0-5A programmable bench power supply works. This is of-course a 120V powered device so it was easy to think of powering it from the inverter... which is powered from the same battery bank where you want to rebalance by topping up one cell. This creates a manual version of an active balancer, where you take power from cells which you put into one cell.

Let’s say, by experience and readings on a battery and/or cell monitor you find that this cell comes 6Ah short of the avg of the other cells. You can just set the charger to 3.65V and 5A, connect it to the cell in question (making sure it’s negative output post is not grounded... remove the link to the ground post if installed) and run that for an hour or so.

You have to account for the power that is drawn from the cells in addition to what you put in. For a 4S configuration, for every 3Ah you put into a cell, the other three cells give up 1Ah (plus efficiency losses).

Please comment with your thoughts? This is the little psu I used: https://www.ebay.com/p/28014018703

[GRIT]

I'm not sure what you mean by removing the negative ground. If you unground the cell, you'll lose the power for the inverter. Perhaps I'm being dense.

Cheers.
Paul.

[tanglewood]

Originally Posted by s/v Jedi

So I worked on coming to a decision on a manual balancing method that works well on boats.

During my initial top balancing I noticed how great my little cheapish 0-30V 0-5A programmable bench power supply works. This is of-course a 120V powered device so it was easy to think of powering it from the inverter... which is powered from the same battery bank where you want to rebalance by topping up one cell. This creates a manual version of an active balancer, where you take power from cells which you put into one cell.

Let’s say, by experience and readings on a battery and/or cell monitor you find that this cell comes 6Ah short of the avg of the other cells. You can just set the charger to 3.65V and 5A, connect it to the cell in question (making sure it’s negative output post is not grounded... remove the link to the ground post if installed) and run that for an hour or so.

You have to account for the power that is drawn from the cells in addition to what you put in. For a 4S configuration, for every 3Ah you put into a cell, the other three cells give up 1Ah (plus efficiency losses).

Please comment with your thoughts? This is the little psu I used: https://www.ebay.com/p/28014018703

This is exactly how I topped up a couple of cells at 18 mo of operation, having never even done an initial balancing (I wanted to see how they behaved with no intervention).


I didn't have a way to know how many Ah behind they were. All I could see was they their voltage lagged behind by 10s of mv when the other popped at the end of charge. I took a guess and added 6Ah, and grossly overshot. So I then added to the others to catch them up. The end result was that the low cells were only behind by 3Ah out of 360Ah. It was a tiny amount, and helps show how few Ah of capacity there are in the tail of the charge voltage curve as it pops up, i.e. stopping at a 3.45vpc vs 3.6 vpc sacrifices very little capacity.

[tanglewood]

Originally Posted by GRIT

I'm not sure what you mean by removing the negative ground. If you unground the cell, you'll lose the power for the inverter. Perhaps I'm being dense.

Cheers.
Paul.

I think he's talking about the power supply to be sure the negative output isn't tied to ground. My PS has a floating output, which I confirmed carefully before connecting.

[GRIT]

Ah. Thank you.

I think sometimes my clutch slips.

Cheers.
Paul.

[T1 Terry]

You have kinda got the idea right. There are single cell chargers available that will stop charging at 3.7v+/- about 1vdc, so anywhere from 3.6v to 3.8v.
When to top balance a cell is the critical part of the whole operation and will make the difference between a successful balance and a nightmare that will take quite a while to sort.
Do the single cell top balance if the difference between the highest and lowest cell is greater than 150mV, otherwise, leave well along. This out of balance will only be seen when the cells are full, well the cell with the high voltage is full anyway. This is the time to do the top balance, plug the single cell charger into the inverter, then connect the positive wire to the positive of the cell you want to charge and the negative wire to that cell negative.
The light on the single cell charger should go from green to red to indicate it is charging.
If you are putting 5 amps into one cell of a battery you are pulling the power from to run the inverter and the charger, the electrical energy is coming from all 4 cells and being put back into a single cell, less losses in the inverter and charger.
5 amps x 3.6v (the end target voltage) = 18w plus at least 10% for losses = 20w approx. 20w divided by 4 cells =5w. This means 5w came out of the low cell but 20w went in, so a gain of 15w. Roughly 4 amps gain or 4Ah per hr the charger remains connected.
This bit is very important to keep in mind, if you forget about putting the charger on the cell it will continue to gain 4Ah more than the other cells for every hr you forgot to disconnect it ......
Easiest method is to do the balance charging when the high cell reaches 3.6v or higher. Connect to the low cell till the low cell voltage is equal to the high cell voltage while the whole battery is still charging. As soon as the charging stops, so should the balance charging stop. Wait till the next charging cycle has reached the cells full stage and see if the out of balance is still greater than 150mV, if it is, then charge the lowest cell the same way, but if the out of balance is now less than 150mV, leave it along. More fiddling will only create more problems than it fixes, those that have done it will testify to the headaches too much messing creates, it ain't broke now so don't try to fix it until it is broke :lol:
The cell voltage differential will float up and down at the top end of charge on a regular basis, often not the same cell goes high or the same cell goes low and often the 150mV is a one off event and doesn't happen at the end of the next charging cycle.
With our customers, we suggest writing down which cell is high and which cell is low, check it again at the end of the next charging cycle and see if it is the same cell that is high and the same cell that is low when the 150mV differential is reached/passed. Write it down if it is and do the same thing at the top end of the next charging cycle. If the same cell is high all 3 times and the same cell is lowest all 3 times, then do a balance charge. Quite often the gremlin sorts itself out, they can't balance between cells in series, but they do between cells in parallel and sometimes it is just a short term issue with a single cell that causes the problem, it corrects itself and all goes back to normal .... unless there was outside interference .......


T1 Terry

[s/v Jedi]

Originally Posted by tanglewood

I think he's talking about the power supply to be sure the negative output isn't tied to ground. My PS has a floating output, which I confirmed carefully before connecting.

Indeed. Many power supplies come with three terminals: black, green and red. Some, like my Mastech 3030 have a link installed between the black and green terminals so that the negative is grounded, which must be removed.

[s/v Jedi]

Originally Posted by T1 Terry

You have kinda got the idea right. There are single cell chargers available that will stop charging at 3.7v+/- about 1vdc, so anywhere from 3.6v to 3.8v.

Ha! Single cell charger, right

[T1 Terry]

Originally Posted by s/v Jedi

Ha! Single cell charger, right

We sell them with every system we install, complete with directions on how to do the balancing, with photos and all :lol: We sell them for about AUD$40 but the freight out of Australia would be frightening. No doubt they come out of China, just like everything else, so tracking down the source would be the go. We buy them through our Australian supplier for Winston cells, but the freight costs outside Australia would be the same, prohibitive generally.

My wife wrote some info on our website t1lithium.com.au and the single cell chargers and all the blurb on how to use it.
Margaret wrote the website info so it didn't go straight over everyone's head, I had to explain it to her till she understood, then wrote in plain speak

T1 Terry

[markcouz]

For my DIY 12v 400ah lifepo4 bank I am using a 4 pole continuous balancer product HA-02.

The HA-02 will "switch on" at 2.4v and then balance continuously. I prefer this method as the top and bottom balancing products that I have seen use voltages higher/lower than I like to expose my cells to, meaning that they would seldom if ever be balanced.

Since installing over a year ago I have never seen 10mV imbalance exceeded - very happy with the product which cost me ZAR650 (less than USD40).

[GoneDiving]

Originally Posted by tanglewood

My PS has a floating output, which I confirmed carefully before connecting.

How did you confirm this? I assume just checked for continuity between -12VDC and AC Ground while turned on?

[s/v Jedi]

Originally Posted by markcouz

For my DIY 12v 400ah lifepo4 bank I am using a 4 pole continuous balancer product HA-02.

The HA-02 will "switch on" at 2.4v and then balance continuously. I prefer this method as the top and bottom balancing products that I have seen use voltages higher/lower than I like to expose my cells to, meaning that they would seldom if ever be balanced.

Since installing over a year ago I have never seen 10mV imbalance exceeded - very happy with the product which cost me ZAR650 (less than USD40).

Idon’t know exactly how this product works, but the 2.4V setting must be for NiMH cells because it also has a 3.6V setting for LiFePO4.

[GoneDiving]

Originally Posted by T1 Terry

Easiest method is to do the balance charging when the high cell reaches 3.6v or higher. Connect to the low cell till the low cell voltage is equal to the high cell voltage while the whole battery is still charging. As soon as the charging stops, so should the balance charging stop.

Are you able to balance one cell while it is providing current to the inverter supply because your single cell charge has no -VDC to AC Earth link? Is there any harm in disconnecting the low cell if a particular bank has numerous cells in parallel ?

[markcouz]

Originally Posted by s/v Jedi

Idon’t know exactly how this product works, but the 2.4V setting must be for NiMH cells because it also has a 3.6V setting for LiFePO4.

I have attached the Chinglish (oh-oh!) User guide - the HA-02 can be used with 2.4v/3.6v/6v/9v/12v cells - or effectively any cell between 2.4V and 12V nominal.

The point is that they switch on and start balancing continuously once the terminal voltage is above 2.4V, which with LiFePO4 should be all the time as you would certainly not like to see your cells below 2.5V (or below 3V in my own system).

I previously used them with a 48V 500Ah lead acid bank made up of 24x 2.4V cells on a home PV system, they worked so well that they effectively hid how rubbish the batteries were because they were always in balance.

Needless to say my experience with the 500Ah lead acid bank cured me of lead acid use for ever (except for start battery where they are the right choice).

[s/v Jedi]

Originally Posted by markcouz

I have attached the Chinglish (oh-oh!) User guide - the HA-02 can be used with 2.4v/3.6v/6v/9v/12v cells - or effectively any cell between 2.4V and 12V nominal.

The point is that they switch on and start balancing continuously once the terminal voltage is above 2.4V, which with LiFePO4 should be all the time as you would certainly not like to see your cells below 2.5V (or below 3V in my own system).

I previously used them with a 48V 500Ah lead acid bank made up of 24x 2.4V cells on a home PV system, they worked so well that they effectively hid how rubbish the batteries were because they were always in balance.

Needless to say my experience with the 500Ah lead acid bank cured me of lead acid use for ever (except for start battery where they are the right choice).

That’s interesting, they even have versions with display on smartphone apps as well as 8s and 16s versions.

I wonder how they do it, must be relatively simple as there doesn’t seem to be any settings for cell voltage. If I find out, I may include that in my bms design.