Top Balancing in Series

RossA

Solar Enthusiast
Joined
Dec 2, 2021
Messages
35
Okay, so I top balanced my 8 EVE 304 AH cells today. I don't have a constant voltage charger like the ones so many people use, so instead I tried something that I think should work pretty well.

What I used:

8s JK BMS
MPP LV 2424 Hybrid Inverter charger
Battery shunt that I found on Will's website (not victron, but his budget version. I can't remember the brand.)


The JK BMS has an active 2A balancer within it.
I was able to program the inverter charger (plugged into a 30A plug) to only charge with 2A.
Set the BMS over voltage protection to 3.65v per cell.
Set the inverter charger to a bulk and float charge of 29.2v (3.65v per cell)

Then I monitored what happened through the BMS bluetooth app and a battery shunt that I have.

I noticed that I had a few cells that had seemed to be with all the others, until the voltages started getting higher. On cell in particular was about 0.3 v below the top cell ( roughly 3.36 and 3.65 respectively). Since my inverter charger was just trickling in about 0.6 A (it was using some power to power the inverter itself so 2 A became 0.6A according to my shunt) The active balancer was able to semi keep up. Realistically since the programming of the balancer isn't perfect at choosing which cell to add power to or remove power from, the charger would charge at 0.6A for maybe a minute, and then it would shut off and the balancer would continue balancing for a minute, then the charger would turn back on and the cycle would just keep repeating itself. The BMS was successfully cutting the charging off whenever a cell popped up above the 3.65v. Then the balancer would move some of the power around and the cell voltages would settle around 3.64v before the charger then turned back on. This on off on off, and the fact that the current in was less than 1 A, the battery voltage wasn't spiking too much due to the charging current. Over about 2 or 3 hours of it doing this, all cells were in the 3.64-3.65v range. At the very least, pretty close to fully charged and balanced albeit not 100% perfectly.

I figure even though there is still a slight voltage difference between the top and the bottom cell, it is so small that the active balancer in the BMS isn't even kicking in to attempt to balance.


I just felt like I would document what I did to top balance my DIY battery pack since I wanted to avoid needing to buy a charger in addition to everything else that I had already purchased. I'm curious what you guys think of this method for top balancing? After the cells all seemed top balanced, I dropped the float voltage down to 27.6v and then disconnected the batteries from the system to see how voltage drops overnight. I will change the charging and voltage parameters in the inverter charger and the BMS settings to have 3.45v be seen as 100%. I just wanted to be sure that the cells were reasonably balanced first.

Curious to hear your thoughts!
 

740GLE

Solar Addict
Joined
Apr 30, 2021
Messages
1,114
What was the timeframe from the first cell hitting 3.65 and when the entire pack was balanced?
 

RCinFLA

Photon Sorcerer
Joined
Jun 21, 2020
Messages
1,714
When you do top balancing with inverter/charger and BMS it is better to start with lower average per cell absorb total voltage. 3.50-3.55v (x8) charger setting will give a little more balancing time before a high SOC cell trips BMS cell overvoltage limit.

It is usually better to set start of balancing cell voltage to 3.40v.

Below 3.40v balancing has high occurrence of cells getting balancing in wrong direction due to any pulsing high current variations by the charger.

With BMS cell voltage sampling multiplexed time variance, it can result in cell voltages being taken at differing amount of cell charge current causing variation in cell voltage reading which impacts BMS balancing decisions. Below 3.4v cell voltage, in flat portion of cell SOC voltage, it doesn't take much cell voltage reading variance for BMS to make the wrong balancing direction decision.

The 2 amp active balancing current has a very diluted duty cycle. The BMS stops balancing current periodically for a moment to take cell voltage readings without influence by small gauge sense wires balance current induced wire voltage drop. Then there is the fact it only does a 'from' to 'to' pair of cells at a time.

Lowering charge current greatly reduces the possibility of BMS cell over-voltage shutdown but you pay for that in increased time to balance. If you watch rise in cell voltages you can keep greater charge current until first cell hits 3.4v, then reduce charging current.

Using an inverter/charger for series top balancing often requires a lot of attention to avoid BMS cell overvoltage shutdown because it shuts down inverter/charger and potentially causes unattended restart issues, like inverter/charger remaining in standby mode after restart until you manually re-enable it.

A shunt/battery monitor for high amperage has poor accuracy at low current so would not put too much faith in the 0.6 amp reading.
 
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RossA

Solar Enthusiast
Joined
Dec 2, 2021
Messages
35
When you do top balancing with inverter/charger and BMS it is better to start with lower average per cell absorb total voltage. 3.50-3.55v (x8) charger setting will give a little more balancing time before a high SOC cell trips BMS cell overvoltage limit.

It is usually better to set start of balancing cell voltage to 3.40v.

Below 3.40v balancing has high occurrence of cells getting balancing in wrong direction due to any pulsing high current variations by the charger.

With BMS cell voltage sampling multiplexed time variance, it can result in cell voltages being taken at differing amount of cell charge current causing variation in cell voltage reading which impacts BMS balancing decisions. Below 3.4v cell voltage, in flat portion of cell SOC voltage, it doesn't take much cell voltage reading variance for BMS to make the wrong balancing direction decision.

The 2 amp active balancing current has a very diluted duty cycle. The BMS stops balancing current periodically for a moment to take cell voltage readings without influence by small gauge sense wires balance current induced wire voltage drop. Then there is the fact it only does a 'from' to 'to' pair of cells at a time.

Lowering charge current greatly reduces the possibility of BMS cell over-voltage shutdown but you pay for that in increased time to balance. If you watch rise in cell voltages you can keep greater charge current until first cell hits 3.4v, then reduce charging current.

Using an inverter/charger for series top balancing often requires a lot of attention to avoid BMS cell overvoltage shutdown because it shuts down inverter/charger and potentially causes unattended restart issues, like inverter/charger remaining in standby mode after restart until you manually re-enable it.

A shunt/battery monitor for high amperage has poor accuracy at low current so would not put too much faith in the 0.6 amp reading.
Thanks for the reply. I guess I forgot to mention that originally the cells were at a lower state of charge originally and I did use 30A to charge till they were at a high SOC. After reaching a high SOC I turned it down to 2 A so it was only a few hours that I had to wait.

Its good to know that the BMS cell balancing should be turned off until the cells reach a higher voltage because it can balance in the wrong direction. I will change the settings. Would 3.35v be sufficiently high for the BMS cell balancing? I don't intend to charge over 3.4v/cell much.

My BMS did trigger overvoltage several times and fortunately the inverter didn't seem to have an issue restarting.

I appreciate all the input. That's exactly the type of post i was hoping for.
 

RCinFLA

Photon Sorcerer
Joined
Jun 21, 2020
Messages
1,714
Just about everyone I have heard with JK/Neey active balancer BMS that sets start of balancing lower than 3.4v ends up having cell overvoltage shut down problems when attempting a full charge.

Mismatched cells will have differing internal impedance causing more or less cell terminal voltage slump under inverter load/charge current which BMS has no control over. This can cause more mis-direction balancing during flat portion of LFP discharge curve.

Having differing amounts of bus bar/terminal connection resistance will cause balancing problems when there is large inverter discharge or charging currents happening. Youtube 'Ray Builds Cool Stuff' has several good videos on attention to detail on bus bar connections. Only thing I would add is keep your salty, dirty fingers off the cleaned contact surfaces.

There are two reasons for not starting balancing until cell is above 3.4v. During charging, this is where cell voltage starts to rise quickly making contrast in cell voltage easier to detect. If there is any moderate amount of discharge current from inverter, the cell will almost instantly drop below 3.4v, regardless of how close to full charge it is, shutting down balancing so you don't have to worry so much about larger discharge current and series resistance variance causing variance in BMS cell voltage readings from screwing up balancing direction decisions.
 
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time2roll

Photon Sorcerer
Joined
Mar 20, 2021
Messages
3,639
I would expect if the cells are balanced and fairly well matched the the 15 to 20 mV spread to initiate balancing should not even come into play until 3.4 vpc. Yes if there is no spread threshold then waiting for charging to hit 3.4 vpc is best.

And BTW I far prefer the OP method to balance. Although I may have set the charger at a lower voltage initially to minimize the over volt shut downs.

While the top balance in parallel works it does seem many still have troubles. A bit slow and obsolete IMO.
 
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