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Balancing troubles...
#1
Hi folks. I'm running into a few troubles where I can't seem to bleed off enough power to get 2 low cells to come up.

Attached are a couple of screenshots. I charge to 55.5v absorption setpoint and I have a 12 minute CV period (absorption timer) before I drop to float. I use an outback FM60 with no RTS compensation.

I use auto levelling at >85% SOC but I'm not sure what I should set my balance voltage to. I would expect, given I have my setpoint at 55.5, my cells would settle on 3.46v/cell when it is at absorption. But often I get a few cells start to nudge above that even when the blockmons are running at full capacity and cell 6 and 14 don't come up. 

Cell 6 and 14 aren't particularly weak, they hold up well when subjecting the pack to reasonable discharge loads (50a), they just don't want to come up very easily.

I've been playing with the balance voltage this summer, tweaking it up and down a few .1's of a volt, at the moment it's at 3.49 iirc. Does this seem sensible?

Cells are 10 year old CALB 210aH but they had low cycles on them - came from an Allied electric peugeot boxer that had a failed BMS.



Current system: 9.6kWh wet Nicad batteries, 16S1P Calb LiFePo4, 210aH, Batrium WM4, Outback vfx3048 inverter, mx60 mppt controller, flexware 500 mounting hardware, 2.4kW solar array, 6kW lister diesel genset.

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#2
You gotta unpinch the busbar when current is going back to the packs. Otherwise the little angry pixies get bunched up on the wire Tongue


That is interesting, if they handle high discharge current fairly well. Have you tried taking the pack out of service and testing it directly? Or at least inspecting it for damage. It's possible you could have a loose connection on a few cells and when dumping, you're really pulling from less than your parallel count, but there's enough of a connection that the cells will still balance out. Then when charging, there's a lot of resistance to go back in and being burned off as heat, making the cell appear to be lagging.

Just a thought.
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#3
(05-23-2019, 11:28 AM)Korishan Wrote: You gotta unpinch the busbar when current is going back to the packs. Otherwise the little angry pixies get bunched up on the wire   Tongue


That is interesting, if they handle high discharge current fairly well. Have you tried taking the pack out of service and testing it directly? Or at least inspecting it for damage. It's possible you could have a loose connection on a few cells and when dumping, you're really pulling from less than your parallel count, but there's enough of a connection that the cells will still balance out. Then when charging, there's a lot of resistance to go back in and being burned off as heat, making the cell appear to be lagging.

Just a thought.

I am wondering if I have a high resistance joint somewhere... I will wait for an opportune moment to run the generator and bypass the pack, and check the busbar straps on 6 & 14.

Although, I dropped absorption down .1 of a volt this morning and things look a little better now:



There is a .5-.7V difference between what the outback sees and what the watchmon sees, I guess due to the crappy 10 bit A2D that is in the atmega's that outback use. I've learnt to overcome that.

EDIT: Things settling down nicely about 5-6 mins into the CV stage:
Current system: 9.6kWh wet Nicad batteries, 16S1P Calb LiFePo4, 210aH, Batrium WM4, Outback vfx3048 inverter, mx60 mppt controller, flexware 500 mounting hardware, 2.4kW solar array, 6kW lister diesel genset.

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#4
I would say you are close enough for call them in balance. Mine looks the same and have done that for 3 years but still they perform from full to empty as they should. You might have some self discharge on those packs and over the day they discharge same amount as you balance the rest. As long as the system keeps up and doesnt fall behind its ok

You can check in the reports how much you balance each cell and by doing that you can get a feeling on what cells tend to go higher than other. The ones never balanced are the ones with self-discharge-symptom.
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#5
I've just watched it complete an absorption cycle and it is sitting nicely now at float:



I dropped float by .1v as well this morning, hence the slight discharge you see above. I might need to tweak that up again to make the net current across the batteries 0mA, but I tweaked it down because spending 4-5hrs in float would cause cell 2 to start to creep above 3.45 by the end of the day.

The joys of having a completely disconnected solar charge controller and BMS Sad
Current system: 9.6kWh wet Nicad batteries, 16S1P Calb LiFePo4, 210aH, Batrium WM4, Outback vfx3048 inverter, mx60 mppt controller, flexware 500 mounting hardware, 2.4kW solar array, 6kW lister diesel genset.

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#6
The cells #6 & 14 that are "not coming up so well" might have drifted a bit in SoC (few % max) & just need time to catch up again.
The balance current the blockmons can do is only small for this size & age pack IMHO so you may be on the edge of being able to keep it balanced.
You could add more blockmons (double them up) or maybe consider adding a set of 1s balancers across the pack.

The 1s units can balance higher currents but can also disguise an issue though.

For my LiFePo4 packs I've actually changed the display range of the Watchmon s/w to be 3.65V max scale, 2.75V min scale (click gear wheel on bottom right)
This gives me a more zoomed in view. The log files would catch any extremes - I really wouldn't want my cells near either end of these 3.65/2.75 values though!
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#7
(05-23-2019, 12:26 PM)Redpacket Wrote: The cells #6 & 14 that are "not coming up so well" might have drifted a bit in SoC (few % max) & just need time to catch up again.
The balance current the blockmons can do is only small for this size & age pack IMHO so you may be on the edge of being able to keep it balanced.
You could add more blockmons (double them up) or maybe consider adding a set of 1s balancers across the pack.

The 1s units can balance higher currents but can also disguise an issue though.

For my LiFePo4 packs I've actually changed the display range of the Watchmon s/w to be 3.65V max scale, 2.75V min scale (click gear wheel on bottom right)
This gives me a more zoomed in view.  The log files would catch any extremes - I really wouldn't want my cells near either end of these 3.65/2.75 values though!

I was concerned that the blockmons couldn't dump enough for this size of pack, shame they don't make the new active balancer watchmon capable of handling 16S
Current system: 9.6kWh wet Nicad batteries, 16S1P Calb LiFePo4, 210aH, Batrium WM4, Outback vfx3048 inverter, mx60 mppt controller, flexware 500 mounting hardware, 2.4kW solar array, 6kW lister diesel genset.

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#8
Lower the charge current when bypass starts, actively cool the mons as you'll kill them running them so hot, and lengthen the absorb time to hours not minutes.

Fully balancing takes a long time with large cells, which might not be very well matched in terms of capacity and a fairly limited amount of passive bypass current.

And don't forget you can reduce the bypass current if you don't want to cook the mons and don't want to actively cool them either.
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#9
(05-23-2019, 12:52 PM)Sean Wrote: Lower the charge current when bypass starts, actively cool the mons as you'll kill them running them so hot, and lengthen the absorb time to hours not minutes.

Fully balancing takes a long time with large cells, which might not be very well matched in terms of capacity and a fairly limited amount of passive bypass current.

And don't forget you can reduce the bypass current if you don't want to cook the mons and don't want to actively cool them either.

I have no way to reduce the charge current when bypass starts - you can't connect a WM4 to an MX/FM60

Think I'll leave things as they are now I've dropped it .1v on absorb... Definitely need to tweak the float back up though, and get a dump load sorted out. So much sun at the moment, so little demand.
Current system: 9.6kWh wet Nicad batteries, 16S1P Calb LiFePo4, 210aH, Batrium WM4, Outback vfx3048 inverter, mx60 mppt controller, flexware 500 mounting hardware, 2.4kW solar array, 6kW lister diesel genset.

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#10
Do they not do voltage based charge reduction anymore ? WM4 can trigger limited charge signalling, which is designed for when the mons go into bypass.

Your only problem is management of the mons temperature and an overly short absorb period - you don't want to be dumping energy that could usefully be stored in a cell.
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