Balancing troubles...

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HughF

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Jan 1, 2018
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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 wouldsettle 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.


image_bawvqy.jpg



image_pyobgx.jpg
 

Korishan

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


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.
 

HughF

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Korishan said:
You gotta unpinch the busbar when current is going back to the packs. Otherwise the little angry pixies get bunched up on the wire :p


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:


image_gvqsez.jpg


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:

image_smwgqo.jpg
 

daromer

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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.
 

HughF

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I've just watched it complete an absorption cycle and it is sitting nicely now at float:


image_pvoaqc.jpg


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 :(
 

Redpacket

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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!
 

HughF

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Redpacket said:
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
 

Sean

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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.
 

HughF

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Sean said:
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.
 

Sean

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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.
 

HughF

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Sean said:
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.

I meant I have no way of telling the FM60 that the WM4 is starting to bypass and it should start reducing charge current. There is no can support on the outback Mate. I think you can do monitoring over the sunspec protocol with an ASX port that converts from outback's specific network to modbus, but it won't allow parameter changes.

Mon cooling is a must, I will look at adding this. The short absorb time is set specifically because I am cooking my mons and they are reducing their bypass current to stop themselves burning up. As they reduce their bypass current the high cells drift higher and we hit HV cutoff at 3.75 and we end up critical cycling. Not that this happens often, but it does happen probably a few times per summer.

The MPPT controllerwill only reduce it'scharge current naturally once it hits it'sabsorption setpoint. Set that too low, and at low charge rates (winter time), it drops into absorb too soon, requiring a longer absorb time. Ideally the FM/MX60 would be responding to the 'remote' commands from the WM4, but sadly it isn't possible.

I've done a year and haven't killed my batteries or burnt my house down, yet. Think I'll stick with the current settings.
 

Sean

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Can your MPPT cease charge based on a tail current, the next Victron update will feature this apparently.

You can also use the charge complete signal from WM4 to cease PV input, but that'sbypass complete conditional.


image_geczku.jpg
 

HughF

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Sean said:
Can your MPPT cease charge based on a tail current, the next Victron update will feature this apparently.

You can also use the charge complete signal from WM4 to cease PV input, but that'sbypass complete conditional.


image_geczku.jpg

It can, set to 10Apresently, left over from the days when I was charging wet nicads (urghhh)

It's called Absorb end amps. it will drop to float based on either the absorbtimer expiring, or the current dropping below that value.
 

daromer

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You can have the batrium to engage a load when soc or voltage is above threshold. Easiest to dump energy you have. Im y case i export all excess like for instance now. SOC 100% and exporting almost 8kW right now.
 

Sean

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You can use that to curtail charge on Li cells as well, based on charge isn't complete at x volts, it's when current has tapered down to y amps.


daromer said:
You can have the batrium to engage a load when soc or voltage is above threshold. Easiest to dump energy you have. Im y case i export all excess like for instance now. SOC 100% and exporting almost 8kW right now.

When my soc = 100% I export to a couple of Leafs, and then resistive storage heaters - nothing escapes.
 

HughF

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Sean said:
You can use that to curtail charge on Li cells as well, based on charge isn't complete at x volts, it's when current has tapered down to y amps.

I might have a play with this setting then...


daromer said:
You can have the batrium to engage a load when soc or voltage is above threshold. Easiest to dump energy you have. Im y case i export all excess like for instance now. SOC 100% and exporting almost 8kW right now.

This would be preferable to using the float relay in my FM60 diverting into water heating via a PWM'd SSR? It can do PWM float relay control to match the solar input

Actually, not entirely sure what I would do with a tank of hot water that was 100m away from the house, in a shed. And the house doesn't even have wet central heating to couple into. Really not sure what the best diversion load would be for us given we heat/cook/heat hot water with a rayburn and have no radiators. And I'm the only driver, and I charge my EV at work.
 

daromer

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You can divert the load remotely. I have so i engage an ssr 100s meters away. yes i convert the solar energy to AC for that but still better than nothing.
 

Sean

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HughF said:
Actually, not entirely sure what I would do with a tank of hot water that was 100m away from the house, in a shed. And the house doesn't even have wet central heating to couple into.

Perhaps this rather retro use of hot water .....


image_gixbta.jpg
 

HughF

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Sean said:
HughF said:
Actually, not entirely sure what I would do with a tank of hot water that was 100m away from the house, in a shed. And the house doesn't even have wet central heating to couple into.

Perhaps this rather retro use of hot water .....


image_gixbta.jpg
Genius....


Just looked in the expansion outputs - what's the best one to use to switch on mon-cooling-fans? I don't see a '1 or more mons are bypassing' option?

would 'cooling required' do the same thing, or does that just work on cell temperature?
 

Redpacket

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I cool my 'mons with a 12V fan triggered by the "cooling enabled" state with the default output relay.
My WM1 doesn't seem to be able to switch the MOSFET outputs based on temp (would prefer this but it didn't seem to work).

With the last pics you're showing this shows a mis-match between the voltages the Outback MPPT & the blockmons are set to.
The pack is in balance & you're still pushing the blockmons quite hard.
You need to back down the charge voltage a bit.

It's a bit of a pain, you need the balance current but don't want to cook the blockmons too hard/too often/too long.

You really, really need to stop the cells getting to 3.75V thing, those cells will die soon.
 
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