Renogy - odd parallel behavior

[Decto]

Hi all,

Joined today so I could view some of the images, so really didn't expect to be posting a question this soon.
I have setup a solar shed as a project to get some experience over the next 12 months or so before commiting to a full house solar + battery install.
Bought this setup about 6 months ago, however only up and running 2 days ago due to deciding to fully insulate the already very full shed before starting, some illness and a pretty cold and wet UK winter.

Fired up 2 days ago, everything seems to be working fine, both MTTP charge well, hitting 40+ amps depending on time of day, Max approx 60A combined.
Multiplus seems OK, using battery / solar as priority until <30% SOC (I think, this needs some optimisation).
Have run the thermal camera around a few times, no hot joints etc.
Initial settings.
LiFePO4
Absorb 14.2V, 1 hr
Float 13.5V


The issue.
It's dark so no PV input, Multiplus inverting, not charging.
Randomly tonight I remembered I have a clamp meter, I had be wondering about how to monitor parallel DC batteries to check end of charging etc.
With my load of ~8A running I found one battery was proving all the current, the other was zero.
I checked the terminal voltage and both batteries showed 13.32V, smart shunt and inverter agreed.
Measured on the crimp lug and on the battery terminal.

I disconnected the negative of the zero amp battery then remeasured.
The zero amp battery now showed 13.10V (the same as both when delivered), the other battery remained at ~13.3V
I applied a 3A LiFePO4 charger from other small batteries to the zero amp battery and immediately battery voltage hit 14.4V and it said full.
On disconnect and 5 minute hold it was back to 13.10V.

I powered down, then swapped so the system was connected only to the zero amp battery.
On applying the 8A load, the voltage of this battery increased to ~ 13.50V before dropping slowly to 13.31V over the next 10 minutes where it held.
I then disconnected the load and let both batteries rest for 30mins.
Voltage of each was 13.30V + 13.31V (zero amp) so I reconnected in parallel.

On a restart and reapplying the 8A load there was a 50/50 current split between batteries as I'd expect.
My meter actually reads 13A DC at the multiplus and 6.6A DC from each battery though the multiplus and smart shunt say ~ 8A.
Load is a little under 100W so 8A seems closer, main point is that it is now splitting the load equally between batteries.

I did have a battery voltage spike today which showed ~ 15.5V for a single reading on the smart shunt.
This happened after applying a 20% overload (heater) to the multiplus for a couple of minutes to check connections with my thermal camera.
The multiplus dropped into charge mode, when I was actually expecting it to switch to AC throughput.
On disconnecting the load, there was a battery voltage spike for a couple of seconds.
Possibly this may have disconected the BMS on one battery, however I don't understand why this would still have a terminal voltage of 13.1V without supplying current. Unsure how to avoid this in the future if using high loads.

Realistically, the max loads will be ~ 200W, typically 100W or less. The multiplus was sized to allow startup of inductive loads such as non inverter fridges and freezers.
From what I'd read the 800VA would alarm with overload in some instances, while the 1200VA did not.

So, it's odd. I ended up with what seems like a battery in idle, not a BMS behaviour I've seen in the small stuff I've played with before.
Unfortunatly these have dumb BMS so there is no data on what is happening.

Some more testing to do, but curious if anyone else has similar experience.
I think I'll also be building an arduino project to monitor current per battery with some CT clamps.

Image below

 

[sunshine_eggo]

Hi all,

Initial settings.
LiFePO4
Absorb 14.2V, 1 hr
Float 13.5V

I would increase absorption to 2 hour to enable more balance time.

The issue.
It's dark so no PV input, Multiplus inverting, not charging.
Randomly tonight I remembered I have a clamp meter, I had be wondering about how to monitor parallel DC batteries to check end of charging etc.
With my load of ~8A running I found one battery was proving all the current, the other was zero.
I checked the terminal voltage and both batteries showed 13.32V, smart shunt and inverter agreed.

Two batteries in parallel will essentially always measure the same voltage.

Measured on the crimp lug and on the battery terminal.

I disconnected the negative of the zero amp battery then remeasured.
The zero amp battery now showed 13.10V (the same as both when delivered), the other battery remained at ~13.3V
I applied a 3A LiFePO4 charger from other small batteries to the zero amp battery and immediately battery voltage hit 14.4V and it said full.

This implies the battery was in charge protection mode even while at a low-ish state of charge.. The charger didn't see a voltage, so it spiked to its set absorption voltage.

On disconnect and 5 minute hold it was back to 13.10V.

I powered down, then swapped so the system was connected only to the zero amp battery.
On applying the 8A load, the voltage of this battery increased to ~ 13.50V before dropping slowly to 13.31V over the next 10 minutes where it held.

This makes no sense.

I then disconnected the load and let both batteries rest for 30mins.
Voltage of each was 13.30V + 13.31V (zero amp) so I reconnected in parallel.

On a restart and reapplying the 8A load there was a 50/50 current split between batteries as I'd expect.
My meter actually reads 13A DC at the multiplus and 6.6A DC from each battery though the multiplus and smart shunt say ~ 8A.
Load is a little under 100W so 8A seems closer, main point is that it is now splitting the load equally between batteries.

I did have a battery voltage spike today which showed ~ 15.5V for a single reading on the smart shunt.

That's typically the result of high voltage charge protection where the BMS cuts off the charge and the charger spikes before it can react to the loss of load (batteries are loads to chargers).

This happened after applying a 20% overload (heater) to the multiplus for a couple of minutes to check connections with my thermal camera.
The multiplus dropped into charge mode, when I was actually expecting it to switch to AC throughput.

It did. The MP can't charge AND inverter at the same time. If it's charging, it's in AC passthru mode.

This also implies the MP sensed a voltage below cut-off. Do you have virtual switch or some assistant managing AC fail-over?

On disconnecting the load, there was a battery voltage spike for a couple of seconds.
Possibly this may have disconected the BMS on one battery, however I don't understand why this would still have a terminal voltage of 13.1V without supplying current. Unsure how to avoid this in the future if using high loads.

Odd.

Realistically, the max loads will be ~ 200W, typically 100W or less. The multiplus was sized to allow startup of inductive loads such as non inverter fridges and freezers. From what I'd read the 800VA would alarm with overload in some instances, while the 1200VA did not.

Victrons can be remarkably tolerant of overloads. They can perform above rated current provided temperature, voltage and frequency stay in tolerance.

So, it's odd. I ended up with what seems like a battery in idle, not a BMS behaviour I've seen in the small stuff I've played with before.
Unfortunatly these have dumb BMS so there is no data on what is happening.

Some more testing to do, but curious if anyone else has similar experience.
I think I'll also be building an arduino project to monitor current per battery with some CT clamps.

Image below

IMHO, you should float the batteries at 14.2V for about 48 hours to give them time to balance and re-evaluate the batteries.

 

[Decto]

I would increase absorption to 2 hour to enable more balance time.

Will do

Two batteries in parallel will essentially always measure the same voltage.

I was checking if somehow there was a connection issue where I'd see different voltages

This implies the battery was in charge protection mode even while at a low-ish state of charge.. The charger didn't see a voltage, so it spiked to its set absorption voltage.

The charger applied 14.4V across the terminals, so must be the BMS not accepting charge, hence the decison to apply a load to see if that would reset the battery.

This makes no sense.

Agree, which is what lead me to post here.
I can only put if down to the BMS reconnecting/activating when a load was applied, possibly due to the voltage drop... not sure so speculating.
The battery was charging at the time of the voltage spike so the 30min rest voltage is in the ball park.

That's typically the result of high voltage charge protection where the BMS cuts off the charge and the charger spikes before it can react to the loss of load (batteries are loads to chargers).

I can only assume the charger suddenly lost at 1200W load which increased the battery voltage causing the BMS to trip.

It did. The MP can't charge AND inverter at the same time. If it's charging, it's in AC passthru mode.

Ok, thanks. I thought it would just switch a relay to pass through the AC ... but actually it was buzzing and the fan was running full speed.

This also implies the MP sensed a voltage below cut-off. Do you have virtual switch or some assistant managing AC fail-over?

No, nothing extra, I did set the shutdown voltage to 12V in case I screw up as nothing currently running is critical. I'll have a look at the smart data tomorrow to see if there was a low voltage at the time.

Odd.

Indeed, it's the 13.10V that which strange as it would not respond to a charge so I tried to opposite and applied a load.
I'd expect a zero for a BMS in protection.

Victrons can be remarkably tolerant of overloads. They can perform above rated current provided temperature, voltage and frequency stay in tolerance.

Yes, I went over my budget somewhat to go Victron, no regrets. Just need to get the hang of it all.

IMHO, you should float the batteries at 14.2V for about 48 hours to give them time to balance and re-evaluate the batteries.

Thanks for the recommendation and comments above.
I'll update the settings in the morning and take another look at dusk tomorrow and extend as needed.

 

[sunshine_eggo]

I was checking if somehow there was a connection issue where I'd see different voltages

This is always a good first step, and I still can't seem to remember to always list it as the first step...

The charger applied 14.4V across the terminals, so must be the BMS not accepting charge, hence the decison to apply a load to see if that would reset the battery.

Yep.

I can only assume the charger suddenly lost at 1200W load which increased the battery voltage causing the BMS to trip.

Shouldn't work like that. If a battery is in the circuit, it's resistance will almost always allow a charger to adjust in time to avoid an over-volt spike. If it hit a single cell over-voltage, then yes, that could happen.

Ok, thanks. I thought it would just switch a relay to pass through the AC ... but actually it was buzzing and the fan was running full speed.

The same transformer inverts and charges - it can't do both simultaneously. One would normally hear the ATS switch over, but charging buzzes too, and a hot inverter is going to run the fans even if it's not inverting.

No, nothing extra, I did set the shutdown voltage to 12V in case I screw up as nothing currently running is critical. I'll have a look at the smart data tomorrow to see if there was a low voltage at the time.

12V on LFP is plenty deep. I wouldn't call it excessive.

Indeed, it's the 13.10V that which strange as it would not respond to a charge so I tried to opposite and applied a load.
I'd expect a zero for a BMS in protection.

Yeah, that is really unexpected. If not 0V, then some absurdly low V for a 12V like 1-5V.

Yes, I went over my budget somewhat to go Victron, no regrets. Just need to get the hang of it all.

You made the right choice.

Thanks for the recommendation and comments above.
I'll update the settings in the morning and take another look at dusk tomorrow and extend as needed.

I missed in your initial post that you bought them 6 months ago. This cinches it... it's almost certainly an imbalance issue. I double down on the recommendation to float at elevated voltage for an extended period. LFP batteries sitting for extended periods will lose balance over time. They were also likely shipped to you at no more than 30% charged.

 

[Decto]

I missed in your initial post that you bought them 6 months ago. This cinches it... it's almost certainly an imbalance issue. I double down on the recommendation to float at elevated voltage for an extended period. LFP batteries sitting for extended periods will lose balance over time. They were also likely shipped to you at no more than 30% charged.

So charging seems OK today, quite a dull rainy day so charging ~20 to 40A, though peaks of 70A+.
The charge current was ~ 20-30% higher for the battery that had the issue yesterday throughout the charge cycle.
The absorbtion started around 11am, at 1pm the MPPT's were floating at 14.2 with the 'issue' battery charging around 8A for the next couple of hours in float before the charge tailed to nothing. I confirmed this with the clamp meter at regular periods, 1 battery taking no charge(assume full) and the issue battery charging for another 2 hours. From 3pm to ~8pm it floated @ 14.2V until there was insufficient PV power.

I'll let it float @ 14.2V tomorrow, then reintroduce the load which should pull around 80AH every night and then PV recharge during the day. Shallow cycling at approx 25% of capacity for now and I'll keep the extended absorbsion time while the pack stabilises.

Overall, I added a little under 2kWh today with no loads which would confirm the pack was relatively low. Each battery is a little over 2kWh on a full cycle.

Learning for me, I'd assumed as the new batteries were at 13.1V on install they were ~ 40% charged so would be fine to go straight into the system and allow the MPPT to charge to capacity, with the internal BMS sorting any imbalance over a number of charge cycles.

Next steps will be some load testing when I get a few hours to run down the batteries, or I may just let it run a couple of nights without the PV and see where the voltage settles.

The only really odd part is the 13.1V on the battery while not taking a charge or contributing to the load.
I'll have to keep an eye on that, as it may be a BMS behaviour.

Thanks for the help.

 

[sunshine_eggo]

I don't think enough emphasis is placed on it, but IMHO, any branded LFP battery needs to be fully charged and floated at absorption for 24 hours before deployed in a working system. I don't think they are consistently top balanced, AND I don't trust that they won't lose their top balance in the time elapsed between the manufacturer and the end user, especially if that timeframe is measured in months.

 

[DIYrich]

I don't think enough emphasis is placed on it, but IMHO, any branded LFP battery needs to be fully charged and floated at absorption for 24 hours before deployed in a working system.

If you are adding a new rack of batteries, can you charge and float them together, or does each one need to be done individually?

 

[sunshine_eggo]

If you are adding a new rack of batteries, can you charge and float them together, or does each one need to be done individually?

I still say individually as this gives one the opportunity to catch outlier behavior. Wiring a bunch up in parallel means errant behavior wouldn't be noticed.

If one devises a means of evaluating them individually, it could be bypassed.

 

[Decto]

So to follow up... my issue seems to be an increasingly dud battery.

I checked today and the smart shunt had reported ~ 13V battery voltage in the night after both batteries had been floated all day at 14.2V.
No loads other than the MPPT and Smart Shunt, inverter was in the off position.

Split the pack again and one battery is at ~ 13.3V and one at 13.1V.

With just suspect battery connected to the inverter I powered it up and the voltage slowly reduced to ~12.8V with just the idle current of the inverter ~1A. So then added 100W load, and the magic voltage jumped back to 13.3V while running the load though was falling fairly quickly.

I then incresed the load to ~600W which resulted in a rapid voltage decline to 12.3V and falling so I quickly disconnected.

On attempting to charge the multi tried to jump in and charge at 50A so the battery immediatly disconnected giving my a ~ 18V spike on the smart shunt.

I then trickle charged at 2A which was slowing increasing the voltage, I bumped to 5A and it quickly hit overvoltage, again I think this is the BMS tripping.
I reverted back to 2A after pulling a little more current to reset and get it to accept a charge then it charged at 2A again for ~ 30mins before charge current was zero as it had hit peak voltage.
Tried setting absorb to 14.6V (max spec) to see if this helped... but no, it just sits there at zero volts.

So I think this is a dead battery, was bought from Amazon UK so let the Renogy warranty dance commence.

 

[sunshine_eggo]

Concur. Bad battery. BMS or actual cell(s) failure.

 

[Decto]

Concur. Bad battery. BMS or actual cell(s) failure.

After another couple of days testing, the battery actually seems fine.

Having left the 'good' battery installed, I found when the chargers (MPPT and Multiplus) were off it also settled at 13.1V post sustained float charging. Really odd to have both batteries showing the same behaviour so I decided to capacity test the 'bad' battery, partly as evidence for Renogy.

Same odd behaviour, on load connection there is a momentary drop in voltage from 13.1 before the battery seems to 'reconnect' voltage jumps to ~ 13.3/13.4 and then starts to gradually reduce. The long term resolution of the smart shunt loses the detail but the spike are visible in the real time.

For the 150W load, it ran for ~13hrs resulting in the smart shut recording 156AH.
Not a full discharge as the discharge floor of 10% cut the inverter with a minimum voltage (Shunt) of ~12.1V

Charged up again fine at a constant (limited) 20A but capped the Absorbsion voltage to 13.8V to see if that would stop the BMS 'disconnecting' but it still did the same 13.1V idle before an dip then increase when the load is applied.

Second test from 98% charge was 600W which ran for around 3:15 hours before the SOC% disconnected at a calculated 5%, though the voltage was ~ 12.2V as read by the shunt and 12.37V accross the terminals. This pulled 149AH.

Battery now happily charging at 30A via the multi.

So I can't really fault tha battery capacity, if I'd run to the floor voltage of 10.5V I would have squeeze out some more, plus I noted on the 600W run I had a .25V drop from battery to inverter, or ~0.15 battery to shunt at least half of which is the ANL fuse.

The odd issue is the BMS disconnect on charge hence the 13.1V which makes the battery seem part charged... hence refusing to take any more charge.

I suspect part of the issue is with a 'dumb' battery (no bluetooth), I can only see a post BMS voltage rather than cell voltages so likely there may be small amount of mosfet losses etc preventing me seeing the full cell voltage.

I also note Renogy specs this at nominal 12.8V (3.2V) vs most other manufactures who spec 13.6V (3.4V) nominal. Is there a potential difference in nominal voltage for the cylinderical cells used in the Renogy 26650 vs pouch or prismatic designs.

Anyhow, I'll capacity test the other battery this weekend and then hopefully be able to just use it.
Still not sure one battery sitting idle until there is sufficient demand is an issue, however I suspect this is part of the reason Renogy stress 'Not in series' parallel with same type of battery only.

Bonus, my Venus PI is not running, at least for the inverter and smart shunt so I don't need to be 6 ft away to connect.

 

[sunshine_eggo]

After another couple of days testing, the battery actually seems fine.

I also note Renogy specs this at nominal 12.8V (3.2V) vs most other manufactures who spec 13.6V (3.4V) nominal. Is there a potential difference in nominal voltage for the cylinderical cells used in the Renogy 26650 vs pouch or prismatic designs.

I was with you until you said this. All 4S LFP batteries are 12.8V nominal (3.2V/cell). Some specify a float voltage of 13.6V, but not nominal. If they labeled it as nominal, they're wrong.

The next step up is 3.6-3.75V/cell for the various fiery/explodey chemistries.

The BMS cut-off may create issues - voltage spike that trips the inverter over-voltage protection, etc., and you'll NEVER want to put them in series for 24V.

Anyhow, I'll capacity test the other battery this weekend and then hopefully be able to just use it.
Still not sure one battery sitting idle until there is sufficient demand is an issue, however I suspect this is part of the reason Renogy stress 'Not in series' parallel with same type of battery only.

Bonus, my Venus PI is not running, at least for the inverter and smart shunt so I don't need to be 6 ft away to connect.

Essentially, a battery manufacturer must meet two criteria at a minimum:

Specified charge voltage.
Specified capacity.

If either can't be met, you have grounds for a return.

If you really need it to be put in service, hold it at 14.2V for a week. If it doesn't correct the BMS behavior, I'd return it.

 

[Decto]

Essentially, a battery manufacturer must meet two criteria at a minimum:

Specified charge voltage.
Specified capacity.

If either can't be met, you have grounds for a return.

If you really need it to be put in service, hold it at 14.2V for a week. If it doesn't correct the BMS behavior, I'd return it.

Capacity >90% at >12.2V under load end of discharge seem fine.

Once the BMS cut trips in, the battery won't accept any power, I tried this with a bench PSU at 14.4V and no current would flow so I don't think floating will help as the battery just sits until it gets a significant discharge.

I'll try cycling this one for a week, I can set the multi to jump in at 20% SOC and then top up with solar each day. I have a daily load which can pull down to ~the 20% and then it can charge on solar and float. Possibly the balance only happens in the tail end of charging, I've asked Renogy support to confirm.

No plans for Series, well aware this wasn't an option when buying.

 

[sunshine_eggo]

Capacity >90% at >12.2V under load end of discharge seem fine.

Once the BMS cut trips in, the battery won't accept any power, I tried this with a bench PSU at 14.4V and no current would flow so I don't think floating will help as the battery just sits until it gets a significant discharge.

Right, but what you can't see it the cell balancing that's happening. If you were to log the process you would see voltage increasing/decreasing and current cutting in and out as cells oscillate between over-voltage and reconnect. You'd be forcing the cells to balance all the time.

I still recommend holding at 14.2V for a week.

 

[Decto]

To close this out for anyone finding this thread later.

Batteries are now working as expected.
The issue seems to be a BMS lockout as per snip below. The battery would detect a cell overvoltage and partially disconnect resulting in a terminal voltage of 13.0-13.1V which fell below 12.7V when drawing current before the battery would then 'reconnect' and increase to a normal full battery terminal voltage >13.3V.

Battery A (battery I suspected was faulty) was cycled a number of times to <20% SOC with charging limited to ~0.1C (20A) and after a few cycles was OK, no more disconnects, terminal voltage post charge OK.

Battery B (also with the 13.1V issue), was held for several days at 14.2V on a bench PSU while Battery A was cycling. On swapping back into the system to replace battery A, it still had the 13.1V issue on initial consecutive charge cycles. Cycling a few times below 50% DOD and capping the charge to 20A has cured this battery as well, though it took a few discharge / charge cycles same as the other battery.

Both batteries now 'graciously' complete charging with a taper of current, no sudden cease of charge current and voltage spike.
Post charging, in float, both batteries were still consuming some charge. (0.3-0.5). Will bump the float voltage over the next few days as I think this is some passive balancing going on so I want to allow that to happen gradually.
On physical diconnection, both batteries are now 13.4-13.5V at the terminals vs 13.1V before.

Seems the batteries are now in a position where the BMS can now manage the cell imbalance, these will now be runing ~50% daily DOD for a period which should allow plenty of charge cycles to futher balance.

I'm no longer limitting the charge current, not sure if this helped, however I read passive balancing occurs only when there is cell voltage seperation in the final few % of charging so I used a less agressive charge cycle to 'slow the final charge stage' and give the batteries a better chance to passive balance, given the passive balance effect is limited.

Renogy customer service have not been very useful... can't provide any info on how the BMS operates.
If they had this in an FAQ, would have all been sorted a while ago.