Charging LiFePO4 batteries with Classics

[keppelk]

Hoping for some assistance setting up my Classic 200 and Classic Lite 200 to charge a 16 cell Sinopoly 400Ah LiFePO4 battery. I'm unsure what battery setting to use - basically none of the defaults seem to fit.

Battery pack is 16x Sinopoly 400Ah cells in series. Fully charged voltage of 3.8V x 16 cells = 60.8V. Fully discharged voltage 2.5V x 16 = 40V. Battery datasheet calls for constant voltage/constant current charging. I'll add a safety margin to both of these limits as it seems LiFe batteries like to be 'half-charged' rather than totally full or empty. My question is what are the best settings to give the Classics to charge this pack?

Additional system info:
12x Benq 255W poly modules on each controller (24x panels in total)
BMS installed on pack with separate input/output ports - each with own contactor to isolate any load/charger before battery is damaged.

I could toy with this and eventually get it right, but since the battery pack is worth a small fortune I'd prefer to get this right from the start. Happy to provide any additional information if needed.

[Westbranch]

don't have any but there are 14 threads here if you search for  LiFePo4

hth

[zoneblue]

gidday. Well we hope that youre not going to "toy" with those lithiums.  Having taken on what amounts to an "advanced setup", youre going to have to get right up to speed with all the (extensive) ins and outs of this new technology. Then you can teach us, LOL.

The classic itself wont have any trouble, with regards setpoints etc. But youll need to be careful in a couple of areas.  If you set up a low voltage disconect contactor, the classic wont like having the battery removed with the pv still connected. If you do this youll need to remove the pv first. Alternatively, you can use aux2 as an input to tell the classic to stop charging.

Balance or not to balance, top v bottom, high setpoints low. Start balanced, use the 20 80 mid zone, tricks to slightly overcharge,  these are all things that are being hammered out by the enthusiast community at present. No clear winner so far as far as i can see. Early days.

Do you have experience with lead banks?

[keppelk]

The classic itself wont have any trouble, with regards setpoints etc.

I have some lead experience. I don't 'need' absorb, equalisation etc for LiFePO4. The Sinopoly batteries are happy to be charged at 1C (400A @ pack voltage) up to almost full charge. I can't supply anywhere near enough power to charge like that though. Can bulk charge almost right up to the setpoint. The mains chargers terminate the charge once the current reduces to a certain limit (I think 8A per cell from memory). Looking for suggestions of exactly what settings others would use on the Classic. Won't be blindly entering these, but since others have far more experience with the Classic's I thought I would ask for some educated input.

Guessing a very short absorb time with absorb voltage only fractionally higher than the Bulk charge voltage. No equalisation charging necessary so set to 'manual'. Float voltage set to my maximum - still well clear of the BMS disconnect. Does this sound reasonable?

I'll be setting the Classic voltages safely away from the BMS disconnect voltages, so the Classics should never see an over-voltage disconnect (battery being removed while charging). Similarly the inverter will do it's low voltage disconnect before the BMS. BMS is only there in case of major failure that risks cooking the pack.

If in the future I start charging the battery closer to it's full capacity then I may need to get the BMS to signal the Classic's to stop charging. Currently just the Classic setpoints will have charge stopping significantly before BMS approaches disconnect voltages.

[dgd]

These cells show constant voltage between 20% and 100% full so voltage is not really an effective indicator of SOC.
If the charger ends charging at 8A then it should be easy to set the Classic Absorb set point to the manufacturer's recommended charging voltage and use the Classic's ending amps, set to 8A, to terminate absorb. I would set float at cells normal resting voltage.
I understand the proper way to recharge these cells is to use a battery  monitor that effectively counts the Ah out and Ah in to achieve charged status.
With the Classic and the WBjr the ability to terminate Absorb stage on Ah replacement count will be the way to go. Not sure if this is yet available or if it is likely.
In meantime if you are trusting the 8A ending amps figure then that would appear to be the way to go.

dgd

[Cniemand]

I know this is an old thread but wanted to post to any newcomers that are reading about LFP cells. Unless Sinopoly version of LiFePo4 are somehow drastically different... You should not charge your cells up to 3.8 volts each. That is too high on their charge curve. In the beginning days of LFP batteries hitting the market in 2008 they used to say you could go to 4 volts but as the years have gone by they realized that really you shouldn't be exceeding 3.5-3.6 volts per cell. These type of cells have a very flat curve in the center and they either shoot up at the end very quickly or on the bottom end... drop like a rock. Best to keep them between 3.1vpc (80%DOD or 20%SOC) to 3.5v.

[keppelk]

Sinopoly cells (per the manual I received with my cells in early 2014) need an initial charge to 3.8V/cell. My initial post wasn't clear on this sorry. Fully charged voltage is 3.65V/cell, which seems to be fairly standard across the different manufacturers. Worth noting that when I received the cells the Sinopoly datasheet charged voltage was 3.7V, but this has since been reduced to 3.65V inline with others. Discharge limit is 2.5V/cell. The 3.1V to 3.5V range seems to have been adopted by most alternative energy users. A small reduction is stored energy is an acceptable trade-off to improve cell lifetimes.

I charge a 16 cell Sinopoly 400Ah pack to 56V (absorb 20 mins) then float at 55.2V with no regular/automatic equalize. This equates to 3.5V/cell absorb and 3.45V/cell float. If I turn off my background loads (fridge, freezer, electric fence unit) the Classics will reduce charging current to zero, and hold both voltages without overcharging the battery.

[Cniemand]

Companies producing Prismatic Lithiums have changed their charge set points a lot over the years since being released in 2008. They originally had them all the way up to 4v.

I would counter that even 3.65 is high. 2.5v (0%SOC) 3.1v (20%SOC) and 3.5v(100%) are your main useful numbers.

Not sure where you come up with Absorb of 20 minutes? Lithiums being different than Pb cells do not have special needs for Absorb times. The specific voltage set-points and ENDAMP settings are most important. They are full based on reaching an ENDAMP of 0.05C (5a for 100aH cells) while maintaining their ABSORB voltage (3.5v per cell).

If it times out on an ABSORB of twenty minutes then you are not quickly charging them but rather charging them at a slower rate while holding them at a higher FLOAT voltage.

If you move your ABSORB time to 5 hours or even three (doesn't matter as once it hits ENDAMP (0.05C) then its full) you will be able to charge them at a faster rate with the higher voltage. Get a Battery to a true Float stage early in the afternoon, leaving you the rest of the day to Float with free juice.

My own LFP pack is ABSORB/BULK to 55v (could choose 56v for 3.5vpc in a 16 cell series) and ENDAMP=5 (I have 100aH...with WBjr to accurately terminate regardless of background loads) FLOAT = 3.375vpc or 54v in my 16 cell case.

[keppelk]

I don't have a WbJr, and would need to do some substantial rewiring to add one. Thus using end amps to terminate charge is not an option.

I use the 20min absorb as replacement for end amps. Trial and error has showed 20mins to be a good amount of time. Using 5mins previously and the battery was still accepting charge at the end of the absorb timer. 10mins was usually done, but not always. 20mins seems to always get to 0A (allowing for background loads) before dropping back to float.

If it times out on an ABSORB of twenty minutes then you are not quickly charging them but rather charging them at a slower rate while holding them at a higher FLOAT voltage.

The speed of charging doesn't change between bulk, float and absorb. All 3 modes will throw the maximum available current at the battery until set points are reached. Unsure what your "slower rate" comment is referring to?

[Cniemand]

I am referring to that if your FLOAT set point is high enough to still get "maximum charging" as you say, then you run the risk of actually hitting a full battery and over-voltage it. A LFP at rest is 3.34v. The purpose of bulk and absorb is to hold a higher voltage to allow a larger current go into the battery and it pulls them up. LFP aren't like lead acid cells where it is a time of Absorb to get them full but rather an actual decline to a rate of amps going in to determine full. IE ENDAMPS.

When I did not have a WBjr I would add a few amps to my ENDAMP number to compensate for background loads in addition to not fully utilizing to system until it when to FLOAT. I choose 8 amps rather than the 5 needed to accomplish it.

The purpose of FLOAT isn't to be a charging regime but rather an maintainer. With LFP you would choose something just above their rest voltage (as the charge/discharge curve is so flat and a float of 3.34 would actually be slightly draining them) of say 3.375 like I have chosen.

You CAN charge a bank on FLOAT by picking a voltage higher than their resting voltage but that is sort of squirrely when you do not want to over-charge them in the process. (If you are charging at a fast rate as you would get with Absorb, ie a high enough voltage to allow more amps into the bank ... it would hit 54v and throttle the amps much sooner than say 55v. At 54v it would take longer to fully charge as to maintain that voltage less amps could be put in at a time)

I would rather BULK/ABSORB at a higher voltage (55-56v or 3.5vpc) for a couple hours and be full at 11am rather than FLOAT at 54v and take all day....

[Cniemand]

Example : 55v would allow the bank to take 30 amps for quite a while before it starts to cut back to maintain 55v. (That is the battery would come from 20% to 80 or 90% before ramping down the current in)

54v would take that 30 amps until the bank is ... say 50-60%SOC and then have to cut back as it ends up causing it to over-voltage. It would then cut back the current in to maintain 54v. Say 20 amps.

With 54v at around their rest voltage the bank will easily reach that voltage at a low state of charge (the charge curve is so flat for lithiums) and then only take 5-10amps in order to hold it there. That wouldn't be far above what the background loads are. (Mine is 2-3amps) so you would spend all day charging the pack at a rate of 3-7 amp per hour.

[dgd]

I don't have a WbJr, and would need to do some substantial rewiring to add one.

I have seen a shunt/WBjr combo connected to the -ve battery terminal on a row of six 300Ah LiFeYPO4.
A very short piece of copper bar, two holes drilled in it, one end on the -ve terminal and other end to the shunt. On the other side of shunt was cable that was originally on the battery -ve
Very neat, no DC power cable cutting necessary, just needed the purple wire made longer to reach Classic.

dgd

[Cniemand]

DGD: Agreed. Shouldn't be hard to install a WBjr to a standing system. It is on the battery side before all the "Mess".

Six cells? That is an interesting setup. At a nominal 3.2v per cell... a 4 pack would be 12v equivalent. What was the idea behind having 6?

Cloud

[dgd]

oops, that was sixteen, all in a row, its a 48v bank

[Cniemand]

AHH. Yes. I have a 16 cell in series setup as well. 48v.