Lithium battery charge profiles

[5 Minutes 2 Midnite]

Five/Jim : Last month marked my 4th year running a full time offgrid LFP setup. Surprisingly I havent had any issues and have gone all this time without owning a generator. My stance is still the same of voltage setpoints and bottom balancing. Through both of those, the system has remained hands off (aside occasionally taking a multimeter to the bank for verification) With that, I am not 100% certain what my battery capacity is relative to day one. One of these upcoming sunny days I need to fully discharge the bank from 100% to ~0% and then back to Float with the house inverter off to get a more accurate measurement of what went into the bank minus charging losses. From an anecdotal perspective, I do not notice any difference in capacity. That is, my regular routine doesn't bring the SOC any lower. Typically I do not use more than 50-60% DOD. My usage could be considered mild to moderate. If we are to take Calb paperwork concerning cycle life, mine would mimic closer to 5000 cycle life. My lifetime Amp-hours currently is 104,356. Take that into my 100aH bank, I am sitting at 1000 cycles. Still running well without noticeable degradation of capacity.

Thanx for the update. Happy to hear your system is doing so well. So, still the same charging strategy. The main thing I noticed about your charge profile is that you are setting float close to the array's resting voltage. When I went on my hunt for the best LFP profile for the Classic I got back "set bulk=absorb=float".

When I went to program the classic there is of course no bulk setting per sey. And you must set float at least .1v below absorb. Still, I have gone with that strategy so far. And I use a whiz bang to terminate absorb (go to float) when end amps = 1. So specifically, I set absorb to 3.45 (55.2) and float to 55.1.

But your notion of dropping float down to near resting voltage is intriguing. It would be interesting to run some load tests and see how much difference in total capacity there is between the two strategies. If any? I'm assuming that my batteries are still charging to 90% or so at the top end (depending upon which charts one consults).

So, why am I setting end amps to such a low value, you ask? Using that often quoted formula of charging to 0.05C, my 144AH pack should have end amps set to 7.2. However, for now, I'm going by the battery manufacturer's spec sheet, which says to stop charging at 1A (see attached). These are also 3500 cycle rated cells, which is why at the last minute I bought them in lieu of the Calbs. Should I believe the spec sheet? Well it all depends upon whether is was assembled by the engineering department or the marketing department. Who knows?

http://www.electriccarpartscompany.com/New-Energy-72Ah-Aluminum-Encased-Battery

By the way, what setting are you using for 'battery efficiency %'? The default is 94%.

/jim

[Cniemand]

Each manufacturer is different, I guess. Using the 0.05C on LFP chemistry appears to me to give a good ENDAMP number. I use that and with 5amps it charges them without seeing voltages shoot up from one another. I would be concerned with my own system of CALB 100aH that if I used a lower number they would start to overcharge and fall drastically over balanced. Afterall, the chemistry being different than PB and bottom-balancing vs TOP. PB wanting always to be topped off. I, personally do not care what the capacity and voltages are so long as between 0%SOC and 100%SOC stay within an acceptable mv range/balance. 55v or ~3.4v per cell and 5 ENDAMPS for mr over the years seems to keep them charged up to the point of highest capacity given versus before one or two start to really break away and "over charge" versus the rest. What I am shooting for is that when the bank is nearly spent that they are as tight mv wise as I can get them so no cell has any great potential to drive another beyond recovery. Now, I rarely will discharge the bank beyond 60%DOD for longevity/max cycle life. As i see it, according to data sheets, I should attain 3000 Cycles (at constant 80%DOD usage) and between 5000 cycles (50%DOD). IDK. That is a lot of years. So far, I am at 1004 cycles 4 years in. Not bad for such a small bank (5 KwH nominal) with big screen tvs, vacuums, space heaters to burn excess, hot plates etc. With a largish Array, you are free to use gobs of juice during the days.

Over the first year or so, I found that a float voltage that is just slightly above the battery true resting voltage is perfect. It keeps you from either over voltage (so long as you are constantly bringing the battery down over night, rather than a non attended outpost) and slowly discharging it before the sun goes down, leaving you less than 100%. The "problem" with LFP is the linear voltage curve. If you set your voltage below the resting voltage, then you may end up with a battery that floated all day and left you with 80-90%SOC at night rather than 100% based on the linear voltage curve alone.

That is why my voltage setpoints are all very close eventhough each mode: bulk/absorb/float are all completely different profiles within a cc charge parameter.

To get the fastest charging possible, without overvoltaging/pushing the individual cells out of balance permanently, and holding them sligtly above a true resting voltage to maintain 100%SOC to be used once the sun sets.

I have messed around with Efficiency a bit. Once when I first got the Whizbang Jr and then a year or so later when I had a number of cycles on the battery to see if there was much degradation. Both times I come around 97%. It seems to be a decent number to get the Net aH in versus Net aH out to zero out relative to when my 0.05C Endamps switches the system over to float.

- Cloud

[Cniemand]

BULK : Really is a voltage number your Battery bank can ramp up to safely that will allow as many amps to come in until it hits that safe high voltage. With LFP you can dump in a lot of amps to fill it up fast until it hits around 80+%SOC. 55v for my 16 cell bank. You could do 56 if you wanted. I leave it the same for absorb.

ABSORB : Once you reach that 80% + point, the voltages start to creep up beyond safe limits so the charge control starts to throttle the amps in to maintain a safe voltage. At 55v ( again for my specific 16 cell battery bank) it gives it just enough voltage headroom to charge the bank fast without overvoltage. Once it throttle back to 5 amps (the endamp setpoint) the individual cells are starting to widen their voltages between each other as they get to the sharp ends of the charge curve. BUT. At that point the endamp setting causes it to go into FLOAT.

FLOAT : Purpose is to keep a full battery bank while you still have access to sunlight. Giving you a full usable battery for the night. Due to LFP having such linear charge curve, if you set it below the resting cell voltage of 3.34v it will slowly drain the battery rather than keeping it full. Using 54v for me or 3.375v keeps the charge up and doesn't add much throughout the day to slowly overvoltage the system because it is always drawn back down on a daily basis ie at night.

[5 Minutes 2 Midnite]

Very good. More helpful info.

btw, a few days ago I actually re-configured my charging profile like yours (floating just above resting volts). I don't want to take any chances as I haven't balanced my array yet. Although I did do a "parallel cell balance" when I first received them, which was simply the act of using the bus bars to configure all the cells in parallel and let them sit for a day or two.

I have a specific reason for not balancing yet. Nor have I installed the BMS. I wanted to run the system for a couple of weeks and "get to know" the pack. I'm taking measurements when the cells have discharged in order to spot any "runts" in the litter. Obviously I'm monitoring that no cell falls below 2.7v or rises above about 3.5v.

One reason for this approach to has to do with the cell balancers I bought. I purchased balancers that use a distributed management architecture. There is no central control board, and no connection to the Classic.

Rather, these balancers all daisy-chain together. Instead of one balancer bleeding off voltage to keep a pre-set cap on cell voltage, all cells are talking to each other and are aware of pack voltage.

This seems a unique approach, and I am going to skip bottom and top balancing for now to see how well these devices maintain uniform voltage at all charge points.

http://www.electriccarpartscompany.com/3V-1S-Lithium-Lighted-Battery-Balancers

Thanx Cloud!

[Cniemand]

  NP

[Robin]

Did I mention that we are looking very seriously at making a B17 type of unit that is actually plug in Lion batteries?
Envision this: A B17 on top and two or three more B17 looking units below. That makes for a nice compact system.

[ClassicCrazy]

Did I mention that we are looking very seriously at making a B17 type of unit that is actually plug in Lion batteries?
Envision this: A B17 on top and two or three more B17 looking units below. That makes for a nice compact system.

How many AH of lithium are you talking about ?

Larry

[Cniemand]

So, the battery bank wouldn't be separate? More of an all in one easy peasy "tesla" power wall idea? Or did I miss the bus on this one...

[mahendra]

Large Lithium type batteries pretty much already come with Battery Management System  (BMS) integration which is necessary to
balance all of the cells so that any one battery doesn't get hot or hog a charge.

What I've seen is that with this already in place, the controller simply has to either have the Classics' Absorb time
set to nearly zero OR, the BMS may have a control output that tells the Classic to stop and start its charging
via the Aux 2 terminal which now also acts as an input for this function.

Question if a MNwbjr is on aux2 can this still be used with two signal feed from a bms with the feature

Pretty cool, huh ??

We'll try to post some links for some of these as they pass by.  I have one or two somewhere already.

boB

[mahendra]

Would the whiz bang still function if a signal from a bms was placed on the same aux port ? Or would the whiz bang have to be removed?

[CDN-VT]

 whiz bang as I know is the only one to use that port , There is another AUX port in a Classic .

One WBjr per Aux port , but on the shunt , you can have 2 WBjrs & any other unit all piggybacked.

VT