ad: MyersEng-1

Man Portable, RF Quiet, LiFePO4 Solar Charge Controllers for /P

Discussion in 'Amateur Radio News' started by OH8STN, Jun 18, 2018.

ad: L-HROutlet
ad: l-rl
ad: L-MFJ
ad: Left-2
ad: abrind-2
ad: Left-3
ad: Radclub22-2
  1. W5LMM

    W5LMM Platinum Subscriber Platinum Subscriber QRZ Page

    Dereck, THANKS!!! I had no idea that I was supposed to do this! I will have to disassemble the pack and
     
  2. AI5DH

    AI5DH Ham Member

    You are quite welcomed. I am surprised you did not know cells have to be pre-balanced. You got lucky you did not over discharge and destroy a cell or two. Here is the issue and challenge. When you buy bulk cells. they are not shipped pre-balanced. They arrive at various SOC levels. Just using an example lets say you buy 100 AH cells. It could be 10 AH just pick you values that float your boat. You can have as much as 30% difference in SOC or 30 AH in 100 AH cells. On a 100 AH BMS, the typical Balance Current is 200 mah and for a BMS to balance that with a a commercial AC charger running 24 hours a day, to balance a 30 AH difference with 20 mah = 30 AH / .2 AH = 150 hours or 6.25 days. On Solar where you only have 3 to 5 Sun Hours per day you are talking 30 days to balance with the BMS. When you buy a commercial Lithium battery pack, the manufacture has already done the initial balance. Take away here is a BMS can only make very small corrections.

    So here is how you do it. Place all your cells in parallel and walk away for a day or two. This is going to Mid Balance the cells to an unknown SOC which you could care less what the SOC is at that point. Take note they WILL NOT BE THE SAME CAPACITY and capacity will be unknown. The reason you wil not know the capacity is because bulk cells have not been matched, and capacity tolerance is -10 to +5%. So that 100 AH cell at 100% SOC capacity can range from 90 to 105 AH. So at 50% SOC the capacity would be anywhere from 45 to 52 AH.

    So once you have them Mid Balanced time to go to the Top or Bottom. Going to the TOP can be tricky if you do not have a good regulated 3.6 volt power supply of sufficient current. Method is the same going to the Top or Bottom. Leave the cells in parallel and pick a direction. Me don't care which direction you go. If you go to the TOP you must use a BMS to monitor individual cell voltages. Just make sure you understand these 2 points.

    a. In a Top Balanced system you only have 1 reference point and that is at the TOP. When fully charged all cell voltages are equal, and capacity is unknown. All you know is SOC is 100%. If the Weakest cell is 90 AH on a 100 AH string, then you only have a 90 AH battery pack. Where this can bite you in the butt is on discharge. In a Top Balanced system cell voltages will NOT be equal as you discharge. Once you reach the Bottom, the Weakest cell voltage will fall off a cliff like you experienced. You had 2.7 to 3.2 right? 2.7 volts was roughly 2 or 3% SOC, and 3.2 was around 30% if those voltages were OCV. This is one reason you need a BMS in a Top Balanced System to monitor individual cell voltages and disconnect when the weak cell crashes.

    b. In Bottom Balanced System you have 2 reference points. At 2.5 volts all cells are at the same SOC and capacity of ZERO. So in a 4S system a pack voltage of 10 volts is dead and in no danger of over discharge. You also have a 3rd reference point which comes in handy. Since the cells are in series, when charged all capacities are Equal and cell capacity is determined by the weakest cell. Take note in a Bottom Balanced system cell voltages are not Equal when charged, they are only equal at the Bottom. When charging a BB system the first time or two is monitor individual cell voltages. The cell with the highest voltage is your weakest cell. Stop charging when the first cell reaches 3.55 volts and note Charger Voltage at that point.

    There is a 3rd way to Top Balance if you do not have a 3.6 volt charger and that is try to use your BMS, but it could take considerable time. Once you get the batteries Mid Balanced, connect them in series and connect BMS. I do not recommend you use solar, you want to use a commercial battery charger set to 14.4 volts on a 4S battery. You can even use a Power Supply if you can set voltage and limit current as that is what a battery charger really is. Set the power supply to 14.4 volts and limit current to C/2 of the batteries. As soon as the first cell reaches 3.6 volts and the Balance Board turns on, limit current to the value of your BMS bypass current say 200 ma. Otherwise if the pack is still taking 50 amps, the fully charged cell is only bypassing 200 ma and 49.8 amps still flowing on a fully charged cell until the others catch up and current tapers as the pack voltage comes up to charger voltage.
     
  3. W5LMM

    W5LMM Platinum Subscriber Platinum Subscriber QRZ Page

    Thanks, Dereck, I do NOT have a 3.6v charger, and bottom balancing is fine by me. That also means I can get rid of the BMS?
     
  4. KD2JIP

    KD2JIP XML Subscriber QRZ Page

    This is a great discussion with lots of useful information. Julian, maybe I'm one of the people you referred to above. We discussed my issues briefly by email last October. The main reason why I would build a DIY pack is because my current pack disconnects from my GV-5 near end of charge. It does this at 0.02C due to a low charge rate disconnect feature. When the BMS disconnects, the GV-5 sends panel Voc to all terminals and signals an over voltage condition. And yes, as you said, the battery manufacturer recommended using their charge controller, which I found to be too big and heavy for man-portable operation.

    I have found that the low charge rate disconnect APPEARS to be remedied by placing a buck boost converter between the GV-5's load terminals and the load. With a converter in place, I'm finally able to power the load with panel power when available, supplemented by battery power as needed, and recharge the battery from excess panel power as required. All of this happens with smooth transitions and no over-voltage alarm on the GV-5 at end of charge. I added a voltage regulator simply to protect the load from panel Voc; the disappearance of the over-voltage alarms was unexpected. I wonder if I could be damaging the battery somehow.

    Also, Julian, I'm clueless as to how you choose your BMS boards from among the sea of Chinese products, most of which don't provide complete specs. I noticed that the boards that you've used in your builds, which I think are manufactured by BesTech Power, use a balancing method that's different from my current pack's method. The BesTech boards all seem to perform balancing at end of charge, whereas, as explained by tech support, my current BMS balances continuously during charging and discharging. I saw a blurb on batteryspace.com about end-of-charge balancing that recommends letting the battery sit for a half-hour when fully charged to allow balancing to occur. This seems inconvenient. I'm interested in your comments on this. Also, you mention in your 5Ah 26650 build that you use a different BMS than in your Headway build, but I find no info about that board on your site.

    Thanks for all of the great info here.
     
  5. KD2JIP

    KD2JIP XML Subscriber QRZ Page

    Julian, also, is RF noise a problem with some BMSes, and if so, how do you choose one that's quiet?
     
  6. KE0WVV

    KE0WVV Ham Member QRZ Page

    @KF5LJW

    Dereck, I have some questions for you. In the reading I've done, i'm leaning towards using bottom balancing for my LiFePO4 battery pack, but want to know a little more about what you have referred to as "float" charging .

    So, if I bottom balance my pack, and find that I have a cell hit 3.55 volt when the charging voltage is at 13.7 volts. Is it then recommended that I set my power supply at a CV of 13.7 (and current limited appropriately) for charging every time? (You mentioned 13.6V charging voltage a lot here, then discussed this 3.55V in your weakest cell bit that threw a curveball at me)

    If I leave it attached to the charger at this 13.7 V for an extended period of time, will this do damage, or will "float" charging at this voltage be safe and non-detrimental for the pack?

    If the cell capacities are not well matched, would you recommend using the 3.55V of any one cell as the limiter, even if another cell is still below 3.4V?
     
  7. AI5DH

    AI5DH Ham Member

    How did you know I bottom balanc
    How did you know I bottom balanced? Or did you figure out who I am? :D

    For the moment I assume you do not fully understand what Float Charged is. When it comes to charging there are basically two algorithms being Constant Current (CC) and Constant Voltage (CV). Example you may here Bulk or Boost charge are CC mode. Float, Equalize, and Absorb are all CV.

    So Float is a constant voltage mode exactly what a DC Power Supply does. It is a fixed voltage. However the voltage chosen must be considered to be optimum for battery type and chemistry. When you Float a battery, it is a voltage that will hold 100% SOC or something less if you want. It is a voltage that neither charges or discharges. Its a perfect fit for the battery. Every telephone company, utilities, CATV, wireless use DC battery plants on a Float Charge. They never turn the chargers called Rectifiers. Very common for Pb.

    OK lithium LiFePo4 aka LFP for short can be Float Charged providing you meet two conditions.

    1. Voltage = 3.45 vpc or 13.8 volts on a 4S LFP
    2. Never charge more than 90 to 95%. Never ever charge to 100% SOC.

    OK there is only one thing different between Pb and LFP. Voltage wise they are equal. Both charger well at 14.2 volts and Float at 13.8. What is different is a mentality breaking the habit of keeping a battery fully charged, or stuck in a Pb world. Pb you want to store on Float @ 100% SOC. If you avoid going to 100% and shoot for 90% will double to triple your cycle life. LFP prefers to be ran in Partial State of Charge, not 100%.

    So if you Float the batteries like you would a commercial radio, you Float @ 13.8. Batteries will never provide power if the charger stays on in Float. At that point it is nothing more than a DC Power Supply. If AC goes out, radio stays working.

    Question is do you know how to BB?
     
  8. KE0WVV

    KE0WVV Ham Member QRZ Page

    So, a few posts up (and last year) you explained bottom balancing. I did have further questions about it though as I'll layout below.

    Bottom balancing only assures voltages are equal at the bottom balance voltage (2.5VDC), correct? So, as you charge, those voltages can diverge, dependent on battery capacity, correct?

    Now, after balancing, and while charging (with the cells in series) I plan to monitor/log the voltages of each cell and the input voltage. I charge until the first cell reaches 3.55V, correct?
    From that point on, do I set my charge voltage to be the input voltage I saw when my weakest cell hit 3.55? (Maybe repeat this a few times and take the average?)

    What if I hit 3.55V before one or more cells even hit 3.4V? Seems like I would lose a lot of capacity, right? I would venture to say that the cells are really poorly matched and I should just look to replace the weak cell, but I would think that if I'm ditching the weak cell, I might as well take it to a higher charge with each cycle (within specs still) and then replace the cell eventually with one that is better matched for the pack and bottom balance again. Agree?

    Anyways, I feel like I'm starting to get it all down and even think I stumbled upon a thread on a solar forum that had you discussing this same thing (https://www.solarpaneltalk.com/foru...ler-and-lifepo4-battery-for-backpacking/page3). I appreciate you taking the time to bring us up to speed on these things.
     
    Last edited: Nov 14, 2019
  9. AI5DH

    AI5DH Ham Member

    Going good until here. Just gotta change your POV. What you have accept is a chain is only as strong as it weakest link. No different with batteries. Example lets say you buy 100 AH cells. You measure the capacity of each cell, it spans 96 AH on the low end (weakest cell), and 105 AH on the high end. No matter how you charge it only gives you a 96 AH battery pack with them in series.

    Now answer your own question. Does not make a bit of difference if the other cells do not charge fully up? When you BB, all the cells have the same AH capacity at any given moment. So if 96 AH is the weakest cell, all cells have the same AH charge of 96 AH. The danger comes when you Top Balance, capacity is not equal, and that is where the danger comes from. The weak cell going flat, while all the others still have a good charge will cause the dead cell to go into polarity reversal and game over. With BB you eliminate that risk because all cells will be discharged the same time. None of the adjacent cells have any energy left top damage its neighbor.

    When you Top Balance, you only know one thing, where 100% SOC is at. You do not know the capacity. Only SOC is equal at the Top. When discharged SOC of each cell widens. Without a cell monitor you do not know exactly when to stop. With BB you know where 0% SOC and 0 AH Capacity is at 2.5 vpc. As we charge, all cells have the same capacity.

    If you take away anything remember this. Pb batteries operate best at 100% SOC and should be stored at 100% SOC on a charger. Lithium operates best at Partial State of Charge PSOC. You put a lot of stress on the cells charging them to 100% SOC. No reason to go to 100%. Store them at 60% SOC.

    So get out of the Pb battery box you might be stuck in.
     
  10. KE0WVV

    KE0WVV Ham Member QRZ Page

    Ah yes, thanks. It's so obvious now that I read it in somebody else's words haha! Though, I could keep charging it up knowing that I'm stressing the weak cell more to get better capacity from the pack with the knowledge that I will be replacing that cell eventually. In reality, I would swap it sooner rather than later and repurpose the cell into some other project.

    Ok, so I will be playing around with my cells this weekend, I'll be doing the BB for them. I plan on running a capacity test on the cells before I put them in a pack. Would you recommend I do that from 100% SOC to 0% SOC (10%?), or more like 90% SOC to 10% SOC, where I plan to use them? They are used cells, so I am curious to see what their capacity is compared to original specs.
     
  11. AI5DH

    AI5DH Ham Member

    Run 10/90
     
    KE0WVV likes this.

Share This Page

ad: Sussex-1