LiFePo4 BMS testing

Discussion in 'QRP Corner' started by KC3AJV, Mar 18, 2021.

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  1. KC3AJV

    KC3AJV Ham Member QRZ Page

    I am trying to put together a cheap QRP setup. I am using what I have along with some stuff I purchased. I am not terribly concerned with weight, but it was a consideration.

    I bought a 12Ah LiFePo4 battery off Amazon.

    I am trying to determine if it actually has a BMS internal.

    I bought this charger. I needed a better SLA charger, and this one said it does lithium. I'm not so sure it actually does......

    Because I thought it would be neat, I bought this battery monitor.

    I actually bought two of the batteries. One for my kids' power wheels, and one for radio. I labeled the batteries #1 and #2. Out of the box, both batteries read 12.4V. I connected #1 to the charger set to "Lithium Battery" and the charger displayed 12.8V and 0.0A. I tried batt #2, and got the same results. I let the charger sit like that for a few hours, and there was no change.

    I got to thinking that maybe the batteries were already fully charged (I know they should be higher than 12.4V, but I thought maybe they were bad, or damaged etc). I figured I would try to discharge them, and then charge them up.

    I connected batt #2 to a 14ohm 100W resistor, and let it discharge for about 2 hours. I had not yet connected the all-in-one battery monitor so I connected one DMM for V and another for A. I saw a load of about 750mA and I let the voltage drop down to 10.9V. At that point, I got scared and disconnected the resistor. The battery recovered to 11.098v. I decided to push my luck and reconnected the battery, and I let it drop to 10.800v, got scared again and disconnected - the battery recovered to 11.016v. The TalentCell website states the battery can provide as low as 9V. It doesn't specifically mention a BMS, but talks about PWM.....ok, cool.

    So, my question is, does anyone know of a way I can SAFELY determine if there is in fact a BMS in these batteries?

    I did end up charging batt #2 using the charger, set on "Regular Battery" because the TalenCell website says the batteries can be charged using an SLO charger. If that is good for them....I dunno. Also, I connected the battery monitor.

    Attached Files:

  2. KG7WGX

    KG7WGX XML Subscriber QRZ Page

    Battery specs say Input 14.6 v / 6 A.

    Charger specs say 10A peak for 12V, 7.5A peak for 24V. This looks like the charger can exceed the battery recommended charge rate.

    Charger description says "Normal" position includes LiFePO4 and also says "Lithium" position is only for Polymer batteries, (aka LiPo or Lithium-ion.) Looks like "Lithium" may not be the correct battery type for LiFePO4 chemistry.

    I understand that LiFePO4 batteries don't like "pulse repair charging" or anything like it.

    Finally, since one of the reviews said this,

    It works for charging batteries, my problem is I bought it to charge batteries to back up power for my amature radio station and it causes horrible noise in the HF spectrum. My only current solution is to unplug it while I'm on the air and plug it in when I'm off the air.

    it sure looks like trouble in River City.
  3. KC3AJV

    KC3AJV Ham Member QRZ Page

    :eek::rolleyes: Thanks....I did see that review, and figured I would do the same while on the air.

    Welp, looks like I've got some issue. I'm into another problem now. I tried using the battery to power my 706MkIIg, and it won't do it. As soon as I plug it in/flip the switch on my battery system, it cuts power. I guess there is some kind of battery circuitry internal, and it is killing the power. I know the 706 says 20A in the specs, and this battery says 15A max, but I figured the 20A 706 spec was for full power rf TX. Could it possibly be trying to draw 20A at turn on?
  4. KG7WGX

    KG7WGX XML Subscriber QRZ Page

    It is unlikely the rig draws 20A when you power it on, unless the PTT switch is stuck.

    The LiFePO4 battery probably has charge equalizing / protection circuitry in the battery. Not necessarily a charge controller, just a protective system.

    What does a voltmeter show when you attach the radio and turn it on?

    The battery monitor shows 3.38 Ah. If this is supposed to be the battery capacity, it is only a fraction of what the battery should be able to do.

    Looks like it didn't charge, but that's a guess.
    Last edited: Mar 18, 2021
  5. N1RBD

    N1RBD XML Subscriber QRZ Page

    Did you try contacting the manufacturer? There are a number of Q&As answered on Amazon and they all say it does have a BMS.
    US7IGN likes this.
  6. KC3AJV

    KC3AJV Ham Member QRZ Page

    No, I didn't reach out to the manufacturer yet. That would probably be good thing to do.

    I read the comments, but based on those, I wasn't convinced there was a BMS. I guess there is some kind of circuitry going on.
  7. KC3AJV

    KC3AJV Ham Member QRZ Page

    That was my thought as well, however I turned it on without a mic connected - doesn't mean it's not still stuck internally.

    2.2v and drops down to about a volt.

    Nah, that's this silly monitor. I got this stuff far too hastily. I typically do a better job researching gear, but I am trying to put a QRP station together quickly, and cheaply (recipe for failure) so I didn't read as many reviews or go to manufacturer websites before purcahses.

    This monitor counts UP mAh regardless of charge or discharge. Pretty useless as a capacity monitor. The visual battery indicator is just a user defined set of parameters. I input 11v as the low value and 14v as the high value, so the indicator scales the number of bars up or down based on voltage out from battery. It's my understanding that LiFePo4 batteries have a fairly steady output, so my indicator is gonna tell me, "Good, Good, Good, I'M ABOUT TO DIE". Which is why I set it at 11v. I know the battery can go lower, but I don't want to try to power the 706 with less than 11v.

    So, the 3.38Ah reading is the Ah it has discharged since charging. I forgot to mention that I hooked the battery back up to the 14w load after I installed the monitor, so I could see what it did.

    One thing I will try next is hooking the DMM up to read Amps from battery to radio at turn on, and also try disconnecting the monitor and switch, and try powering the radio straight from the battery.

    Thanks for all the help. I'll report back when I mess with it some more.
  8. KG7WGX

    KG7WGX XML Subscriber QRZ Page

    FYI, most DMM's have a 10A limit for measuring current. You can calculate current by measuring voltage across a known resistance without connecting the DMM in series with the load.

    Also, a fuse is a good idea when connecting "problem" loads to a power source.

    If there is something wrong internally with the battery, it may not like being "messed with"...
  9. K7WFM

    K7WFM Ham Member QRZ Page

    Most, if not all LiFePo BMS require 14.2-14.6 VDC to allow the cells to accept a charge.
  10. KC3AJV

    KC3AJV Ham Member QRZ Page

    I forgot to mention an important detail. With my installed kill switch, I can disconnect power from the battery to the monitor, LED and power out from power pole connectors. With the switch on, the monitor screen is lit.

    As soon as I plug in the radio, without even trying to turn it on, the screen dies. I read the voltage under this situation, and as soon as I plug in the radio is when it drops the voltage.

    FYI, most DMM's have a 10A limit for measuring current. You can calculate current by measuring voltage across a known resistance without connecting the DMM in series with the load.

    Also, a fuse is a good idea when connecting "problem" loads to a power source.

    If there is something wrong internally with the battery, it may not like being "messed with"...

    I hooked the meter up with the inline automobile style fuses. I have 10A fuses there. I knew it was a risk, but I figured if it didn't blow the automobile fuses before it "likely" wouldn't blow the meter fuse. I read about 80mA when I plugged the radio in.

    Something about plugging the radio in kills the battery output.

    The battery worked fine yesterday, but one change I did make was installing the kill switch and green LED. I'll try hooking it back up to the 14ohm resistor and see what happens.

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