LiFePO4 and How much current can I safely draw?

Discussion in 'General Technical Questions and Answers' started by W3KW, Jul 6, 2021.

ad: L-HROutlet
ad: l-rl
ad: Left-3
ad: Left-2
ad: L-Geochron
ad: MessiPaoloni-1
ad: abrind-2
ad: HRDLLC-2
ad: L-MFJ
  1. W3KW

    W3KW Platinum Subscriber Platinum Subscriber QRZ Page

    Since my station is in shambles, I’ve been using all my mobile and QRP gear. Today, I broke out my FT-891 and a 20Ah LiFEPO4 battery which reads “20A max continuous discharge.” I used a Turnigy 130A load meter and noted the following current draw at 13.36v:

    100w = 13.9 amps
    50w = 10.9
    25w = 8
    5w = 5

    So, what’s interesting is that the manual for the FT-891 indicates a draw of 23 amps at full output. So, something is amiss. I switched to my Powerwerx SS-30DV power supply for more analysis. Alas, same power draw indicated from there. I found my Drake W-4 power meter and with the FT-891 set at 100 watts out, I’m getting just over 100 watts on CW. Hmmm. Conclusion: I think my Turnigy 130A is faulty or not reading correctly.

    What is a simple and safe way to determine how much current I’m drawing on the FT-891? Keep in mind, my shack is completely dismantled and in boxes in the garage due to a flood restoration project. I might be able to find a multimeter.

    Current station (Into a 43 foot telescoping pole using end fed and 9:1 balan)


    current draw indicated on receive:


    Current draw indicated at 100 watts output using the Powerwerx power supply:


    I’m a hobbyist, not an EE. What am I missing?

    Thanks in advance.

  2. W3KW

    W3KW Platinum Subscriber Platinum Subscriber QRZ Page

    Update: Well the set up works. Just had a 59 report from Val, IU3BTY, in Italy on 14.239. Nice to know I’m getting out and the bands will support a compromise antenna!
    IU3BTY likes this.
  3. KB0MNM

    KB0MNM Ham Member QRZ Page

    An engineer might say something about source and load resistances, plus note that the ammeter adds some- as does the cable and fuse. While the battery may be capable of supplying more current than the 14 amps seen at the radio, under the conditions of load- the radio seems to draw less current and manage to produce 100W RF output. While it may seem to be distressing that the radio is unable to draw 'full output' current, the 23 Amp draw is the theoretical maximum where the radio would hopefully be limiting current. The more distressing fact is that the battery will not take very long to discharge below the threshold at which your radio shuts down, unless it is being charged during use. You may wish to keep the transmit output at 5 watts unless more is needed, and explain early on in any conversation that your battery power is limited. Of course, simple signal reports result in less 'duty cycle', conserving battery power. A typical battery charger for older batteries will recharge at a rate of 10% of battery capacity, thus if you have 2 amperes going into a 20Ah battery- it may not heat up in such a way as to lead to battery destruction. Lithium batteries should be charged only with chargers designed for the purpose- modern battery chargers typically monitor the temperature to prevent failure. Some Lithium battery technologies have been banned from commercial air flights due to the danger of flame if one overheats in a carton. As to the meter, I might be more inclined to think that the switching power supply is rated for 30 amperes peak, not continuous.
    Last edited: Jul 7, 2021
    W3KW likes this.
  4. WB2LBV

    WB2LBV Subscriber QRZ Page

    So for an output of 100W the radio is drawing 185W from the supply. Seems about right, that's a bit better than 50% efficiency. At 23A and the same voltage it would be drawing 307W which is less than 30% efficient.
    The maximum current draw numbers in the specs are just that, maximums. In practical use you will never see those and it will be much less.
    AK5B, W3KW and KB0MNM like this.
  5. K7JEM

    K7JEM Ham Member QRZ Page


    If you look at these results, they are close to what you are seeing, if you account for the fact that you seem to have higher voltage than what is being used in the video.
    W3KW likes this.
  6. W3KW

    W3KW Platinum Subscriber Platinum Subscriber QRZ Page

    YouTube! I didn’t think to search there. Thanks for the input fellas. I think that 20Ah LiFEPO4 is fine at full output. I’m really only interested in about 40 watts or so though. The way I operate, I could get tons of life out of it. Mystery solved.
  7. W4EAE

    W4EAE XML Subscriber QRZ Page

    One thing to know about the Bionenno battery is that it will not let you draw too much. The battery management system shuts the battery down if the current stays too high for too long.

    I found that out when it happened. I had an HF rig and a VHF rig connected to a Bioenno. I forgot to turn off APRS on the VHF rig, and it sent a packet while I was transmitting on the HF rig. Everything immediately went down. Once I realised what had happened, I reset the PCM and the battery has functioned perfectly ever since.
    W3KW likes this.
  8. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    That battery has a 144WH/12AH rating per their literature, based on 1/C and indeed Bienno builds in protection mechanisms to prevent damage and fires/explosions if the max rated current is exceeded.

    If your rig actually draws 15A at full power, the battery "should" last about 48 minutes of "transmitting" time at steady-state full power (FM, RTTY, etc.) but SSB and CW have much lower duty cycles. SSB without heavy processing is usually only about 25% D/C so the battery might last about 4x longer at 100W PEP with voice modulation; and of course nobody transmits all the time. If you transmit 25% of the time and "listen" 75% of the time, that would extend it again.

    Probably last a whole day of intermittent use.

    I have a Bienno similar to that one which I've used for portable (SOTA) operations with my little 20W PEP output Xiegu G90 and the battery has never run down even if I used it a few times before recharging it. That rig only draws about 4.5A transmitting at full power and about 1A receiving with the volume turned up.

    Their discharge characteristic is spooky...unlike a lead-acid battery, their terminal voltage doesn't observably drop with discharge until they're about "dead." Stays at 12V right until the very end, then it's gone.:p Monitoring the terminal voltage isn't much of a clue about its charge state.
    AK5B and W3KW like this.
  9. WA2EIO

    WA2EIO XML Subscriber QRZ Page

    Yes, measuring battery voltage does not really give you any information on remaining 'life' in this type of battery chemistry.

    The in-line meter also measures Ah used, so you can try watching that, but once the battery is disconnected from the meter, it resets itself to zero. That means you would have to note the Ah used (discharge) after first use, then and add it to the Ah discharge after the next radio operation is completed. It is definitely not convenient, and not very accurate, but it may be a way to give some indication of the battery's remaining 'life' before it needs to be re-charged.
  10. K8JD

    K8JD Ham Member QRZ Page

    Is the current you measured in The CW mode key down or on SSB saying "Test, test, hi there" ?????
    Try the FM node with ptt ?
    Currents you stated are nothing like my Rigs at 100 W CW RF output levels. They are all pretty close to 20 amps.

Share This Page