Zener diode based LiFePO4 load dump

oil pan 4oil pan 4 making green energy payPosts: 328Solar Expert
Not being able to find a LiFePO4 charge controler has lead me here.
Only problem is I cant find any 14.x rated zener diodes. Seems like everything over 10 volts is rated in hole volts. At that point it becomes 14 or 15 volts.
But I found lots of 1 to 10 watt rated 15v zener diodes, there are boat loads of them for cheap.

15 is a little high for charging serries of 4 LiFePO4 cells.
But if I use a blocking diode between the solar cells and the battery that blocking diode will naturally drop something like 0.2 or 0.3. Then with the zener diode draining off anything over 15 volts the batteries should see a max of 14.8 volts which is the proper charging voltage for LiFePO4.

My solar panels are real small I have only been able to measure about 10 watts coming off them going into 12v lead acid setup, manufacturer propaganda says they are "rated" for about 14 watts.


  • BB.BB. Just some guy Posts: 23,746Super Moderators admin
    Re: Zener diode based LiFePO4 load dump

    I would not suggest diodes as a "dump load"... They are just too "non-linear" and the chances of overheating (xx amps * 14.x volts = lots of watts) is high. Also, having diodes failed "shorted" is not unusual either (need fusing/breaker--but then opens on something going wrong, and your batteries risk damage).

    If there was a PWM based controller that could dump to a resistor bank of some sort--That would be interesting... Many of the Morningstar (Xantrex? and probably other) controllers can be configured as dump loads--I would look at how they are configured/operate to see if they are compatible with your needs.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Joe_BJoe_B New User Posts: 318Solar Expert ✭✭✭✭✭
    Re: Zener diode based LiFePO4 load dump

    Zener diodes are not rated that way because a number of factors effect the zener knee, the primary one being temperature. since you are using this device as a shunt regulator, the temperature will rise as the diode goess into conduction. I am by no means an expert on LiFe battery charging but I dont think a shunt regulator is the way to go.

  • oil pan 4oil pan 4 making green energy pay Posts: 328Solar Expert
    Re: Zener diode based LiFePO4 load dump

    I contacted morning star and they did not recomend any of their charge controlers be used with lithium.

  • waynefromnscanadawaynefromnscanada New User Posts: 2,997Solar Expert ✭✭✭
    Re: Zener diode based LiFePO4 load dump

    Lithium batteries are tricky, and if not handled (electrically) and charged properly - - downright dangerous.
  • oil pan 4oil pan 4 making green energy pay Posts: 328Solar Expert
    Re: Zener diode based LiFePO4 load dump

    I have been using them for a while now and so far so good.
    I started using them because they deliver the high amp draw I need for about 1/10 the weight of lead acid and take up less than half the space. Reserve capacity is a lot less than lead acid but I don't need very much capacity.
  • RCinFLARCinFLA New User Posts: 1,280Solar Expert
    Re: Zener diode based LiFePO4 load dump

    First order issue is LiFePO4 should have charge terminated when they reach about 3.65 vdc per cell (14.6 vdc for four) and the charge current then drops off to about 1 or 2% of their rated C as charge current. A simple continously voltage shunt would overcharge the battery as LiFePO4 don't like continuous trickle charging. The fact that you are doing it with solar is helpful but if the battery is not discharged some then day after day charge topping will degrade batteries.

    You can get electronic shunt regulators that will have a sharp defined limit voltage. You can also get BMS controller for small cells. Just do a Google search and you should find several.
  • john pjohn p New User Posts: 814Solar Expert
    Re: Zener diode based LiFePO4 load dump

    Lithium ion phosphate can be safely charged to 4.2v with no problems for them but plenty for a inverter connected . As most 12v inverters wont work over about 15.2 v.
    The useful maximum that they should be charged to is 3.8v . There is very little extra power availiable from them above that point.
    Any good PWM charger like the old Steca ones can be set to 14.8. LEAD ACID AND NO EQUALIZE CHARGE
    The most important part is the cells are NEVER ..NOT EVEN ONCE not discharged below 2.2 v but its much safer to have some automatic low voltage switch off set to 2.4 v . For 4 cells thats still a low 9.6 v .Most inverters wont opperate below 10v anyway.
    But the low voltage switch is necessary if any other devices are connected to the batteries.
  • oil pan 4oil pan 4 making green energy pay Posts: 328Solar Expert
    Re: Zener diode based LiFePO4 load dump

    I find the right mix of parts to get the right voltage on my LiFePO4 batteries.
    I have solar panels set up in a 2x2 conviguration that put out 15.5 volts at around 0.8 amps max. I added a simple blocking diode in with the panels because I figured it would drop panel out put voltage by around half a volt.
    So far it appears to work, after sitting in the sun most of the day, the charge leveled off at 14.79 volts and stayed there for 2 hour with full sun.

    I have a deep cycle lead acid charge controller coming in that should cut power once it hits 14.8 but it has not shown up yet.

    When its cold out they don't seem to be able to get a full charge off from a standard 14.4v vehicle charging system.
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