Back to the drawing board.

Discussion in 'Gen 2 Prius Main Forum' started by Prius92, Jul 13, 2022.

  1. dolj

    dolj Senior Member

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    Yes, that's what I'd do. However, it might be pretty hard to do in reality unless you were there at the moment you reached the cutoff voltage as the voltage will have bounced back (a reverse surface charge - would that be surface discharge??? - happens) so if you check the voltage after some minutes to some hours later you might see the voltage up a lot higher than your cutoff voltage. You might even start to think the discharger is not working properly. LOL. Being there will save time idling i. e. where no charging or discharging is taking place.
    I wouldn't try to put these two things together, it is just a waste. Manufacturers have way more sophisticated equipment where they can accurately measure capacity and mAh. Way more than a simple hobby charger.
    What you're observing is surface charge bleeding off. I don't know all the ins and outs of why it happens, but just about every battery chemistry does this. I don't see any benefit of waiting any amount of time between charge/discharge or vice versa. A known technique to bleed off surface charge is to apply a load for 15 - 30 seconds then wait a couple of minutes for the voltage to stabilize, and this is what will happen if you start a charge after a discharge or a discharge straight after a charge.
     
  2. Prius92

    Prius92 Member

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    It sucks they don't make a programmable hobby charger where you could set the current charge/discharge to lower to another amount once it hits a certain voltage, but I'm sure one made that way is pretty pricey.

    Also..while this is a bit in the future, what do you recommend for load balancing? I see some people using a load like a headlight bulb to try and take all the modules down to around 7.2v, and I also see people switching the modules around so all the negative and positive terminal posts are together, wiring them up, and letting it sit a day or two. Supposedly the excess voltages of modules "drain out" into the lower voltage modules back and forth until all are "stabilized".

    Also since the computer only sees "blocks" which are two modules together..is it better to stick a slightly higher capacity module with the lowest capacity module to "hide" it, or try to match modules of roughly the same capacity in each pack, for example one pack of a 3200 and 3250 module, another pack of a 2900 and 2925 module, etc?
     
  3. dolj

    dolj Senior Member

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    I would not bother with any of it. If you've done your job properly then the resting voltages will be the same or within 0.1 or 0.2 V of each other. If not, you'd ideally find replacement modules that met the requirement. In any case, once you install the battery in the car and start using it they will settle at their natural voltages.
    I have no opinion of this method. Again as above, if you've done the job properly it becomes a bit academic.
     
    #23 dolj, Jul 16, 2022
    Last edited: Jul 16, 2022
  4. Prius92

    Prius92 Member

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    I received the new discharger today and holy heck..it's 5 times faster because it's a constant current.
    Oddly enough..under the NIMH section in the manual...for a 6S cell it says the min discharge current is 27 Amps.
    That seems a bit high, don't it?
     
  5. Prius92

    Prius92 Member

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    In one hour the new SkyRC discharger has drained 2,000Mah off a module, while the hobby charger is only at about 400 and 387 (two channel capacity).
    Only downside is that it runs off the module, so I doubt it could be left on overnight.
     
  6. alftoy

    alftoy Active Member

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    Select cutoff voltage and discharger will cutoff?
     

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  7. Prius92

    Prius92 Member

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    Ya, learned that the hard way. Since the discharger is powered by the battery, it goes blank after reaching the cutoff voltage.
    I'm going to wait until the hobby discharger finishes two modules then move everything near my PC so I can install the software. I am not sure if the discharger will continue to run if connected to the USB port or not.
    It is a beast though and I recommend it to speed things up, still not sure about why the manual says the min discharge current for a 6S NIMH module is 27A, that seems like it would short out the battery at that high of load.
    But from reading online, supposedly each module can discharge at a max rate of 10C (65A), is that true?
     
  8. mr_guy_mann

    mr_guy_mann Senior Member

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    That discharger can log data to a laptop when it's connected. (I don't recall if it turned off afterwards, but I had the log so I didn't care)

    The prius battery modules can handle very high discharge and charge rates for a very short period of time within the "normal" 40-80% SOC range. I have seen 100A charge and 170A discharge for some seconds during hard driving.

    That said, the longer you apply more current, the more heat you're going to get in the modules. If you want to hit some modules with lots of amps at the extreme upper or lower voltage limits, go for it and let us know how it goes.

    I had (and took) lots of time to do my pack 2 years ago, 1.5-2A charge, 1.5-2A discharge. Pack still works, so...

    Posted via the PriusChat mobile app.
     
  9. Prius92

    Prius92 Member

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    Curious..is there a datasheet on these cells? You'd think there would be but I can't find one online.
     
  10. dolj

    dolj Senior Member

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    +1. Just what I was going to highlight as well.
     
  11. Prius92

    Prius92 Member

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    Wouldn't that be 170 amps across all 28 modules tho? Or about 6A per module.
     
  12. Prius92

    Prius92 Member

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    Here's a screen shot of a spare module I discharged. You can see how quickly the voltage plummets after it hits around 6.6 or so volts.
    [​IMG]

    While I cranked the discharge up to about 4 amps, when it was done the module was still quite cool to the touch.
    This discharger also only goes down to about 5.4 volts, so one would need a hobby charger to go below that on one module, or with two modules put together, but as other posters mentioned, you don't want to discharge as fast anyway below the 6V cutoff voltage. So this is a great tool for quickly draining up to 3 modules at once down to the cutoff, or 18V in this case, as the discharger will handle up to 21S, or 3 Prius modules. So it's a time saver for sure.
    But the only downside is it has to be connected to a PC the entire time, or you have to babysit it until it's down.
    As you can see the module discharged 3.3Ah off the module in 49 minutes. This can take around 7-8 hours with the hobby charger.
     
    #32 Prius92, Jul 19, 2022
    Last edited: Jul 19, 2022
  13. dolj

    dolj Senior Member

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    Nope, connected serially, the 170A current goes across each module.
     
  14. Prius92

    Prius92 Member

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    How can each module take 100A of charge if charging mine with only 3 amps can cause them to swell? Or is the 100A only for a couple minutes?
     
  15. dolj

    dolj Senior Member

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    It is being charged at that rate in the 40% to 80% band and generally only in short bursts of high current.
     
  16. Prius92

    Prius92 Member

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    Also I wanted to ask, it is my understanding that once you go beyond the low and high voltage cutoff points, the battery tends to not like higher amounts of current for either discharging or charging.
    Now on the site for the prolong discharger..it shows swapping a series of light bulbs until the modules are down to 3 volts.Using a 25W bulb between 5 and 3 volts. But I can't imagine a 25W lightbulb producing under half an amp at 5 to 3 volts.
     
  17. dolj

    dolj Senior Member

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    I'm not so sure the battery doesn't like it so much as it is a safer method.
    A note about the HA lightbulb discharger, in order to use the standard 110 V incandescent bulbs on voltage up to 245 VDC, they are wired in series which splits the voltage with half the line voltage going over each bulb. Of course, the current is the same over both bulbs.

    The second thing is that the voltage is the pack voltage so the 4.8 V per module is 134 V and the 3 V per module is 84 V.

    There is no need to imagine what the current might be, simple maths will tell you what's what (pun intended!).

    A Watt is a unit of electrical power (P) and is defined as 1 W = 1 V x 1 A. The formula used to calculate Watts is P = V x I or simply P = VI

    The bulbs are rated at 25 W with 110 V applied over them so you can work out the theoretical resistance in Ohms (Ω) using the formula P = VI adjusted for the known variables P and V to calculate I. It, therefore, becomes P ÷ V = I

    So 25 W ÷ 110 V = 0.227 A

    From that you can calculate the resistance of the 25 W bulb using Ohm's law V ÷ I = R

    So that calculation is:

    110 V ÷ 0.227 A = 484.58 Ω

    Again using Ohm's law (rearranged for unknown current to V ÷ R = I) you can calculate the current for the respective 134 V and 84 V

    134 ÷ 484.58 = 0.277 A or 277 mA
    84 ÷ 484.58 = 0.173 A or 173 mA.

    It is not so straightforward in reality. Due to the voltage decreasing, the filament gets cooler and because of this the resistance of the bulb also decreases. I'm sure there is a formula to work out the change in resistance however, I don't know what that is right now. What I do know is the actual current measured at 134 to 84 volts which is a range of 220 to 190 mA. This would suggest the resistance is a little higher than theoretical. Or maybe my equipment is just not accurate enough at these low currents.
     
    #37 dolj, Jul 19, 2022
    Last edited: Jul 19, 2022
  18. Prius92

    Prius92 Member

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    So what is generally considered to be the pass/fail in regards to discharge capacity? I read somewhere 2500MAh but not sure if that is true or not.

    Here is another module I discharged, notice it maintains a nice smooth downward trend over the discharging cycle
    [​IMG]
     
  19. Prius92

    Prius92 Member

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    Well this is interesting..I connected two modules together to discharge faster, and set the discharger to 8v, 4v per module.
    But when it finished, one module was at 5v, and one was at 7v.
    It seems doing it this way does not evenly withdraw from each module.
    Not only that..but when done discharging, the modules seem to instantly gain a volt or two within minutes.
    So I am curious how grid discharging works, because if you took the entire pack down to say 84V, how on earth would all the modules be within the same voltage?
     
  20. mr_guy_mann

    mr_guy_mann Senior Member

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    Chemistry is a bit sloppy by nature, so having some "bounceback" or cell recovery after load is removed is normal.

    Grid charging tries to equalize cell state of charge during the "top charging" phase where the pack is (gently) overcharged. AFAIK the discharge phase is where you try to "improve capacity".

    I'm no expert, but really, it's similar to what you do with hobby chargers on individual modules (you can't monitor and evaluate individual cells, just the modulel.) Likewise, all you can do when grid charging is look at battery ecu data afterwards to see how things went.

    Posted via the PriusChat mobile app.
     
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