1. Attachments are working again! Check out this thread for more details and to report any other bugs.

Recondition HV Battery: is it really just a simple equalize charge at ~1.5V per cell??

Discussion in 'Gen 3 Prius Technical Discussion' started by PuckSR, Jun 21, 2021.

  1. PuckSR

    PuckSR Junior Member

    Joined:
    Sep 18, 2014
    35
    9
    0
    Vehicle:
    2011 Prius
    Model:
    Two
    I'm recently having trouble with my battery, so I started looking at all of the rather exotic devices people are using to treat their batteries. It all looks like a pretty simple 1.5V equalize with a deep discharge to .7V or something similar.

    I dont have a lot of experience with NiMH cells, but I work with industrial UPS and have used a number of different batteries.

    If I understand it correctly, all I really need to do is hook up a 250Vdc constant current source and charge these up until the monitored current is close to 0? Is that the long and short of it?
     
  2. PriusCamper

    PriusCamper Senior Member

    Joined:
    Mar 3, 2012
    10,767
    4,370
    0
    Location:
    Pacific Northwest, USA
    Vehicle:
    2007 Prius
    Model:
    Two
    I'm pretty sure this is what they call balancing from below, which is what you do with Lithium-based batteries...

    Close... The idea is that NiMH self discharge voltage through heat when they're charged to max voltage. So you balance them from above by adding voltage that fully charged cells can dissipate excess voltage as heat while they weight for other cells of lower voltage to catch up.

    The trick is NiMH can be easily ruined by overcharging, so you have to trickle charge your 250v down around a 1/4 to 1/3 of an amp to prevent from causing damage. You also need to make sure you're using a cooling system to take the heat away so nothing gets too hot.

    More info: Build Hybrid Battery Maintenance Gear For Under $100 | PriusChat
     
  3. PuckSR

    PuckSR Junior Member

    Joined:
    Sep 18, 2014
    35
    9
    0
    Vehicle:
    2011 Prius
    Model:
    Two
    I think I confused you with my statement. This isn't describing bottom/vs top balancing. I'm describing a few things simultaneously with my statement.

    1. From what I've read, people take the batteries through charge/discharge cycles.(Yes/No?)
    2. Safe voltage levels: all batteries have a safe range for charging/discharging. I always discuss this per cell rather than per battery, because in my line of work, the number of cells/jars changes a lot. It is simple math. A prius has 28 jars with 6 cells per jar. That means 168 cells.
    So, I am saying that you would charge at ~250Vdc (and discharge down to ~120Vdc if performing a cycle) (Yes/No?)
    3. Different batteries have different tolerance for charge current. Lead-acid batteries do fine with 100A of current. Lithium-Ion can charge at high current, but prefers a regulated current. From what I've seen, most people are doing 300mA, which is also convenient because a lot of LED drivers operate at 300-350mA and 250Vdc(I think I got this one right)

    Equalize/Float are terms for the charging voltage for batteries. You obviously don't want to put 600V on a 250V battery. The "Equalize" voltage is the level for the charger when attempting to equalize cells.
    NiMH
    Float=1.2V
    Equalize=1.4-1.6V according to Wikipedia

    From your response and what I saw on that thread you linked is that a meanwell 250Vdc 300mA constant current driver will charge these pretty well. The only problem is that you don't want to leave it on "equalize", as NiMH doesn't like long-term equalizing. Lead-acid are far more tolerant of this behavior. Your 12V battery in your car is normally fed by an alternator with 15Vdc.

    Top vs bottom charging
    I had to look that term up, as I wasn't familiar with it, but it has more to do with charging/discharging cycling of individual cells vs the whole battery. We are both describing top balancing. Bottom balancing would require me to discharge all of the cells and then install them when they had nearly equal voltages.

    Heat
    That makes sense and honestly I hadn't thought about it before. I normally don't even get out the cellcorder unless the batteries have been equalizing for 3-5 days, though that is lead-acid batteries. They have a fairly high internal resistance
    From what I know, NiMH have a low internal resistance.
     
  4. PriusCamper

    PriusCamper Senior Member

    Joined:
    Mar 3, 2012
    10,767
    4,370
    0
    Location:
    Pacific Northwest, USA
    Vehicle:
    2007 Prius
    Model:
    Two
    The other aspect is the quality of your equipment... For example my buddy who's been rebuilding packs for many years and ships all over the world just replaced his worn out $30K rig from 8+ years ago with a new $24K rig that can manage as many as 80 modules/jars simultaneously and the equipment can recondition without ever having to go below 6v per module/jar and his modules are always the best ones in the packs I rebuild.

    For us poor folks who use a light bulb discharge and trickle chargers we have to spend way more time discharging and charging. I always get my best results when I discharge down to 0.6v per module, but Hybrid Automotive recently boosted those numbers because modules nearing their end of life don't handle that so well, especially the sustained heat from the recharging part, which is when having a thermal camera helps find cells that are running too hot.

    Anyways, here's the latest chart for deep discharging from hybrid Automotive's website: Light Bulb Discharger User Guide | Automated Prolong Battery Discharger | Hybrid Automotive

    Screenshot from 2021-06-21 17-14-55.png
     
  5. Grit

    Grit Senior Member

    Joined:
    Aug 22, 2017
    6,103
    4,032
    1
    Location:
    Wilkes Land
    Vehicle:
    2012 Prius
    Model:
    Four
    like that one feller who said the excess voltage would just bleed off has heat while the other cells catch up voltage charging.

    another feller his brother did the same thing and the batteries in his pack bulged up.
     
  6. PuckSR

    PuckSR Junior Member

    Joined:
    Sep 18, 2014
    35
    9
    0
    Vehicle:
    2011 Prius
    Model:
    Two
    That wouldn't be excess voltage.
    That would be excess energy.
     
  7. PuckSR

    PuckSR Junior Member

    Joined:
    Sep 18, 2014
    35
    9
    0
    Vehicle:
    2011 Prius
    Model:
    Two
    Actually, I said that backwards. NiMH have a very high resistance