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Lithium battery BMS Question with Flexmax80

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  • #46
    I agree. Before bypassing a BMS you have to be very clear about the distinction between a BMS failure and a BMS responding to a battery failure.
    SunnyBoy 3000 US, 18 BP Solar 175B panels.

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    • #47
      BMSs can't prevent or put out a fire but it can give you early warning that a cell is getting overcharged or over discharged.
      9 kW solar. Driving EVs since 2012

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      • #48
        Originally posted by Ampster View Post
        BMSs can't prevent or put out a fire but it can give you early warning that a cell is getting overcharged or over discharged.
        Sorry BMS are the #1 cause of battery failures and fire.

        MSEE, PE

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        • #49
          Originally posted by Sunking View Post
          Sorry BMS are the #1 cause of battery failures and fire.
          Haha. So you are saying a BMS is not required? If you were a newbie, one of the moderators would ban you for saying that.
          I also know that is what Jack Rickard has been saying for years.He has changed his tune since you last saw him. He is now selling a BMS for used Tesla modules.

          I am looking to better understand the specific cause and effect relationship that you speak of. Can you give us an example of how a BMS has caused a battery to fail and consequently catch fire? That might be useful for the 400 lurkers that frequent this forum and never post.

          The great fireworks show of the guys that bought a Disney cart that was powered by Tesla Batteries did not have a BMS. That fire, as far as I can tell, was caused by overcharging the modules and not paying attention to the voltage of the charger. I am looking for an example of a BMS causing a fire. I do not consider the case of a BMS failing to shut off a high voltage cutoff as an excuse for causing a fire, when there was no other high voltage cutoff in place. In that case it was the high voltage that caused the fire. The failure of the BMS was contributory but not the main cause. Maybe that is all you are saying. .

          I think I know the answer but your experience will carry a lot more weight, especially for the lurkers here who rely on our posts but never participate. I am also not looking to engage in a debate about the benefits of bottom balancing.
          Last edited by Ampster; 03-06-2019, 12:25 AM.
          9 kW solar. Driving EVs since 2012

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          • #50
            Originally posted by Ampster View Post
            Haha. So you are saying a BMS is not required? If you were a newbie, one of the moderators would ban you for saying that.
            Nope I am not saying a BMS is not required exclusively. Not required for all applications. Not worried about being banned, the MODs know where I stand.

            Originally posted by Ampster View Post
            I also know that is what Jack Rickard has been saying for years.He has changed his tune since you last saw him. He is now selling a BMS for used Tesla modules.
            Jack has not changed his tune, he has sold BMS since he went into biz. That is called hedging and playing both sides to make MONEY This is where application comes into play. If you are going to use the most dangerous lithium cells like Tesla does using NCA cells at 100S give or take a few cells, you had better be using a BMS. Those cells are so dangerous Telsa has to use thermal management on top of energy management.

            Originally posted by Ampster View Post
            am looking to better understand the specific cause and effect relationship that you speak of. Can you give us an example of how a BMS has caused a battery to fail and consequently catch fire? That might be useful for the 400 lurkers that frequent this forum and never post.
            Not so hard to understand, All the BMS made for DIY consumers use Vampire aka Bleeder Boards that turn on when a cell is fully charged and stay on until it bleeds the voltage down. Very simple dumb devices consisting of a voltage comparator, FET, and power resistors. Very prone to fail and when they do fail, fail Shorted in the ON position which will discharge the cell. As soon as the cell is flat, it is destroyed, and when you use power drives the cell into reverse polarity and thermal runaway. If not detected quickly can result in fire.

            It comes down to application. If you are running 4S or 8S using LFP cells, then a BMS is not required. If you bottom balance, most all of the risk are eliminated. All the DIY BMS are designed using the same flawed principle. They do not work unless you intend to fully charge a battery. Same logic used for Pb batteries. Last thing you want to do is fully charge a lithium battery. On say a 4S Bottom Balanced pack you Float charge to 13.8 volts, and disconnect @ 12 volts. On a BB pack would be impossible to ever over discharge a battery. If there were a bad cell in a 4S or 8S configuration you are going to know it by monitoring the pack voltage. If three cells were at say 3.2 volts, and you see less than 12, you know something is wrong, bad wrong. In say 100S like an EV you could have several flat cells and could not tell it from pack voltage. Only way to do that is with a BMS that actually monitors cell voltages.

            Keep in mind EV and commercial manufactures do things DIY cannot do. To start the do not use Top Balance or Vampire Boards. They use Active Mid Balance. If there is a high and low cell, they take energy away from the high cell and transfer it to the low cell. They never allow the user to ever fully charge a battery, otherwise they would go bankrupt with warranty claims when the battery dies in a year or two. Additionally the lower their liability exposure by not fully charging a battery. Lastly they match cell capacities and use a lot higher quality cells than you can get. They match cell capacities and resistance and Middle Balance so in effect the BMS they do use rarely does anything.

            So for 95% of DIY, yes use a BMS because most likely you do not know what you are doing to begin with. Manufactures really like that because they know you will destroy your battery a lot faster and have to buy a replacement sooner rather than later. If you know what you are doing, you can double battery life an dkeep some of that money in your pocket. That is why some of us Bottom Balance and mimic what EV manfactures do to extend battery life and minimize risk.
            MSEE, PE

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