sunnuntai 20. tammikuuta 2013

How To Bottom Balance

Warning! This article is about LiFePO4 cells. Some of it may or may not apply to other chemistries.

My earlier post on bottom balancing was long and mainly about why to do it. So I figured I might do a little step by step for those who just want to get it done without delving into the philosophy behind.
  1. Drain all of your cells down to 2.75 volts. Note that the cells will bounce back after you disconnect the load. Repeat the discharge until each cell standing alone stays at 2.75 V.
  2. Connect your cells to each other. Do not connect anything else between the cells but the bars, straps or fuses connecting cells to each other. Do not install BMS. Do not connect any other wires. Strictly cell interconnects only.
  3. Charge the battery pack to a voltage of 3.5 to 3.55 volts times cell count. Examples: 25 cells = 87.6 volts = 3.50 per cell, 33 cells = 116.8 volts = 3.54 per cell. Use CC/CV profile which is the default in many LiFePO4 chargers. End charge when total voltage is reached.
  4. Always discharge the pack as a whole. Always charge the pack as a whole, the same way as described above. Never put any load on only a part of the pack, no matter how small. Never install cell level monitoring or BMS which put any load on individual cells.
If you do this right you shouldn't need to repeat it. If you want, you can periodically drain your pack and measure the voltages. They should all be about the same. If you notice irregularities they are most likely caused by bad cells. Replace bad cells and repeat the bottom balancing for the whole pack.

Bottom balancing is not negligence. Bottom balancing is utmost cell care. You drain them one by one, manually or with a device like Revolectrix PowerLab. While you do that you will spot bad cells quite surely. When you charge them you end the charge well before overcharge and the charge is controlled by the one device which does it best, the charger. As an extra precaution you may also consider adding a meter which disengages the main contactor if pack voltage is not within limits.

On discharge you set your controller to a low voltage limit of over 2 volts per cell (for example, 60 V for 25 cells, 80 V for 33 cells, perhaps even lower). This will prevent any cell going too low, since they all empty at the same time. Since they're all empty they can't harm each other.

Fires happen when cells get overcharged. Bottom balancing doesn't cause overcharging.

Disclaimer: All of my battery ramblings are based on my own experience and Jack Rickard's original work on the subject. They are applicable to CALB SE- and CA-series cells. Other cells and chemistries may at least require different voltages. I take no responsibility for any problems or damage caused by anyone.

9 kommenttia:

  1. I have one of the Powerlab 8's on order for the same reason. It is also good that you mentioned not putting any extra load on any part of the pack.

    So for example trying to get 12 volt supply from your pack is only possible with a proper DC-DC converter.

    1. Yes, and make sure you get one that is isolated. The cheap ones are not. They have only three wires with common minus which results in pack negative in your chassis. Very bad. An auxiliary battery for the 12 volts system is probably needed as well. I'll probably have a lead acid aux battery simply because you can just charge it with the DC/DC converter and not worry about anything.

      Here's a converter I'm considering:

  2. "Always discharge the pack as a whole. Always charge the pack as a whole, the same way as described above. Never put any load on only a part of the pack, no matter how small. Never install cell level monitoring or BMS." I have an rc charger that can take up to 8 cells. I'm using headway 8ah, so does this mean I can't bottom balance if I charge in an 8s2p configuration and discharge in a 16s1p configuration?

    1. You can bottom balance, but charging the pack in anything but the 16s1p you discharge in will ruin the balancing. You'll need to charge the pack as a whole to keep it balanced.

  3. Two comments (and why is your Wordpress in Finnish, BTW? :-) --

    1 - you never use the words "serial", "parallel" or "disconnected, one at a time" anywhere in either of these posts, and they seem important

    2 - you say "nothing except other batteries"... but I assume that you do *not* mean "voltmeters", "chargers" or "test loads" -- since you mean to be exhaustive here, you might want to edit the relevant terms in where appropriate -- especially since this is the number one google response for "bottom balancing". :-)

    1. Lots of things are important, but I try to keep some of a limit on things I cover. I don't really like packs with cells in parallel, because it multiplies the number of connections. And if something's going to fail it will probably be the connections.

      Yes, I suppose if you're going to charge the pack and use it, you might need to connect it to a charger or perhaps even the motor controller or inverter. I don't think it has to be spelled out as such.

      Actually you can discharge the cells to 2.75 V either one by one or by connecting them all in parallel. It can be a little easier and less watching paint dry if you use the parallel method, but on the other hand you won't get to know your cells and may miss a bad cell.

      Something I've learned since writing these posts is that once you have the cells bottom balanced at 2.75V, you can leave them sit for a while and monitor their voltage. If a cell doesn't stay at the voltage or creep up a litte, but instead goes down in voltage, it probably has an internal soft short and needs to be discarded.

  4. (Correction: you do say "pack as a whole" in item 4, but that seems targeted at *use* of the pack; step one does not say whether to discharge cells one at a time.)

    1. You can discharge the cells to 2.75 volts one by one or in parallel when bottom balancing. Both works. Just make sure that you disconnect the parallel cells when you're done and let them rest disconnected. Only then you can be sure they really did get to correct voltage and don't have soft shorts.

  5. Nice simple post. Thanks.

    A lot of people over complicate this sort of thing.