Saturday, February 11, 2012

Faster Initial Charging and CellLog8

Episode 02 of my video blog:


While charging the first cell, it quickly became clear that the 5A output of the bench power supply wasn't enough to get charging done in a sensible amount of time.  The cells appear to be delivered with 50% charge and so I have to put another 200Ah into them.  At 5A, this would take 40 hours at least.

I'd also had some concern expressed by Jack Rickard about such a slow charge rate being effectively a trickle charge that might not even be able to push the cells to the top and could even overcharge them in some way by trying for too long.

There was some conflict in advice, in that Jack has been using these cells for a couple of years and has never done an initial charge on them and never charges them to 4.00V.  His method is to stop charging at 3.60-3.65V (but often charging the cells in series), safely under charging them, with no active BMS present (or required).  I take his advice seriously, as he's tested lots of cells in his lab and blown up a few when over charging them on the bench!

However, the Winston Battery operator's manual is quite clear (and insistent) that this initial charge to the full 4.00V is done before the first discharge.  GWL echoed this in their user FAQs.  So I contacted the Technical Manager at GWL for advice on whether I should do the initial full charge, whether the 5A bench PSUs are suitable and whether any harm comes of doing the initial charge too slowly.  The Winston Battery manual recommends a charge rate of between 0.1CA and 0.5CA, but this is out of my reach as it means a PSU that can deliver between 40A and 200A!

The response from GWL was go with the full 4.00V CC-CV cycle and it's ok to do it slowly, so long as you terminate the cycle properly when the current has reduced in the CV phase.  They have posted a technical paper (PDF) warning about using shunt BMS systems that hold cells for too long at the terminal Voltage (4.00V). However, this was in the context of holding cells at that Voltage for long (cumulative) periods after the charge acceptance current of that cell has effectively reached nothing.  Not the same case as doing a very slow charge and terminating at the end of charge acceptance of the cell.

So, on balance, I'm going with the advice of the manufacturer and the supplier of the cells on this.  At least that way, if it turns out to be bad advice, they are on the hook for giving it to me - if it comes to a warranty claim.
I'm using these new 80W constant power SMPS PSUs from Maplins.  They can run in parallel with a master/slave link and they also have a remote sense input so that the constant Voltage dialed in is measured at the cell terminals and can compensate for the Voltage drop in the heavy current power leads.  This way you dial in 3.97V and you know the charger will hit the mark.

The unit is small, light and efficient.  Doesn't get hot in use, has no need of a fan and is power factor corrected.  At £100 each, they're not cheap but also checking RS, Farnell and Rapid didn't turn up anything better value.

After the initial charge, the manual advises that it's ok to subsequently charge within any part of the cells safe operating range from 2.50V to 4.00V.  So I'll adopt Jack's strategy of making sure all the cells are bottom balanced (at 2.75V when "empty") and then never fully charging them, to avoid the problems and need for active top balancing and current shunting and all that expensive and failure prone junk.

The CellLog8 should be all I need to prevent over discharge of the pack or individual cells in the pack.  It can monitor each cell individually and I can set an alarm for any cell that goes below 3.00V or the whole pack goes below 24.0V.  It can also alarm if any cell differential Voltage (the difference between individual cells) exceeds a limit.  This will tell you if the pack is out of balance -even before it gets to being near full or empty.  The CellLog8 has a beeper and an open collector transistor output that can be used to provide an "inhibit" signal to my inverter to shut it down until the solar array can recharge the pack.

Of course, the required 9 pin header cable for the CellLog8 didn't come with the thing, so I had to source that from a radio control models shop (Electriflyer).  They also sell the Junsi lithium battery chargers that use the same JST-HX balance lead (model BW-9-11).  Just under £5 delivered.
The cable is actually meant to connect a Junsi iCharger 208b charger to a balance board (hence the 11 way header on the other end) but you can either cut off the big plug or use it.  The CellLog8 doesn't have a proper JST socket so a plug with more than 9 pins will fit;  the spare ones will stick out over the end of the 9 pin header on the device.  I might cut the lead in the middle and then have two usable plugs for the gadget.
Another thing I had to order was some terminal lugs.  These are also a bit harder to track down because they are M14 stud holes.  I found some on eBay from Clarik Engineering Supplies.  They take up to 120mm2 cable (too big for my 35mm2 cable).  I might have to use a blow torch or my pipemaster plumbing soldering iron to attach cable to them.  These alone were another £7 delivered... Everything is expensive when you have such massive terminals on a battery!

6 comments:

  1. Morning Outta,

    I have a couple of 35mm2 terminals with a 10mm hole. The terminal is 21mm wide on the flat so plenty of room to open out. PM me your address on Navi and I'll post em out.

    Keep up the good work, Camillitech

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    1. Thanks for the offer! I think it'll be ok with the ones I've got now though. Truth be told, The 10mm holes on yours would only be a problem, as I haven't got a 15mm drill bit :D

      I had to spend another £4 on a 22mm socket for my torque wrench just to do these massive terminal bolts up!

      At least with the oversized cable entry on the lugs I have now, I'd have the option of jamming more than just the one 35mm2 cable in there!

      I'm planning to make up just short fliers from the battery pack to a pair of 8mm studs I've got in some wood as a connecting point for everything else.

      All my other lugs on the existing cabling are 8mm stud so I need to downsize from M14. I can use the short flier cable as a shunt to measure net battery current.

      BTW, had a gander at your blog site. What a place to live... amazing!

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  2. Beware soldered lugs.

    A) in case of over-current, the solder can / will melt, and the wires fall out.

    B) Solder wicks up the strands, and creates a "fulcrum" where the wire will bend and eventually break. Try it with a bit of soldered stranded speaker wire - you'll see what I mean.

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  3. Thanks again Mike,

    Fortunately my battery lugs were *only* M14 size, so I could just buy some :D

    It would have to be some pretty amazing over current to melt solder in a lug with 35mm2 copper cable attached. It took a twin element pipe soldering iron just to get it to solder in there in the first place!

    But I have, ermm, "crimped" the doubled over 35mm2 wire in the ferrule by hammering it down on the wire before soldering, just in case.

    I also have a 200A breaker on the battery positive and fuses for the inverter and charge controllers from the terminal block board, so it shouldn't ever suffer a plasma generating kinda short circuit :D

    I hear you about the flexing damage at the end of the solder but to be honest, these cables are never going to move again after being bolted down so there's no flexing going on here. Granted it would be an issue in a electric car, with the thing bouncing down the road all the time.

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  4. Seems off to have to charge them all the way up only to need them back at the bottom for balancing then back up to 3.5. I cannot understand the logic to it.

    Is there any sort of physics/chemistry principals at work that makes sense here? Or is it a throwback to companies being familiar with different chemistry properties entirely?

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    Replies
    1. Granted it is a bit of a chore, but you do ideally have to make a 4.0V conditioning charge the first time.

      Stressing the electrode material a bit seems to improve its life (according to the makers). You only need to do it once and then on subsequent charges it is advised to NOT charge to such a high Voltage, as it is close to the potential at which the electrolyte breaks down.

      The makers of the cells do not do a full charge at the factory. They charge to about 40% and leave the rest to you. Also, they charge to 40% because that is the ideal long term storage level of the cells and if they charged to full, they'd then have to waste more time discharging the cells to the storage level.

      The rest of the process of draining the cells to the bottom and balancing them there and then charging up again are just a fact of life if you want the pack to be bottom balanced. You only have to bottom balance the pack once and then you can charge it as normal, so it's not that troublesome.

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