Thursday, March 29, 2012

CellLog8s Hardware Mods

In researching the firmware issues with the CellLog8s, I discovered on the RC Groups forum that there are a couple of hardware modifications that can be done to the device to make it work properly when using more than 6 cells.

The OEM, Junsi, posted an item in the thread here (links to the post) describing how the CellLog8s only draws power for the CPU and LCD from the first 6 cells connected.  This will cause a 7 or 8 cell pack attached to have a charge imbalance over time as cells 7 and 8 are not loaded.  It's only a few dozen mAh per day but for a solar pack connected 24 hours a day, it all adds up.

I've copied the photo from the forum, so you can see it here more easily.  Click on it to see it bigger or follow the link I gave above to see the whole article posted by Junsi.

The red line denotes where the link wire has to go and the resistor is circled in pink.
The cure (for users who only want to use more than 6 cells all the time, like me) is to add a wire link and replace one microscopic surface mounted device (SMD) resistor.  The resistor is an optional modification for those with excellent soldering skills (or the right tools) :D. 

Changing the resistor from the "202" (2k2) value to a "103" (10k) value reduces the CellLog8 power consumption when used with more than 6 cells.

Note that the modifications make the CellLog8 unable to function on just 2 cells, so it's not a full fix for the issue.

Of course, applying this "fix" may well invalidate your warranty from the supplier (especially if you muck it up and fry the thing).  But given that the device has a known hardware "bug" out of the box, you could always argue for a refund on the basis that the device was "not fit for purpose" in the first place.  So anything you do to remedy this has no bearing on the initial situation.  Either that, or you could ask the supplier to make the modification for you, again citing that the device was not fit for purpose (if your purpose was to monitor 7 or 8 cells).

On the plus side, at about 28 Euros, they're cheap if you fry one :D

7 comments:

  1. Hello,
    your blog is very interesting. Thank you very much for all the information. It was very important for me to learn, that I can’t mounting the cells on their sides.

    I’ve got a big motorhome with a 690 W Solar on the roof. Actually I’ve 3 EX’IDE-GEL 140, 12V Sys-tem. I think the next should be 8 each of WB-LYP200AHA from GWL-Power (my problem is the high from max. 30 cm).

    My info from your blog is: LiFePO4 is: nothing for amateurs. You have problems with the balancing, with the hard- and software and you are testing different configurations. That’s nothing for me.

    You don’t take the balancing- and battery-management-system from GWL? - what´s the reason?

    Best Regards
    Gerd

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  2. Hi Gerd,

    Thank you for your comments. You can mount the cells on their sides but it is best to mount them standing up. If there is a serious problem and the cells try to release gas through the valve in the top, some liquid may be squirted out if the cells are mounted on their side.

    As for not using a BMS, I don't need one because I have bottom balanced the pack (at the lowest Voltage of 3.00V per cell) and will never charge the cells to 100% full, so have no need for active top balancing (as provided by most BMS).

    In fact, an expensive top balancing BMS charger would mess up my carefully calibrated bottom balancing. Bottom balancing is more important as it allows the battery to keep working when close to being empty (as no one cell gets to the cut-off Voltage much before the others).

    This does mean that I'm only using 80% of the total battery capacity but that's more than enough for my needs. Using less than 80% of capacity also makes the cells live much longer (5,000 to 8,000 cycles). Top balancing BMS systems are only needed if you are trying to charge the battery to 4.00V per cell for 100% charge.

    I do have a battery LVD (Low Voltage Disconnect) system to protect individual cells from over discharge damage. Over discharge is a much bigger risk if you leave large loads turned on all day and night, like my fridge freezer).

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    Replies
    1. Hello,
      thank you. If I do understand right, you don’t need an overcharge-protection because you don’t charge with 4.00 V per cell, and your fridge freezer is always running.

      What is the maximum of capacity you can charge in your LiFePo with photovoltaic and with your Morningstar? 80 %, 90 % or more?

      Not actual for you, but a situation from me: What would happen, if you have for 3 weeks no discharge and always wonderful days with a lot of sun? The Morningstar every day, from morning to the night, charges with 3,65V per cell. Could this be a situation for overcharging?

      If I discharge my Exide-Gel up to 50% I’ll get 2520 Wh. If I use 8 WB-LYP200AHA (12V) between 20% min. and 90% max. (= 70% of the max. power) this would be 3584 Wh.

      Please, can you explain me the adjustment of the Morningstar solar-charger?

      A duration of live from 5,000 to 8,000 cycles – ok, I´m reading these days, the Lithium-battery will lost every year 10 % of its capacity. I think the time is working against 5,000 or 8,000 cycles.

      Perhaps an information from Germany for you:
      New balancer for Lifepo4 von ECS:
      http://www.photovoltaikforum.com/inselanlagen-f57/neue-balancer-fuer-lifepo4-von-ecs-t76433.html

      What do you think if I mounted the cells on their short sides (not on her long side)?
      Then I can use 4 each of LYP400AHA LiFeYPo4
      If they stand up, I must install 8 each WB-LYP200AHA

      Best Regards Gerd

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    2. I would say that I'm charging the cells to about 80% at 3.50V per cell. There isn't much extra capacity to be gained by charging to 3.65V and nearly none between 3.65V and 4.00V (maybe only 10Ah on a 400Ah cell). It's not worth the hassle and expense of an active top balancing BMS for 3% capacity.

      I discharge to no less than 3.00V. At low current discharge this can be about 5-10% remaining charge.

      I use about 5kWh solar energy per day (24 hours, excluding the water heater). I haven't measured the amount of battery used only at night but it's in the region of 2kWh with the rest provided from PV during the daylight time.

      I DO have over charge protection. The two chargers do constant current (limited to 75A) and then constant Voltage (limited to 28.0V or 3.50V per cell average). They use temperature compensation above 25C to reduce the target Voltage at high temperatures. Below 25C they do not increase the Voltage.

      The main "protection" is confining charging to the linear part of the charge Voltage curve and avoiding the unstable saturation zone at the end.

      The GWL cell protection modules prevent cells from rising above 3.60V. As I'm charging to 3.50V, the cell protection boards are not required. The highest cell reading I've seen in the data logs since February on daily charging is 3.53V.

      Limited calendar life (capacity reducing by 10% per year) is a feature of older lithium chemistry cells. I hope it's not the case for these new LiFeYPO4 cells. But nobody knows because these cells have not existed for more than 2 years.

      Also, the so called "ageing" of older lithium cells from laptops cannot be compared to my situation. Heat (40C in a laptop) causes premature ageing. Keeping cells on constant high (>3.65V) float charge damages them.

      I am keeping my cells at under 25C, only charged to 3.50V and 80% "full" for between 1 and 30 minutes per day and then up to 18 hours a day discharging. A VERY different pattern of use to laptops.

      In my blog I've described the Morningstar settings used but in relation to over charging when not loaded for a few days, I set the first absorption timer to only 1 minute. So, if the battery is not discharged each night, it will only charge to 3.50V per cell for 1 minute per day. It would take a long time to cause cumulative over charge at that rate. If I were to go away for a long time, I'd isolate the battery from loads and chargers.

      You asked about what happens if you charge to 3.65V "all day" with the solar chargers? The answer is "Don't do that!". Charge to 3.50V for between 1 and 30 minutes and then float charge at 3.375V for the rest of the solar day. Then you'll have an 80% full battery at night.

      Looking at the German forum link, it appears to be another over-complicated battery management system (BMS) :D.

      I can't read German, so it's hard to tell but the photo shows the typical unnecessary "micro-controller per cell board" and does not appear to have any heat sink on it for the dissipation of the large amount of heat energy wasted in top balancing cells. Solar chargers have little energy to waste so don't bother using top balancers that just throw huge amounts of energy away (as heat) to charge just less than 3% of capacity between 3.65V and 4.00V!

      Effective per cell low Voltage disconnect can be done with the CellLog8 (one micro-controller) and about €10 worth of relay electronics... About €40 in total to control over discharge protection for any sized battery from 1 to 8 cells!

      You can mount these cells in any position apart from upside down, but the "best" position is upright, according to the makers instructions. If you install on the narrow edge, any free liquid will collect only at one end of the cell and may not be evenly re-absorbed into the plates (see my blog entry http://solarbodge.blogspot.co.uk/2012/02/mysterious-liquid.html about free liquid in some of my cells).

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  3. hi.ive heard on other forums, that once you make such mods(jump lead and resistor replacement), you cant get the data from the cell-log 8s usb port. Would you please confirm it...Cheers...

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    Replies
    1. I can get the data out of my CellLog just fine with the mods.

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