Chris
Great information.
Whatever the stated capacity it was achieved under the optimum temperature and at a very low discharge rate.
Measuring available capacity under a working load and conditions (on balance a cool) should produce a significant drop in capacity over the rated capacity.
The rule of thumb for lithium cells suggests they at 5% in 24 hrs. However two thirds of that occurs in the first 8-12hours so that can knock the available capacity compared to immediate use. On top of that you are meant to be loosing up to 4% per year just to aging. So 20% after 4 years seems brilliant.
So how do the latest tests compare to your first tests or were they done under different load conditions?
Look forward to hearing more
Andrew
Hi all
Its now four years since I took delivery of a pack of 16 x 200 ah thundersky cells and you might be interested in how they have performed. These are Lithium Iron Phosphate, or LFP cells, which are not prone to thermal failure like the lithium cobalt chemistry.
A month or two back I did a full discharge to check the capacity now and my tests showed they are around 80-85% of the their nominal 200 ah.
Its a 48v pack that I use in conjunction with a Torqeedo Cruise 4 on my trimaran, Current Sunshine. When first installed them I was using this pack with an Epod, which I later removed in favour of the Torqeedo.
In the early days when I first got them I did a couple of complete discharges, just to learn more about their capacity and my monitoring equipment. But I never did see 200 ah in any test. As far as I can tell they never had 200 ah capacity. This doesn't match the experience of others, but it is my results. At the time I wondered if it was because the pack was not properly balanced and the low result was due to my bad execution of the test. Now my view is that they never did have the rated capacity.
Anyway, this how I tested the capacity... I charged them fully to about 60 volts for the pack, which is 3.75v per cell, and this is higher than I normally go. So I'm sure they were full. After a charge to this to this level they naturally fall back to around 55 to 56 volts anyway.
I then used the motor for a trip which used up about 1/3 the capacity and then on the mooring continued to run the motor until the voltage dropped to below 3 volts per cell. I have a BMS that warns when the voltage of any cell falls below 2.5 volts and I kept going till this warning. At that time all the cells were below 3 volts and all pretty close in voltage. So it means they all have more or less the same capacity and it is around 160 to 170 ah.
I measured the ah capacity by means of a Link 10 battery monitor, and the Torqeedo throttle control. The Link 10 showed 20% remaining at the end of the test and the Torqeedo throttle showed 15% remaining. And both were calibrated to a 200 ah pack.
At about the same time that I took delivery of these batteries I also bought four Dekka AGM batteries. These are good quality AGMs which were used mainly as house batteries in parallel but also sometimes for testing the 48v motor before I had properly installed the lithium batteries. I threw these batteries away about a year ago more or less dead. I gave them the same degree of care that I did with the lithium cells. Except that as far as I am aware I never fully discharged the AGMs. I have discharged the lithiums completely maybe 6 or 8 times in their life. (that is, down to around 3 volts per cell).
I now have some Optima AGMs which I use as house batteries and am soon to exchange them for another lithium pack.
I have also installed a set of LFP batteries on a solar boat which I will write about in another message. Four cells in this installation are used as a 12v house battery and they were left connected to a small load which completely discharged them. So low that one cell was showing .01 volts and the others around 0.2 volts. I have since recharged these and they seem to be ok although I have not done a capacity check and expect it will have dropped.
In my experience LFP cells are quite robust and I will only use these cells from now on. The apparent cheapness of AGM is an illusion and when you consider life cycle and not just first cost LFP cells are good value.
After using a cell level BMS on both installation I now think it to be unnecessary. The BMS was used for the initial balancing and since then has served no useful purpose, except the low voltage warning. And if instead I had bottom balanced the pack manually, I could have used pack voltage as the low voltage warning. In future installations, I will not use a cell level BMS. I would still however have a BMS which just monitors pack voltage and provides a warning or relay disconnect of load or charging in case of low or high voltage.
I may change my existing installations to do away with the cell level monitoring. But this is a low priority.
I hope this information is of interest...
Cheers
Chris
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