Lithium facts in practice

[nut17]

Just a quick update on my original post. You may or may not have noticed in a couple of the photos the presence of a Sidewinder DC -DC charger which was a carry over from the AGM charging regime and initially was put into service to charge the new lithium battery. However, it appeared to not be compatible as there are a number of blackened components inside and it no longer provides any charge current. Sidewinder in Brisbane intimated that their charger was unable to cope with providing a maximum charge current for an extended period, and the Lithium battery with a very low internal resistance will happily take a large charge current until full and would not cover the warranty under those conditions. Sound advice from a clever person on the Caravaners Forum saw me powering the on board C- Tek mains charger by running a dedicated 600w inverter from the Sidewinder supply feed from the tug. This mod is auto switched and works superbly.

My understanding is that the cells that I am using are a different chemistry to both those used in power tools and a Toyota Prius. The 10 year warranty and quoted life expectancy of 15 years plus depending on their use were major contributing factors in my decision to run with lithium.

My 200 Amp / Hr battery pack weighs 22kg. This needs to be compared to at least 400 amp / hrs of AGM or conventional lead acid batteries that would come in at well over 100kg. My battery is slightly larger than an NS70 starting battery.


Cheers Chris

[T1 Terry]

Hi John,
Better get a cold beer, this is a long one
As Chris has already mentioned, the lithium batteries we use are not the ones in power tools and up until this year, not the ones in mass produced electric vehicles either. We use lithium ferrous cells, LiFeP04, actually lithium yttrium iron phosphate, LYFeP04, the other chemistry often used in electric tools and electric vehicles in lithium cobalt oxide or lithium manganese oxide, neither of these have a long service life, 1,000 cycles is considered a good run.
The next thing to consider is how they are charged and more importantly how the individual cells are monitored. This is extremely important in any battery because this is the biggest killer of batteries.
All batteries die one cell at a time, either from:
(A) Being dragged dead flat or even reverse charged because current from the other cells is dragged through the fully discharged cell.
(B) Being over voltage charged.
With lithium cells it's the chicken or the egg, one will cause the other if a poor charging technique is used.

I'll try and explain how it happens.
Lithium cells are full when they are full, just like a glass of water, you can stretch it just a bit and get the ever so slightly over full and get away with it, the next drip has more than that drip running over the sides.
As the cells are mass produced there is a tolerance in capacity, some have more capacity than others, good cell manufacturers under rate the cell capacity to ensure you get out what they say you will, it saves bad press and unwanted claims. Say there are 10 cells, each 100Ah, one maybe 101Ah and another 106Ah and the others some where in between, if we try to take 100Ah out we still have between 1Ah and 6Ah left, depending on the cell, every thing will be fine as long as every cell reaches 3.5v when recharged, that full plus a little bit.
If we try to take 102Ah out, one cell has had 1Ah dragged through it backwards, this severely reduces it capacity. Say it lost 3Ah, recharging using only terminal voltage as the limit, 3.5v per cell, 35v, 9 cells require 102 ah to fill up to the top and reach that 3.5v, but one cell has lost 3Ah, it only needs 98ah till it's full, once it's full the voltage climbs rapidly.
Nine cells at 97Ah capacity or less will be 3.35v x 9 cells = 30.15v, that means for the charging to stop at 35v the already damaged cell will have 4.85v across the terminals, this boils the electrolyte and capacity is lost. It won't take many more cycles like that before this cell is completely stuffed, every deep discharge involves reverse charging to a greater extent each time and each recharge involves being held at a higher and higher voltage while the other cells partly recharge, they never reached their 3.5v the first time so they aren't full, each time there is less and less capacity in those cells so the voltage per cell is less and less by the time the 35v cut off is reached..

Tool manufacturers, electric vehicle manufacturers and even some suppliers it seems don't monitor cell voltage. By limiting charging to say 80% and discharging to 20% it would take a while before the cells got that far out of balance, higher than 3.5v per cell recharging.... it only takes one cell to be 0.5Ah ahead of the others.... remember the water in the glass, you can't put more in than full or you have a problem.

T1 Terry

[ucdailoi]

A very interesting topic....

I like the fact that someone else is paying to experiment with technology so I can benefit without the associated costs later on.

I hope you can come back and post information about your batteries and any issues you may have as time goes on.

My question is.....

1.. Why would anyone drag any battery down to the absolute minimum voltage, especially after having lived with the knowledge that anything below 12 Volt is an early death to a battery.

I know that lower limit applies to conventional batteries, but with years of experience drummed into me I would start to sweat and want to turn everything off at such a low voltage... The other aspect about pulling the batteries down so low, means that the battery capacity is inadequate for the task expected.

(SWEATING NOW AT THE THOUGHT)

[T1 Terry]

the catch with lithium ferrous batteries is, the discharge voltage curve is virtually flat untill the vey end, here is a 100Ah cell, the 3 lines are, top line 50 amps discharge rate, second line is 100 amps, third line is 300 amps and the forth line is 500 amps, out of a 100Ah cell, 4 of these in series makes a 100ah 12v battery. Without a battery monitor that actually measures amps in and amps out there is no way of knowing when that sudden drop off at the end will happen. In a 4 cell 12v battery a 11.8v cut off would be reasonably safe to say no cell would be dragged to zero voltage but an 8 cell 24v battery, even at a 23.6v cut off there is a risk of a cell going very low, at 18v, the common electric tool voltage a cell can be dragged flat without you knowing, in electric vehicles with 48 cells or more in a series string it's very easy for a cell to be dragged below zero volts under high load and reverse charged, looking at the charge, a 90% discharged cell would drop below 0v with a 300 amp load, they pull up to 500 amps or more on take off.

WB LYP 100AH Discharge chart.jpg

T1 Terry

[oldtrack123]

HI Terry
Are you prepared for a shock ---------
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------------p--
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Those last two posts are the best & most detailed & in MHO the most accurate statements, I have seen on ANY forum regarding
LiFeP04


Peter

[T1 Terry]

Well that's it, I'm out here
Thank Peter

[oldtrack123]

Well that's it, I'm out here
Thank Peter

Hi Terry
Bugger,
We cannot get to lovey, I'll have to reread them very carefully,I may find just one thing to fault,but I think I may have to dig deep
Nope had another read still as per previous post


Peter

[Dot]

Even I am impressed with you two boys

[dapope]

Always made sense to me! you can only get so much in the glass, and you can empty it as fast or slow as you like!!
House settled today. Battery shopping coming up when I find trev's email address

[dapope]

Even I am impressed with you two boys

hope there is no intimate meetings planned