Battery Vehicle Society

Thks Beemer & all, It was a question on not what I might or might not know but to ask a wider group & I thank you all. I am about to fit come headways 38140S to replace a L/A starting battery on my boat. So the interconnection, BMS & cell monitoring safety has my attention just now even though it is only 12V. It is any cell failure that I was looking at i.e. just stored in a box not being used. (Stored safety issue)

Regards Bill

ahh. Specifics!

Hi Bill,
Put it this way. You can buy automotive Lithium batteries that are a straight swap for lead batteries on motorbikes, cars, campers and whatever.
i.e.:-
http://www.ev-power.eu/LiFeYPO4-batteries-12V-1-1/

They do not need nor are fitted with a BMS or anything because modern charging systems will never over Volt them. The only watch out is to never let them become discharged (<10V) if you leave the boat with any parasitic loads on the battery. That will at least affect the capacity or cause nasty issues if loaded.
Charge Voltage i.e.:- 15V/4 = 3.75V.

We see a similar thing with laptops. The brick supplies 19.6V for six cells. that's under 3.267V each. No BMS, only a bit of a circuit for state of charge for the screen & low voltage warning/cut off.

Kept clean and no parasitics and you will see no self discharge on these things at all. It's also good house keeping not to run anything at max load just because someone has tested it to that once. Specs might say 3C max and 0.3C standard for our LiFePo4 cells. Work for half that.

As goes storage.
Jack Rickard of EVTV (again) has several times proven on camera by opening filthy old boxes boxes containing LiFePo4 cells from 2009 and comparing the voltage from brand sponking new cells. There is no self discharge found by noting voltage change on a USD$1000 meter to three decimal places on cells stored for three years. These are amazing but if you have a bad cell, you will know soon after using it..

A good early warning is one of these:-
http://www.evdl.org/pages/battbridge.html
But it's parasitic; never leave it in circuit if stored. Even "cell log8's" have been noted with differing awry loads on the monitoring wires. After a few months they had caused expensive problems.

Andy

That's not an issue with A123 unless you go berserk and heat the tabs up recklessley. The issue is soldering aluminium which is tricky. I managed it with 50 cells.

In a nut shell, storing the cells in a box not connected to anything should be perfectly safe. The most dangerous time is during charging. If you leave them unattended and they overcharge you risk a fire which is why I started this thread. Another risk is that you leave them connected to some load and the cells get drained to destruction. I am not aware of any fire risk in this scenario, but you will probably brick the cells.

Using lithium as a starter battery.
I will presume that your boat has an alternator to keep the battery charged. So you need to find out the float charge voltage for the cells. This can be hard to find out, many lithium spec sheets don't list a float charge voltage. Once you know what the float charge voltage is you must make sure that the alternator doesn't exceed this. I heard a report from someone who had left a lithium cell connected to a 3.6 volt source for a few days (he didn't mean to do this), and when he did realise, he found that the cell had swelled considerably and was ruined. The potential risk then for your boat is if the alternator overcharges and you don't realise (or you've let someone else borrow the boat and they notice the warning light and decide to ask you about it when they get back ) you have a problem. A lead acid battery will just vent gasses (which are explosive), a lithium could vent it's electrolyte which is a flammable liquid.

In this instance the lead acid battery may be the better, or at least easier option.

I heard of that lad who supposedly left his cell connected to a 3.65v supply. Unless it was constantly pumping in a high current I do not believe him at all. He had a financial interest in making his point.

The only reason to chuck in higher currents is to hit a charged state quicker. Leave charged cells alone and they drop down to 3.45v

You can't have constant current and constant voltage at the same time. If you supply a constant current to a cell, in time the voltage at the cell will rise above 3.65, and keep rising until something goes bang. Supply a constant 3.65 volts to the cell and the current will start off high (if the cell is discharged) and drop to near 0 as the cell reaches full charge. It is impossible to keep supplying a high current to a fully charged cell and maintain 3.65 volts at the cell. My understanding is that by maintaining the cell at 3.65 volts for days on end it was ruined. So if you are going to keep lithium's on a float charge it would be worth while to do some testing and see what happens at different float charge levels. It may be that 3.4 volts is fine, but 3.6 volts could be a problem. I don't know the answer to this, but every battery has a float charge voltage and if you exceed the float charge voltage it will shorten the life of the battery as a best case scenario. Worst case with lithium is much worse than it is with lead acid batteries and they can be pretty dangerous.

On my Matiz I considered using lithium for the 12 volt side. In the end I am using the original starting battery with a 13.5 volt DC-DC from the traction pack to keep it topped up. 1 year on and it works perfectly. You need to use the best chemistry for the job and for a staring battery on a boat I would be inclined to stick with lead acid.


I recently received four headway 40160s cells. Out of the box terminal voltage was 3.7 volts on each cell which I found quite surprising. It will be interesting to see how they behave after a few cycles

Those white Thunderskys on ebay were 3.75V on his voltmeter but I put them down to being LiCo's

Greg,

I never stated constant current AND constant voltage simultaneously while at 3.65V. Unless the cell was shorting that would be impossible.

Never "float charge" Li cells! They are not lead acid! It's pointless and leads to tears. No Li charger uses "float charging", ever. If the cell has 3.65V and the charger is 3.65V then there is no exchange of current. No exchange of current does not boil electrolyte. Like I said, his failure makes no sense.

A CC/CV charger limits the current and voltage. When the cells charge initially; full allowed current. The current drops when the cells gain voltage due to the CV. The charger knocks off the charge typically when the current drops to around 0.05*AH.

Maybe the voltage limiting is what you call the float charge?

Use a sealed lead battery in the boat so there are no dangerous gas issues. Its got a short life against LIFePo4's but is cheaper. Horses for courses. I made a presumption for light weight and long term reliability when left alone for long periods.

A lot of electric cars have parasitic loads when switched "off" so that also means the main pack needs to keep the 12V battery topped up. Making bad, worse. The bricked Tesla Roadster's are a fine example.

There is exchange of current if when you switch off the charger the cell voltage drops to 3.45 volts. The current will be small but never the less that current is what is holding the cell voltage high. Doing this means you are slowly overcharging the cell. At some point it will become a problem. Now it may be that you need a voltage higher than 3.65 volts to cause a problem, but there will be a voltage that does cause a problem and you need to know what this voltage is.


Wikipedia has a good definition of float charge. http://en.wikipedia.org/wiki/Trickle_charging

If you are going to use lithium cells as a starting battery then you have to consider the possibility that you may be slowly over charging the cells if your alternator is always holding them at 3.65 volts. Just checking cell voltages to make sure they don't exceed some value probably isn't enough. What really needs to be done is some testing of cells at various voltages and see what happens. Hold 1 cell at 3.65 volts, another at 3.75 volts, another at 3.85 volts, etc and see what the long term effect is to the cell. Until someone does this and publishes the results you can't make blanket statements that 3.65 volts or any other voltage is safe to hold the cells at when running them in a vehicle with an alternator charging them. Without doing these tests, the best you can do is look for a float charge voltage. When I have found them on Lifepo4 cell spec sheets it has been around 3.40-3.45 volts. But mostly the information isn't there suggesting that the manufactures either don't know what the float voltage is or don't want you to float charge the cells.



The solution for this problem is simple and is what I have implemented in my Matiz. I have a 12 volt lead acid battery that gets charged from the traction pack by a DC-DC converter. When the car is switched off a contactor in the middle of the pack opens meaning that there is no parasitic load on the traction pack from the DC-DC or anything else. The 12 volt battery handles the parasitic loads from the car and if it is parked long enough to flatten the 12 volt battery I can just recharge it. This has never happened. The only other parasitic load would be the bms cell monitors which is tiny and can easily be unplugged if necessary, again I have never had the need to do this.

Greg, he boiled the electrolyte over the weekend when unattended. There is no "if" and "disconnects" to this 3.65V. He did what he did, as I said.
Yes, the voltage settles down but the current drop is as you know very small. The top knee is more like a capacitor, a waste to enter.

The very people who created and defined the Li cell never trickle charged them. They simply replaced the charge in a constant current then cut it off at the AH required. It's all over their graphs, seminars, lectures and video's. I know you can recall Prof. Whitacres video........

Flattening a lead acid battery is a bad idea as any other.. I left my motorbike over this winter and it cost me £50 a year early.

People have unbalanced packs with cell log8's!! Their drain is not constant over each sensor wire. Some have had wires inexplicably burn up on them.

I don't even want to have a DC-DC converter in my trike never mind any other items that can and do fail. I'm working on using mains wiring units, 110v leds, alarm horns etc. This goes with all failure points.

Power to those who want them. Not me.