Batteries and New Battery Technologies

Hi Jonathan,

Good question. First at all, if we speak about NiMH or NiCd, and you have 40 batteries in series, they are all charged with a constant current and the absolute total Voltage shall not exceed 40 x 1.76 Volt = 70.4 Volt. This is the emergency pull the plug voltage of a NiMH. Some batteries may thus have gone over the 1.76 Volt and are damaged. But if we use the value 40 x 1.45 = 58 Volt, yes indeed one or two batteries may be just more than 1.45 Volt, however as we are charging 0.1C, it is irrelevant. No damage will be done to the batteries. However if we charge with 1C, all hell breaks loose, if one or two batteries exceed the Voltage level, as the battery will very quickly heats up and lifetime of the battery is reduced.

With 16 LiFePO4 in series, I dare do not charge at 1C, because every cell has to be monitored. Either with a thermistor, or temperature sensor and each battery needs to feed a wire to the outside world.

However what I like to try out is to charge at 0.1C only, first constant current charge and then switch over to constant Voltage charge. LiFePO4 are not so finity as all the other Lithium types. Also no danger for explosion.
But I will not charge to 100% but charge the batteries only to 90 - 95%. O.K. I hear what you say, what a waste, but by adding a few extra batteries I will still have the required power I need. Therefore I like to try the simple "Compare with other batteries system" out instead of millions of wires all over the place.

You know Jonathan, My Mercedes did 330.00km, why?, because I never did the speed as shown on the speedometer, 220km/hour, but at the most 130km/hour. But my Triumph Spitfire was wrecked after 100.000 miles, because I drove flat out 190km/hr most of the time.

What I try to say is, if I charge the batteries till 90% I probably will have very little problems with them and can probably get away with the comparison BMS.
Bert

 

Here is some information on lithium BMS:
http://www.lithiumbalance.com/WhatisBMS.html

This company, Lithium Balance, is set up with the sole objective of producing BMSs for lithium batteries.

As lithium batteries develop they may be less sensitive to charge and discharge conditions but at the present state of the art it seems individual cell monitoring is required for good battery life. I know the battery in my portable has individual cell monitoring.

Rick

 

Hi Rick,

Having used batteries for many years for solar energy applications and electric vehicles, from experience I learned that battery failure often starts with a single cell that starts building up internal resistance. This reduces the apparent voltage of the cell demanding more current from the battery charger than the other cells. This causes the weak cell to heat up, accelerating it's demise as it sheds electrode material and further increases resistance.

If the defective battery cell is being charged in series with other cells, those other cells would tend to get the voltage/current that the weak one is calling for, causing the entire bank to go on a downward spiral. It seems to me that there must be a way of selectively tailoring the charging profile of each cell to avoid catastrophic failure of the battery bank. I am sure this is possible, but is it affordable? Would such a scheme add an intolerable additional expense to these already expensive batteries?

Jon

 

Jon
You will see from an early post above that there are companies dedicated to just making lithium BMSs.

My little charger has a balancing function that I have not yet tested. I expect it is able to either add a little extra charge to a lazy cell or remove a little from the better cells.

If you are paying $5k for a battery then a few hundred dollars more for electronics that ensures safe reliable long-term operation is money well spent.

The little micro-processor controlled charger I have cost USD24. It is able to charge/balance 7 cells. With 4V per cell this gives 28V so not quite good enough for a decent electrical system. The biggest cell count I have found with the RC stuff is 8S:
http://www.hobbycity.com/hobbycity/...uct_Name=IMAX_B8_Charger/Discharger_1-8_Cells
Note the plugs for the various size cells. So this gets up to about 32V.

There would be more complication with higher voltages but not excessively so. I will be looking around for a 48V system eventually. The technology is growing fast.

I read recently Warren Buffet has bought USD250M shares in a Chinese lithium battery manufacturer. He is usually good at picking the trends early.

Right now the price is set by the limited manufacturing capacity. We should see gradual reduction in prices as the supply starts to meet the demand. A bit like what has happened with flat panel TVs.

Lithium miners do not seem to be benefitting so the production has not hit the fundamentals yet. Unlike lead a few years back.

Rick

 

An additional point is that a boat battery needs to be able to accept a reasonably high charge rate.

The system I am considering would have a battery able to cruise for about 10 hours without charging. The charger, through solar or wind, would both be able to hold cruise speed in good conditions. So at anchor on a sunny day in a good breeze the battery could be charging at twice the normal discharge current - of the order of 0.2C.

Rick

 

One addendum for my case Rick, on my build with a 48V battery bank, I do not wish to run my genset (capable of 100A@60+VDC = 6000W - peak) for more than an hour or two at most each day and complete the top-up using 1600W (nominal) of solar panels as I like the quiet... so with this technology and BMS requirements, it seems I still have problems...

 

Brian
Most of the lithium batteries will take a higher rate of charge than lead-acid.

I thought you had settled on lead-acid though?? You should be able to get a standard charger for your bank but you would need lithium if you want to charge at 6kW unless you have a massive battery.

Rick

 

Yes pretty well still lead/acid, as in spiral wound AGM, but still open to developments... Decision date is around March 2010 - All I have is the genset - so - batteries, PV panels, inverter, and even the Torqeedo electric outboards will wait till then... Seeking better $price, performance, reliability, suitability etc... I will continue searching until the acquisition then installation & acceptance....

 

I think that a lot of the systems seen in home brew automotive projects basically monitor on a cell by cell basis, and burn off unwanted current as heat before it enters which ever cell is lagging behind. (Does this make sense, or have I got it wrong?) These are the simple/wastful systems I mentioned earlier, because there is no central or intelligent control needed, and the systems is drawing constant power from the AC outlet, some of which has no chance of ever being used. The LEDs seen on these packs and mentioned earlier light to indicate when an individual cell is done. When the whole pack lights up like a Christmas tree you're ready to go. Also, watching the time delay between cells gives an indication of which ones are lagging, and by how much.

The thing to keep in mind here is that these systems are using large prismatic cells, and generally only need less than 50 per-cell-monitors. People still end up spending ~$,1000 USD on these systems. Now, the packs cost a lot because they are using the 100aH+ cells, so the cost disappears.

I need to clarify that I don't own a pack like this, and haven't paid super close attention to the subject, because I'm going lead acid. So, if something here doesn't make sense, or is wrong, please correct me.

 

I think it is worthwhile leaving it as long as you can with the batteries.

Interest in lithium is very high and I expect will be gradually eating into the lead-acid market.

I guess 50% of the vehicles in China are electric and they are producing lithium in quantity. From the reports I have read they are roughly doubling the range of scooters by changing to lithium so it is a big incentive.

Strong demand for lithium will reduce the demand for lead-acid. The LME stocks for most metals has leveled out or even dropping but lead is still climbing. This should keep the lead price subdued.

The Japanese car makers have underestimated the demand for hybrids and full electric. I think the car world will change quite fast over the next ten years.

Already fuel prices are starting to kick up so it is an ever present dampener on economic activity. This time I do not see people forgetting high fuel prices too soon.

Rick

 

rick, there are also 10cell chargers in hobbycity. schulze makes one for 12s iirc but it had some issues (balancing error) at least until recently.
then there is the two i mentioned above.

big brushless and esc have dropped 2-5fold in the last year or so... probably chargers will, too?

bertku, at .1c maybe a resistor is ok and a constant voltage and low current cutoff (and maybe a timer)... even better may be IGTB / mosfet PWM switcher.. less waste, and also is healthy for the cells to be pulsed, I hear. what is the power source(s)? not sure about the balancing thing..

here are some trying different ways to make a charger: diy charger

 

The 12S Shulze chargers get to the voltage I need but the biggest one goes to 10A. I need about 100A charging capacity.

It may be possible to use the balancing function while charging through separate lines with some shutdown for say the 90% charge state. Just trim above that.

I am interested in what you mean by the price reduction on the large brushless stuff. What is "big". The Mars motors I have are the same price as last year. I did notice Kelly controls are now selling scooter size units. THey have an 800W PMSM for USD149.

Rick

 

Adapter > to adapt > "anpassen" in your language.
Ordinary 2-wire adapter as opposed to an adapter with feedback or monitoring leads like used in a Makita pack that is fully recharged in 45 minutes.

 

do you realy plug your torch into an AC outlet with 220 V@50 Hz?
do you realy think so?

 

Hi Capt Vimes,
Yes CDK is doing just that. However the tourch has some electronics incorporated like bridge-rectifier, capacitors, maybe a little transformer or floating capacitors, if tourch is isolated from mains. And the charging will take place at probably 16 Volt DC.

Bert