Why are my lithium batteries dying after only a month?

a few corrections,

the loaded voltage of these cells are 3.6vpc not 4.2vpc.
4.2vpc is the max charge voltage.

so what you really have from 3 cell is 10.8v.

so to get this straight, when you are at 21.6v, you are only using 6 cells?
and for 32.4v, all 9 cells?
its not possible to do series/parrellel for 21.6v with just 9 cells ( you need 12 for two strings of 6)

chances are you are over-discharging the cells, hence the reduced run time.
how are you charging them? individuall? with balancing?
if you are string charging them without balancing, you are overcharing the weaker ones, again reducing life.

Matt

D3AN,

I am sorry to inform you the charger you are using is NOT a balance charger is is a string charger. The only thing about this charger that is balanced in the name. This is one of those advertising loop holes. You can name something anything you want like I could name a screwdriver "The Hammer" but it is still a screwdriver.

The charger you have selected does not monitor each cell in the pack, that is what a balance charger does. This can NOT be done with only 2 wires.

Here is an example of a Battery Management System (BMS) which can monitor each cell of a 10 cell pack. Notice it has more wires to monitor each cell.

Grandpa Chas S.

I stand corrected. The link you gave has no information that I saw regarding multiple wires for balancing the batteries.

I went back and read your first post. You said there is no controller and that is your problem. Lets look at what you are really doing. You apply power directly to the motor. Motor = very long piece of wire. So you are placing a short on your batteries every time you run the motor. You said in the last post you read 17A on new batteries during a load. The no load number means nothing for this application other than the motor needs .5A to spin, which is normal.

Your batteries are 2400mAH or 2.4 AH. 17/2.4=7083333... This means you are drawing 7C from these batteries and I don't believe they are rated for more than about 3C which is 3x2.4=7.2. So you should not pull more than 7.2Amps for the batteries. As you can see you are trying to draw far too much power from the batteries at one time. When you have the 3 battery strings in parallel you should be able to draw 3 times 7.2 but when you place one string in series with the other 2 then the max becomes 7.2 again. You are also stressing the single string to try to keep up with the double string. I can see why batteries are not lasting very long.

The best thing you are doing is using a charger that does seem to balance the batteries.

The best thing you can do is get a controller which can limit the current flow so you don't damage any more batteries by shorting them out.

Grandpa Chas S.

You are welcome, sometimes I have to either read something again or get more info. Rereading and your information on the charger was helpful to me too.

Later,
Grandpa Chas S.

if all 3cell packs are in parrallel, you have 10.8v, not 21.6v.
If you wire a 21.6v string in parrellel with at 10.8v string you have fireworks.
can you draw a circuit diagram (in paint will be fine) to show exactly how you have wired it up?
if you have put 2 strings in parrallel and one in series, you will have a 21.6v pack, where half is rated to 4.4AH and the other half 2.2AH.
If you use the pack like this, than the 2.2AH cells will die first, and continued usage beyond this point results in cell reversal (although it seems these cells have a cut-put feature to prevent this).

you would probably be best buying a controller. controllers sized for your application are fairly cheap ($40-50)
The RC groups are usually the place to look.

Matt

PS were all here to help :)

Wow if you are pulling 7c from those batteries, there's your problem. Doubling the sise of your pack would help a lot, lowering the strain on each cell.

Uhhhh.... you're just letting the skateboard die without LVC protection for the battery? Sorry to say, but over-discharging lithium is like a death sentence to them(unless you have some premium batteries that are designed to take that kind of abuse, like a123, but even they'll be irreversibly damaged below about 2v per cell.).

Your a murderer

A muderer?! Should we give him the electric chair? yuk yuk!

Hello;

Well you have someone who destroyed a battery or two. Not intentionally of course.

I think what you need is a shunt type of regulator for a BMS if you are putting quite a few of these batteries in series.

The shunt will keep the battery voltage at 3.7 volts during the charge cycle. This will ensure that the battery voltage does not run away.

Also your BMS should have a temperature sensor to shunt the battery if it starts to overheat.

A site you can visit is Metric Mind and take a look at Victors Shunt Regulator. It does not handle temperature issues. But will give you a start.

How I have destroyed my batteries is not keeping tabs on the Voltage limits in the specs provided by the manufacturer. Draining the batteries is one of the worst things you can do to the little critters.

A BMS can be built for under $2.00 per battery. It all depends how much protection you want to have.

In my unit that I built to use with the Thunder-Sky batteries. The B.M.S units use a PIC16F684 micro-controller allowing the host controller to poll each battery individually as well. I have not found any B.M.S. units commercially available to perform this function. The Micro-controller uses Nano watts of energy so there is NO drain on your batteries.

The PIC16F684 micro-controller can monitor both voltage and temperature very well and is a forgiving little guy for you to get your feet wet on.

I have the PIC16D84 controlling a TIP142 Darlington Transistor through a 40 amp fuse and a 1.8 Ohm wire wound resister.

If the battery overheats it is shunted right away. As the voltage increases the Microcontroller throttles down the voltage gradually

I poll my batteries so I know how each unit is doing. Also my controller keeps my battery case at a set temperature.

My controller not only polls the BMS’s but can have the battery identify itself, throttle down the charger until all batteries are charged and then turn the charger off.

The charger was a project in itself. If it was not for the controller I could not have built the charger. The Charger I built is a PWM charger to control the voltage and current. For that project I used an ARM-7 microprocessor. This may be beyond your grasp.

If I were you I would use a commercial charger that is able to be controlled digitally. You may want to look at BRUSA,

I hope this helps.

Hi UnixGuy,

couple of questions,
how does shunting a cell help with a temperature problem?
im not quite sure what you mean, are you bypassing the cell?

and how hard are you pushing the cells to get them to overheat?
i discharge my 40AH cells at 50A continuous and 120A on acceleration on a 40 deg C day, and they dont get hot.

Matt

Hey Antiscab;

The shunting is only used during the charging cycle. Not during normal use. The Shunt does bypass the battery keeping the voltage for a cell at the proper level as other batteries in the series reach their charged level.

Shunting has no value for taking power from the batteries. Although the BMS does report when a cell is is below it's operating voltage and will shut the system down.

I am also putting Boost caps on my van as soon as I can afford them. This will allow me to have a shirt load of power when I need it.

The UnixGuy

P.S. The worst thing D3AN could have done was take the protection off the batteries. That is a cardinal sin in the Lithium-Ion world. During a charge the batteries could flame out or even explode.

D3AN, are you happy will all the toes God gave you, or are you willing to sacrifice a few of them for your learning experience. You only have 10 chances if you only blow off one toe at a time. Your chances will be less if you blow off an entire foot or two.

If anyone is interested;

I dug up the Shunt Regulator that Metric Mind uses.

Victor says it is free for anyone to use as long as you mention his web site.

So I'll mention it. Here it goes. . . .

http://MetricMind.com

Whew that was a close one!

Edit :

Here is a shunt I found that may work for you.

You can find it on this web page.

http://carrott.org/cgi-bin/twiki/view/BMS/EVBonesBMS

Good luck.