Self charging Li-Polys

[all_fly_all_day]

Hey all,

I want to be able to charge my Li-Polys once i order them without spending money on a charger. I know this is risky but I am just dying to get one of these! I know this much, You have to keep the chatge rate at 4.2v and you have to moniter* them for peak charge of 4.2v. I have a voltagemeter* and a power supply unit. I can put out 0 to like 50v with that thing. And i have some 100mah adapters im not sure what the voltages are. But i think i might have a 3 or 4v one lying around somwhere. Anyway, my questions are. Do you have to stop charging them the minute they reach 4.2v? does the charge rate HAVE to be 100mah? What voltage should i charge them at? Should it be 4.2v? And what risks are involved such as blowing up the pack damaging it etc...

Any extra things you might know about them will be a great help!

[Dave Robelen]

Hi Friend,
The approach you are thinking about will most likely be destructive to the cells. You need a charger that will maintain a constant CURRENT that is no greater than the 1C rating of the cell (145 ma for a 145 mah batt. etc,) The charger must be able to sense and shutdown when the cell reaches 4.2V. This type of charging generally requires a unit with microprocessor control.
Take care, Dave

[all_fly_all_day]

Thanks Dave. I guess its time to cut some more lawns and get a job at Mc Donalds! lol

[CalmAir]

Dave is right,

It would be mighty difficult to maintaing the proper charge rate that way.

A good lithium ion charger will operate in the following manner.

1. It will first check the voltage of the cells. If they are below 2.75 volts per cell it will place a trickle charge on first. This rate is 1/10 the bulk (quick charge) level. Once the cells are above the 2.75 voltage threshold it will go into the bulk charge state.

2. The bulk charge state can be up to 1C rate, but most manufactures recomment .6 to .8C bulk charge rate. At this point the charger is in a constant current mode and will maintain the constant current until the pack reaches 4.2 volts per cell.

3. When 4.2 volts per cell is reached the charger will go into the constant voltage mode. The voltage will be maintained at 4.2 volts per cell, plus or minus 50 millivolts max. During this time the current will slowly drop from the bulk charge rate as the voltage is maintained. A 60 to 90 minute timer is also started at this time.

4. When the current drops to 1/10 the bulk rate the pack is considered fully charged and the charger will shut off. If the timer expires before this time it will also shut off the charge.

No matter what size lithium you run it will take 1-1/2 to 2 hours for full charge.

There are simple lion chargers that perform the constant current and constant voltage, but they do not look at voltage before charging or make any other decisions. Make sure you at least have one with a cutoff timer.

Yes it could be done with a variable power supply and good voltmeter and ammeter, but you would be very busy and give up a lot of time trying to do it manually. Keeping the supply within the plus or minus 50 millivolts would be very difficult.

Use your money on the right charger and not on damaged battery packs.

Dale