how do you charge lithium betteries

6451 Views 13 Replies 9 Participants Last post by  ElectriCar
i was hoping to find out a little more about how the charger actually charges the batteries.

is it as simple as setting the max voltage and then pumping that many volts into the battery untill the battery reaches that level.

for example if i have a lithium battery that i want to charge to 4v could i just pump 4v into the cell no matter what the voltage of the cell is. even if the cell is discharged to 3v.

or does the charge voltage need to be set at a slightly higher level then the battery voltage.
for example
cell voltage = 3v charge voltage =3.1v,
cell voltage = 3.2v charge voltage =3.3v
etc

there are two reasons i ask. Firstly I've seen a couple of people use dc/dc converters to charge cells individually and these can't adjust the voltage. Secondly i have been pondering the idea of putting in a permanent petrol generator, which I'll rectify to DC and then use that to charge the batteries on the go.

So will a constant charge voltage damage the batteries?

thanks you for all you help
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Here's something I posted on the BMS thread. I thought it may be useful and shed some light on this subject so here it is. I have Calb cells coming with a max V of 3.6. I'm not going to go that high to extend the cell life and since I'm not using a BMS, a charge voltage less than 3.6V will make the possibility of overcharging the lowest capacity cell much less likely. When these batteries are full the voltage spikes rapidly and you don't want to go there or you overheat the electrolyte and reduce the ah capacity. Nobody wants that!

I'm not experienced with the new batteries yet but my "plan" is to charge them with a voltage at 30 amps that will charge them sufficiently but not overcharge them. I'm expecting the batteries to be completely charged when the voltage averages 3.45 or so per cell.

Here's something that comes to mind that I teach my employees about electricity. For current to flow in a circuit, there must be a difference in potential. That's the definition of voltage. In this case, the charger is creating a voltage higher than the pack voltage. Because the pack resistance is extremely low, it will accept a charge as long as the voltage is only slightly above the cells terminal voltage.

Here's an example. The batteries I ordered were measured at around .00025 ohms. Multiplied times 50 batteries that I'm running and that's a total of .0125ohms for the pack. So in order for 30 amps to be able to flow into it I only need .375V more than the pack resting voltage using the formula E=I x R. E is voltage, I is current and R is the resistance in ohms. So you can see it doesn't take much voltage to charge one of these batteries.

That's how it's possible to bottom balance the cells all at once by simply tying them in parallel before you install them in the vehicle.
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