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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:D
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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.
Usually, but only because a charger usually has a current limit built in. So with a cell at 3.0 V, you apply say a 20 A charger set to 4.0 V, and the charger immediately adjusts the voltage to say 3.02 V so that the current stays under 20 A. As the voltage of the cell rises, the output voltage of the charger rises, until it hits the limit you set of 4.0 V. At that point, the charger will be putting out little or no current, to maintain the cell at 4.0 V.

or does the charge voltage need to be set at a slightly higher level then the battery voltage.
It does, but the current limit on the charger does that for you automatically.

Firstly I've seen a couple of people use dc/dc converters to charge cells individually and these can't adjust the voltage.
If these don't current limit, then they're not suitable for battery charging. All EV DC/DCs will have a current limit that reduces the voltage as needed to respect the current limit.

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.
For that, you'll need some sort of current limit. Fortunately, it should be pretty easy to set up a circuit that adjusts the field voltage of the alternator to maintain about the right current into the batteries.

So will a constant charge voltage damage the batteries?
If you connect a power source with no current limit and there is a significant voltage difference, then uncontrolled current will flow. Lithium Iron cells do not like to be charged too fast (i.e. at too high a current), and will lose life if charged with too much current.

The other issue is that no EV pack is a single cell; there are always cells (or groups of paralleled cells) in series. Just because the average cell voltage is say 3.8 V, does not mean that no cell is exceeding 4.0 V, and that's bad for the cell. That's what battery management systems are about.
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so if i try and charge a 3v cell with 4v (and 20ish amps) the charge voltage will drop to (around) 3.2 and work its way up until the battery voltage reaches the charge voltage. So in a way the voltage will adjust to what i need it to be, to charge the batteries?
Yes, but only if the charger or other power source is inherently current limiting. If you just connect a 144 V pack across a 120 V pack with a contactor, the current won't limit, and both packs would be damaged, and possibly the contactor as well.

If you connect a generator that doesn't have current limiting designed in, it could possibly attempt to charge at too high a current, and stall the engine as well as give a pulse of high current.

I'm not very technically minded so what ever i come up with has to be fairly simple. the generator can only put out so much current, and i plan only use it while i drive when i can watch what is happening. as long as the battery voltage doesn't go to high the generator shouldn't produce enough current to do any damage right?
I don't know the characteristics of a generator. It might be OK to just connect to a battery and it may be self limiting, but my suspicions are that it won't work and will just stall without suitable current limiting circuitry.

I could put a 30amp fuse between the generator and batteries or find a way to shut off the generator automatically if the voltage or current went to high, would these options be a suitable failsafe?
That should save the batteries, yes; one or two pulses of high current won't do much damage. But if the generator's engine stalls every time to try to charge when the pack is low, that's no good either. Batteries aren't like light bulb loads.

what happens if i floor it and the battery voltage sags while i'm charging, i'm guessing the current will just go into the motor, and not into the batteries
Yes, that's pretty much how it will work.

like what dc braveheart said i'll be in the middle of the cell range to increase safety, i'll also have a decent BMS.
I'm not a fan of limiting the charge voltage as a proxy for cell safety. Sure, if you only have 6 cells in a module, like with a car starter battery, then you usually don't have to worry about one cell going too high in voltage (though lead acid car starter batteries have other equalisation methods, involving gassing). But a typical pack is at least 30 cells in a series string. Even if you limit the volts per cell to say 3.4 V, which is pretty conservative, it doesn't take much imbalance (say ten cells down 0.1 V from the average) for one weak cell to suddenly have an extra volt across it, and 4.4 V will ruin a cell quickly.

You recognise the need for a BMS; that's great. I say don't limit the average volts per cell as a sort of "extra insurance"; give the cells what they need (and a little more, if the BMS has bypass capability), so you don't needlessly prolong charge times. I work on 3.65 VPC, since our cells (Sky Energy/CALB) are listed as 3.60 V maximum. Our BMS can bypass 1 A, and communicates with the charger to limit charge current to 0.9 A when a cell goes over-voltage.


sorry to bombard you with questions but i guess that's the curse of knowing what you talking about.[/quote]
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