Building a DIY Lifepo4 charger

[brandnewb]

Would it be feasible/advisable to DIY a LifePO4 battery (pack) charger?

I am considering DIYing the following;

240V/50Hz(AC) power outlet -> 2 string cable -> Full Bridge Rectifier -> DC step down transformer (Transform DC Voltage to a voltage that matches with the battery pack down the line) -> charge battery pack with the power that is available from the power outlet.

If that is a good idea then can I do the same concept but then use a 3 phase source?

400V/50Hz(AC) power outlet -> 3 string cable -> Full Bridge Rectifier -> DC step down transformer (Transform DC Voltage to a voltage that matches with the battery pack down the line) -> charge battery pack with the power that is available from the power outlet.

Or would any of those ideas result in bad stuff?

 

[Don B. Cilly]

Transform DC Voltage?

 

[brandnewb]

Step down I meant I guess

 

[brandnewb]

But I'll admit that I am very new and may be widely misguided

 

[Don B. Cilly]

No, honestly. Two salient characteristics (drawbacks) of AC and DC: AC you can't store, DC you can't transform.
So obviously you'd want to put the transformer before the rectifier bridge.
After which you'd have a vaguely unregulated pulsating current.
After which :·) you'd want to
a) smoothen the pulsating current down - at least a bit - and
b) regulate it to charge the battery properly.

One way to do that (smoothen it) would be with capacitors.
One way to do the other (regulate it)... could be... if you manage to get the pulsating current smooth enough, feed it into an MPPT solar controller, which will take care of the proper charge - providing it's properly configured for your battery.

The main question - to start with - would be: what amperage would you want?

 

[Samueru]

Would it be feasible/advisable to DIY a LifePO4 battery (pack) charger?

I am considering DIYing the following;

240V/50Hz(AC) power outlet -> 2 string cable -> Full Bridge Rectifier -> DC step down transformer (Transform DC Voltage to a voltage that matches with the battery pack down the line) -> charge battery pack with the power that is available from the power outlet.

If that is a good idea then can I do the same concept but then use a 3 phase source?

400V/50Hz(AC) power outlet -> 3 string cable -> Full Bridge Rectifier -> DC step down transformer (Transform DC Voltage to a voltage that matches with the battery pack down the line) -> charge battery pack with the power that is available from the power outlet.

Or would any of those ideas result in bad stuff?

I assume that by DC step down transformer you mean a DC-DC converter. Buck converter in this case.

This would be a non isolated power supply. You can do it but a failure in the converter will very likely result in massive damage to the battery and everything that's connected to it and a fire.

Also being non isolated means that there is no galvanic isolation, so all terminals in the battery, plus whatever is connected to it should be treated as live at mains voltage (Shock hazard), so it would require the entire system being inside an insulated enclosure.

Also your DC should not be grounded, because otherwise depending on how you connect this power supply AC side you will cause a short circuit as well.

 

[brandnewb]

The main question - to start with - would be: what amperage would you want?

I am not sure. I am thinking of charging my battery pack at 0.5C (140Amps)

 

[brandnewb]

I assume that by DC step down transformer you mean a DC-DC converter. Buck converter in this case.

valid assumption. I guess you are correct

 

[brandnewb]

You can do it but a failure in the converter will very likely result in massive damage to the battery and everything that's connected to it and a fire.

Could you please give an example of one of the ways the converter can fail?

 

[brandnewb]

Also your DC should not be grounded, because otherwise depending on how you connect this power supply AC side you will cause a short circuit as well.

ok, no grounding then. basically I am trying to create a crude Switching power Supply with massive force. Not at all fine tunable and the switching aspect will be changing the buck converter

 

[brandnewb]

Also being non isolated means that there is no galvanic isolation, so all terminals in the battery, plus whatever is connected to it should be treated as live at mains voltage (Shock hazard), so it would require the entire system being inside an insulated enclosure.

thank you for the heads up. well understood. I can make an enclosure

 

[brandnewb]

@Don B. Cilly I like your rig on your blog. it has the touch of tinkering which I really respect

 

[brandnewb]

I actually meant that non sarcastically!. Before one misread it ;(

 

[brandnewb]

One way to do that (smoothen it) would be with capacitors.

I have 2 caps

 

[Don B. Cilly]

A DC-DC (step-down) converter - unless it just dissipates the excess voltage, which is as primitive as it gets - what it does is convert the DC into AC, "transform" that, and then re-rectify it. You can work out for yourself how intelligent and efficient that is, if you have AC to start with.

Now, 140A is big. More like huge. At... what (nominal) voltage? 24?
I wouldn't really try. You could aim for... 40-50 and still have to deal with some serious components, some heat dissipation, wire sizing, quite a few hurdles. What sort of battery are you planning to charge? From what source?

Man, I had a 50A charger some 15 years ago. It had wheels :·)

[EDIT] When I say "transform", they don't, really. Usually they use PWM (Pulse Width Modulation) which takes the AC square wave and makes the "periods" (squares) longer or shorter... look it up. Still, it's absurd to do that if you have AC to start with, and can just use a good transformer.

----------------------------------------------------------------------------------

The stove rig, it was very tinkerish, I agree. But effective. I had the pressure cooker. The valves were shot. I though, hey, why don't I put some tubes in there and run the hot water somewhere else. It took a bit of experimenting, made it work.
.

 

[brandnewb]

You can work out for yourself how intelligent and efficient that is, if you have AC to start with.

I am not known for intelligence ;( I'd assume that transforming AC to DC to AC is just a step too much with all the losses that go along with it.

Now, 140A is big. More like huge. At... what (nominal) voltage? 24?

Any. wether it is to charge a single LifePO4 cell at 3.2V or a pack 16S (48V) or what ever. I have asked on a different thread whether there is a use case for a 400V battery pack but the jury is still out.

The stove rig, it was very tinkerish, I agree. But effective.

Yes. Effective is what is the most important

 

[brandnewb]

Now, 140A is big. More like huge.

Yes I tend to agree. This is how I like to do things before my stupidity gets me killed ;(

 

[brandnewb]

Yes I tend to agree. This is how I like to do things before my stupidity gets me killed ;(

or even worse someone else around me. But yes. I am aiming high and I think there should be no fundamental reason not to other than safety and what have you.

 

[Ampster]

Would it be feasible/advisable to DIY a LifePO4 battery (pack) charger?

When you consider that any charger is just a power supply with a Constant Current phase and a Constant Voltage phase then it is feasible. I do not know if it is practical especially if you wanted to be able to adjust the settings and add features found in many charge controllers or chargers already integrated in many inverters.

 

[Don B. Cilly]

Well, say you aim for 40A. Still pretty big.
Say you use a double-coil toroidal transformer, that makes 70V AC on one coil and 15 on the other... forget it actually, to charge individual cells you'd have to make a separate one. Say you have a transformer that makes some 60V AC. Rectify it and smoothen it out. Plug that into a 50A MPPT and see how it deals with it. If it doesn't like it, smoothen out the pulsating current some more.

Or, just feed the pulsating current to the battery, at some 10V over nominal, through an ammeter, and watch it like a hawk. In that case, you could use a double-coil transformer - or two of them, one for 48V, one for 24.
If the MPPT works, it will deal with higher voltages, so you might get away with just one coil.

In any case, you would need to have fuses/circuit-brakers, properly sized wires, dissipate some heat, and have a decent basic knowledge of electricity.