Help with DIY charger for multiple EV3 batteries using PC PSU

[le60head]

Hello!

My plan is to use an old PC PSU some kind of power supply to charge multiple EV3 batteries at the same time.

Since I am no expert, I wanted to make sure the following idea would be safe. If you are familiar with the EV3 rechargeable battery, i would really appreciate it if you could share some thoughts on this matter.

The specs of the official LEGO charger (# 8887 ) used with the EV3 battery are:

10V - 700mA - 7W

From what i know those are the minimum requirements for the EV3 battery to be charged properly, and that it can be charged safely using any voltage between 9 v and 12 v.

Let's say i want to charge 5 batteries at the same time. If I use a 100 watt Computer PSU and connect the battery terminals to the 12-volt power supply would in that be fine?

Also do you know whether the EV3 batteries have built in protection and automatic switch-off, so that i can charge them safely using the example above?

I would be very thankful to hear what you think about this idea! Thanks in advance!

Edited May 29, 2014 by le60head

[Philo]

9V is clearly not enough, these batteries start charging at 9.5V. It is probably safe indeed to charge from 12V, but you can't be completely sure, some components in battery charger integrated inside the battery pack could be tightly specified...

Let's say i want to charge 5 batteries at the same time. If I use a 100 watt Computer PSU and connect the battery terminals to the 12-volt output, would in that be fine?

One problem is that these PSU are generally not well regulated when drawing too little current on all outputs (here your 5V output would not be loaded at all). Check this carefully...

Also do you know whether the EV3 batteries have built in protection and automatic switch-off, so that i can charge them safely using the example above?

Yes. No problem about this.

See this page, which includes information on NXT battery (EV3 one is extremely similar except for mechanical considerations).

[le60head]

Many thanks, Philo!

I'll make sure to put some 5v load, and i'll see how to cut the voltage from 12v to 10v.

That should be enough for safe operation.

[le60head]

Here's what i found in this topic:

http://www.eurobrick...showtopic=66415

Using a larger transformer does not result in any extra heat at all. The rating on the transformer is the maximum amount of current that it is capable of delivering, but it will only actually deliver what the load calls for. The battery charger only draws 0.48A when the voltage is 10V, so literally any 10V transformer with at least that capacity will deliver exactly the same total power to the battery. No difference at all.

Now if you used a charger that charged at many amps, that would be a different story. You would destroy the battery immediately. But in this case the charging circuitry is part of the battery so you don't have to worry about that. The transformer is only supplying a power source for the charger. The charger will take care of setting the right charging voltage and current.

If that is true, do you think that the difference between 10v and 12v in the transformer is neglectable?

Edited May 26, 2014 by le60head

[Moz]

A PSU is not a transformer, so that comment isn't relevant. Somewhere inside the PSU there's almost certainly a transformer, but that's not the same thing. It's like, inside every car is a motor, therefore I can treat a car just like a motor.

The PSU will have effectively a "master" output that everything else relies on. Normally this is the 5VB line, since PCs typically draw a lot of 5V current. The design assumption is that "make the 5V rail work, then once that's up everything else can assume that 5V is available". So they make a lot of the electronics inside the PSU run off the 5V rail.

If you don't draw much or any current off the 5V rail, some stuff inside won't work properly. Normally you see a combination of ripple and poor regulation on the other lines. I suspect that to kill a Lego battery all you really need is to get the 12V rail up to about 15V for a millisecond of two but obviously I haven't tested this. Send me a (working) battery and I'll let you know :)

[le60head]

Thank you for your thoughts on the matter!

So in other words a normal PC PSU's 12v output voltage is not truly regulated.

I'll look for information on how to lower the PSU's output voltage from 12v to 10v, and how to regulate the output.

Edited May 27, 2014 by le60head

[le60head]

One last question:

Can anyone tell me for sure whether the EV3 battery itself has its own built in voltage regulator?

I guess it is more likely so, because all the charging electronics seem to be inside the battery, not the LEGO 8887 charger.

[OzShan]

I can add to the suggestions of placing a dummy load across the 5V output to keep the 12 happy. Caused a lot of head scratching when I found it by mistake a while ago!

A big chunky wire-wound resistor 5.6(ohms) capable of 5W should do the trick.

[le60head]

Thanks for the tip, OzShan!

[Captainowie]

I've read sources suggesting that even the load of the PSU's cooling fan is sufficient for this purpose. Your mileage may vary though.

I did something similar a few months ago - write-up is here

Owen.

[Philo]

Can anyone tell me for sure whether the EV3 battery itself has its own built in voltage regulator?

Yes it does! A Li-ion charging circuit, TI BQ24123 or similar.

[le60head]

Thank you Philo and Owen!

I ditched the idea of using a computer PSU and purchased a regulated 12v 5a power supply.

This is the exact power supply i got:

http://www.sunny-euro.com/data/files/252018c1f45bbd78664b27eb1d50b0e7.pdf

Thank you for all your feedback. It helped me realize that a computer PSU is not the best solution.

After plugging in the 45501 EV3 rechargeable battery, the two led light up indicating that it is charging. Seems fine so far :)

After a few minutes i checked the battery, i noticed it has become warm ONLY in the area near the two LED indicators.

Can someone please confirm whether it is normal for the battery to become warm during charging?

Edited May 29, 2014 by le60head