transformer

[fred]

Want to replace a transformer wih these specs input 100-240v 50/60Hz 0.5A Output 25.2v 0.8A

Is this output 25.2v@ 0.8A critical? Would 1.0A suit?

I'm complete neophyte here :-(

[GB]

Yes, anything over 0.8A should be fine.

[Fredxx]

This is a power supply and not a raw transformer. 1.0A would be fine if the voltage is the same.

[Peter Able]

You'd have to tell us more about what the application for the transformer is:

Would 1.0A suit?

Maybe, maybe not. See above.

The spec you've given is a bit wonky. A transformer that has input tappings for 100 to 240v will not draw a set current for a set output.

And if it has no taps - Yikes!

Presumably the 0.5A is for when the transformer is driven from something near to 100v - and is fully loaded - but that's a presumption! Still

50W in for only 20W out!

Remember that the mains voltage varies, so such a precise expectation for the output voltage (to three figures!) is very wonky ;-}

Beware of anyone giving you "sure" replies.

PA

[Paul]

This is a transformer. AC in and AC out. Just a bunch of wire, plus steel laminations dipped in lacquer to prevent core eddy currents from flowing. When used as a power source, "transformers feel heavy". And if this was an adapter of some sort, just the weight-in-hand, tells you whether a massive transformer is inside.

Some wall adapters produce regulated DC. The device you are powering, will have symbols printed in the plastic, next to the barrel plug hole, and they will indicate AC or DC. DC is a pretty common thing for electronics (laptops) these days, when it comes to wall adapters. These normally feel relatively light.

A little extra output capacity won't hurt. 0.8 versus 1.0 isn't going to be an issue.

Paul

[Rod Speed]

Fredxx [e-mail] wrote

That 25.2v is weird tho if its not a typo

[Sam Plusnet]

Interesting transformer. With anything between 100 and 240v on the primary, it promises to offer a precise 25.2v on the secondary.

[Joe]

Looks very much like a switch-mode. The uninitiated often refer to them as 'transformers', particularly in the field of LED light driving.

[Fredxx]

It's a standard voltage of a Li-Ion battery pack. I'm now wondering if this was a purpose built charger for such an animal, and how smart it might need to be.

I think more info from the OP is required on the application.

[Rod Speed]

Fredxx [e-mail] wrote

It wouldnt charge a Li-Ion battery pack.

More likely meant to be a replacement for a Li-Ion battery pack and it makes no sense to be that precise about its output voltage.

Yep

[Theo]

Yes, in that case the current does matter. Such chargers are often constant-current/constant-voltage (CCCV): when the output senses a voltage below the rated voltage, they output a constant current (0.8A in this case).

When the voltage sensed has risen near the rated voltage (25.2V) they taper off the current, and continue supplying that fixed voltage at a low current when the output has reached near there.

So if you swapped in a 1A charger to replace a 0.8A charger, you'd be applying a charging current 25% higher than rated. That may not matter, depending on the kind of cells being charged but it could also shorten their life, especially if they get hot. On the other hand, if it was something like a 5Ah drill battery then charging at 1A likely wouldn't stress the cells too much (it could be the original PSU was just cheapskate).

If you swapped in a straight 25.2V PSU, you may not get the constant current part of the charging curve. The PSU would try to supply its maximum amps, and possibly shut down because of an overcurrent condition when it measured its output and found it was below the target voltage. This would either shut down the charger or cook the battery.

(I would hope there's a BMS that would detect an overtemp condition and shut off charging to avoid it actually catching fire, but won't do the longevity much good)

Theo

[Rod Speed]

Theo <theom+ [e-mail]> wrote

Much more likely to be intended as a replacement for a Li-Ion battery pack with the designer choosing to play safe by using that precise voltage to minimise the risk of what it will be powering deciding that it need charging and so stuffing up its use that way.

But not if its meant to replace a Li-Ion battery pack.

[Andy Burns]

Is it for a segway or similar escooter? if so, it's not a transformer, it's a power supply

e.g.

[Brian Gaff]

I'm not quite sure about your input, one assumes its a tapped transformer that you have to set up accordingly. As long as the output volts are the same as the old one an it can sink more current than it needs you can use that. The usual problems is finding the space to put a bigger one. Are you absolutely sure the transformer is at fault, it hads been my experience that the rectifiers, capacitors or any voltage regulator stage go bang most often. The transformers often have a low temperature soldered sensor that springs open with over heating and on many designs can be resolded, but if there still is a short......! Brian

[fred]

Its for a childs electric scooter. This the item I wish to replace

is the item on Amazon I think I should be ok with this Thanks to all for their helpful input

[GB]

That's got the correct polarity. No idea whether the plug will fit into the scooter.

[The Natural Philosopher]

+1.

[The Natural Philosopher]

I think theo has hit the nail here

4.2v is a fully charged li-ion cell., and that's 6 of them (25.2V) . The 0.8v is a charge rate limiter. It suggest something like an 8AH 4 cell LIPO so that is what you need a charger for.

And it needs to BE a charger

Not a transformer.

google '6s LIPO charger mains' and see what pops up

[Paul]

Like this maybe ?

There are more than fifteen different barrel plugs like the one in the picture, each with different dimensions.

As near as I can determine, this is not a conventional wall adapter. It's not a power source with only one voltage.

It implements this.

Stage 1 is constant current.

The constant current is fed into the battery, until it hits the "full" voltage. 4.2V (a certain lithium ion cell chemistry) times 6 cells = 25.2V

This means, the charger will charge at 1.0A constant current, until the voltage at the barrel hits 25.2 volts.

Then, during Stage 2 charging, it switches to constant voltage charging. It is feeding 25.2 volts to the six cells. Current is flowing. But, it's no longer 1.0 A. The current flow declines with time, during the "topping phase" during Stage 2 charging. At around say 0.1 A, it will decide to stop charging. Maybe there is a LED indicator somewhere, to indicate charging is done.

*******

One of the original questions was, is a 25.2 1A *charger* suited to a 25.2 0.8A *charger* replacement.

It it is charging the device a little faster than before. It will take a few minutes less time to charge. The batteries can only accept charge at a certain rate, related to "C".

It really depends on whether the 18650 cells have an honest rating for their C value. If it was 3000maH, then the charger difference would not matter. But if the cells were inferior units, say 700-800maH, now you're charging the inferior cells at a pretty high rate for their "size".

So partly, you have to look at the behavior. How many hours did it take, to fill with the 0.8A charger ? Does the charging rate seem consistent with the value printed on the shrinkwrap sleeve of the cells ?

If the cells were 3200maH and the charger was 0.8A or 800ma, that should take four hours. If it only took one hour to charge the machine, then the cells don't have the expected capacity. Moving up to a slightly higher charger then, is a bit more risky. Remember that eBikes have caught fire. There is no particular reason to put quality cells in anything, when you think about it. Certain companies *do* make reputable cells, and that's what you would hope is in the scooter. Maybe a Samsung (Korea) or a Panasonic (Japanese) cell would have a correct value printed on the plastic sleeve.

If the charging circuit was inside the scooter, then all of the electrical safety would be "the responsibility of the scooter company". By craftily arranging the charging function into the wall adapter, now the sub-standard cells that caught fire in the scooter, can be blamed "on that charger you substituted".

Here for example, they recommend selecting a charger with C/3. So whatever is on the sleeve of the battery, you charge at C/3 of that. If it was 3200maH, then charging at 1.0A is a bit less than "C/3".

Now, this one makes me nervous, because the "slow start" cycle implies it will charge a multi-cell pack all the way from 0 volts. A charger is not supposed to charge a Li pack any more, if the voltage has dropped too low, because the activity that got it down to zero volts, invites reverse bias and metallic Li plate-out.

Notice also, the ambiguity in the voltage specification of the charger. Why could they have not labeled the f****ng charger "6 cell Lithium Cobalt" or "7 cell Lithium Iron Phosphate", so we would know for sure what their intention was ??? I want labeling on my devices, that guarantees success.

*******

Summary:

Go ahead, but keep the scooter in a "fire proof" area. So that, if there is an issue, it can't burn the house down. There have been incidents with eBikes already, and you can research those incidents so you understand the risks better.

There is zero risk... if good cells were used.

If lower-than-normal capacity cells happened to be used, then the risk is growing now.

If the scooter does not have "low-voltage cutoff", now we're toying with cell damage when the child rides the scooter for too long after the scooter announces the battery is low. A BMS in the scooter, should be taking good care of the batteries and working to prevent an issue.

If the scooter charger does in fact have under-voltage cutoff (part of the *charger* design), perhaps your charger is not dead after all... and it's the damn cells that are now too low for the charger to charge them.

Lithium is a rat-bastard for consumers.

And the dishonest Chinaman is laughing all while you deal with this.

If I was standing in the room, I would at least want to see the area in the device where the cells are stored. To see what design features are there, what cells, what brand, what maH rating "claimed" on the sleeve and so on.

If your scooter has been recharging way too fast, from being absolutely dead, this is not a good sign in my book.

Paul

[Paul]

It appears the wall device is a "Lithium Ion Charger", intended for a six cell or seven cell pack. The "smart charging algorithm" is handled totally within the wall adapter device. It has a CC phase and a CV topping-up phase.

That's the kind of device it is. It's supposed to be a precision charger, that cuts off charge current when the battery is full. And this is so nothing catches fire in the scooter.

Paul