Power supply vs. dedicated charger

[Battery Guy]

Hello All

I am afraid that I am about to ask a really dumb question, so please be gentle.

I have been trying to decide on a multi-purpose battery charger that would allow me to tweak charging conditions for a wide variety of different battery chemistries. While doing my research on the options, I stumbled upon a variety of "bench-top" power supply units, like the B&K unit here.

So, it seems to me that a decent power supply like the B&K, combined with a timer and a basic understanding of how to charge batteries, is all that one needs. However, it is apparent from this forum that most people choose to buy dedicated chargers.

I have a simple question: why? It seems to me that I decent, relatively cheap (<$150) power supply provides a lot more flexibility for the hobbyist.

What am i missing?

Cheers,
Battery Guy

 

[Stereodude]

It's possible. For example you set the power supply to 4.15V and set the current limit to .5A. Then you hook up a Li-Ion and watch it charge. Basically it will stop charging once the cell voltage reaches 4.15V.

However, I'm still not sure how good of an idea it is though.

 

[M@elstrom]

Far better to combine the bench top power supply with a charge controller/regulator like the Turnigy Accucell-6 (or similar/better) IMHO this will still allow you to charge a variety of battery chemistries but with a significantly higher degree of safety whilst also providing you with a better "overview" of the cell's state-of-charge :thumbsup:

 

[Justin Case]

A programmable bench supply with two decimal place or better current and voltage displays will probably cost a lot more than a good hobby charger, unless you get a really premium one like a Schulze.

And you will still have to program in charge algorithms like -dV for NiMH. For Li-ion, CC/CV charging can be very easy to implement, perhaps even without programming if you babysit the charging.

But you still won't have common hobby charger capabilities like discharge testing, and balance charging.

I do use my bench supply for occasional charging duty, mainly for my cordless drill pack. But mainly I use the bench supply for testing and diagnostics of drivers, LEDs, etc.

 

[Mr Happy]

As others have mentioned, advantages of a dedicated charger include:

• Automatic charge termination while unattended;
• Can charge several Li-ion in series with charge balancing;
• Can run discharge tests.

However, a bench supply does offer a lot of flexibility beyond the features of a dedicated charger. Ideally you might own and use both. (I have neither actually, but that is mainly because I don't actually need them and I try to ration my purchase of new toys.)

 

[LumenHound]

Far better to combine the bench top power supply with a charge controller/regulator like the Turnigy Accucell-6 (or similar/better) IMHO this will still allow you to charge a variety of battery chemistries but with a significantly higher degree of safety whilst also providing you with a better "overview" of the cell's state-of-charge :thumbsup:

Couldn't agree more. This will give you the best of both worlds and it won't break the bank while doing it.

 

[45/70]

I was going to say that you can, but then Justin beat me to it.

The problem is, unless you have a programmable PS like Justin mentioned, that you'd have to "babysit" a CC/CV charge near the end, so that the charge could be terminated before the current dropped below 0.03C (or higher, if so desired). I have no idea how you would use a non-programmable PS for nickel based cells. Seems like towards the end you'd have to keep your eyes glued to the voltage, to catch the -dV, for instance.

Dave

 

[TranceAddict]

when the voltage is about to reach CV, it will takes forever to reach that setpoint due to current decreases exponentially.

what dedicated charger do is using PWM to "push" current in by higher voltage, then it stops at every fix interval to check for maximum voltage and deltaCurrent/deltaTime, if any condition meet, the charge will be terminated.

 

[Battery Guy]

Thanks for all of the responses. I am truly a noobie when it comes to hobby chargers, and am having a hard time wading through the marketing/technical information.

It seems that the biggest concern with using a simple bench top power supply is charge termination. Why can't I simply plug the power supply into a timer?

Sorry if this all sounds silly. I just have a hard time with the concept of putting a battery into a box and pushing the "NiMH charge" button.

Cheers,
Battery Guy

 

[Battery Guy]

Far better to combine the bench top power supply with a charge controller/regulator like the Turnigy Accucell-6 (or similar/better) IMHO this will still allow you to charge a variety of battery chemistries but with a significantly higher degree of safety whilst also providing you with a better "overview" of the cell's state-of-charge :thumbsup:

Sounds like a fantastic solution.
Exactly how does the Turnigy interface with a power supply?

Cheers,
Battery Guy

 

[Justin Case]

Exactly how does the Turnigy interface with a power supply?

The hobby charger I am considering uses banana plugs to connect to a bench power supply.

 

[Justin Case]

It seems that the biggest concern with using a simple bench top power supply is charge termination. Why can't I simply plug the power supply into a timer?

For NiMH you can use a timed charge. But that's not optimal. It is most likely very slow, since you'd probably want to use a slow charge rate, such as C/10. Ten or fifteen hours later, voila! Finally charged up.

If you try to use a higher charge rate with a timed approach, then you risk overcharge unless you know how much capacity is left in the cell so that you know how long you need to charge the cell. You could avoid this by fully discharging the cell before doing the faster recharging. But your bench supply isn't going to have a discharge capability.

In contrast, good hobby chargers can use both -dV and temperature sensing to terminate NiMH charging.

And what are you going to do with Li-ions? Timed charging seems like a disaster waiting to happen the first time you miscalculate how much charge to jam into the cell and overcharge it.

 

[Justin Case]

when the voltage is about to reach CV, it will takes forever to reach that setpoint due to current decreases exponentially.

what dedicated charger do is using PWM to "push" current in by higher voltage, then it stops at every fix interval to check for maximum voltage and deltaCurrent/deltaTime, if any condition meet, the charge will be terminated.

That sounds like a charger such as the Ultrafire WF-139. But the hobby charger I'm evaluating does not use PWM to push a float voltage greater than 4.20V.

It does not take "forever" to reach the float voltage set point when using CC/CV to charge Li-ions. A good charger will deliver the charge current set for the CC charging phase (e.g., 1C) until the float voltage is hit. Then the charger will switch to the CV phase, during which the charge current then falls exponentially. Is this what you are referring to?

Charging termination occurs simply when the charge current finally falls to some relatively low, preset value, such as C/10. There is no need to push a higher float voltage than 4.20V to get to that charge termination point.

 

[Justin Case]

I have no idea how you would use a non-programmable PS for nickel based cells. Seems like towards the end you'd have to keep your eyes glued to the voltage, to catch the -dV, for instance.

You can use a slow charge rate such as C/10 and do a timed charge. Or you could first discharge the cell down to 1.0V, and then use a faster charge rate such as C/3 or so and do a timed charge.

 

[Meterman]

To me charging batteries is a range of different jobs:

Everyday charging should be as comfortable as possible. The battery has to be "thrown" into it's specialized charger without need to set any values and after an acceptable time it has to be ready, even if the possible capacity is not reached at full 100%. Safety must be self-evident.

For simply comparing batteries or charging exotic batteries I use my Orbit Pocketlader to be able setting literally all necessary parameters. Disadvantages are to carefully regard all these parameters and to find and use different qualified cradles. Advantage: results are usually better and sometimes anyway possible only this way. Safety is fairly good here.

When it comes to measuring batteries, only stringent adherence to the manufacturer's standards can provide reliable results. So Nixx batteries are charged at constant 0.1C from discharged state during 16 hours, switched by setting in the Orbit Pocketlader or a timeswitch when a PSU is used. For LiIon batteries only a Lab PSU gives correct results, as CC and CV can be set precisely to the mA / mV and are independent of the inner resistance of the battery. A good PSU will even provide 4-wire connection and thus improve the accuracy. The process runs self-controlled, only the cut-off must be handled manually at the right moment (current). The problem of the cradles is existent here, too. For safety purposes you should have your eye on the set from time to time.

So there is nothing like combination pliers for all tasks - and the applied device depends on the intended result. As I own all of them (and that in different variations) I'm often the victim of the agony of choice. :hairpull:

Wulf

 

[Battery Guy]

And what are you going to do with Li-ions? Timed charging seems like a disaster waiting to happen the first time you miscalculate how much charge to jam into the cell and overcharge it.

For lithium-ions (and the new NiZn PowerGenix cells), the charge is typically CC/CV, which is very easily accomplished with a power supply by setting the voltage and current limits. A lithium-ion cell held at 4.2V will only take enough current to reach full charge. You cannot overcharge a lithium-ion cell by float charging it, unless you set the voltage limit too high.

Cheers,
Battery Guy

 

[Justin Case]

There you go. You fat finger the float voltage, or mis-read the float voltage that you set, or whatever. IMO, it is harder to make this particular mistake with a hobby charger since you often just select "LiPo", "LiIo", or "LiFe" and the float voltage is pre-programmed. If you have a programmable bench supply, then you can achieve the same thing. But hobby chargers often also have other built-in levels of protection, such as temp cutoff, max charge capacity cutoff, and a safety timer.

I recently blew my linear bench supply charging an IMR26650. Not sure why the supply blew. All I can think of is reverse polarity (with flat top cells, incorrect polarity IMO could have a relatively high probability of occurrence), although I thought that I made sure that I had polarity correct. Hobby chargers often have reverse polarity detection and protection.

Errors occur and the more manual steps that are required, IMO the greater the chances of a procedure error.

I was fortunate in my case. The cell got very hot but didn't go high order. The power supply blew a 1N5408 rectifier, which was easily replaced.

 

[45/70]

......A lithium-ion cell held at 4.2V will only take enough current to reach full charge. You cannot overcharge a lithium-ion cell by float charging it, unless you set the voltage limit too high.

While it is true that a Li-Ion cell held at 4.20 Volts in the CV stage will never overcharge, as far as voltage is concerned (actually, it will never be able to obtain an open circuit voltage of exactly 4.20 Volts either), charging a LiCo Li-Ion cell beyond the 0.03C charge current limit can cause damage to the cell, and possibly a "vent with flame" incident. Float charging of LiCo anyway, Li-Ion cells is not recommended. From BU:

[FONT=Verdana, Arial, Helvetica, sans-serif]No trickle charge is applied because lithium-ion is unable to absorb overcharge. A continuous trickle charge above 4.05V/cell would causes plating of metallic lithium that could lead to instabilities and compromise safety.[/FONT]

This is why when using a proper algorithm, Li-Ion cell charge termination is determined by a point in the diminishing charge rate current during the CV stage, rather than a specific cell voltage. Unless you can program the PS to terminate at 0.03C of the charge current, you will have to be there and "eyeball" the termination, and stop the charge manually to properly charge a Li-Ion cell with a bench supply. I might add, been there done that.

This is also why I consider CC only chargers that terminate charge by cell voltage alone, to be bad for the cell's health, and possibly unsafe. With a well seasoned, or damaged cell, these chargers will charge the cell well beyond the 0.03C termination of a proper algorithm, oxidizing the electrodes and possibly plating out metallic lithium, until the termination voltage is reached.

You can use a slow charge rate such as C/10 and do a timed charge. Or you could first discharge the cell down to 1.0V, and then use a faster charge rate such as C/3 or so and do a timed charge.

Yes, of course. I have used a bench supply for a number of years, before I obtained a hobby charger, to charge welded NiCd and NiMH packs using the 0.1C method. I've never felt comfortable with any other timed method however, as I wasn't confident that I wouldn't either overcharge, or undercharge the cells in the pack.

Dave

 

[M@elstrom]

Sounds like a fantastic solution.
Exactly how does the Turnigy interface with a power supply?

A simple matter of "hooking up the power wires"

Due to the wide input voltage range of these type of hobby charger (designed for in field use) a stable computer PSU with suitable amperage on the 12v line could also be substituted for a bench power supply, I was going to use an old CB radio power supply but figure the old PSU could supply the higher amperage rates needed for series/parallel charging :thumbsup:

 

[Battery Guy]

There you go. You fat finger the float voltage, or mis-read the float voltage that you set, or whatever. IMO, it is harder to make this particular mistake with a hobby charger since you often just select "LiPo", "LiIo", or "LiFe" and the float voltage is pre-programmed. If you have a programmable bench supply, then you can achieve the same thing. But hobby chargers often also have other built-in levels of protection, such as temp cutoff, max charge capacity cutoff, and a safety timer.

But that is exactly what I don't like about the hobby chargers I have seen. Simply hitting the "LiIo" button and putting my faith in the charger is kind of hard for me. Also, what if I only want to charge my lithium-ion cells to 4.1V instead of 4.2V? Can some of these chargers be adjusted to customize the cut-off voltage and other charge parameters?

Cheers,
Battery Guy