Using a WF138 to charge multiple cells?

[Capo_au]

Hi all

My question is this:

If I was to modify an UltraFire WF-138 charger so that each charging channel was connected to two Protected UltraFire 18650 batteries in parallel, would this have any major drawbacks?

After doing bit of reading in to how the charger performs over at this post I would expect a charge time to be around 4-5 hours, which doesnt bother me.

However my main concern is that even if I used the two identical batteries from an equally discharged stating point that over the course of a charge, or repeated charges, they would become uneven. Would this be the case?

If it works I intend to repackage the chargers internals into a new housing with a 4 core cable connecting from the housing to a 4 cell (2cell's per channel) pack. The goal being a sealed separate battery pack that doesn't require the removal of the cells for charging.

I'm trying to get that end result on the cheap, so if you have any alternatives that are equally cost effective I'm all ears.

This is the first of what I hope will be many posts on this forum. So thanks in advance for any advice and with a bit of luck I might be able to return the favor one day. :)

-Capo

 

[Capo_au]

Alternatively, does anyone see a reason why i couldn't use the internals of the same charger mentioned above to charge two 26500 4000mAh 3.7 batteries?

Far as I can tell the cut off voltage should be the same, it would just take longer to reach it.

Obviously I would need to need to add some protection when making them into a pack.

If I can use that charger then I should be able to make an 8,000mAh pack weighing approx 15grams with external AC smart charger and over current protection for under $34USD..

Im just shooting in the dark here so any thoughts at all would be great =)

 

[Black Rose]

Honestly you would be better off buying a charger that is designed for larger cells.

Trying to charge a pair of large Li-Ion cells in parallel with a WF-138 seems like a recipe for disaster.

 

[mdocod]

Trying to charge a pair of large Li-Ion cells in parallel with a WF-138 seems like a recipe for disaster.

Actually, provided the cells are put in parallel in a similar state of charge, there is no danger to charging as many li-ion cells in parallel as you want. Chargers like the DSD have their channels in parallel right out of the box (it's a single channel charger with 2 bays.)...

The problem I see is not the wiring, but rather, the quality of the charger and the charging methods and charging speed. If I were going to go through all the hassle of gutting a charger to put in a custom application, I would want to start with one that handles charging correctly. The WF series of chargers use a charging algorithm that is less than desireable, to top that off, many of them do not actually terminate the charge when "the light goes green." (some trickle after termination)

You can get a li-ion cell hobby charger with more ideal charge rates and better charge termination methods for ~$50. Often they'll do ~1-4+ li-ion cells (for series packs with balance taps), but you can just set it to run in single cell mode for a parrallel pack and crank the charge rate way up.

-Eric

 

[donn_]

You can get a li-ion cell hobby charger with more ideal charge rates and better charge termination methods for ~$50. Often they'll do ~1-4+ li-ion cells (for series packs with balance taps), but you can just set it to run in single cell mode for a parrallel pack and crank the charge rate way up.

-Eric

Eric...can you name a brand for such a charger? I also need one which will handle LiFePO4 cells. All of the charging will be in parallel.

 

[Capo_au]

The WF series of chargers use a charging algorithm that is less than desireable, to top that off, many of them do not actually terminate the charge when "the light goes green." (some trickle after termination)

There are two main reasons that i chose this charger, one because of its low cost and secondly because of this post which showed that it does indeed cut off once charged..at least thats the plan.

Trying to charge a pair of large Li-Ion cells in parallel with a WF-138 seems like a recipe for disaster.

After giving it some more thought I agree. If the pack is sealed then no doubt after a number of charges they are bound to become uneven..My hope is by using only these two cells (one per channel) then the only difference apparent to the charger will be the charging time.

Hobby chargers are great, but a tad on the expensive side of things. Anywayi guess the only way to be sure if this will work is to buy the bits, put it together and give it a try.

I will be sure to post if/when i do this and let you know how i go.

Cheers,

-capo

 

[mdocod]

Hi capo,

Based on the thread you linked to, we can clearly see that the charger reached charging voltages of 4.35V during a constant current only charge.

I haven't yet found a data sheet for a 3.7V cell that recommended or even mentioned this charging method, I know of no consumer devices that exceed 4.25V during charging of LiCo cells.

granted, since the capacity of your pack will be much greater, the differential between charging voltage and cell resting voltage will be much lower, so you probably won't exceed 4.25V during charging, so technically, it will be fine...

What concerns me most, is that there have been many changes to the designs and the circuitry in the WF chargers over the years, some better than others but all having some problems. Not to mention, UF batteries have about worst track record of all of them out there.... Maybe the 138 that was tested in that thread is not too bad, but who's to say that the next batch won't have the same very dangerous over-charging problems that many of the 139s have had?

I'm not saying you are going to spontaneously-combust with that configuration, but as far as presumed safe configurations go, this would be one of the least safe in actuality. (cheap cells combined with poor leaning on possibly very poor charging methods).

-Eric

 

[Capo_au]

Hey Eric thanks for your post.

I'm still pretty new to all this so the heads up regarding charging safety is appreciated. Now thinking that I settled on an idea a little prematurely i'm going to hold off buying anything for the time being. After all, not much point if my "cheap" solution goes up in smoke..or worse.

Undeterred though i'm still going to push on to try and find a budget solution that is both safe and useful for my purpose.

I will post when i come up with something.

Thanks again,

-Capo

 

[mdocod]

A few minor tweaks to the "build" will make things substantially better and safer...

UltraFire brand cells have had a pretty lousy record until very recently, while TrustFire's have been actually pretty good. Similarly priced too... If I had to suggest a really cheap cell that I felt a bit more comfortable with, trustfire would be it at this time.

Then maybe just a different charger.... Here's a cheap one that my gut tells me would be a bit better as an example:

http://www.batteryspace.com/index.asp?PageAction=VIEWPROD&ProdID=4862

 

[Capo_au]

Thanks for the link, i will check it out.

I'm also now looking into the Yoho-122 charger (bottom of the page) which has been getting some good comments and pairing it with a couple of protected 25500 3.7v cells.

I'm not having much luck finding information the batteries so that may be a dead end yet. The unknown/no name brand of the batteries also is a worry.