Rechargeable Cells

[dee]

I've been using NiMH cells for a while now but don't like the way they discharge over time.

Can anyone tell me if NiCD and Lithium Ion/Lithium-Ion Polymer do the same (are LiPoly available in standard cell sizes)?

Also, is it OK to wire Li cells in parallel where standard cells are in series (given the 1.2V vs 3.7V of the two chemistries) or will this damage the cells/light somehow.

 

[shiftd]

All rechargeable batteries discharge over time, it is how rapid they discharge that is different. People say that the NiCd hold its charge better than NiMh, but still, fairly high self discharge. Li ion, IMHO, is better than those two, but still, they self discharge too.
li ion polymer is used because of it can give high capacity even though it is of smaller size. But the use of polymer was only for low drain purposes, like a couple (to tens, maybe) of miliamps.
I think it is ok to wire them in paralel, as long as you did not mix diff type of batteries, mix old and new; also, you need to be sure you get the same voltage as the previous arrangement.

 

[JSWrightOC]

To add to shiftd's statement, all batteries (including primary disposable) have a self-discharge. This is the number one reason why Alkalines are stamped with a "best if used by" date.

To the best of my knowledge, Lithium primary cells have the slowest self-discharge; After that, I would say Alkaline, Li-Ion/Li-polymer, NiCd, and then NiMH. I'm not sure where lead-acid fits in.

It is perfectly acceptable to connect cells of the same chemistry in parallel, as long as they are identical in rated capacity, and it helps if they are of the same age or better yet, brand new. Also, it would be better, for example, to wire a battery pack of eight cells in a parallel-series configuration, where you have four pairs of parallel-connected cells in series, instead of having two quad-cell series wired sets in parallel. I hope that makes sense. /ubbthreads/images/graemlins/smile.gif

Conversely, it is NOT a good idea to connect cells of different chemistries in any configuration. /ubbthreads/images/graemlins/tongue.gif

 

[eluminator]

Connecting cells in parallel could have some advantages. The thing I would worry about is accidentally connecting two cells in parallel that had considerably different voltages, like a charged NiMH with a discharged NiMH. That might cause a large current flow.

The self-discharge is temperature dependent. I'm told the rate doubles for every 10 degree C temperature rise. That's why I keep a charged set of replacement batteries in the refrigerator.

I believe NiCad self-discharge at about half the rate of NiMH. But they have somewhat less capacity and shouldn't be disposed of in the garbage because the Cadmium is toxic.

 

[dee]

Recharging Li-Ion

Thanks folks, all useful info - think I might get a couple of Lithium-Ion cells and see what I make of them.

One more question though, can cells (specifically Lithium-Ion) be charged in parallel or would they need to be rearranged into a serial configuration?

 

[CM]

Re: Recharging Li-Ion

You definitely don't want to charge them in series. I don't know about parallel charging, my inclination is to charge each cell individually. Someone can pipe in here whether it's advisable to charge them in parallel as I have no experience with this. I just got some LTC4054-4.2 single cell chargers and wired one up. Works just as advertised and each one cost $1.66 not including shipping which is $7.00 I think. For the DIY, this is a great product from Linear Tech.

CM

 

[JSWrightOC]

Re: Recharging Li-Ion

I know for a fact that many laptop and camera batteires hard-wire the Li-Ion cells in a parallel fashion, charging and discharging. The two big concerns with Li-Ion are the overcharging state (no higher than 4.1-4.1V) and the low voltage cutoff (2.5-3V, depending on who you ask). I would say charging Li-Ion cells in parallel is safe, but it would be good practice to charge them separately if they are in dissimilar charge states. CM is right about not charging in series _unless_ you have a Li-Ion pack controller (found in any Li-Ion pack greater than 3.6V) that measures each cell's voltage independently and cuts off charging when the first one reaces 4.1/4.2V. Even so, it's healthier to charge them in parallel or independently, as it allows the cells to equalize, improving performance and extending cell life.

eluminator, there are NiCd/NiMH chargers on the market that configure all bays in parallel with each other, and have no mention of the hazards of connecting a charged cell directly to a discharged cell. I personally don't think it's good practice, but you have to consider that no one has issued any recalls regarding such products. I'm sure it does add to cell stress and reduces lifespan somewhat, so that's why I try to avoid it.

 

[dee]

Re: Series Charging Li-Ion

I'm using a Maha C77Plus charger. At http://www.mahaenergy.com/products/prosumer/mhc777plus.htm one of it's features is listed as
[ QUOTE ]
Supports a wide voltage range of 1.2V to 14.4V (1 to 12 cells) for NiMH & NiCD, and 3.6V to 14.4V (1 to 4 cells) for Lithium Ion.

[/ QUOTE ]

Surely this implies that a Li-Ion pack made from cells connected in series can be charged as is (certainly this is true for NiMh as I currently charge a bike light built using 13 cells in series)?

 

[JSWrightOC]

Re: Series Charging Li-Ion

Every factory-manufactured Li-Ion pack has a charge limit controller built into it, which will stop the flow of current if any one cell reaches 4.1/4.2VDC. The Maha charger is probably accounting for this, unless it specifically says it can charge bare cells in series.

 

[CM]

Re: Series Charging Li-Ion

JSWrightOC is correct. If any cell reaches 4.1/4.2V, then that cell is fully charged. If you have weak cells in the series connection, just monitoring the series voltage could spell disaster for that one fully charged cell (potential KB!) and possibly yourself. For safety's sake, I would stick to single cell charging. I know it's a PITA, but I'd hate to be party to any incident involving accidents with these things.

Your other option is to go with Pila's since each cell has a built-in charge/discharge protection. They're pricey but more user friendly than bare cells. Each Pila charger can do two cells simultaneously.

CM

 

[Ray_of_Light]

Re: Series Charging Li-Ion

Commercial Li-ion cells (like the PILA, or those used in camcorders or cell phones) have a built-in electronic device to end the charge and disconnect if the battery is overdischarged.

If you are talking of bare Li-ion cells, you can charge them in series or parallel, with a termination voltage of 4.1 volt. THe charger HAS to disengage when the charging current falls under C/50, of you may destroy the batteries.

I do not suggest the use of bare cells in chargers born for different use.

Also, even a single overdischarge will transform a bare lithium Li-ion in a paperweight.

Anthony

 

[PaulW]

Re: Series Charging Li-Ion

[ QUOTE ]
CM said:
. . . Your other option is to go with Pila's since each cell has a built-in charge/discharge protection. They're pricey but more user friendly than bare cells. Each Pila charger can do two cells simultaneously.

CM

[/ QUOTE ]

I have ordered a Maha 777 plus2. It's on its way. Can I deduce from what you say that I can safely charge Pilas in series in that charger?

Paul

 

[CM]

Re: Series Charging Li-Ion

Paul, I would verify that operation just to be sure. Do you have a voltmeter? You can discharge one cell and then try to charge it in series with another fully charged cell. Monitor the voltage across the fully charged cell to make sure it does not exceed 4.2V. If the charger attempts to charge that cell further, then the scheme will not work and you should take the cells off the charger an do them individually. Of course if the cell properly cuts out and prevents any current from flowing, then the second cell will be prevented from being charged.

One more important disclaimer. I have no experience with series charging, so please take what I say with a lot of grains of salt and exercise some caution. If someone knows what they're doing, please speak up. What I suggested above is what I would do myself but I've been wrong many times before. As long as you make sure the cell voltages do not exceed 4.1/4.2V, then I think you're OK.

CM

 

[PaulW]

Re: Series Charging Li-Ion

CM,

That's an excellent way to test the charger's capability. Before using it to charge series Pilas, I will run that test. I will also read the instruction manual with care. Oh, and I will not hold you responsible for any possible mishaps. /ubbthreads/images/graemlins/smile.gif Thanks.

Paul

 

[CM]

Re: Series Charging Li-Ion

Paul, can you report back on your findings? Stay safe now /ubbthreads/images/graemlins/grin.gif

CM

 

[PaulW]

Re: Series Charging Li-Ion

CM,

I can't promise to report back. I'll want to play with the charger for a while. And then maybe I'll get the Pilas . . . maybe only one. Maybe two or more. I just don't know yet, for I'm still in the information gathering phase.

By the time I have findings, this thread may be just a memory or an intuition that before I charge Pilas in series I have to test the process to be sure that the voltage across any one cell doesn't exceed a maximum amount. When I go searching for that amount, I may find this (by then ancient) thread. I may find another.

That's just a long winded way of saying that at my age I don't make any promises about remembering anything. But if I do remember I'll report back.

Now, what was I saying? Someone wrote a report about something . . . ?

Paul /ubbthreads/images/graemlins/grin.gif

P.S. No need to worry about me. I'm the guy who, after buying a chainsaw, went out and spent more than the price of the chainsaw on protective clothing.

 

[CM]

Re: Series Charging Li-Ion

/ubbthreads/images/graemlins/grin.gif /ubbthreads/images/graemlins/grin.gif

 

[Christoph]

Re: Series Charging Li-Ion

I need to find out this information to use these in a modded mag. charging would be simpler. But I do have the protective circuit with these so that makes it simpler
Chris

 

[dee]

Re: Series Charging Li-Ion

Just to back up what's been said above, the following comes from maha tech support re: the 777plus2 charger

[ QUOTE ]

We highly do not recommend charging cylindrical Li-ion batteries without the protection circuit. Without it, charging Li-ion cells can become very dangerous.


[/ QUOTE ]

 

[Christoph]

Re: Series Charging Li-Ion

I am using a triton R/C charger made to charge naked LI-ION cells. Thanks
Chris /ubbthreads/images/graemlins/grin.gif