Complete noob question about a DIY battery pack

[jam]

I'm going to put together a rechargeable lithium battery pack. I've not done this before, so please forgive me if any of these questions seem kind of naive (and I strongly suspect that they will).

I'm putting this together to drive a triple Cree XPE light using a bflex delivering 1A, and need a total of 14.8V and around 5Ah. I'll be charging this with a Turnigy Accucel 6.

What sort of protection does this pack need? So far I'm thinking that I'll just put a polyswitch in each series. Would a 2A polyswitch be correct, or should I be using a higher/lower rated polyswitch?

Do I need a PCB in this somewhere? For charging, can I just rely on the Accucel charger to cut off safely? For discharging, I'll simply turn off the light when the bflex gives a low voltage warning. Hmm... on second thoughts, I can imagine missing the low voltage warning... PCB is probably a good idea.

How should I charge this pack? Should I charge it as one entire unit, or should I charge each series separately? If it's as two separate series, then that means that I should use 2 PCBs, right?

 

[yellow]

a protection PCB is a MUST, believe it or not

f.e. such ones here: http://www.batteryspace.com/index.asp?PageAction=VIEWCATS&Category=1376

keep the pack together, charge and use it together, do not align in parallel "on the fly"
(f.e. if You make 2 packs and then connect them in parallel to double capacity: when there is a different state of charging, the voltages get a "forced evening out". In fact thats a short

 

[jam]

Thanks for that. Is there any concern with using a charger to charge a 4P2S pack? The charger can apparently deal with up to 6 cells in series, but makes no mention of parallel setups.

 

[Mr Happy]

Note that what you have drawn is a 4S2P pack (4 in series, 2 in parallel).

To help keep the pack balanced, you should cross-connect each of the intermediate parallel pairs, not just at the ends. If you connect two Li-ion cells in parallel it behaves like a single cell of twice the capacity. You then put four of these "double cells" in series.

It would be a very good idea to connect balancing taps from each of the 3 intermediate connections and use a cell balancer when charging. Otherwise the charger will control the overall voltage of the pack, but it will not know if any individual cell is exceeding its voltage threshold.

 

[LuxLuthor]

Words of Wisdom: Take your time up front and do reading on the fundamentals. Start by reading the entire sections at www.batteryuniversity.com

Then learn about the chemistries, what can go wrong, what "S" & "P" means, how to construct them, what kind of chargers to use, PCB circuits, etc.

 

[jam]

Mr Happy- thanks. Just to be clear, are you suggesting that a 2P4S arrangement would be better? I'll look up cell balancing- thanks for mentioning the term.

LuxLuthor- I'd already read most of the articles at batteryuniversity. Very interesting reading, but unfortunately it stops somewhat short of the more practical aspects that I need to learn in order to actually put a pack together with some degree of safety. Any sites that you point me towards that would cover how to charge parallel batteries would be much appreciated- everything I have found so far only deals with single cells.

 

[Mr Happy]

...it stops somewhat short of the more practical aspects that I need to learn in order to actually put a pack together with some degree of safety.

To be frank, you are not demonstrating the requisite knowledge to put a lithium ion pack together with any degree of safety. I would advise you to put your plans on hold until you have acquired the missing knowledge.

Here's the deal: anyone can play with lithium ion cells and they might not blow up on you. However, the lack of fireworks in such experiments is not evidence of safe practices. It is simply evidence of being lucky so far.

You can be lucky many times, but you only need to be unlucky once.

 

[LuxLuthor]

Jam, there are a number of threads at the RC Groups forum such as this one that can add to your knowledge.

Parallel cells are treated as a single cell (with larger capacity). They should have relatively close voltages before bridging a parallel set (i.e. two cells joined together in parallel of 3.90V & 3.95V is fine; 3.85V & 4.15V is not--they should be manually individually charged/discharged to be closer to each other before joining parallel).

You need to also learn about "balance tap leads" which are illustrated in the above post. These smaller wire tap leads are used with a Lithium balance charger that compares one cell (or a parallel set of cells) in series with another cell's voltage, and bleeds off higher cell voltage so all the cells in a series pack are close to each other. Again, somewhat similar to parallel discussion, try to get cells in series relatively close before making into a pack.

It is not recommended to solder Lithium Cobalt cells due to their explosive risk. There is much less risk with NiMH, or the new safe chemistry Lithium cells. See this post I did a while back which is in pinned topics of interest thread at top of this battery section.

 

[monkeyboy]

I've never liked the idea of building li-ion battery packs myself. There are a lot of things to consider; balancing connections, PCB, cell matching (very important). Not to mention the danger involved in soldering (best to get tagged cells). You also need to make sure you don't block any vent holes.

Even the ready built packs on batteryspace seem a little dangerous. Many don't have balancing connections meaning that imbalances can develop over time between cells. Li-ion packs are not like NiMh/NiCd packs that can simply be trickle charged overnight in order to balance them.

By comparison, modern li-ion packs found in laptop computers have highly sophisticated protection and balancing circuits.

Jam, Have you considered using individually protected cells in a battery holder and then charging individually? I know this would be a PITA but at least it would be safer and the cells should last longer too.

 

[LuxLuthor]

I've never liked the idea of building li-ion battery packs myself. There are a lot of things to consider; balancing connections, PCBs, cell matching (very important). Not to mention the danger involved in soldering (best to get tagged cells). You also need to make sure you don't block any vent holes.

Even the ready built packs on batteryspace seem a little dangerous. Many don't have balancing connections meaning that imbalances can develop over time between cells. Li-ion packs are not like NiMh/NiCd packs that can simply be trickle charged overnight in order to balance them.

By comparison, modern li-ion packs found in laptop computers have highly sophisticated protection and balancing circuits.

Mostly all valid...but the Lithium Manganese (one of the "safe Lithium chemistries) actually do pretty well without regular balance charging and/or balance/protection circuits--within reason.

 

[jam]

Thanks guys. I appreciate the pointers, as well as the blunt-but-tactful advice about how wise an this idea is.

I know that I am missing large chunks of requisite knowledge... as I see it, the only way that I'm going to be able to learn about this is asking (initially stupid, hopefully less so with time) questions and reading as much as I possbly can. If I think that I'm getting a grip on this, I'll go ahead and experiment under a controlled enviroment- obviously this area is not as simple as I initially thought it would be. However, I really do want to learn about this.

Monkeyboy- Yep, I'm already using individually protected cells and will continue to do so.

 

[TheSwede]

First of all, thank you for some very useful pointers, Lux Luthor. :thumbsup:

I have wandered between prebuilt NiMh-packs, prebuilt Li-Ion packs, RC-helicopter LiPo packs and DIY Li-Ion packs for quite some time now.

After reading the posts you linked to I have two questions regarding building a Li-Ion battery pack:

1) If using a charger with balancing functionality, like the GT Power V5 to charge a pack with integrated protection circuitry (4 Li-Ion cells with this PCM) will the charger's balancing conflict with the PCM in the battery pack? Or is it just plug-and-play(pray?)?

2) Is soldering wires to the poles of a Li-Ion cell totally out of the question even if flux and persons with high soldering skills is used?

Regards,
Anders