Battery packs: parallel or series?

[The Light!]

Hi all, just a quick question, since I tried a CPF search but didn't quite come up with the answer I needed:

What would give you the most runtime out of any battery pack? One in parallel or one in series? Can you even weld the batteries in parallel without worrying that one is going to die from being overdischarged/spontaneous a-sploding?

Thanks!

 

[Norm]

If your light is optimised to run either way, it shouldn't make any significant difference.
Volts go up current comes down and vice versa.
You really need to post more info about the setup you have in mind.

 

[The Light!]

Well, this isn't for a specific light atm, just an idea floating around in my head.

Setup will be both 12V incans (think osram, WA, or Hikari) and LED's. I reckon the max estimated current draw would be 15-20A with everything turned on (with everything turned on... yes I know that's a LOT! But the battery pack should handle it, if I use big enough cells for capacity).

Basically what I want to know is this:
If I wire the cells in parallel, and one reaches max discharge, will it be overdischarged by the other cells still running, like if it was wired in series? And what other effects would having a parallel-welded battery pack produce compared to a series-welded one (current, voltage, discharge rate etc)?

 

[SilverFox]

Hello Chris,

We almost have enough information to help you...

12 volts at 20 amps is a good start, now, how long do you want this to last?

A 50 Ah automotive lead acid battery (6 cells in series) should give you between 45 minutes to 1 hour of run time.

Tom

 

[steve6690]

Depends on the battery type. I don't think it's recommended to charge Ni-Mh in parallel. Anyone confirm that ???

 

[SilverFox]

Hello Steve,

That's correct...

Tom

 

[odessit]

Parallel vs Serial configuration choice also depends on your driver (if you are using one)
Some drivers will work most efficiently at certain voltage (90% efficient at 6v and 75% efficient at 12v - you get the idea)

 

[LuxLuthor]

Probably good to know if you are looking at Lead Acid, Ni-Cad, NiMH, or Li-Ion (of which there are several chemistry types). You need to know your voltage & current requirements, then how much space for batteries.

• Parallel increases capacity (mAh rating of battery) while keeping voltage the same...so it will run longer.
• Serial increases voltage, keeping capacity limited to that of single cell.
• Serial + Parallel (i.e. 2s2p with 4 cells) doubles capacity and voltage.

As Tom said, you cannot charge NiMH that are in parallel, but you can discharge them in parallel. No such restrictions on Li-Ion.

 

[45/70]

As Tom said, you cannot charge NiMH that are in parallel, but you can discharge them in parallel. No such restrictions on Li-Ion.

If this is true, and I've heard it before, how does the C. Crane charger work? It would seem to be a parallel charger. Maybe not?

Sorry The Light!, maybe pushing the limits of staying on topic here.

Dave

EDIT: After checking the C. Crane with a DMM and looking further into the link above, the C. Crane is, in fact, a parallel NiMh charger. Hummm......

 

[The Light!]

Don't worry 45/70, everyone (including me! ) has done their share of off-topic stuff!

Battery types will either be NiCad F size batteries from Batteryjunction, or D size NiMh (not sure about battery life on these? I have the impression NiCads have a longer battery life EDIT if cared for properly and a good battery zapper /EDIT), probably with an auxiliary battery made with the safer type of Li-Ion (I think it's the IMR types from AW, with Li-Mn chemistry, is that correct?).

The incans will be direct drive, and the LED's will need some sort of buck/boost driver so they dont burn out.

• Parallel increases capacity (mAh rating of battery) while keeping voltage the same...so it will run longer.
• Serial increases voltage, keeping capacity limited to that of single cell.
• Serial + Parallel (i.e. 2s2p with 4 cells) doubles capacity and voltage.

OK then, so to get 12V I'd have to wire them in series, and for long runtime, parallel... Well that makes it easy to build a 12V battery pack with long runtime!

So how does the serial + parallel battery pack work? Seems a bit confusing. Anyone got a diagram I could look at?

 

[SilverFox]

Hello Dave,

The C. Crane charger was such a good idea that it is no longer being made...

However, it did have some unique features.

I believe it is able to charge a single cell. When you put the second cell in, it charges the two cells in parallel. The charge is terminated when it reaches around 85% full charge, then the charger drops the charging current back to a top off charge to bring the cells back up to full charge.

Also, you are given the option of discharging the cell(s) before charging. If you start with fully discharged cells and only charge to 85%, you can probably get by with parallel charging. Although, if you took the extreme case of charging a completely discharged cell with a cell that is only about 5% discharged, you will end up with an unbalanced pair of cells, unless you let them trickle charge for a few days.

If there is no other option than to charge NiMh cells in parallel, the usual method is to go for a 16 hour charge at a 0.1C charge rate.

Tom

 

[The Light!]

Just as an add-on to my last post, there is still something I'm confused about: if the cells are wired in series, why do they only have the mAh of one cell? Wouldn't it be:
mAh cell1 + mAh cell2 + mAh cell3 + ... = total mAh of pack

 

[45/70]

mAh cell1 + mAh cell2 + mAh cell3 + ... = total mAh of pack

What you describe here is correct for parallel, and the Volts would be the same as one cell.

For series it's, Volts cell 1 + Volts cell 2 + Volts cell 3 + ... = total Volts of pack, and the mAh is the same as one cell.

Dave

 

[The Light!]

OK, now I understand! Thanks! Will be editing this post soon to include a diagram of how the serires-parallel battery pack might look.

EDIT: is this how a series-parallel battery pack would look?

 

[45/70]

The C. Crane charger was such a good idea that it is no longer being made...

Thanks, Tom. Is that a SA remark? For it's time, I really think it wasn't a bad charger. The three (or is it four?) stage charging process seems to work quite well. I only use mine for charging D cells. It beats my, even older, Rat Shack 500mA 2s+2s charger, because with that one, while it has some sort of charge termination detection, it also has an 8 hour "safety timer" so 10 Ah cells take 2 restarts.

Yes, you can charge 1-4 cells with the Crane however, the charge rate is proportional. 1800mA for one, 900mA for two, etc. (for D's and C's). So, it's not any faster than the Rat Shack one, but I don't have to remember to restart it twice!

It's always done a good job for me. I don't have too many D's anyway. I was considering buying a Maha 808M for the D's and being able to charge 8 cells (AAA-D) on one charger, but with the deal on the C-9000's, I went that way. With the one I have already, and two more on the way, I'll be able to say:

"It's like having twelve charger-analyzers for AA and AAA NiMH/NiCD batteries"!

Dave

 

[Mr Happy]

EDIT: is this how a series-parallel battery pack would look?

No!!! That's a fire!

If you remove the green wires then it's fine.

The short answer is this: two cells have twice the energy storage of one cell, and this remains the same whether they are wired in series or parallel. The difference is that energy = volts x capacity. If you wire them in parallel you get energy = volts x (2 x capacity) whereas in series you get energy = (2 x volts) x capacity. The final answer is the same either way.

You can wire two cells in series OR in parallel, but not both at the same time. To do both at the same time would result in getting something for nothing, and since this would violate the laws of physics it is impossible.

 

[LuxLuthor]

The Light, it may seem confusing, but with batteries in series, the current output capacity remains the same. Remember you are now getting twice the voltage output with two cells in series. It is good when you have a chance to read some of the fundamentals at www.batteryuniversity.com

I don't believe that C.Crane charger would work properly with NiMH in parallel, unless it had sensors in each battery bay/slot, which I doubt. The heart of the problem, unique to Nickel based chemistries is that when any one cell in parallel drops their internal resistance, they heat up, more current will flow through it at a given voltage. It can lead to a NiMH thermal runaway explosion. In effect this one cell steals charging current away from the cells that are not yet charged. Can this lead to problems? I'll let you be the judge.

It may be theoretically possible to charge NiMH in parallel using lower trickle charging voltage following higher voltage...but there needs to be a reliable way for the charger to sense the termination drop in voltage to signal end of charge, or a timer cutoff. Neither are going work properly in that type of charger, nor will it be able to tell without individual cell sensing the degree of imbalance when charging starts.

Imagine you put one of four cells that was fully charged. It is going to be subjected to the higher current flow because the other three empty cells have more impedance. I don't see anything "good" about that Crane design, nor does it appear to be emulated by any other reputable charger design I have seen for NiMH.

 

[SilverFox]

Hello LuxLuthor,

Actually, the C. Crane charger seems to work very well...

There must be a little magic in their algorithm.

Tom

 

[LuxLuthor]

Hello LuxLuthor,

Actually, the C. Crane charger seems to work very well...

There must be a little magic in their algorithm.

Tom

You may be right, but based upon the fundamentals of Nickel charging that I have read so far & correcting charge current for impedance variations among parallel cells, color me "skeptical." Why is it no longer made, and not shared as a viable technology among other Nickel chargers?

I can't see how it could safely charge 1 full cell & 3 empty of various mAh capacities. It has to sense ΔV to either change charge current stage, or terminate.

 

[Mr Happy]

You may be right, but based upon the fundamentals of Nickel charging that I have read so far & correcting charge current for impedance variations among parallel cells, color me "skeptical." Why is it no longer made, and not shared as a viable technology among other Nickel chargers?

I can't see how it could safely charge 1 full cell & 3 empty of various mAh capacities. It has to sense ΔV to either change charge current stage, or terminate.

A lot of industrial designs are based on very conservative design approaches. It takes a strong commercial case for anyone to innovate and deviate from the norm. With NiMH nobody is going to get fired for using a series connected -dV charging approach. It doesn't necessarily mean it is the best or only way, but it is a way that is known to work.

However I am quite confident I could come up with a parallel charging approach that would also work, and from the snippets I am reading about the C. Crane charger in this thread my thoughts seem to resonate with their design. I think it would certainly be necessary to sense voltages or dV/dt to select the charging phase, but there are ways of doing things with diodes and voltage summers for instance that would be safe and regulated.