I'm an amateur with batteries and circuits.

I'm working on a project that requires a 5V output. The cells I'm using are dictated by the space I have, and I have enough for 6 18650 lithium ion cells. There are efficient V-regs out there that can produce 5V whether I put these in parallel (3.7V) or serial (22.2V). If I parallel them, I estimate that I can get 90% efficiency in the regulator. If I put them in serial, I estimate 86% efficiency.

I plan to use simple, single cell battery holders (with integrated PCB) for each 18650 cell. Beyond the PCB incorporated into each single cell holder, I don't plan to use any other PCB. I'm probably going to take each cell out of the PCB to charge each cell individually, so the holders are only used for discharging. I'm just going to do a very simple wiring job in terms of connecting the 6 single cell holders, and again, this is only for discharging.

The 18650 cells I'm using are all Panasonic and they're all the same model but they were purchased at 2 different times and some were used more than others, so they're not exactly matched. This is a relatively low current application, maybe 5V 800mA output.

Based on the above information, would it be better to put them in serial or parallel? For simplicity reasons, I prefer not to do a combination of serial and parallel, unless it can be demonstrated that this is a better choice.

Thank you.

 

IMO A 'sacrifice' of 4% efficiency is more than compensated by obviation of the not inconsiderable 'hassles' attendant to paralleling of the cells...

Best regards
HP

 

It is always safer to put batteries in series.

 

It is always safer to put batteries in series.

I understand and I've heard this before. I've actually used these same 18650 cell holders before where I connected 3 of them in series, and they worked well.

I've found them to be extremely safe regardless of what you do to them. They cut off automatically if there's a short and they limit current to 5A max (both in charge and discharge). They also cut off automatically at 2.5V, which IMO, is probably too low for my taste. In summary, I've never been able to do anything to these cell holders (accidentally or otherwise) to get them to do anything dangerous, but I can't say that I've tried everything or rule out that one of them is going to be a dud.

Something I forgot to mention is that the 3.7V regulator cuts off automatically at 2.9V, which is probably better for cell longevity than cutting off at 2.5V. If I put the cells in serial (22.2V), the only cutoff I'm going to have is the 2.5V cutoff of the individual cell holders. So, in essence, I can probably get a better/safer cutoff voltage if I put them in parallel.

One of my concerns with parallel though is that there's going to be an uneven current delivery between the cells which could somehow affect the longevity of the cells. That may not be the case if I had a "master" PCB that controlled all the cells, but as it stands, that's beyond what I can do.

I'm leaning toward serial, but parallel is tempting for the reason of the 2.9V cutoff. But I'm pretty sure that serial will give me a more even current for every cell than parallel.

So, I'm still confused, but, again, leaning toward serial.

Thanks

 

Look at the LTC3300.

http://www.linear.com/product/LTC3300-1

 

As lestraveled indicated put them is serial reason:
No cell is absolutely identical hence one will unload quicker than the other result one is loading the other result all batt's flat after a certain time.
the only way, if parallel is mandatory you should couple them with fets used as diode's disabling above effect. However that will create a charging as block problem.
Picbuster

 

As lestraveled indicated put them is serial reason:
No cell is absolutely identical hence one will unload quicker than the other result one is loading the other result all batt's flat after a certain time.
the only way, if parallel is mandatory you should couple them with fets used as diode's disabling above effect. However that will create a charging as block problem.
Picbuster

Okay, since it's unanimous, I'll go with serial (unless someone else can provide a compelling argument for a parallel arrangement).

Thank you all for your help.