Basic Battery Knowledge Forgotten

[Stu :)]

When building battery packs do you place the cells inside in a serial then parallel OR parallel then serial configuration? Picture attached.

I'm building some new batteries using four Panasonic CGR18650As (Lithium-Ion, 3.6v @ 2000mA) in order to get myself a 7.4v 4000mA pack.

...I hope this isn't one of those old age signs :eek:

Stu :)

[Kmot]

series-parallel

Try your setup with some AA's to verify your pack.

[nick_75au]

When I pulled apart a laptop battery it had cells with the same part number, it was wired as your second diagram, ie parallel each pair of cells(increase mAH), series each parallel pair (increase voltage).

The end result electricaly will be the same no matter which way you do it double voltage and capacity. Only if batteries require balancing then you have to use the second diagram and add a tap. I believe these batteries are much more robust than li-poly and balancing is much less of an issue.

The laptop battery used parallel series for the balancing connections. Ni-cads and NiMH don't require individual balancing of cells so the first diagram is used for those packs as its probably much simpler to make.
Regards
Nick

[Ghost 2501]

with lithium batteries, they have a reputation of being finicky, best place to ask would be the battery firum in the plane area, they know all that is to be known about lithium tech batteries.

[nick_75au]

The batteries Stu is using is different chemistry to Li-po (Lithium_Polymer) they are Li-Ion (Lithium Ion) which are much less temperamental.

The first Lipo (and Last) I had puffed the first time I used it.

The laptop batteries I found in a dumpster, had no charge at all, charged each cell direct off a 12 v battery for a couple of minutes, left them for a few months because I got scared from all the Li-po hype, got a li-po/li-ion capable charger charged them and use them regularly now.

They don't have the "C" rating of the Li-po, mine are only 2 c, 1350 mAH. They appear to have a safety feature, they cut out if too much current is drawn but start working again. They are similar in chemistry to the A123 batteries but have lower charge and discharge capability's.

Ghost is right about the air plane forums, those guys are the pioneers when it comes to battery technology.
Regards
Nick

[E-Challenged]

Lipo batteries do not normally puff up and become useless unless they are defective or discharged to less than 3 volts per cell. They do not often explode or burst into flame unless they are punctured/crash damaged or grossly overcharged i.e., a 2-cell lipo hooked to a charger mistakenly set for 3 cells. Lipoly packs can be damaged by being charged fully and allowed to sit without being discharged for long periods. Lipoly packs can be damaged by being charged with cells that have become unbalanced, the cells with higher voltage than the rest become overcharged while the lower voltage ones may never reach full charge. Good quality Lipoly cells can give good performance for hundreds of cycles if kept in balance, charged and discharged properly. Occasionally, a lipoly pack from a good manufacturer can be defective and won't give good service. Name brand suppliers will often replace bad packs for free or at reduced costs.

[Stu :)]

Thanks guys for you're answers and you're quite right. I'll go ask this question with our resident aviators, E-Flight stuff seems be forefront of motor and battery technology.

I source my CGR18650As from laptop batteries, I go along to the odd computer auction and pick up two or three no guarantee / un-tested packs for for about $10 and pull them apart. Normally there is two or three cells that are completely dead and cause the pack to "fail" in the laptop, but the rest are fine; just recondition each cell using my intelligent battery charger (http://www.multiplexusa.com/RC-comp/Chargers/LN-5014.htm) and volia! Cheap, light and pack some punch, although not quite the same dump as you would get from a high performance NiCd pack.

Stu :)

[Stu :)]

Got a reply with a VERY good link that has to be shared!...

http://scriptasylum.com/rc_speed/_lipo.html

It even shows where the taps need to go :D ...right where's the soldering iron.

Stu :)