Choosing a battery...

[BrianG]

My "racing" revo has a limited space for batteries so I'm looking for a pack that will deliver 16v (or more) and fit in a space of the following dimensions:

Width: 2" (50mm)
Height: 1.5" (38mm)
Length: 5.5" (140mm)

I was planning on using Emoli cells (which would fit), but might just try Lipos (against my better judgement) for weight reasons. However, I can't find Lipos that will work. I'm using a fairly light (10lb) Revo with a 10XL motor with a final gear ratio of 16, and am looking for something around 4-6Ah. So I figure a steady current rating of 80A should suffice, but can't find anything that will work. Any ideas?

[MetalMan]

Is that the total amount of space? Or are there two of those? If there's only one battery spot, then that's going to definitely limit your space, and there might not be any choices.

[BrianG]

Yeah, that's it. I'm sure you've seen it, but this is where it is going (3rd pic down): http://www.rc-monster.com/forum/show...4&postcount=41

Like I said, I originally planned to use those Emoli cells. However, I forgot I could use those LiMN cells over at Boom Boom Batteries. Those will fit too I guess. So, it looks like those are my only two choices.

I've looked thoroughly at Maxamps and BoomBoom. Any other good places to look?

[MetalMan]

It kind of depends on your budget. The lightest weight/highest energy and power density will be Thunder Power. Then there are the Hyperion/Polyquest/Poly RC/Flight Power/etc. packs that are made from Enerland cells (the 20C packs) that would be another choice.

To be able to get a capacity of 6Ah with 5s Lipo (if that's the voltage you're shooting for) you'd need more space. If you go with less capacity with 5s Lipo, then it'd be more feasible. With 4s Lipo, you can get a higher capacity (obviously).
Here's a pack that would fit (Enerland cells):
http://aircraft-world.com/shopexd.asp?id=3765

[Batfish]

It should also be noted that RC-Monster.com happens to carry lipo packs that are very similar in size:
http://www.rc-monster.com/proddetail...2S_3700&cat=18

143mm X 50mm X 15mm (approx)

They may be something to consider.

[MetalMan]

Woops, forgot about those :019:. I'm pretty sure that those packs use the same Enerland cells.

[BrianG]

FYI: I changed by setup so it has the following battery dimensions now:

Width: 2" / 50mm
Height: 2.5" / 64mm to be reasonable, but can go higher if need be.
Length: 5.75" / 147mm

I am still eyeballing those EMoli and/or M1 cells. I think I asked the question before if it was OK to create two 5s packs, and then wire them in parallel on the truck, or if it was better to run five sets of 2p, and then wire those in series. I believe I was told the latter was better (5 sets of 2p) for balancing purposes, but then I read the following over at aircraft-world.com (BTW: that page has some pretty nice info):

Should I use a pre-asssembled "unit" pack or wired individual packs?

We recommend that individual packs be harnessed or wired together, in series or parallel, whenever you need to increase voltage or capacity. Compared to a single "unit" pack, harnessed packs have these advantages:

1) Easy to check, easy to balance, easy and safe to charge: Two packs harnessed in parallel - for example 2200-3S to achieve a 3S2P, 11.1V 4400mAh pack - can be disconnected at any time, so you retain the ability to check each individual cell through the multi-connectors, and to charge safely by separating the packs and using PCM Guard or Balancer devices. If you have a "hardwired" parallel pack in a single unit, you don't have the ability to clearly diagnose a single weak cell in the pack.

2) Easy to separate for use as a single pack: If you decide to retire that big model, you can instantly revert your 3S2P pack to a pair of 3S packs, for use in a smaller model.

3) Easy to re-wire. Need to convert that 3S2P pack into 6S? With a harness, it's a snap. A hardwired pack would likely be damaged in the arduous job of re-soldering the individual cells, as required in such a conversion.

4) Easy to arrange. Different models require different pack shapes to fit and balance the model. Harnessed packs allow you to choose the optimal arrangement - front-to-back (inline), side-by-side, or whatever you need.

If I am reading this correctly, it says to create two 5s packs and use them in parallel, but charge seperately?? That makes sense to me, and as long as each pack is balanced to exactly 4.2v/cell, it should be OK right? The reason why this would work well for me is I can buy one Dewalt 36v pack (contains 10cells) and rearrange them for two packs of 5s1p. This would allow me to use them as 5s2p in the truck, but use the factory charger (with a little modding of course) wired as 10s1p. Or am I really off base on this?

[BrianG]

I got tired of doing all the various calculations long-hand for the different battery types, so I made this. Maybe someone else has a use for it...

[MetalMan]

You could take two 5s packs and run them in series, and charge them individually. But, the A123 cells are most likely connected together using thin (welded) nickel strips, so you'll want to rebuild the pack. Just make sure that if you do this all of the tap wires go where they were before you disassembled the pack.
I haven't read anything special about the A123 cells. What I have read people seem to be saying that these cells are similar to the Emoli cells in that after a certain current is passed, the voltage starts to sag (~35amps for the Emoli cells). The A123 cells are a lower voltage, their max charge voltage is 3.6v, compared to Emoli/Lipo of 4.2v. That puts the nominal voltage of the A123s ~3-3.2V. And I still haven't read anything on them about LVC voltage if it's necessary and if so at what voltage it should be set to.

[BrianG]

Thanks.

Actually I was talking about running 2 x 5s1p packs in parallel on the truck, but then charging them as 10s1p using a different Y cable.

I read the information about the M1 cells too. That low 3.6v charged voltage is probably the only thing keeping me away. However, if I use the "stock" charger, all should be well. Still, they're bigger than the EMoli cells, have less voltage, and less capacity. The only reason I'm eyeballing them is for the safety aspect and the fast charge time is nice too. In the end, I'll probably end up with Lipos. I just wish the "Holy Grail" of batteries would come out soon, but that won't happen until the day after I make my purchase. :rolleyes:

[MetalMan]

I thought the A123 cells are smaller than Emoli cells? Emolis are 26mm x 70mm, A123 are 26650, which is 26mm x 65mm.

[BrianG]

Originally Posted by MetalMan

I thought the A123 cells are smaller than Emoli cells? Emolis are 26mm x 70mm, A123 are 26650, which is 26mm x 65mm.

I assume you are referring to the spread sheet? If so, you are right. I must have switched them around for some reason (probably when I named the cells at the top). From what I've read, Emoli's are 3Ah @ 26.5mmD x 70mm @ 99g, and M1's are 2.2Ah @ 26mmD x 65mm @ 70g. I'll fix that on the sheet when I get home tonight. Sorry. :dft001:

[Edit]: Spreadsheet fixed and uploaded...