Talky McNeighborson

Hello,

So as usual I've tried to google and search threads for some help here but I can't really make heads or tails of this. I Just have a simple question. I have a Team Associated Rivla running the factory brushless 2000kv Reedy Motor. I've been trying to sniff around for a while and find some decent info on what LiPos people have been replacing the weak factory NiMH batteries that come with my truck. The problem is even after reading the basics of LiPo rating, discharge rates, milliamps, voltage, yadda yadda yadda, i'm still at a loss for which battery pack(s) I can use for my truck, I've got a basic question I was hoping someone could answer for me:

Team Associated says the truck is compatible with two...TWO...ridiculously expensive lipos at 2S and 7.4v. The problem is they leave it at that with no hint as to what discharge rate or even mAh. Is there any way I could buy ONE battery pack at 4S and 14.8v and keep from having to spend an arm, leg, and a few fingers for change for a battery pack? I still have to buy a LiPo compatible battery charger and I'm on kind of a tight budget here.

Any advice or info on what you folks are running would be greatly appreciated

Maj_Overdrive

My Feedback: (11)

Sure you could get a 4s brick pack but you'll be on your own figuring out how to secure it in the truck. A 4s brick pack is going to be too tall to fit on either side of the chassis with the stock battery hold down. Plus the battery leads on the esc are made to hook up two batteries in series. You'll either need to modify it or buy a battery connector to make a series bypass plug. To make the bypass buy the appropriate connector that would be used on the battery and assemble it with a piece of wire connecting the positive and negative side together. Plug it into the esc's battery leads and then you can use a single 4s pack.

As for the lipo ratings you obviously read about c ratings and how when you multiply the c rating by mah you get how many amps the pack can sustain. Well for a 1/8 monster truck you want to be able to sustain at least 100amps, preferably more like 150amps as c ratings are inflated by all manufacturers. My recommendation is to get 5000mah packs with a 30c rating or higher. 5000mah is a common size, provides good runtime, less mah means less weight but in a monster truck it doesn't make a noticeable difference. Higher than 5000mah packs can get pricey all because they're higher than the "standard" 5000mah. Don't go lower than 30c. You're looking at lower prices packs and I guarantee you'll waste $ with lower prices 20-25c packs. They'll puff after a few runs because you'll be pushing them too hard.

I'm not sure where you're shopping but Hobbyking, Hobbypartz have some of the cheapest lipo prices. I've had good luck with Turnigy (Hobbyking) and Gens Ace (Hobbypartz and other places) and the I haven't heard anything bad about Sky lipos on Hobbypartz. SMC and SPC are supposed to be a good value as well. Check out cheapbatterypacks.com as well.

Theres lots it's of cheap chargers on both the Hobbyking/Partz sites. Just be aware that most AC/DC chargers have a lower charge rate when powered by AC. My advice is to get a DC charger and find a good used power supply or convert a computer power supply. Also you don't need a dual charger to charge two packs at once, just a 4s capable charger and a simple series harness for the main battery leads and another for the balance plug. I've seen them pre made but they're not hard to make yourself.

Talky McNeighborson

Awesome, that clears pretty much most of my confusion up. Thanks alot man!


Also, didn't know about the AC/DC thing, I'll definitely look into that as well as the PC power supply, i think i've still got an old 350w laying around

Maj_Overdrive

My Feedback: (11)

Yeah 9 out of 10 AC/DC charger have two separate output ratings. The AC power supply they put into AC/DC charger is a rinky dink little thing often 90watts or less. Actually 90watts is the highest I've seen, the lowest I believe was 60watts. For lipos you want to charge them at 1c which means for a 5000mah (which is 5.0amps) you want to charge at 5.0 amps. A 90watt max charger is fine for a 2s or 3s as max voltage for a 3s is 12.6v so with a 90watt limit you can charge at up to 7 amps. 90watts divided by 12.6v = 7.something amps. But step up to a 4s pack at 16.8v and the maximum a 90watt charger could put out is 5amps. So you'd be maxxing out the charger when trying to charge at 5amps, heating up the charger and it's likely the output would actually be a little less than 5amps anyway.

So yeah, going with a DC power supply is definitely the way to go. If you have a on old PC power supply on hand I'd go with that. There's lots of info out there about converting them for uses like this and you'll be saving $ over an AC/DC charger.

Talky McNeighborson

Okay, Cool. I ended up going with two Turnigy 5000mah 2S 40c Hardcase LiPos from HobbyKing, following the C rating by mAh formula they should be right at 125amps If i'm not mistaken.

Maj_Overdrive

My Feedback: (11)

You multiply the c rating by the mah and that tells you the amp draw the pack can supposedly sustain. Mah is milliamperes, so 5000mah is 5amps. 5 * 40c = 200amps.