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Battery Questions / Input (speed, distance)


#1

Here are some calculations i made. Would someone more knowledgeable than i review? Did i do this correct? The size motor does not effect distance? is this because you will get same distance but at different speeds? seems like a different motor couldbe less or more efficient, but i guess that is what the weighted section is for.

i used this formula for distance and the superblog esk8 calc for speed.

r spec @6s

r spec @8s

Sk3 @ 6s

Sk3 @8s

If these Calculations are correct i will be leaning to the 8s. Im just trying to narrow down some more parts so i can get things ordered.

I also Found these batteries. Im looking to run 4 of them. 2 in parallel and then in series to give me 8s@10Ah they have a continuous 25C rating with a peak at 35C (10 secs)

If someone could give this the old once over. If i go to 8s i will need to get a new charger since the imaxb6 will not work. I checked the commonly used parts thread and there are not chargers listed. Any standby 8s chargers? in the meantime i will be on hobbyking. Thanks for any input and info.


#2

You Could still discharge in series and charge in parrallel.


#3

The grey quote above looks familiar… hehehe.

Yep your calculations look correct. A good example of the better efficiency you get with higher voltage. 8Ah vs 10Ah and realistically the 1km difference will be negligible.

I too would discharge in series, but charge in parallel - so your charger should work fine. But you’ll need to at least disconnect to charge. Or go with BMS if you wan in-board charging. I don’t mind the extra step as it also lets me inspect my lipos and keep an eye on them when charging (vs in board). In pyrex container etc.


#4

alright good info! Keep it coming. i don’t mind taking them out to charge as well. it is only one more step and i plan on putting them in an easily accessible container. also if something should happen while charging the whole board doesn’t wind up in flames. ;] will this take forever to charge them this way? it isn’t a huge deal as it will be for after work cruises, just trying to get my ducks in a row. not crap i have no battery let me charge at the office.


#5

charging - that’s another whole can of worms. Or down the rabbit hole or something…

Your charging is going to depend on a couple key things.

1 - pack life vs quick charge. Your battery will have a charging “c” rating. Just like the discharge rating of the battery - it lets you know max charging amps.

2 - more complex setup with AC-DC PSU and you’ll need a Parallel balance board to utilize the need for more amps.

Let’s look at the specs for the Turnigy battery:


So 2 C max charge rate. 2x5Ah (5000mAh) = 10 Amps max charge rate! That’s quite a bit and will give you a fast charge. Like if you get home from work, want to charge and get out the door to ride before sunset!

BUT - nothing’s free. Higher amp charging and discharging decreases pack life. Usually decreasing the # or re-charges it will do before IR (internal resistance) or a cell will go (discharge before the rest) - which leads to puffing and all sorts of bad things (why i look at my packs before charging).

So to extend battery life, you usually want to do a lower C charge - like .25 C or .5 C - so on this turnigy 1.25A or 2.5A respectively. You can also charge just below “full” 4.2v per cell which extends life further - but decreases range.

Back onto chargers - if you have a b6acv2 charger (a budget charger i usually recommend) - it maxes at 5A charge rate - across all batteries you are charging. So 2 batteries would be getting 2.5A ea. That’s good for pack life, but going to be slow sometimes when you want to ride NOW!

A better charger like an iCharger (206/306 etc) can charge at 20-30A respectively (the first two numbers of the model is amps, the last number is # in series it supports). So the 206 could charge two of the Turnigy’s at 10A each for 20A total - FAST! or a slow .25 C charge for better battery life. A great option.

The downside of most chargers like this (iCharger or similar) is that they require DC voltage. So a separate AC-DC power supply is needed. You can get several really nice variable ones online, or do what i did and get a Dell Server (or other brand HP/Lenovo/etc.) power supply and modify (or buy already modified) to switch on and supply a huge 12v power source. I have two 750w in series for 1500w 24v to supply my iCharger 306b and Hobbypartz Thunder 1220.

A setup like this as an example of AC-DC PSU -> 8s Lipo charger:

And in order to need 20A+ you’ll need to connect enough batteries in parallel to utilize (also convenient and decreases swap and charge time) the full potential of a setup like this. The best ones i’ve found (will depend on which connector you utilize on your packs) is BuddyRC’s paraboard:

Nicer chargers typically also have a temp probe you can utilize. with an elastic band they sit against the pack while charging and will auto-shut down the charge if the temp of the pack spikes while charging. Heat almost always happens before a catastrophic failure on Lipo. So an inexpensive failsafe (usually a few bucks for the temp probe if not included).

Told you it’s a rabbit hole discussion and a ton of info to digest. Hope it’s not too long winded and makes sense. let me know any questions i can explain or help further.

GL!


#6

that is quite alright, never too much info. Besides i am already in the rabbit hole just have to find my way out. i might research the modded psu solution as i build computers from time to time and have a few spares laying around. I appreciate all the info and will attempt to digest it all. :joy:


#7

Alrighty then - more info for you to research:

Great forum thread w/ TONS of info:
http://www.rcgroups.com/forums/showthread.php?t=1292514

Also can check out this guy’s tutorial/steps:

I was going to do this with some server power supplies from work… then found these on ebay for cheap already modified for 24v series. Did a quick search and don’t see that seller right now, but here’s a cheap dell PSU - i’d check on rcgroups if the modification steps are listed! If so, <$20 you have a 750w 62A 12v PSU!!!

be very cautious of series for 24v:
http://www.rcgroups.com/forums/showthread.php?t=1799197
Some steps to help understand if you go this route.