Method for calculating battery capacity???

[longballlumber]

OK, so here it goes, I am looking to better understand how to identify the proper battery capacity needed for setups. So far my only way of knowing where to start is asking others that I trust won’t give me wrong direction. For this discussion let’s assume the batteries being used are of good quality/brand vs. some of the “value” type battery brands. Let's also assume this will be ONLY for heat racing situations (less than 2:00min runtime)

Why run 10,000mAh if 7,000mAh will be sufficient

Let’s take a look at a set up I plan on running this year.

• P Sport Hydro
• PT Stealth Hull
• Neu 1518 1.5D (2350kv)
• Castle Controller (ice 240 or 200 lite converted to water cooling), or maybe a T-180
• I would guess that starting off with a 45mm prop would be reasonable

How do you know what capacity of batteries to use? Will 6000mAh be ok or do you need 10,000mAh?

I look forward to your input.

Mike

[properchopper]

Mike,

I was just asked this question yesterday. The best answer I could give was to do an in-the-water test given that paper and pencil calculations may not take into account relevant field data. Each boat has its own dedicated current demands based on all the usual running parameters which contribute to draw on the batteries. One way around all the formulas, head scratching and speculation is to do a one minute timed run at full throttle, then re-charge and observe how much mah capacity was put back in to restore full charge. Then, knowing how much is used for one minute, multiply by your desired run time to find out much you'll need ; e.g. a mill + 6-lap on the 1/6 mile (plus, if you drive like me, a penalty lap) and you'll get a close count on how many mah are needed. Then add however many mah you want left to conserve battery integrity/safety.

[ray schrauwen]

Yeah.... pond time. Wish I was in California because everything up here is frozen for a good month yet.

I've been asking what Mah do I need of said X quality lipos to run a 1527 1y in a Whip 40. One guy in his build thread said he settled on 6000mah total running a X455 prop. That's not a lot of Ah's to me but, I'm still just skinning the boat up and want to get cells for it soon.

Sorry Mike, I too would like to know what Mah is required for P-Sport but, a different setup, bigger hull, bigger motor, 1521 1.5D in ML335 and big prop like 447 x3 modded or sumthing. It's been a looong time since I ran a P-Sport.

[Fluid]

There are always compromises. I run 6500 mAH packs in my Q Sport Hydro and the average use with my setup is 70 percent in six race laps plus mill. Plenty of capacity so as not to hurt the packs even after a penalty lap. The downside is that the lower you go past half capacity the lower the pack voltage. Maybe only 0.1 less per cell less, but less regardless. Does the lighter weight of the 6500 packs make up for the lower voltage the last couple laps? Depends. So far I have been very successful running 6500-6600 mAh packs in three open class race boats. Others may see different results.


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[Flyguy55]

Credit Dave Newland for this formula. I used 100 amps for 2 min heat in this example .

N to T, formula is the same.

3600 / 120 (seconds) = 30 ( 3600 sec. in 1 hr, divided by 120 secs. - 2 min )

100 amps / 30 = 3.333 amps, or 3333 mAh. X 1.20 for safety factor = 3999 mAh. ( 80 % )

3999 mah is 80% of a 5000 mah pack

You need to make an assumption or measure amp draw .

I hope this helps

Dick Roberts

edit of my post to be more clear
3600 is seconds in 1 hr . 60 min X 60 sec
120 sec is 2 min to finish a heat
adjust numbers that fit your boat

[ray schrauwen]

Thank you Mr. Roberts

Credit Dave Newland for this formula. I used 100 amps for 2 min heat in this example .

N to T, formula is the same.

3600 / 120 (seconds) = 30 ( 3600 sec.in 2 min. divided by 120 secs. in 2 min )

100 amps / 30 = 3.333 amps, or 3333 mAh. X 1.20 for safety factor = 3999 mAh. ( 80 % )

3999 mah is 80% of a 5000 mah pack

You need to make an assumption or measure amp draw .

I hope this helps

Dick Roberts

[ray schrauwen]

THanks Fluid, helps a lot!

There are always compromises. I run 6500 mAH packs in my Q Sport Hydro and the average use with my setup is 70 percent in six race laps plus mill. Plenty of capacity so as not to hurt the packs even after a penalty lap. The downside is that the lower you go past half capacity the lower the pack voltage. Maybe only 0.1 less per cell less, but less regardless. Does the lighter weight of the 6500 packs make up for the lower voltage the last couple laps? Depends. So far I have been very successful running 6500-6600 mAh packs in three open class race boats. Others may see different results.


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[HTVboats]

The information so far is spot on. Setting up for heat racing there are some compromises. More prop more speed, more battery weight. My experience with my "P" mono (and P tunnels) with a 1515 1Y and an M445 prop was using 3500-4000 ma in a heat. I could get by with 5000ma packs. Better to have some cushion so 6000-6600 would allow me to not have to cut props down to 43-44mm. Using a 1518 1.5 D it seemed to be a couple mph faster but consumed more mah's. Maybe 500-700 in a heat. Again some prop tunning can make things work. On a P adding capacity over 8000mah in my opinion would be a severe weight penalty unless you go to a 1521 motor and then a stouter ESC. Light weight "smallblock" or heavier "bigblock" becomes a personal choice. Racing boils down to a simple weight to horsepower and in our application efficiency of the hull design. In a mono a large motor and more batteries might actually handle better in rough water. In an OB tunnel the weight penalty hurts more. The right combination is the "64 thousand dollar" question and what makes racing FE challenging. Driving style and conditions are in play too. Not as big of an experience pool in FE so still some trial and error. The better racers here do share good information as they know that bringing up the level of competition helps us all and grows the sport.
Mic

[Flyguy55]

Also Newland passed this on many yrs ago . Run your setup for 1 min ,prop ect .
recharge batteries and record mah you put back in . You can calculate the amp draw . There is a little safety margin because of losses in the charger but its a good guide . Most chargers have that feature .

[Fluid]

To avoid all the math, go to the top of the page and click on "Speed Calc". I don't know who developed it, but it's been on Steve's site for years.



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[jfrancisco892]

I know I'm not the most experienced one among this bunch but here are my relevant findings. Here is a heat in LSH with my UL-1 showing pack voltage on the y axis and time on the x axis. This was a 4s1p setup with 2 5000 mah 2s batteries in series and Grimracer 45x68 prop. After a typical heat this would take about 70% of the total capacity out of the packs. As you can see there is a considerable drop in voltage from the start of the heat to the end. IMO you can do all the capacity calculations you want, you'll have enough capacity to get through the race but I think it is much more important to consider the voltage drop associated with the loss in capacity. Obviously the higher the capacity you have the less drop in voltage you will see throughout a heat. If I were to rebuild my P Sport I would run the maximum capacity to have the lowest voltage drop possible and sustain the highest motor rpm possible for the entire length of the race.
LSH Pack Voltage.jpg

[TRUCKPULL]

Here is a chart that I use.
I think this came from Mr. Newland.

mAMP's that you put back into your pack, across the top.
Run time down the left side.
Example - run time 1Min. 40sec.
5000 mAMPs. back into the pack.
= an average AMP draw of 180AMP's

Remember that in the straightaway AMP draw is less then the turns.

Remember the rules say MAX 10,000 mAMP's on board.
If you run 4S2P - your two 4S packs can only be 5,000 mAMP's MAX. NOT the new 6,600 packs

I always use the highest "C" rating and the highest mAMP rating I can in a boat.
These highest ratings = Highest voltage under load.
Our motors are rated KV X VOLTS = RPM = speed.

Larry

[longballlumber]

A big thanks to everyone for their replies! I appreciate the input.

Many of your post validated what I already knew. You really can't make those calculations unless you have the imperial data to support the desired results.

However, I did learn a few new formula's to help with my record keeping on my runs. Especially with setups that I am not very experienced with.

Thanks,
Mike