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Author Topic: battery capacity  (Read 5766 times)

dodgy geezer

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battery capacity
« on: April 06, 2012, 08:56:40 PM »

I'm putting a 50A brushless into a fairly solid 30" Cabin Cruiser, and was wondering about capacity. I'd like to see it plane - which probably means 15v or so, and keeping it light, which makes LiPos an obvious thought - but they cost a lot for not very much capacity.

I wonder if 3 6v gell cells would be similar to the LiPos - lower performance, but still sprightly, and running for longer?

What power supplies and running duration do you recommend for this sort of application?
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triumphjon

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Re: battery capacity
« Reply #1 on: April 06, 2012, 09:19:23 PM »

a 6volt gell cell will be much heavier than even one lipo ! ive a similar sized modified aerokits sea rover ( now 3 " shorter , and with balsa upperworks ) which is running a brushed 600 , a ripmax 20 amp esc , a lipo gaurd , power is from a single 11.1 volt 2200 mah lipo


jon
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john s 2

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Re: battery capacity
« Reply #2 on: April 06, 2012, 09:53:08 PM »

One thing that shortens the life of gell cells is running them down to far. So on paper you have more capacity, But in practise less.So back to Lipos or possibly nimhs. John.
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dodgy geezer

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Re: battery capacity
« Reply #3 on: April 06, 2012, 09:57:16 PM »

Particularly interesting, because I was going to use a Sea Rover as well! What sort of performance/duration do you get from that set-up? I assume you're running at about 10amps? That would give around 15min?

I wasn't sure if that would get a boat planing properly, which is why I thought of a 50A brushless - but now I have the problem of how to power it. The LiPo you are using would run that for 2-4 mins....

NiMh look attractive, but they might be heavy getting up to 15v. For that number the cost of lipos is not that much more, and I don't know whether they will last much longer...
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Mad_Mike

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Re: battery capacity
« Reply #4 on: April 06, 2012, 10:09:41 PM »

what motor have you got and what propeller does it turn?
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triumphjon

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Re: battery capacity
« Reply #5 on: April 06, 2012, 10:17:24 PM »

performance is very good , its quick enough to have the rear two thirds of the hull just below the surface , and airbourne when i hit waves ! as for run times , im getting around ten mins per battery pack when on constant flat out ! the motor is a low drain brushed 600 from fratton models in portsmouth , prop is a 32mm  2 bladed carbon hydro prop from graupner .
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Mad_Mike

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Re: battery capacity
« Reply #6 on: April 06, 2012, 10:29:14 PM »

what propeller has your boat got dodgy geezer? and have you allready selected the motor?
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dodgy geezer

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Re: battery capacity
« Reply #7 on: April 06, 2012, 10:33:58 PM »

what motor have you got and what propeller does it turn?

No prop as yet - I'm just examining possibilities. The motor I have is the A3520-6 50A 840kv SP Brushless Outrunner - you can see it here:

http://www.giantcod.co.uk/200299g-c-25_84_197.html
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Mad_Mike

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Re: battery capacity
« Reply #8 on: April 07, 2012, 01:15:42 AM »

Out runners produce massive torque, that 50 amps is its max current. the dimensions of the motor say 44.7 mm x 72.0 mm i presume that is 44.7mm wide. If you run say a 55mm 2 blade prop on that motor is will barely notice its there and i doubt the continuos amp draw will get over 7!

what is the recommended prop size?
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dodgy geezer

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Re: battery capacity
« Reply #9 on: April 07, 2012, 01:35:11 AM »

It's from GiantCod, who are flyers, and suggest a 10 or 11 x7 airscrew on it. They haven't the slightest idea about boat props...

The Sea Rover is a bit design-constrained to a prop of about 1.5" - 40mm. Not utterly critical, because things can be changed. I just thought that motor was rather tasty at 14 and worth playing with. We always get warned about over-propping brushless in a  boat, so it's refreshing to have the opposite problem...
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Mad_Mike

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Re: battery capacity
« Reply #10 on: April 07, 2012, 01:58:26 AM »

with a 40mm prop on that motor it will barely creep over the no load current. It will produce such massive instantanious torque that the boat will rock!! it wont hurt though, just go steady on the throttle. id recommend lipos for your boat especially if you want to keep the wait down. I think a 40mm prop with 3 6v gells in it would slip. to counter this you could easily up the diameter of the prop but that would then increse the amp draw, mildly though. Ill find some batteries i reckon would suit

what price range you looking for?
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dodgy geezer

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Re: battery capacity
« Reply #11 on: April 07, 2012, 02:28:15 AM »

Hmm... I was really looking for a 'sweet spot' on price/performance.  I have an 80A watercooled esc to use with it, the ESC and motor are rated to 21v, and I've had it running on 12v on the bench. It humms along at 12 - I could easily ramp it up to 15 or 18 if that were to help the planing performance.

Having talked about volts, amps is really about duration. I had hoped to get 30mins+ out of it, but that looked very hard if it were going to draw 50A. If it's nearer 10, that still means a 5000 battery, or 2 2500s...  I'm not sure what the relative benefits of NiMh and LiPo are at those ranges...

It sounds like changing the prop will make a lot of difference to this one - so a range of sizes would come in useful... It also sounds as if I have rather over-engined a 30" hull, but the old Aerokits are not light. I could move it to a Sea Commander, or Swordsman. What equivalent glow-plug would you say this was?
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Mad_Mike

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Re: battery capacity
« Reply #12 on: April 07, 2012, 02:55:48 AM »

on the esc did you buy one those blue esc's from giant cod? ive got one on my current build.

if you go lipo you will need a balance charger, you cant use a standard charger. The advantage of lipo is that a 5ah lipo is a 3rd of the weight of a 5ah SLA, this weight loss will give better handling and performance. also due to the fact you have less weight you wont need to run the motor as hard to get the speed you require, thus less amps used and longer run times.

I would personally use this:

http://www.hobbyking.com/hobbyking/store/__11936__Turnigy_nano_tech_5000mah_4S_25_50C_Lipo_Pack.html

it turns out to be 42 pound inc pnp.

or this

http://www.ebay.co.uk/itm/Turnigy-5000mAh-4s-14-8v-20c-30c-LiPO-hardcase-fits-HPI-Trophy-/320882567193?pt=UK_ToysGames_RadioControlled_JN&hash=item4ab6176819

you could go with nimh too but i cant seem to find a cheap source or the voltage requirments

i cant tell you what the glow plug equivalent is i dont know nothing about them
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barriew

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Re: battery capacity
« Reply #13 on: April 07, 2012, 07:40:04 AM »

Dodgy

I have a Dave Milbourn designed Swordsman running a 1400Kv brushless with a 25A fwd/rev ESC and 7.4v LiPo. It planes! Run time I can't tell as about 15 mins is long enough for me at that speed %%
That was on a 4Ahr battery. I think with your motor it might fly {-) %% O0

Barrie
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dodgy geezer

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Re: battery capacity
« Reply #14 on: April 07, 2012, 12:28:13 PM »

on the esc did you buy one those blue esc's from giant cod? ive got one on my current build.

Yup - overspecced as advised. I will have to work out what the beeps mean, but it seems to work ok...

if you go lipo you will need a balance charger, you cant use a standard charger. The advantage of lipo is that a 5ah lipo is a 3rd of the weight of a 5ah SLA, this weight loss will give better handling and performance. also due to the fact you have less weight you wont need to run the motor as hard to get the speed you require, thus less amps used and longer run times.

Yup - I'm well stocked with balance chargers, including the ubiquitous iMax B6. I was using little Lipos and brushless in EeZeBilt hulls for a while. The problem with those is that the KVM is so high - this is the first time that I've played with a low KVM motor. But it was so cheap I couldn't resist.... I suspect now that I should have gone for a more efficient one (lower no-load drain) at a slightly higher KVM, and stayed with 12v. But you live and learn... and 14 is very good value for this motor - I'm sure I'll find a use for it...

I would personally use this:

http://www.hobbyking.com/hobbyking/store/__11936__Turnigy_nano_tech_5000mah_4S_25_50C_Lipo_Pack.html....it turns out to be 42 pound inc pnp. or this.... http://www.ebay.co.uk/itm/Turnigy-5000mAh-4s-14-8v-20c-30c-LiPO-hardcase-fits-HPI-Trophy-/320882567193?pt=UK_ToysGames_RadioControlled_JN&hash=item4ab6176819
you could go with nimh too but i cant seem to find a cheap source or the voltage requirments

Interesting - I tend to go for Giant Cod - I've never bought a Lipo off ebay - and I notice that two 2600s would give me a bit more Ah at a lower total price. I am more used to NiMhs and would prefer them, but as you say, it's hard to find the equivalent performance, or even a similar one...

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Mad_Mike

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Re: battery capacity
« Reply #15 on: April 07, 2012, 02:57:01 PM »

like i said i got that esc in my current build. I also have the beep sheet too:

http://www.giantcod.co.uk/forum/viewtopic.php?f=9&t=3452&p=12166#p12166

youl need that.

the esc does not have reverse and the other day mine had a moment and forgot its settings  >:-o so looks like ill need to invest in a better quality one if it fails entirely. Id also sugest cutting the black heat shrink off and giving it a good water proofing. Espescially the nipples on the water cooling jacket.  It is allready kinda done but i went to town on it, first i marinated it in epoxy, wrapped it in electricians tape then put it in a balloon. Just to make sure :-))

If you know of a better and cheaper source of batteries then go that way. NIMH batteries will work fine on this motor and even SLA's, youl have to experiment with props if you go this route as i think the added weight will cause the prop to slip. Gens ace batteries from giant cod are cheap but pretty good you know. There are a lot of cheap batteries on ebay but i dont know how good they are. That last post of the turnigy 5ah lipo is a battery i would put my money on to work.
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dodgy geezer

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Re: battery capacity
« Reply #16 on: April 07, 2012, 09:45:09 PM »

Thanks for that.

I had wondered about making a simple crossover switch run by an extra channel to switch two of the motor wires. Ideally, it should cut all wires and have a delay built in. One of those would enable us to use the cheap flyers ESCs with a bit more flexibility....
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Mad_Mike

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Re: battery capacity
« Reply #17 on: April 08, 2012, 11:20:08 AM »

i like the sound of that. just throttle back before you change. youl need a heafty switch like.
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dodgy geezer

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Re: battery capacity
« Reply #18 on: April 08, 2012, 11:59:04 AM »

I was thinking big mosfets.  But I suppose that would cost. As would a 200A relay.....  Ah well....

How about a big knife switch, operated by a slow servo?  http://www.ebay.com/itm/Antique-Edison-era-electric-chair-style-Knife-Switch-HUGE-frankenstein-lab-/320866794532   shows an example.  (howl, and insane laughter in distance....Martin, you need another smiley...)
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johno 52-11

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Re: battery capacity
« Reply #19 on: April 08, 2012, 12:39:30 PM »

If you are looking at the stated capacity of an SLA battery it is worth noting  that the stated capacity is only achieved with a discharge current of 0.05C  or 20 hours. If you look at the manufactures discharge graphs then you will see that if you discharge at about 1C Then you will only get about half of the stated capacity.

Here is a link to the Yuasa site with all the data sheets for some of there SLA batteries http://www.yuasabatteries.com/industrial.php

If you look at this on a power to weight base then an SLA is about twice the weight of the same capacity NiMh but the NiMh will discharge close to what it's stated capacity is at 1 or 2 times C. So this means in practice is that we can get 4 times the usable power from NiMh for the same weight as SLA's.

If you look at the weight of LiPo batteries then they are about half the weight of NiMh for the same capacity so they will give you 8 times the power than your SLA for the same weight.

There is one other thing to consider and that is of course the PRICE.

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malcolmfrary

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Re: battery capacity
« Reply #20 on: April 08, 2012, 04:56:59 PM »

If hoping to switch something in the 200A range, it needs something that is normally operated by a guy with a hump called Igor*.  Something much more modest will work when switching zero current, just something that will carry the required current when it is applied.  Maybe a DPDT push-push switch worked by a mini servo on the throttle channel?  That sort of thing was in regular use a few years ago when electronic controls were rare and expensive.  Reverse was achieved by taking the throttle down past zero momentarily.

*That would be the guy, not the hump.
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Mad_Mike

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Re: battery capacity
« Reply #21 on: April 08, 2012, 05:19:09 PM »

I was thinking big mosfets.  But I suppose that would cost. As would a 200A relay.....  Ah well....

How about a big knife switch, operated by a slow servo?  http://www.ebay.com/itm/Antique-Edison-era-electric-chair-style-Knife-Switch-HUGE-frankenstein-lab-/320866794532   shows an example.  (howl, and insane laughter in distance....Martin, you need another smiley...)

I would agree with malcolm on that 20 amp would do it, like this:

http://www.ebay.co.uk/itm/METAL-TOGGLE-ON-OFF-ON-SWITCH-3-TERMINAL-20-AMPS-CAR-/260983454377?pt=LH_DefaultDomain_3&hash=item3cc3d37aa9
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dodgy geezer

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Re: battery capacity
« Reply #22 on: April 08, 2012, 09:22:01 PM »

If hoping to switch something in the 200A range, it needs something that is normally operated by a guy with a hump called Igor*.  Something much more modest will work when switching zero current, just something that will carry the required current when it is applied.  Maybe a DPDT push-push switch worked by a mini servo on the throttle channel?  That sort of thing was in regular use a few years ago when electronic controls were rare and expensive.  Reverse was achieved by taking the throttle down past zero momentarily.

*That would be the guy, not the hump.

I agree that, if we can guarantee that the motor cables will not be carrying current, the switch requirements are far less onerous - the engineering problem is providing an interlock which guarantees this for a low cost...

Thank you for reminding me about the proper nomenclature for this kind of operation. I don't know if you got a chance to see the ebay reference I provided? I am very tempted to build a smaller version using the insides of a 13A socket and plug - and I will be sure to name the servant servo 'Igor'....
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Mad_Mike

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dodgy geezer

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Re: battery capacity
« Reply #24 on: April 08, 2012, 09:56:56 PM »

...would have thought that could take more than 0.5A....
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