Battery life

[Brian Porter 2Participant @brianporter2]

Just a best guess will help us!

[Brian Porter 2Participant @brianporter2]

We’re building a 93cm K D Perkasa. It will have two 12v motors at 10amps (max), a 12v sound board at 1.2amp (max) and a few switchable low voltage low current items. We add it up to about 22,500 mA with everything switched on & running at max (highly unlikely). So we believe in reality it would be nearer to 11000 mA max or likely less for most of the time.

The proposal is to use two 12v 5000 SC NiMH battery packs in parallel giving 12v 10,000 mAh (from Component Shop) with their parallel power board. The total weight for motors and proposed batteries is about 2.6kg.

So, your general thoughts would be most welcome BUT in addition please – the big one – any guesses from your experience on high much running time we’ll have? Max power would only be in occasional short bursts. I realise there are calculators online but your experienced views would be far more accurate we reckon!

We understand the “length of string” bit etc and only ask for ball park guesses and promise no moans if you’re wrong!

Many thanks

Brian

[Charles OatesParticipant @charlesoates31738]

If your numbers are right, 27 mins. 5 ah Batts, 11 amp draw. It's easy to work out, 5 divided by 11 equals about .45. then divide by 60 mins, hence 27 mins.

You obviously understand that there are many if's but's and maybe's, but that is how to get in the ballpark.

Chas

Sorry, I've just noticed that you said 2 x 5 ah Batts, in parallel so double the 27 mins, ….54 mins.

That will teach me to read the question properly, just like school!

Edited By Charles Oates on 26/04/2021 13:49:36

Edited By Charles Oates on 26/04/2021 13:55:32

[ashley needhamParticipant @ashleyneedham69188]

Brian. The only calculation you can do is for max power consumption. This would give you the theoretical endurance throttles wide open as above. In reality you may get double or treble that depending on throttle usage, chatting time etc.

Ashley

[Dave Cooper 6Participant @davecooper6]

That's quite a big boat – I don't know what your finished (water-ready) weight will be, but, I would also consider LiPo's.

I found when I first started messing with model boats (and aircraft) that my attention span was about 10 min's. After that, I needed to bring it back for a chat /drink etc…so, plenty of time to change batteries if necessary.

The only disadvantage I can think of is that LiPo's are lighter and you may need extra ballast. On the other hand, you can put that ballast where you want it for trim etc.

Properly looked after, LiPo's will last a long time (mine are now about 5-7 years old). The only safety thing I would recommend is that you place them in a fire-proof bag when charging. (My 'FP' bag cost about £3.50 I think).

Just a thought…..have fun.

Dave C

One more safety item – choose connectors which don't allow the main power leads to short each other (I use "XT60's", mainly because they are compatible with my planes and trains and things !) 

 

Edited By Dave Cooper 6 on 26/04/2021 15:55:03

[Paul TParticipant @pault84577]

I get it to 45 to 50 minutes but as others have said this is a very rough estimate as over this length of time using a high draw on the batteries other variables come into play such as battery heat, motor heat and increased frictional drag from the prop shaft.

You might have a lot of excess heat to remove from the system which, of course will in some way have a direct effect on battery life.

[Malcolm FraryParticipant @malcolmfrary95515]

A lot of years ago I was chatting with George Turner at a CADMA show. He was playing with one of his torpedo boats. I did ask what his run time was.

"Oh, almost and hour" he said.

"Unless I run it like this" he went on, turning the wick up.

"I like running it like that".

Temptation is strong. Assume shorter runs.

[Brian Porter 2Participant @brianporter2]

Chas, Ashley, Dave & Paul,

Many thanks for your responses.

Phew! – That’s overall just about what we were hoping for so we’re clearly starting to learn, your help is much appreciated.

Dave – we’ve passed the point of choice of batteries & committed to NiMH for better or worse.

Paul, that’s very interesting & we’ll take that into account. At this early stage we can increase ventilation, thanks.

FYI – we’re really pleased to have got the authentic sounds for x3 gas turbine engines, they sound great. Plus we’ve become well informed on solving RFI interference problems. What a nightmare that was.

We’re learning!

Thanks so much guys.

Brian

[Brian Porter 2Participant @brianporter2]

Thanks Malcolm, message understood!

Brian

[Paul TParticipant @pault84577]

Hi Brian

Passive ventilation might not be sufficient and whilst at the early stages I would advise including a design space for some CPU type cooling fans.

Paul

 

Edited By Paul T on 27/04/2021 13:26:57

[Brian Porter 2Participant @brianporter2]

Hi Paul,

Ok thanks for that. We’ll have two cunningly concealed areas to bring cool air in from the flying bridge so we’ll collect some air from there and push it through. The three large exhaust flaps will be openable so we’ll perhaps use at least one to extract air. The other two may emit smoke but even if they do we can include an air outlets within the approx 25mm or so diameter holes. We have a 3D printer so are able to make a custom exhaust to combine smoke and hot air. Oh yes, we’d already planned & made our own small but very effective automatic bilge pump, just in case!!

Appreciate your thoughts.

Brian

[Paul TParticipant @pault84577]

Hi Brian

It would be worth researching your batteries and looking at the manufactures test results for things like power drop off and core heat generation.

Then look at Internet for independent reviews from testers who have pushed the batteries at full draw for extended periods.

This level of research will better inform you as the results are taken from actual tests rather than a desk top study.

Paul

[Brian Porter 2Participant @brianporter2]

Hi Paul,

Interesting, never done that before but will certainly try. Thanks for the advice.

Brian

[Dave Cooper 6Participant @davecooper6]

Another good test would be to find out where the BEC (Battery Eliminator Circuit) cuts in – assuming you're planning to use one that is.

This is essential on RC aircraft to get them down safely after the motor(s) has been cut. On boats, less so perhaps, but, could save your boat hitting the bank /another boat etc. whilst it still has substantial way….

I think I would set a timer on the Tx to chime in about 10 minutes, or so, before the BEC is due.

The experts may disagree !

[ashley needhamParticipant @ashleyneedham69188]

I would not disagree with any of the previous comments as I am not an expert. However.

Nimh batteries tell you when they are low as the boat slows down. Usually you have to then get the boat in straight away, however the battery voltage is unlikely to dip below that required for the BEC to work, especially if you conserve battery voltage by using minimal throttle to get back to shore.

A wattmeter is invaluable here and will tell you more than any amount of theorising….not that you don`t want to theorise, far from it, but having an actual sight of whats going on is a must.

The Component shop sells them, they are inexpensive. or e-bay

Ashley

[Brian Porter 2Participant @brianporter2]

Dave & Ashley,

You're going to have to slow down now, remember this thread is in the Beginners section!! Wattmeters I can understand but the BEC cutting in is, as yet, a bit beyond our understanding. I know they're in the two Mtronics ESCs but that's it. If you're able to explain the basics here that would be great. We're slowing advancing up this ladder one run at a time!

Not sure what you both think of this but as a visible warning, our electrics are being designed so the continuous working radar (a microscopic 3v motor) automatically stops, as a visual indicator, when the main 12v batts drop to 10.5v at which point we can shut down everything except motors to bring her home.

Also, in the early days of our marine adventures we'll be experimenting with her use & bring her home often to check the batts so to understand the battery life.

Thanks guys, all advice & criticism welcomed plus any micky taking accepted!!

Brian

[ashley needhamParticipant @ashleyneedham69188]

Brian. Scroll down on the right to members articles, click on that, scroll down to hints and tips and there, somewhere, is an article the late Dave Milbourn regarding electrics. He was the guru and I think you may find all the answers in the scribbling.

Basically the BEC (battery elimination circuit) in an esc provides power for the receiver at approx 5v through the centre red wire, and this saves you having to use an external battery pack for the receiver. If two esc are in the circuit, one on the ESC red wires must be disconnected for reasons unfashionable to me but just do it.

As the BEC voltage is about 5v it stands to reason that if the main feeding battery voltage drops to around, or below this value, the esc will not be able to provide 5v and so the receiver will not function.

You can over complicate things however, and after a few runs you will know when it is time to come in without a lot of gizmos. However, obviously the gizmos are very handy so I am not saying you should not go down this route.

Ashley

Edited By Colin Bishop on 29/04/2021 12:23:58

[Brian Porter 2Participant @brianporter2]

Ashley, Ah, got it. I thought it was more complex than that!

Thanks for the info & will also take a look at the article.

Brian

[Dave Cooper 6Participant @davecooper6]

Hi Brian,

I forgot to welcome you to the forum (talk about getting things backwards !).

Yes, Ashley has it in a nutshell. The BEC is just an electrical safety feature. Some of them are 'programmable' but, that's getting ahead of ourselves, and, as you quite correctly mention, this section is for beginners (that's me too !).

The alternative is to have a separate battery supply for the receiver (Rx). Not a bad idea actually although I think wiring etc. is best kept simple in the early stages…

One thing I would include is a suitable fuse in the main power lead. I think Dave M gives chapter and verse on this in his article.

Have fun and do ask about anything that's not understood,

Dave C

[Brian Porter 2Participant @brianporter2]

Hi Dave,

Don’t worry about the welcoming, I (we) have had the best welcome possible – plenty of help & very informative advice from supportive members.

Programmable BECs – woh there, we’ll learn to walk first!!

Battery for the RX – Although I tried it during our first attempt at set up when we innocently wondered blind into the world of interference (what a nightmare & what a learning curve that was!) we now no longer need it. Point of interest though, Carson say not to (14ch RX).

Fuse – Proud to say we’ve included it in our design plans – impressed? No? Ah well…..

Brian

[Dave Cooper 6Participant @davecooper6]

Hi Brian

Looks like you're on the right road ( / lake).

Keep on 'trucking'…and, enjoy your build,

Dave

[Dave Cooper 6Participant @davecooper6]

As a matter of interest, what size fuse are you planning on using ?

Edited By Dave Cooper 6 on 05/05/2021 22:49:11

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