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Battery life

Just a best guess will help us!

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Brian Porter 226/04/2021 12:56:37
20 forum posts

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


Charles Oates26/04/2021 13:44:26
616 forum posts
50 photos

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.


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 needham26/04/2021 14:08:52
7121 forum posts
206 photos

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.


Dave Cooper 626/04/2021 15:39:43
285 forum posts
28 photos

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, 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 T26/04/2021 20:27:20
7231 forum posts
1226 photos
2 articles

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 Frary26/04/2021 20:38:59
993 forum posts

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 226/04/2021 20:51:30
20 forum posts

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 Porter 226/04/2021 21:01:30
20 forum posts

Thanks Malcolm, message understood!


Paul T27/04/2021 13:26:17
7231 forum posts
1226 photos
2 articles

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.



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

Brian Porter 227/04/2021 14:10:34
20 forum posts

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.


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