Timer/alarm??

[Terry Chapman 2Participant @terrychapman2]

Ive almost finished my first boat build (Diva) and I would like to know how I find out how long the battery last? I would like it to go flat in the middle of the lake?

Thanks

[Terry Chapman 2Participant @terrychapman2]

[Charles OatesParticipant @charlesoates31738]

To calculate that, readers will need to know the battery type and capacity, and the current drawn by the motor. If you haven't a way to measure the current, tell us the battery, motor and prop type and size.

Many people buy a low voltage alarm, this is ( in my view) essential with lipo batteries, and useful for other types.

Charles

[Terry Chapman 2Participant @terrychapman2]

Posted by Charles Oates on 27/02/2021 15:07:11:

To calculate that, readers will need to know the battery type and capacity, and the current drawn by the motor. If you haven't a way to measure the current, tell us the battery, motor and prop type and size.

Many people buy a low voltage alarm, this is ( in my view) essential with lipo batteries, and useful for other types.

Charles

Hi Charles,thanks for reply.

400 brushless motor, 3000 mAh 7.2 battery,30 mm diameter 3 blade prop. Batteries are not lipo.

Thanks

[Charles OatesParticipant @charlesoates31738]

The 400 brushless covers quite a range of motors with different kv values and current draw. That said, we can guestimate a bit, typically 6 to 10 amps drawn for many of these motors. Your 3 amp hour battery, presumably a nimh pack would run approximately 30 mins at 6 amps, and 17 mins at 9 amps. This is only ball park, it will vary if you change the running speed, as most of us do, and vary with the efficiency of the pack, as well as other more subtle electrical issues.

I presume you're just trying to get some idea of what to expect from the model, so maybe that will help a bit. In practice, running a quick model with nimh batteries is more forgiving than lipos, just come straight in to the bank as soon as the model starts to slow down a bit. Never run the model untill it's going a lot slower than normal, it knxxxxs the battery.

I hope you enjoy the model, it's a great looking boat.

Charles.

[Richard SimpsonParticipant @richardsimpson88330]

I completely agreed with Charles, although there are those who would advocate hammering around the pond until it dies then bring it in with a recovery boat. That is still no guarantee however as amounts of charge and conditions will always vary. Allow yourself a reasonable factor of safety for any unforeseen circumstances and go with The guidance from Charles.

[Malcolm FraryParticipant @malcolmfrary95515]

With NiMH, you note how much of the boat rides out of the water on a full charge at full throttle. When less of the boat rides out, its time to come in.

Doing it that way with LiPo is a bit fraught. There is very little between "yes I can go flat out" and "no, I ain't going nowhere". An alarm helps, but alarms are generally just for tone particular type of battery.

[Colin BishopModerator @colinbishop34627]

This is something covered in my current two part article in the magazine.

You really need to have an ammeter or wattmeter so you can measure the current drawn at full throttle. You can do this in the bath or a suitable container. If the current consumption is, say 5 amps then you might get around 30 minutes at full throttle but the boat won't be going at that speed all the time so you might get around 45 minutes of general use. If it draws 10amps at full throttle then halve that but, as said above, with NiMH cells you will notice a drop off in speed in time to bring the model into the bank.

Colin

Edited By Colin Bishop on 27/02/2021 20:05:33

[Terry Chapman 2Participant @terrychapman2]

Thanks guys,thats really helpful.

Terry

[Trevor Drabble 1Participant @trevordrabble1]

Terry , I believe Leeds Model Shop ( no connection ) 0113 264 6117 , have 3 versions of a low voltage alarm unit which you may find useful . Hope this helps . Trevor.

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