Voltage drop down converter to 12v on Bike battery pack

[eHomer]

Thanks, it's good to get all the pros and cons of each solution.

I've had the T24 in the test tank for just over two hours now, running at speed one, which uses around 10 amps at 12 volts.

This shows up on the source wattmeter as 2.76 amps at the bike battery's 36 volts.

It's the speed I use most on the kayak, about 4mph, which equates to a fast "cruising" paddle.

After two hours, the wattmeter is showing 5.79 a/h used so far, and the pack voltage has dropped from 40.87v on starting, to 38.3v now.

If it stays linear, it looks as though I should get about three and a half hours at that speed, about 16 miles. I'd be happy with that from each 36v pack.

The whole battery box weighs 6.5kg with one 36v battery, and 10.5 kg with both. So I'm getting the equivalent of about 60 a/h at twelve volts, whereas my lead-acid leisure battery was 17 kg and had an advertised capacity of 85 amps, (though I guess not all of that would be usable per charge/discharge cycle).

 

[eHomer]

The duration test was a great success, my 36v 10ah lithium ion bike battery packs do have a built-in LVC. The power cut off automatically after 3 hours and ten minutes of low speed ( 10 amp 12v ) running. The wattmeter showed 8.8 a/hr consumed, and the final voltage under load was 31.95v.

the wattmeter readings from the 36v pack were:

start 40.87v 2.76a
after 30 mins 39.76v 2.76a 1.46 a/hr used
after 60 mins 39.42v 2.76a 2.92 a/hr used
after 90 mins 38.97v 2.76a 4.32 a/hr used
after 120 mins 38.38v 2.76a 5.79 a/hr used
after 150 mins 37.72v 2.78a 7.18 a/hr used
after 180 mins 35.33v 2.91a 8.71 a/hr used
after 200 mins - battery cutout with 31.95v no load.

So I should get 3 hours cruising at 4mph out of each of my two bike batteries.

I then thought about the 40a PWM that I'd bought and modified the casing of the T24 outboard to fit it inside the head unit.

It worked out pretty well, sitting neatly in the top casing after cutting out the web of plastic. It did mean fixing the stubby telescopic handle and no longer being able to retract it, but the facility was of little usefulness, so I won't miss it.

Then there was a big problem, the damn PWM didn't work, so it must have been a faulty one and I stupidly hadn't tested it before modifying it. Luckily they're only £7 from the UK, so I've ordered another. I tested it very carefully with a meter for anything obvious, but it's getting power, and the motor itself still runs ok when the appropriate wires are shorted out.

Further investigation on the web shows that the Watersnake T24 does actually have two dedicated motor tappings, one for each speed, so no resistors are used. It'll still be worth converting to PWM though, because continuously variable speed control will be useful.

 

[IR772]

The switch on the output, it looks like the black wire with the yellow connector (-) output is connected to the same pole as the positive to the motor and the positive supply, the brown wire with the blue connector?

I can not see clearly enough to fathom out sorry.

I would put the switch on the supply rather than load side. Is the blue wire the slow speed?

Testing, do not worry about the (+) as they are common, only the (-) is switched.

Do you have a 60 or 100 watt filament bulb and a reading lamp you can take the plug off, any flex will do.

PWM needs a load for testing, so connect the bulb across the load and switch on, start at max assistance and you should see a DC voltage with your multimeter, plus the bulb should glow a bit and might get warm.

If the magic smoke has come out of this one keep the lamp to test the next one.

Show me a picture of the switch and wires if you can so I can see what is going on with the black wires, not sure if that is where the fault is.

Nice installation again.

I love the idea of the water sloshing about for hours in the waste bin while you test, were you tempted to add a couple of drops of Fairy liquid.

 

[eHomer]

Yes, the water did make quite a sloshing sound, the neighbors probably thought we'd put in a Jacuzzi....

I ran the input leads from the battery direct to the PWM input, then it's output to the original fwd/rev changeover switch.

Yes, the blue is the now defunct slow speed winding.

The motor starts and runs when you bypass the PWM.

I'll try the bulb test you suggest (with a 12v car sidelight bulb) and report back.

Thanks for the suggestions, all input welcomed.

 

[IR772]

Hope the new one works.

 

[eHomer]

So do I !

I've just tried the light bulb test, with a 12v 5w car sidelight bulb, and no joy, it didn't light at all. (Though the board's led light does come on when the rotary switch on the pot closes).....

Confession time - I did desolder the pot to put it on a flying lead, as I'd bought the board mounted version.

I did do it very carefully though, with a temp controlled iron and solder sucker etc.. and I've checked the continuity of my new leads too. That's why I can't return it, (I was so stupid to not test it first....)

 

[IR772]

It was a faulty one, can't see what you did wrong even bird sh#t soldering would not cause a problem and length of wires in this device would not be a problem.

If you leave the bulb connected across the output on the new one with the motor connected, that will give you a visible indication as well as all the sploshing.

Its only 5 W max and will be a good indicator.

 

[eHomer]

It's bad for self-confidence though.

This time I've ordered the flying lead version.....

 

[eHomer]

The new 40 amp PWM arrived today and I've just tested it and then fitted it to the outboard.

I'm relieved to say that it's working perfectly now. When the pot is turned up the motor starts turning very slowly, and without any audible frequency buzzing. As per the helpful suggestions on this forum, I'm sure it will give me maximum efficiency at slow and intermediate speed ratings with minimum battery consumption.

After discovering that the three motor wires are for two separate windings (with no resistors), I thought I'd rethink the wiring again, with the two speed switch as well, so I now have two continuously variable speed ranges, one spread over half the range. I know it's belt and braces, but it gives me a bit of extra redundancy if one winding should ever fail.

I also added a weatherproof inline 30 amp circuit breaker. (The motor draws 20 amps max.)

After testing the PWM, but before fitting it, I took a few comparative circuit resistance readings between it and the original non-working unit.

Everything seems the same except for the potentiometer rating. The new working PWM has a 90k ohm pot, and the previous non-working PWM has a 45k ohm pot, and both are reading those correct resistances end to end, and via the wiper.

The original was with a board mounted put, and the new one has flying leads. I'm thinking that's a red herring though, surely the short flying leads would not require a different pot rating ?