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120 VDC 170mA Nihm charger

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tonyp12

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Trickly charging nimh batteries is pretty safe at 10% of mah rating

I have 100 units of 1.2v 1700mah in series to run a 120v 2hp DC motor.

So what is the best way to get from 120vac to 120vdc at 170ma?


Alternative is to charge at 60vdc as I have a switch that puts 50batteries in pararell
with the other 50 batteries (speed controll)
 
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Put a bridge rectifier on the 120V AC, this gives about 180V positive going peaks, just for charging NiCd's you could use a simple LARGE wattage resistor directly feeding the 120V batteries. It's somewhat complicated calculating the resistor value (particularly as I'm just back from the pub! :D ), but if you stick a good sized electrolytic across the rectifiers output it becomes 180V 'proper' DC, which makes it simple using ohms law.

353 ohms, 10.2W.

I would suggest a 390 ohm 15 watt resistor, bear in mind it will get quite hot!.
 
OK, I get

6A 600V BRIDGE RECTIFIER $0.90
1000 UF 100V Radial Electrolytic $1.00
400ohm 20watt (sand) $ 1.00

Sounds about right??

But should I charge at 180volt dc if they are at 120v (total)??

We are talking NIMH batteries.
 
tonyp12 said:
OK, I get

6A 600V BRIDGE RECTIFIER $0.90
1000 UF 100V Radial Electrolytic $1.00

Only 100V would go BANG!!, you need 200V, as it will be charged to 180V or so - you're not feeding any critical electronics (in fact no electronics at all), so a smaller capacitor than 1000uF should be fine, if you can get 470uF or 220uF at 200V, either should be OK.

400ohm 20watt (sand) $ 1.00

Sounds about right??

Yes, it's not critical.

But should I charge at 180volt dc if they are at 120v (total)??

You're NOT charging at 180V, that's at the other end of the resistor, you MUST have a higher voltage for current to flow.
 
What about using a transformer?
Is it not always good to isolate current?


What about 120vac to 100vac 6amp stepdown transformer ($9)
before the rectifier?
 
>>What about 120vac to 100vac 6amp stepdown transformer
>Why?.

I'm thinking about the reducing the current to 170ma by using a transformer
that have about that output rating.

And maybe at the same time reducing the voltage a little.
 
tonyp12 said:
>>What about 120vac to 100vac 6amp stepdown transformer
>Why?.

I'm thinking about the reducing the current to 170ma by using a transformer
that have about that output rating.

That's not what transformers do, the amount of current supplied is dependent on the load - if you try and take too much the transformer will overheat and die!.

And maybe at the same time reducing the voltage a little.

If it makes you feel happy?, then fair enough - BUT you need considerably higher than your battery voltage in order to pass current to them, and limit the current either via a simple high wattage resistor, or more complicated constant current source.
 
tonyp12 said:
<snip>
I have 100 units of 1.2v 1700mah in series to run a 120v 2hp DC motor.<snip>
Does that work? One horsepower=746 watts. Two horsepower ~ 1500W.
1500W/120V=12.5A, and this assumes 100% efficiency. How long do your 1700maH batteries last?
 
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Don't know how long they last.
Did not finishing builing the proto type yet..

It's a underwater scooter, so 20 minutes is good enough.

The batteries cost $70 so it would be to expensive to double up on them.
 
That's why you wear fins (don't EVER call them flippers). The batteries certainly sound VERY feeble for such a project, but why use a 120V motor?, what's wrong with a lower voltage motor and lead-acid gel batteries?.
 
I think RonH is saying you need 12.5A to run the motor at 2HP.. And you have 1.7A. Might be a short ride.

Have you hooked the batteries to the motor yet and ran it. And your body weight/drag. Hope it works out though, sound like a fun toy..

We have one that pulls us around. It is at least 20AMP (12VOLT GEL), but it does last a long time, it has a trolling motor in it.. Not sure of the power.
 
tonyp12 said:
Don't know how long they last.
Did not finishing builing the proto type yet..
It's a underwater scooter, so 20 minutes is good enough.
You forgot to look at the datasheet for the battery cells. Their charge might last only one minute at 12.5A, the voltage will be very low and they will heat like mad. You cannot use battery holders at such a high current.

The datasheet shows the voltage dropping a lot and the charge disappearing quickly with a 3.4A load. Your 12.5A is nearly 4 times higher.
 

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The batteries will encapsulated around the motor in heat-transferring resin (epoxy)

The resin will molded to be the housing (shaped like a torpedo)

It will power a 2hp Yamaha (7.5" d) outboard propeller



I will also put 4 of the motors on a small boat (total 8hp) using 20 deep cycle 33ah 12volt lead acid batteries.

This should last 2 hours.

Motor stats:
Draws 1.2 Amps at 95 VDC unloaded
1-1/2 Hp @ 95 VDC cont. duty
2-1/2 Hp @ 130 VDC int. duty
Rated to 18.5 Amps
0-6750 RPM
Open enclosure
Shaft 17 mm w/threaded end 1/2"-13 UNC LH thread
Four hole base mount on 4-1/4" x 2-3/4" centers
Overall size 7-3/4" x 4" x 4" excluding face plate
Shpg. 10 lb.
 

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