Powerchair Battery Cables and Batteries - Losses!

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Powerchair Battery Cables and Batteries - Losses!

Postby Burgerman » 15 Feb 2010, 14:55

Att JoeC!
You were talking about thicker cables in your wifes competition chair.

Driven into a wall... Stalled for 2 seconds on a stock f55 powerchair. Amps were 70 to 80 on clamp meter.
Voltage under 80 amp load:
At controller module = 21.4
At battery terminals = 23.7

Off load 26.1 at both points so fully charged.

Couldnt test at motors but would expect another drop due to controller resistance and another smaller one due to shorter motor cables.

Thats horrendous!

With 160 amps (your 2x 80 amp DX controller) the voltage drops will be 2x worse!

This was new (2 months) overnight charged group 24, 73ah Gel MK branded (as good as gels get) batteries. Thats why I fit AGMs...

Its also why we should keep battery cables as short as poss, and thicker than stock!

That means the 26+v we started with will be much less than 18v under full load with a big amp controller. agm's and thicker cables and the latest low loss DX or Pand G controllers will give a huge boost in curb climbing, wheelies etc...
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Re: Powerchair Battery Cables and Batteries - Losses!

Postby JoeC » 15 Feb 2010, 20:50

Thanks for the measurement!

I already cut the stock battery cable as short as physically possible, the long run was a stock 10ga, the shorter parts were 12 ga. For some inexplicable reason, the motor's stock cable is 14ga! One of these days I will most definitely figure out how to change it out to 8ga or better all the way from the controller to the brushes- the robot combat people claim to do it as a matter of course, so it can't be that tricky.

Will let you know future updates- for now I'm cleaning and organizing a house so that we can have dinner guests.
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Re: Powerchair Battery Cables and Batteries - Losses!

Postby Burgerman » 15 Feb 2010, 21:42

Not sure how that ga cable measurement works! But these are 8 square mm marked. They look thinner though! And they have 3x (50 amp) connectors and a 100 amp fuse. And pretty long...

In my own chairs I use a short engine earth strap to link the batteries so only one connector and lots less cable. no fuse. Never measured that though.

But I see I will neet to replace the lot with 10 or 12 square mm stuff. A very big percentage of a stock power chairs power goes missing before the motor gets it! The bean counters strike again! Have a good party! I am alone. GF went to paris to see a game of tennis and I saw her on Eurosport...
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Re: Powerchair Battery Cables and Batteries - Losses!

Postby JoeC » 16 Feb 2010, 20:02

The 14 gauge stuff attached to the motors is equivalent to two square millimeters, and I have about a foot of it between the motor and the controller. My chart says its resistance should be dropping about 0.6 volts at full load, about 50 watts. The free runs of wire away from other heat sources definitely get warm to the touch immediately after heavy use.

If memory serves, the battery cables have about 20 inches of 12 gauge, equivalent to 3.5 square millimeters, and two feet of 10 gauge, equivalent to 5.3 square millimeters. I need to double check that it is not composed of 10 and 8 gauge, with 8 gauge being equivalent to 8.4 square millimeter. Assuming it's the smaller size, it looks like 0.51 volts of loss from the 12 gauge section, and 0.16 ohms from the 10 gauge section. All in all, 0.67 volts lost to battery wires and almost 110 watts. Add this to the motor leads, and we're wasting almost 200 watts at peak current- somewhere around 6% of the power being wasted in the cables.

Also inline with the batteries are some thermal circuit breakers, not fuses. I don't know what their resistance is. I should really repeat your measurement though, in case my wires don't live up to the chart. I'm very surprised that you're seeing 2.3 volts of loss between your battery and your controller- if I'm not doing a lot better than that then this is going to jump right to the top of my list!
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Re: Powerchair Battery Cables and Batteries - Losses!

Postby Burgerman » 16 Feb 2010, 20:53

My figures are right.

My cables are 8 and 6 square mm so much bigger than yours. Are you sure you did the calculation right?
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Re: Powerchair Battery Cables and Batteries - Losses!

Postby JoeC » 16 Feb 2010, 22:31

Example calculation,

Chart source: http://www.powerstream.com/Wire_Size.htm

Using 14 gauge wire, resistance is listed as 2.525 ohms per thousand feet. Divide by 1,000 feet, multiply by three feet (a bit more than the length I have, counting positive and negative), and resistance is 0.00505 ohms. Multiply by 80 amps, and it should be 0.606 volts.

Looking for other charts,

http://www.cirris.com/testing/resistance/wire.html
http://www.thelenchannel.com/1wire.php

Resistance value is the same, and diameter listed is close enough that I never see more than 2.5 mm squared.

I don't doubt that your measurements are right, but I wonder if there might be something in your circuit other than the wires that is causing this voltage drop. Maybe the connectors, maybe the fuse?

I'm reconfiguring someone's seating tonight, but I'll also try to get some electrical measurements while I have my tools out.
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Re: Powerchair Battery Cables and Batteries - Losses!

Postby Burgerman » 16 Feb 2010, 23:01

No, I mean the cable size calculation. 2.5 square mm is tiny. I am sure that that calculation is wrong. It would melt? My 30 amp battery charger uses 8 square mm cables. Some chairs use 6 some bigger.

I will be interested in your volt drop / amps measurements! You will likely be amazed as I was...
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Re: Powerchair Battery Cables and Batteries - Losses!

Postby JoeC » 16 Feb 2010, 23:34

I understand, I'll make sure to use calipers in addition to electronic meters.
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Re: Powerchair Battery Cables and Batteries - Losses!

Postby JoeC » 17 Feb 2010, 05:00

I've just checked the wire diameter, and although my calipers have run out of battery and lack a vernier scale, the marking shows that the conductor diameter is no more than 2 mm, so my motor wire cross section is no more than 3 mm square.

I'll try to get some electrical measurements as well, but after an hour and a half of filling the living room with chair parts and general commotion, domestic tranquility may require this to happen another night. I will update as I can.
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Re: Powerchair Battery Cables and Batteries - Losses!

Postby Burgerman » 17 Feb 2010, 09:47

And they dont melt!???

A mains cable for 30 amp is specced as 6mm square.

You are taking 160 amps from 3mm square? I am amazed.
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Re: Powerchair Battery Cables and Batteries - Losses!

Postby JoeC » 17 Feb 2010, 15:34

No, this is the connection from the motor to the controller- only 80 amps max. They do get warm to the touch! Although the motor, gears, bearings, and brushes seem to be up to the task at hand, the wiring that came with this motor seems a little below par. I wish it wasn't such an involved project to crack them open and get big fat wires all the way to the brushes.

The 160 amp connection is no smaller than 5 mm squared, but I think there is a short section that is smaller.
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Re: Powerchair Battery Cables and Batteries - Losses!

Postby Burgerman » 18 Feb 2010, 14:41

You better measure losses then because the motor wires on mine are 6 square mm as are the battery cables. And the losses and burning wiring smell is quite bad under load!
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