Long balance leads for lipo charging?

[bjosta]

I'm trying to determine how to build/design my battery box, right now I'm leaning towards a "permanent" installation on the bike, in the frame triangle so to say. But doing this would mean my balance leads for charging would be long, say 40-60cm to reach the charger (off bike).
Bad idea?

 

[neptronix]

You want them to be as short as possible, to minimize voltage drop which leads to inaccurate readings.
Stock ones are about 24 gauge, maybe run a 20 gauge extender off em to be safe.

 

[Ricky_nz]

Make sure the leads are well protected from damage as any shorts may start a fire.
Possibly fuse them if it doesn't interfere with the balancing measurements.

 

[neptronix]

Heh, a fuse on every balance lead? that's kinda taking it overboard.

if you aren't using duct tape and glue to hold everything together you should be fine. It's pretty hard to short balance leads unless you're doing something really stupid.

 

[Ricky_nz]

Heh, a fuse on every balance lead? that's kinda taking it overboard.

Yep probably is overboard but may be useful if wires are too exposed.
Thickening the wires up will increase the damage they can do before they themselves fuse.
I think somebody on es did fuse some at one point.

 

[neptronix]

They don't need to be exposed for any reason.
All connectors used should be designed to prevent any unintentional shorts, as most modern multi-pin connectors are designed.. You'd put the female end on the balance harness ( just like they are designed ) to prevent this.

Are you talking about them having weak plastic shielding?
If that's what you mean, add a layer of shrinkwrap.

 

[Ricky_nz]

Are you talking about them having weak plastic shielding?
If that's what you mean, add a layer of shrinkwrap.

I'm concerned that once extended there is more possibility of damage to the wires so yeah adding some tough heatshrink over the wires would help and use wires with reasonable thickness insulation.
I just have images in my head of some of the bikes on es where a 60cm balance lead flapping around could be quite a hazard due to being exposed to nicks and cuts etc.
You comment about the connector is dead on.

 

[bjosta]

Thanks for the replies,
I'll use common sense and try to shield/protect the cables and connectors from damage to avoid shorts.
regarding the voltage drop, would that be a problem? is the drop big enough to make a difference while charging? Going to bigger wiring might help. Or getting a charging solution that has a HVC of say 4.1-4.17 would mean the batteries will never go above 4.2 even if the voltage sensing is affected by the balance leads.

 

[Ricky_nz]

regarding the voltage drop, would that be a problem? is the drop big enough to make a difference while charging? Going to bigger wiring might help. Or getting a charging solution that has a HVC of say 4.1-4.17 would mean the batteries will never go above 4.2 even if the voltage sensing is affected by the balance leads.

You can work out the voltage drop from from the ohms/meter for the size wire you use and the maximum discharge current that your charger uses when balancing the cells using ohms law.
Basically thick enough wires will make it insignificant as most hobby chargers have very low discharge currents.

 

[neptronix]

I wouldn't balance charge over 4.15v anyway. Not much energy there in the first place; it is quickly burned off. And charging to just a bit lower than most chemistry's maximum voltage per cell is good for long life anyway.

That gives you a lot of room for error.

If you don't know about ohms like me, you can measure your voltage drop at the beginning of the wire and at the end.

Discharge from the balancing charger itself is usually in the 300mv range, so that's pretty piddly.
I don't think for that distance, and 20 gauge, you will have any problems. most RC balance lead extenders i've seen are using 22ga, so 20ga for that distance seems good. Maybe overkill but its better than underkill

 

[Ricky_nz]

If you don't know about ohms like me, you can measure your voltage drop at the beginning of the wire and at the end.

oops I tend to forget that not everyone is familiar with ohms law . Been using it since I was 10.
The empirical approach works but is a bit of mucking around and needs a good millivolt meter.

Heres an example if anyone is wondering how to work out the voltage drop in the wire
14 AWG copper wire: 0.008285 ohms per meter from table ( just google for wire resistance)

wire length = 0.6 meters actual wire length
Current in wire = 0.5 Amps

Wire resistance = 0.6 * 0.008285 length * ohms-per-meters
= 4.971 milliohms

Now the ohms law part:
Voltage drop = current * voltage = 0.5 Amps * 4.971 milliohms = 2.4855 milli Volts.

This would definitely not cause any balancing problems.

 

[wojtek]

I'm trying to determine how to build/design my battery box, right now I'm leaning towards a "permanent" installation on the bike, in the frame triangle so to say. But doing this would mean my balance leads for charging would be long, say 40-60cm to reach the charger (off bike).
Bad idea?

hi, i am planning to use those balance ext ensions
http://www.progressiverc.com/index.php/balance-extensions.html

and only have very short cable coming out of the battery box. When charging, i will connect 2 or 3 of those extensions to the main pack balance connector and charge the battery. No need to carry them all the time.

btw here is my battery box being built
http://www.endless-sphere.com/forums/viewtopic.php?f=3&t=25839

good luck!

 

[neptronix]

Give this a look.
http://www.currentsolutions.com/vdrop.php

Voltage drop using 20 gauge wires for 2 feet, 4.2v, at 0.3amps( max output of most balancing chargers ) is:

0.006v
Most balancing chargers are rated to +/- 5mv accuracy

So 20awg would work but not be perfect.
Not that balance charges need to be perfect anyway.

I'm just thinking in terms of how heavy / thick the wire should be. You don't want it to be so heavy that it tears the leads out of the batteries, lol.