Jrxpro

iTrader: (2)

A question for the electrical experts.


Short question:

On the battery end of a balance lead, does it matter where along the circuit the pos and neg wires are placed?


Background and long question:

I purchased a few 2s lipo/lihv hard case batteries. No wires coming out of the case, just 5mm female bullet terminals, and a 2mm balance terminal. They come with a charge lead where the pos and neg wires of the balance connector are soldered directly to the 5mm male Bullets, and the middle wire has a 2mm male bullet that plugs into the hard case.

I made some XT90 adapters (male 5mm bullet to XT90) to make the batteries compatible with our various cars and so the kids can’t reverse the bullets when plugging in a battery.

Now I would like to make a charge lead that connects to those XT90 adapters (so that the XT90 adapters just stay in the batteries semi-permanently). Question is, does it matter if the pos and neg wires of the balance lead are soldered to an XT90 connector instead of directly soldered to the 5mm bullets? The middle wire would still go directly to the 2mm port on the hard case.

In other words, does it matter if there is a few inches of wire and an XT90 connector between the battery terminal and the pos/neg balance wires? In my mind it should work, but I don’t know how balance charging works, like if the extra distance from the battery terminals might make the charger read the levels incorrectly.

If this isn’t clear, I’ll post some pics or a diagram later. Just wanted to type it out while fresh in my head.

Thanks!

Roelof

Extra length of wire does not matter.

cbb

iTrader: (9)

That won't impose enough resistance to matter a bit. Charge away!

Jrxpro

iTrader: (2)

Thanks so much guys. This forum is great.

gigaplex

iTrader: (2)

It will matter a bit, and matters more at higher charge rates. There is a voltage drop across the charge leads as current flows. The balance leads get a better voltage reading of the cells the closer they are to the battery. But if you're only charging at 1C and the length of the adapter is short then it'll still work.

Jrxpro

iTrader: (2)

I do occasionally charge at almost 2C. Adapter is about 4 inches. If there is a voltage drop and the charger reads lower, do you think there is any chance of overcharging?

PDR

iTrader: (31)

The likelihood of overcharging due to the cable is effectively zero. You don't mention what gauge wire it is, but assuming spindly 16AWG multi-strand wire, the resistance over that distance is in the milliohm range. If you crank your charger to 20A, the voltage drop will be single digit hundredths of a volt. However, LiPo chargers will taper off the current towards the end, so the drop will be even smaller, so the contribution to any incorrect charging will be tiny, probably less than the error range of your charger's electronics.

gigaplex

iTrader: (2)

No, if anything it'll think the charge is too high so the charger will slow down. It won't cause damage, it'll just take a bit longer to charge.

Alexv2024

iTrader: (2)

I have charge cables that are 12in-24in and 36in is very common as well. Usually using 12g and 10g wire and are made for racers to charge up to usually 40a on 4000-6000mah packs.

Charging at 2c for 8in charge leads will be perfectly fine....as long as you have quality connections. And yes you have to have each pos/neg connection correct and the sense (thin) wires so that the charger can properly balance each cell. And balancing is important, it'll help prolong the life of the pack. So if possible you should be balance charging and discharging. Not sure if you're doing that right now.

Roelof

Yes and no. At the end of charging where balancing will take over the work the charging current is low.

gigaplex

iTrader: (2)

You're right that it'll still charge safely and accurately but it might leave the CC phase of the charge cycle earlier, making the charge cycle take slightly longer.

biz77

iTrader: (47)

I was once in a pinch for balance lead wire while building a 1s charge cable for a fellow racer and decided to come off the 4mm banana plugs that plug into the charger instead of running the balance leads all the way down to the bullets that would plug into the battery. My friend commented that his battery was trash, reading in the 7-8 mohm range. When I plugged them into my charger, which had the balance cables run all the way to the battery, his battery was reading 1-2 mohm on discharge/charge cycling. I gave him my cable to use on his iCharger and he was then reading the same 1-2 mohms. Needless to say, I rebuilt the cable with the balance leads run to the battery plugs for him and problem solved.

Jrxpro

iTrader: (2)

I was totally going to ask this very question, just out of curiosity, how far away can you go? I guess now we know lol.

Alexv2024

iTrader: (2)

Yeah the whole point of the sense wires is that they see what the battery sees so that accurate voltage can be measured that is at the cells. Attaching them any where but the end of the charge leads defeats their purpose. Maybe an inch away, maybe 2 in away from the battery bullets would be ok, but then you have more solder joints and points of failure and reduce the accuracy. Would it be interesting to see inch by inch how the readings differ? Sure! But it wont change how charge leads are made.