Comparison of Soldered vs Welded Straps

Most commercial battery packs use welded connections. While admitting that welded packs do have higher resistance, some manufacturers claim that soldering gives only a slight advantage that is outweighed by the likelyhood of damaging the cells.

Few hobbyists have access to spot welding equipment, therefore soldering is their most popular construction technique. Many believe that their packs are significantly superior to commercial welded batteries. But is the difference really that noticable? Also, how big do the straps have to be, to keep losses down to an acceptable level?

I decided to do a scientific test to find out just how much better a soldered connection really is. I disassembled a commercial pack constructed with 10mm straps and 8 welds per connection, and replaced one strap with a much narrower piece of de-soldering braid. To measure the resistance, I passed a current of 1 Amp through the battery, and probed the inter-cell voltages with a digital meter. The results were as follows:-

Soldered Copper Braid (3mm width)0.6mV
Welded Nickel Strap (10mm width)1.5mV

 

Clearly the copper braid is better, but both values are very low. So is the difference worth worrying about? At 1A, no. But at higher currents the differences may become significant.

For example, consider a 7 cell battery discharging at 30A. With 3mm copper braid the total strap losses would be 0.11 Volts and 3.4 Watts. Welded straps would lose a bit more, 0.28 Volts and 8.3 Watts. Considering that the battery would be delivering about 230 Watts, this slight power loss would probably be barely noticable.

However, at 60 Amps the welded straps would waste 32W, more than 5W per strap! They could get hot enough to melt through the insulation. Available power has been reduced by about 7%. Furthermore, the pack voltage has dropped by 0.54V, which may cause the LVC to cut in early. 3mm copper braid would also have significant losses at this current, but a wider copper strap should be much better (I usually use 6.5mm braid on this size battery, which only reduces power output by about 1.5%).

Conclusions

At normal current levels, a well-made commercial pack with wide straps should be OK. Low-cost consumer batteries with narrow straps and fewer welds may be unsatisfactory. Even a narrow copper strap has lower loss than the best welded strap. In applications requiring maximum power, soldered copper straps should have a significant advantage.

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