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%).
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.