Although this thread is a bit old, well, I may have a small contribution to it.
Long time ago, I came to the same conclusion tan most mates, and I settled on XT-60. I use them whenever supply voltage is between 6 and 24 V, including my radio gear (I’m also a ham radio operator and tinkerer)
However, soldering them was not easy at first. After spoiling some of them, I came to the same procedure recommended by jonhnf and others:
1.- Always solder connectors mated. I've found pretty convenient to hold them in a mini vise.
2.- I always use 60Pb/40Sn solder, with 3-3.5% core rosin flux. I do a lot of electronics tinkering, including SMD hand soldering, and I never use Pb-free solder.
3.- I first tin the wire, and check that the tinned end fits in the pocket.
4.-Then I tin the pocket. When solder is melted, I quickly insert the tinned end of the wire while keeping the soldering iron in in good contact with wire and pocket. If you feel that there is not enough solder, add more.
5.- Apply some traction to the wire when the joint is cool. Sometimes you get a “dry joint” if not enough heat was applied.
6.- Repeat the same procedure with the other wire.
A closing note: Currently, I settled on subtype XT60-H. No need to deal with heath-shrink tube, resulting in a neater job!
Good luck!

the XT60 bullet connectors are rated 60AMP however I no longer run fast electrics so am content with DEANS which are rated 25AMPS (50A burst). There is also a mini-Deans suitable for water pump, audio & lighting wiring inside.
I hav been a devotee of WELLER soldering irons since age 18(now 75yo). I recently bought a WELLER iron on Amazon which has an adjustable power knob & with the correct solder it handles large joints just fine. The identical iron aimed at the Canadian market the model # ends in CUL which is certified for use in Canada & its 3x the price of the USA/MEX unit

Thanks for all the answer/suggestions. I have a decent soldering station, therefore first step is to purchase some decent 60/40 solder lower temp like 183°c and try again........😂😂😂😐😐

The XT60 connectors are a giant step forward in improvement over the Tamiya connectors. I came across them when my new RC 1/16 scale tank had an XT60 connector on the tank, battery, and charger. Unfortunately, I have a lot of RC tanks/armor, boats, and chargers with the Tamiya connectors installed. Too many to change now, o I made some Tamiya to XT60 adapters.

About the soldering irons, the more expensive ones seems to have a higher quality metal tip on the iron. I see no deterioration at all. The "cheap-O's" tips don't last long and seem to build up a coating that does not transfer heat well unless scraped or cleaned. This possibly could be the problem with melting the insulators on connectors. Food for thought.

Lew
Florida , USA

The attached link is the soldering station I had purchased on Amazon. It is no longer available, but this will give you an idea of what I found to be a great soldering problem solver.

https://www.amazon.com/gp/product/B08DXHQXW8/ref=ppx_yo_dt_b_search_asin_image?i

I nearly bought some of these connectors as the usual white ones can be unreliable, but the number of batteries i have made it impractical.

Last year I bought a similar soldering iron with a temp. read out, excellent soldering iron. I wanted one like my son has and I had tried it and liked it. There were many for sale and so I found one on ebay that also included 'make me an offer'. So I took 10% off and that is what I got it for . I think about £28 used it ever since.

However for large items like soldering battery contacts I have a large 75 watt one which has a large mass soldering head.
I never throw away my duff Nicads, I put them to one side and when there are enough I take them apart and find the still working cells, test for discharge rate and then solder them together as a new battery.

Roy

In addition to the other excellent suggestions, I feel a good quality soldering iron is a partner to good soldering. After many years of using the cheap soldering irons, about two years ago I bought one of the nicer temperature controlled iron for about $50 U. S. Worth the cost.

Lew
Florida, USA 🇺🇸

I am with you Bill and agree with most of the posts regarding these connectors. There is one suggestion though that I would never do and that is solder a connector whilst connected to the battery unless it was the battery terminal that I was soldering.😃

I agree with most of the posting,especially regarding a good flux.
I use what is shown in the attached photo. BAKERS paste.
Very old but great stuff.
Take care all.
Regards
Bill.

Hi Nick
I agree with most of the previous replies. I have made many such connections over the years and always use a good flux paste and NOT lead free solder as that requires too high a temperature. I flux and tin the connector and flux and tin or re-tin any leads that are already tinned to ensure any lead free solder is reduced (you can buy low melt solder to do the job but it costs). I use a temperature controlled iron set to 400+deg and hold the connector and wire with wooden clothes pegs ( or any similar mounting). Heat the wire and put with the iron to the connector until the solder flows ( you may need to add more solder) then remove the iron and leave to cool. As others have said finish off by cleaning (IPA) and push the heat shrink cover over the joint and seal.
The most important part in the process is the flux as this helps the solder flow quickly and as a result reduces the time required to apply the heat
Most batteries are supplied with the connector pre attached. Should it be of a different type to your requirements I would either make up your own conversion lead or buy an adapter. Soldering live wires connected to a battery (especially a LiPo or similar) is high risk and is best left to the experts.
dave976

From what has been said the most important things are:
1. Ensure all surfaces are chemically clean
2. Join the XT male and female connectors together
3. Pre-tin the wire end and the connector
4. Use a 100Amp soldering iron when making the final soldered connection.
Good luck
😉