Resistors for bottom balancing (25W, 2.7Ω) -- where to purchase?

[berksrunner]

Has anyone found a reliable supplier for these resistors that Will recommends?

I think I tried to order them once about a year ago and was told they wouldn't ship for 6 months, so I cancelled my order.

 

[snoobler]

IMHO, unless you've bought matched cells from a reputable manufacturer, AND you've conducted detailed testing on them confirming capacity and internal resistance, bottom balancing is not the ideal solution for the following reason:

Every day you will run the batteries to their maximum charge point whether that's 70, 80 or 100%. In the vast majority of cases you will NOT run the battery to near empty on a daily basis.

If the goal is to have the cells at equivalent states of charge most of their life, there is no other solution than TOP balancing.

Will has also recommended top balancing for cells that may not be well matched.

 

[berksrunner]

IMHO, unless you've bought matched cells from a reputable manufacturer, AND you've conducted detailed testing on them confirming capacity and internal resistance, bottom balancing is not the ideal solution for the following reason:

Every day you will run the batteries to their maximum charge point whether that's 70, 80 or 100%. In the vast majority of cases you will NOT run the battery to near empty on a daily basis.

If the goal is to have the cells at equivalent states of charge most of their life, there is no other solution than TOP balancing.

Will has also recommended top balancing for cells that may not be well matched.

Makes sense. Thanks!

 

[boondox]

Here are a couple:

Resistors | Chassis Mount Resistors | DigiKey

Resistors - Chassis Mount Resistors are in stock at Digikey. Order Now! Resistors ship same day

www.digikey.com

Mouser is another good source

 

[Xerxes]

I am assuming you are going to use a BMS.

If so most of them will destroy your bottom balance by top balancing them passively by trickle drain down every time they are fully charged, some passive have as much a 150 ma drain down. Some folks here use BMS with active balancing that uses 1 Amp or more.

In the old days of those other 3.7V batteries you did not have BMS, just voltage cutoff chargers for your home brew. Bottom balancing allegedly would provide more battery charge (or not....) but did introduce risk of overcharging a weak cell which for the 3.7V batteries can be dangerous. I am at a loss on the benefit of bottom balancing when you use the more common balancing BMS from China. I think Wills position on this has changed over the years from when BMS's that worked were >$1000 and spending that much to protect a $200-$800 battery pack made little sense when you could replace it with a little vigilance, or if destroyed by 3-4 battery packs for the price of one BMS, plus vigilance is easy to do when you live in a van and are looking for things to entertain your day and that battery is 10 feet away from you. Now these BMS are <$100 and reliable so they make sense to protect the $400 battery pack and having them top balance ......... and the whole question of top balance vs bottom balance being better is a bit murky and invites lots of strong minded individuals to get very combative and insist on pounding their chest and saying you are wrong.

For the resistor the Mouser and Digikey parts houses are well known but a bit intimidating for a first time buyer to try to order one resistor when they are set up to sell you 1000 resistors of each type for your project. They will honor and ship a single resistor purchase. You might find eBay/Amazon a bit less intimidating, they have several listed at individual count with easy ordering. Many years back I bought a huge 3 Ohm 100 Watt wire wound resistor that has an adjustable slide to get smaller resistor increments and has had several uses. Others more recently here at the forum are buying those USB battery load testers that have morphed into a computerized datalogger variable 150W load with heat sinks and fans and run from $20-$50 from China or Amazon that can also test your cell capacity-albiet very slowly when you have a 10KW-hr battery pack.