Parallel'ing batteries together is something you should only be doing if you know all the ins and outs already coming into the game. I've been doing it for years but only if I absolutely have to. I much prefer building one big battery than paralell'ing two smaller ones. Mostly because of the risk associated with bypassing the BMS when charging. The issue is that if you charge thru the controller (output) connection, that connection does not have overcharge protection via the BMS, which only provides that on the input (charger) connection. You can buy a BMS that does offer this protection, but its a special order item from your custom battery builder. And if you have a custom built battery you may as well just build one big one in the first place.
The whole charge-thru-the-controller thing happens when you parallel two batteries, then if you use the bike a lot (like, daily commuting with a charge cycle at home and at work... twice a day) you leave the batteries coupled and just charge the bike via one charge connection. The charge to the second battery in line will come in thru the connected controller cables.
On a daily driver bike, its just not practical to disconnect each battery, then go down to your parking spot during the day and switch the charger to the other battery. And God help you if your charging process allows anything more than a few fractions of a volt of voltage difference between the two because when you reconnect the two batteries the current will *instantly* equalize voltage from one flowing to the other and that can be youtube-worthy (and ambulance-worthy).
The solution to this practical necessity (leaving parallel'd batteries connected) is one of these battery equalizer kajiggers. Would I trust one from a Chinese seller on Ali-Express? Hell no of course not. What about the westernized ones? Only if their output matched my output needs. Which they pretty much never do. I parallel batteries on 2wd systems that have current limits in the 60a continuous range and only a very few of those gadgets can be had in ratings that high.
@Kingofgreens you seem to have figured out one of the other absolute musts of paralleling, which is to use identical packs with identical charge cycle counts.
Your analysis on what that kajigger does is a big maybe. Many of them do very different things. Some flip back/forth like you describe. Some pull from one pack, then switch to another when #1 reaches a certain voltage. Still another type draws them both down at once and one I know of even has an option to safely charge both of them thru the battery port via a single input. One thing is for sure... a good one is not going to cost any $40.
BTW that linked Battery Blender is one of the many DATEx variations and that is the one that has all the bells/whistles, as well as the good rep among DIY builders. Here is the only retail seller I know of with proper options. The DATEx web site is the most modern web site of 1994, assuming its still in the state it has been for years. All transactions are email-based.
Upgrade your eBike journey with the DATEx2 Parallel Battery Adapter – the 'battery blender' for seamless rides. Connect up to three batteries, handle diverse types, chemistries, and charge states. Say goodbye to inconvenient swaps! Voltage range: 24V to 84V.
biggamebikes.com
Two batteries and 60a capacity with charger port (which will learn the hard way you want) is $483.95.