Here's a real world example of mixed capacity (in Ah), mixed chemistry batteries of common voltage charging (and not exploding) in peace and harmony.
Anyone who has a camping trailer most likely has this setup and hundreds of thousands are currently on the roads.
I'll use mine as an example.
My truck, 1996 Kia Sportage, has a primary starting battery, 12 volt maintenance free lead-acid flooded cell 65Ah capacity. The alternator is rated 80A @ 1800RPM.
My trailer has a house battery, 12 volt lead-acid flooded cell 85Ah capacity. It is non-maintenance free meaning I can top up the water in each cell.
I have an additional house battery, 12 volt sealed (gel-cell) lead-calcium 24Ah capacity, wired in to the trailer in parallel to the main house battery.
All three are seen by the current source (the alternator) as one battery that is of 174Ah capacity. There will be tiny differences due to lead length between starting battery and house batteries. I only mention this to avoid nitpicking.
When the trailer is hooked up to the truck, all three batteries charge simultaneously. When I use the trailer lights and am unhooked from the truck, the house batteries are drawn down. Upon plugging back in to the truck, the batteries begin equalizing with the starting battery. Imagine three cups with different amounts of water joined by tubes at the bottom, the levels will equalize, simple physics, we've all seen the demo in school. Once I start the truck, all three will begin charging from the same current source, the alternator. This causes no harm to the batteries, the alternator, or to any endangered species. The only difference is in the charging times of the batteries. This is similar to having a garden hose flowing at 1 gallon per minute filling a 5 gallon bucket. When filling one bucket it will take 5 minutes. When filling 2 5 gallon buckets it takes twice as long. There? Three times as long.
The trailer's house batteries are connected by a 20 foot run of 12ga wire run through a self-resetting circuit breaker directly to the alternator output (at the underhood OEM fuse enclosure). There is no need for any extra regulation. The regulator is built into the alternator as is nearly universal in late model cars (1985 - present). Vehicles with an external regulator require no difference in wiring. The charging current is regulated by the terminal voltage of the battery or batteries. When the combined terminal voltage reaches a certain threshold, ~14.0 - 14.4v, the current is virtually nil. The output current, not voltage, is linearly regulated by the regulator, external or internal.
Another good example is a battery charging bank at a battery retailer, a real one. To keep batteries from going flat or to top up batteries that have dropped while on the sales floor, batteries, shitloads of 'em (dozens at a time) are hooked to a common bus bar, both terminals, which is fed by a single 12 volt current source, typically a 12 volt 100 amp battery charger. Batteries that are fully depleted can be hooked up right next to batteries that are fully charged. You cannot overfeed a battery. You just can't do it. You CAN supply too much voltage to a battery, say, 24 volts to a 12 volt battery but then you're just paying the stupid tax. A battery will not draw more current than it needs. It's as asinine as saying you can't use a 12 volt 1 watt bulb on a 100Ah battery.
Espressodude's trailer has a breakaway battery, 12 volt at ~6Ah capacity, that is wired to his truck battery. It is the way it is because that's the only way to charge it. He could have it arranged so that he has to hook up a wall powered battery charger but why? Because it is not necessary and requires more work as well as running the risk of it being dead when it is truly needed. The reason it is the way it is is because it it the correct way to do it and has been engineered that way. Hook his trailer up to any tow vehicle and it will work.
Trust me. Trust him. We have both had long careers in electrical engineering as well as uncountable hours of personal time working with batteries on projects. FOGBANK, the sound cannon, charges from vehicles or wall powered chargers. Both work and are functionally equivalent.
Trust Yiggy and Figgy. They have lifetimes of experience and decades of hands-on project experience with batteries and chargers.
None of this is guessing or "I think it just might work". It's plain simple physics, Ohm's law, and universal truth. If you're getting info that id much different than what I, Edude, Yiggy, or Figgy have put out there, either the source is trying to sell you something or plain doesn't know what they're talking about.
I'm not claiming a special ability or brand of genius here, anyone can do the math and studying and come up with the immutable truth as dictated by the physical constants that rule this realm.
He's a mystery wrapped in a riddle, inside an enigma, painted in hot pants. - Savannah Propane ToysHow to do it wrong: