In reply to redheadedrod's last reply...
No - I am correct about parallel batteries being able to be different. What you are talking about are series connected batteries (24V) and permanently connected parallel batteries (whether 2x12V for extra grunt, or 2 parallel strings of 2 series batteries (with "diagonally opposed +ve & -ve power tale offs) and I specifically exempted them - they are not the isolatable parallel batteries we are discussing.
What you say for those systems is correct. In all those systems, you must replace ALL batteries with matched model and new batteries. To replace just one compromises the life of the lot (though replacing a failed new monoblock/battery is often acceptable).
But that is irrelevant when discussing isolated parallel batteries [which could themselves be series batteries of different capacities etc, only the battery in each string needs to be matched with its series battery(s)].
Even if one battery is at 90% SOC (state of charge) and the other is 70% SOC (hence maybe ~12.5V & 12.3C OC terminal voltage), as soon as they are connected together they get >13V from the alternator, usually 14.2V etc.
They both start charging independently as if they were each connected individually to the same voltage and they charge at their individual rate.
That is actually the fastest way to recover total charge. I mention that because many voltage sensing systems mention "priority charging". That is bullsh - they all have a delay before energising their relay after any disconnection; that is a requirement of their operation. Though that is a temporary dedicated charge to the main battery, it is merely time delay - it has NOTHING to so with the SOC of the main battery.
They can't sense the SOC of the main battery anyhow - that'd would require current sensing else somewhat sophisticated AC injection and analysis, and none of them have that AFAIK.
Besides, why delay the 2nd battery's charge if the quickest total charge recovery is as I described?
Some argue to limit the strain on the alternator. I argue that alternators should not feel any strain and that alternators are self limiting etc (though I know many that blow with faulty batteries or excess short-term loads, but that should not happen; but I dumped Bosch and went back to Japaneses alternators for that reason and I heard some GM alternators do the same, as do many many rewired boosted alternators).
Ironically the best isolator is the charge-light controlled relay. It is also the cheapest and is usually easy for a DIY. A better one would be a proper charge-controlled relay, ie current sensing.
In some rare cases voltage sensing isolators may be more suited and better, but the charge-light isolator (dubbed the UIBI in my more recent writings) wins in almost all typical and common installations. And the UIBI does not have the problems inherent in voltage sensing systems like whether or not to drop out due to a dip or transient, nor the delay each time it (re-)connects. (Some would argue that is a hazard - connecting a fully charged main battery to one that isn't. EG - search google "site:www.mp3car.com simbalage". In one epic thread, Simbalage argues for diode isolators despite his differing voltage problems but he does a good job of shooting down his own statements.)
The UIBI or charge-light controlled system is also easy to extend with switches (and didoes) to disable and manually connect (for jump starting or heavy loads).
Search google for "oldspark uibi" to find my other writings.
And adding a voltmeter should alert to any UIBI issues as it does in standard battery and vehicle charging scenarios. (And we all have a dash voltmeter or alarm systems don't we?)
Re both batteries under the hood, that reminds me of an audio forum twit that reckoned the same "because that provide the maximum battery capacity" from the hotter batteries! So they heat their batteries to extract maybe 20-30% extra capacity and replace them probably 8 times as often due to the heat (not to mention their increased resistance which they all seem to concerned about)!
But batteries should be mounted as cool as possible. And for big loads, as close to the load as possible (hence in the boot for audio systems etc).
Venting wise, it is a common Regulatory requirement that internal "non-engine bay" batteries are sealed whether AGM or gels, or wets in a sealed (externally vented) container.
That applies to the UK, USA, Canada, Australia, Europe, etc.
Vehicles that have internal batteries etc are a different issue. They are covered by the approvals granted to OEM vehicles and hence are not necessarily subject to the aforementioned Regulation(s). But as you say, in all cases they are well vented which is consistent with what I wrote previously.
But in general we agree and I like your examples. They explain well the problems that can happen. And as usual, much more can be written. (Like how fuses should arguably not be placed above batteries (just in case), or how batteries vent hydrogen from ALL cells - not just "its +ve terminal" etc.)
But reconsider the situation you explain with trucks (with which I totally agree!) as opposed to "our" isolatable system.
And diode isolators and "intelligent" isolators have no less issues with differing batteries than the UIBI etc. Arguably they have more - diode isolators are more likely to fail hence requiring full replacement (more so with a faulty battery though) whereas the others merely require relay replacement if that were to occur (though fuses should prevent that).
And as I wrote, "intelligent" aka smart isolators (which are generally voltage sensing types with 3 terminals - the heavy main & aux +12V and signal ground) do not determine battery capacity (as in AH) nor SOC.
Also, batteries usually charge at a similar rate wrt SOC. Bigger batteries usually will take more charge (lower resistance), but their %age capacity recharge rates tend to be similar. That does assume similar battery characteristics, but that does not matter anyhow. Each battery charges as fast as it can given whatever voltage it gets. And in the case of relay type isolators, that voltage is essentially the same for both batteries unlike diode types where the voltages can differ and hence have under- or over-voltage issues on the aux battery.
But see my other threads, I'm just repeating myself here.