That's the generally-accepted rule of thumb that's often quoted, partly because it's so simple that it always works, but it doesn't take cost into account. If all batteries are the same model, size, age, and rating, then life is simple, right?
So what happens if you had 6 identical batteries and after a year or two one of the batteries fails. Now they would be different ages. And what if exact model is discontinued? Do you replace all six? I certainly wouldn't. It can't be that black and white in my opinion.
By the way, I think your 75 Amp-hour 6-Volt battery ratings are likely a mistake (they would be physically tiny -- like roughly 1/3 the size of the Trojans) but we'll go with that.
Your controller can't see that the 75 Amp-hour batteries are full (unless you have a very sophisticated system I'm not aware of) because the controller only sees the battery bank as a unit. In this case it sees 525 Amp-hours at 12 Volts. As a simple example, think about charging the three strings from an alternator that puts out a constant 14 Volts. Excluding cable resistance, each string will see 14 Volts regardless of what charge current each string accepts. Each string would charge at its own rate based on 14-Volts.
I think the biggest problem with mixing batteries of different types comes in because the internal resistance of each battery string that is wired in parallel (in your case 3 strings with two being the same and one different) is not always the same, or more importantly the resistance is not necessarily in proportion to each string's Amp-hour capacity.
Basically the string with less resistance will receive more current (in this case likely your Trojan batteries) while the Deka will probably get less charge current. But if they get 1/3 as much charge current for 1/3 the Amp-hour, what's wrong with that? I think that's what Tfryman was implying. And it could be correct because the battery resistance should be similar per unit size if the chemistry and basic design is similar.
However, real deep cycle batteries are designed and manufactured differently than starting batteries or dual purpose batteries, which means their internal resistance can be different, even when adjusted for size/capacity.
Bottom line is that the Deka batteries "may" charge at same rate (adjusted for capacity) than the Trojans. Or not. You could always test it to see if there is a significant difference. If there is, I wouldn't use them -- particularly if they are only 75 A-h versus Trojan at 450 A-h.
Again, I question the 75 A-hour rating.
If that's all they really are, I wouldn't risk it for such minor additional capacity.
With a multi-voltage converter the problem is obviously a little more complex, but what would remain important to me is whether the 3 strings charge at similar rate or not.
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