Um...no, no and, well...no.
Lead-acid batteries come in sealed or vented (removable caps, can add water). Vented are always flooded (sloshy electrolyte).
Sealed lead-acid batteries can be flooded lead-acid (FLA), or they can have fiberglass fabric between the plates to act as a sponge for the electrolyte. That's called absorbed glass mat (AGM). Or sealed can also come with a gelling agent added to the electrolyte to make the electrolyte a sort of paste. That's called gelled electrolyte (GEL).
FLA, AGM and GEL are ALL "lead-acid" batteries.
In regards to charging, they are basically the same, except that FLA and AGM will charge faster and stay healthy longer if pushed to higher voltages, like 14.4v-14.8v...as long as they don't get too hot when doing it. GEL would also charge faster and live longer if pushed to higher voltages, except that higher voltages and temperatures tends to break down the gelling agent, so most GELs recommend not going over 14.2v.
Optimas are AGMs, with a twist. Literally. (Optima, feel free to send me a check for that new marketing slogan.
) They take the positive plates, negative plates and fiberglass fabric and roll it up. That's called spiral-wound AGM. That's why Optimas look like a six-pack...they ARE a six-pack...of 2v cylindrical cells.
That spiral-wound design has one major side-effect - lower resistance. So a fully drained Optima can be pulled down to a lower voltage than a fully drained flat-plate design. This often fools "smart chargers", which see a lower than expected voltage and then refuse to begin charging. The trick recommended by Optima is to hook up (parallel) a second battery, just long enough for the smart charger to see a higher voltage and start charging, then unhook the second battery.
A lot of Optimas have been returned as "bad battery, won't charge" because of this, when the battery wasn't actually bad. An old dumb charger would have charged it just fine. I can't say for sure, but I would guess that if it can fool a smart charger, it might fool a smart tester as well...
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Power does NOT flow from the engine battery to the house battery, except with a dumb solenoid, when the key is turned on, but the engine isn't running. With an ACR (computer controlled solenoid), the solenoid is not engaged until the voltage rises, indicating the alternator is operating.
When the engine is running and the alternator is operating, the alternator has a higher voltage potential, and a lower resistance than either battery. Power flows from alternator to battery A, from alternator to battery B, but not from battery A to battery B.
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Batteries tied into a permanent full-time bank have to be identical. Batteries tied only during charging DO NOT. Different size, age, etc. is completely irrelevant. They don't affect each other.
What matters is that they both require approximately the same charge voltage, and that the charging source supplies approximately that charge voltage. Mixing AGM and FLA is fine - in terms of charging they are the same.
Mixing GEL with AGM or FLA is fine as long as the charging source doesn't exceed 14.2v. Of course, that's not going to be optimal for the AGM or FLA, both of which would prefer a higher voltage, but it will get the job done. Charging GEL from a charge source with a voltage high enough to keep AGM and FLA healthy and happy, won't be healthy for the GEL - over the long term. Ocassionally exceeding the GEL's recommended voltage probably won't hurt it.
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To the OP,
Something is draining your starting battery when the truck is off. What is it?
It may be pulling the Optima down far enough to fool a smart charger (and maybe even a smart tester) into thinking it's a bad battery.
More likely, is that the battery has actually gone bad due to a common problem called, "chronic undercharging". Basically, a parasitic load keeps pulling the battery down, and you don't drive enough to bring the battery back to 100%. It can take a lot of hours (anywhere from 8 to 36) for a voltage-regulated alternator to bring a lead-acid battery back to 100%.
Every second of every day, a process happens inside a lead-acid battery. The process is called "sulphation". It's very similar to a process known as "rust". With rust, an oxygen molecule binds to an iron molecule to become an iron oxide molecule. The metal loses a molecule - a.k.a., corrosion. With sulphation, a sulphur molecule binds to a lead molecule to become a lead sulphate molecule. Again, the metal loses a molecule - corrodes.
When a lead-acid battery is fully charged, sulphation slows pretty much to a standstill. Below 100%, it speeds up. The futher below 100%, the faster it goes. The more time the battery spends below 100%, the sooner the plates corrode and the sooner the battery fails.
Chronic undercharging eats lead-acid batteries. Literally - eats the lead plates.
If your starting batteries are failing due to chronic undercharging, it won't matter what lead-acid battery you replace it with - FLA, AGM, or GEL - it will sulphate and prematurely fail.
If you've got a parasitic load drawing down the starting battery, you've either got to stop that, or start regularly (weekly at least, daily is better) topping off the battery, either with a shore powered charger or solar.
