Flooded and AGM under the same hood?

[t42beal13t]

My new Disco1 has an Optima Blue Top D34 as its only power source right now. While it's OK for now, I want to add a starting/dual-purpose battery for the primary and slide the Blue Top over to the right side for a secondary battery (exact setup TBD). Do I need another AGM battery (it would be a Sears Platinum; I've read too many horror stories about the new Optimas) to go in front of the Blue Top or would a flooded lead acid battery be OK? I don't mind spending the extra money on the DieHard Platinum, but I don't want to waste money, either. My first choice of a flooded lead acid battery would probably be a AutoCraft Gold.

Also, since it already has a group size 34 battery, would I have to get another group size 34 as the primary or can I go with a 65?


Down the road I'll be installing a winch, a couple Hellas and eventually a fridge when budget allows.

[mhiscox]

I can find you people who will tell you that anything less that identical batteries, down to being the same age, is substandard, and technically they are right. Any differences in the characteristics of the batteries makes things less than optimum. However, not much in life is ever truly optimum, and you can decide if small and hard-to-calculate compromises are worth spending extra money.

Remember that when you connect up the two batteries, they'll equalize their voltages. Thus, when one battery, for whatever reason, charges or discharges differently than the other, there'll be voltage flowing around trying to get things equalized in a way that's probably not optimum for either battery. Does it make any real difference? Probably not in an under-the-hood application. The key point is that AGMs and flooded cells have nearly the same charging characteristics--definitely NOT the case for gel batteries, BTW--and thus they can replace each other. However, every battery has one perfect charging program, and with mixed batteries you won't be hitting it. But in charging off the alternator, you weren't at that level of precision anyway. (As an example, on the Odyssey AGM house batteries in my Sprinter that are charged by my Outback Power inverter, there were about ten individual parameters that had to be set to two decimal places to optimize the charging for those particular batteries.)

One thing to consider is whether the batteries are likely to end up being completely discharged. AGMs will bounce back from many total discharges, but two or three total discharges (and sometimes just one) will kill a flooded battery. If there's a chance that something might happen (lights left on, significant phantom draws, etc.) to totally discharge the flooded battery, it might actually end up cheaper to have an AGM that can recover.

So, as you might suspect, the best would be to invest in two shiny new Platinum Diehards and put your blue top into backup service. But if you're not doing that, it might be best to save money on the current setup so you can have the matched set ASAP. Ideally, you don't want to go through "one's new/one's old" cycles forever. (And whether to put a flooded cell into the mix would have a lot to do with how likely it was to be ruined by a total discharge.)

[dwh]

Batteries rigged in series, or permanently paralleled should always be identical.
Batteries paralleled only during charging can be different sizes/types IF using a constant voltage charging setup (such as a standard vehicle voltage regulator/alternator) or a 2-stage charger.

Using a 3-stage charger on mixed types/sizes/ages is generally going to result in the smaller or older battery getting overcharged.


(And, as Mike noted...GELs can't be mixed with anything except other GELs.)

[Carl2500]

Can you have a gel battery and flooded battery, or other matches (new vs. old) charging off the same alternator, if you're using a battery isolator? Or does it not matter since it's using the same voltage regulator?

[mhiscox]

The gel battery's charging program is not compatible with a vehicle alternator. You'll have to get a separate charger with a gel cell program.

[dwh]

Can you have a gel battery and flooded battery, or other matches (new vs. old) charging off the same alternator, if you're using a battery isolator? Or does it not matter since it's using the same voltage regulator?

Gel batteries are "fully charged" at a lower voltage than AGM/Flooded batteries. Gels typically specify no more than 14.1 or 14.2v and they are very sensitive to being overcharged.

AGM/Flooded batteries are generally considered fully charged at higher voltages, typically 14.4v.

AGM/Flooded are not nearly as sensitive as Gels, so they can be safely pushed to higher voltages, such as 14.6v or 14.8v. Most can even handle short periods of up to 15v.

An automotive voltage regulator will hold the "12v bus" (battery, loads, etc.) at between 13.5v and 14.5v (more or less) by switching the alternator on when the voltage drops below 13.5v and switching it off when the voltage rises to 14.5v.

That will end up driving the voltage of the Gel battery up too high.


Diode type isolators (like the ones you see in an auto parts store with the big heat sink) generally have a voltage drop as the current passes through the diode. Usually around a half a volt. For this reason, they are not preferred since the aux battery will always end up .5v short of a full charge.

However, if your voltage regulator was set to max out the 12v bus at say 14.5v, then the .5v drop though the diode would limit the voltage to the aux battery to 14v - which would (theoretically) keep the Gel charged, but not overcharged.

But I wouldn't do it...