Battery Bank - Mixed Capacities

[twscfltx]

Please share your experience.
I was planning on buying 4x T-105 6V flooded batteries for a series-parallel bank, but I have 2x good 12V 92 Ahr (C20) Intimidator AGM batteries that I have not been able to sell. Now I'm considering lower cost options because I only intend to use this system for 6 more months. I also have a 12V 75Ahr (C20) Optima Bluetop D31M AGM battery. I'm thinking about just buying one more 12V 92 Ahr AGM. This would put me at 176 AHr (3*92+75)/2 minus inefficiencies of combining this mixed capacities and age batteries.

I understand that the 75 AHr battery would overcharge, and the 90 AHr batteries would not fully charge. Then the bigger batteries would charge the smaller battery to maintain constant voltage balance.

What reduction in total capacity could I expect if I combined a 75 AHr with 3x 90 AHr batteries? 85% reduction?

Any significant risk in overcharging the smaller battery? or does this just reduce life of the battery?

Thank you,
Tyler

 

[Lou Schneider]

You can mix different capacity batteries when two or more 12 volt batteries are connected in parallel, the charging current will divide between them according to each battery's state of charge.

Two 6 volt batteries connected in series to create a single 12 volt battery is where the size of each battery is critical because all of the current flows through both of them.  That's when a smaller battery will overcharge while the larger battery stays undercharged.

One thing to be aware of is battery voltage changes as a battery ages.  As it gets older, it takes more voltage to reach full charge and it produces less voltage under load.  This is why it's recommended not to mix new and old batteries because when they're connected in parallel, the current is free to divide between them and the newer battery will work harder than the older one, hogging more of the charging current and providing more of the load current than it's older partner.

But I wouldn't worry about mixing new and old if your present battery is working well and you're only keeping the system for another 6 months.

 

[Back2PA]

Would definitely not buy four Trojans if you're selling in 6 months

 

[John From Detroit]

would not fully charge. Then the bigger batteries would charge the smaller battery to maintain constant voltage balance.

You believe a LIE.. Now I do admit it has a "Kernal of truth" but only a kernel

Battereies in SERIES need to be identical in every way, So if you buy 4 GC-2 Class Golf Car batteries and wire them in pairs for 12 volt the two batteres in each PAIR need to be identical, ideally sequential serial number identical but it does not need to be that close.

The other pair, however, need not be identical to the first pair

Likewise other 12 volt set ups in parallel need not be parallel

Imagine if you will a row of half a dozen water tanks,  All the tanks are conneted by fire hoses at the bottom (2.5" lines Big Fire Hoses) now one tank holds 50 gallons but the next tank, though the same height, is bigger around so it holds 70 gallons, and the next 100, 120, 150 240  ALL the tanks however are the same height (Water seeks its own level) Now you stick a garden hose in one of the tanks and start filling

Which tank will fill up first? (3/4" garden hose)

Answer.. they all get to FULL at exactly the same time

Same with the batteries.. If you have a 75 AH Group 24 in parallel with a 230 amp GC-2 Pair,, they both reach full, 100%, at EXACTLY the same instant.

WHY, because battery state of charge is like the water in those tanks, IT SEEKS ITS OWN LEVEL  IF the smaller battery were to be even 1% more charged, then the voltage on that battery would be higher so a greater share would go to the larger battery till it caught up..  Same, I might add is true of Discharge

All the water tanks ,,,drained through a 1" bung, reach empty at the same time
All the batteries hit 50% at the same time..

This only applies to the parallal banks however

As I said, batteries in series (Each pair of GC-2) Those need to be twins of each other.
And that is where the myth comes from.

 

[twscfltx]

So different capacities are ok to connect in parallel. Would I have any issue with the different style batteries? The Deka Intimidators (2x 92AHr) are plates and the Optima (1x 75 AHr) are "spiral". The Optima blue top says "deep cycle".

Please recommend a fourth battery.

Any risk to Outback Max 60 solar charger or Magnum 2800 Sine inverter if I use mixed batteries?

 

[John From Detroit]

Different types (AGM's including Optima, and Flooded wet) do have very slightly "Full" points, They are close enough that they do OK together, but mixing types is not really recommended.

You may consider the Optima to be Generic AGM however.  Some converters minimize the differences, for example the Progressive Dynamics Wizard controlled units are good for this mix.

One comment on Optima.. They cost more and deliver less, as you can clearly see by looking at the Optima there is a lot of "not battery" there (Air, just air) in fact they are 40% not battery, only contain about 60% (Theroy says more but that's what the web page says) of the amount of battery for the space they occupy.  But they cost more.  So unliess you
1: Are planning on driving them like you would a 4-Runner or Wave Runner (in which case battery live won't be an issue for your RV cause.. Well it won't last that long)
2: You already own them
3: You got one "Heck" of a fire sale price.

I do not recommend Optima for RV use... Now back to option 1 above, I DO recommend them for the devices listed there and their equivalent because that is what they are designed for, But not for House batteries. 

The other thing.... AGM's though many yammer about their long life and such, I have yet to seen evidence of that.. And I tried.. When this RV was nearly a year old I added a pair of MK AGM's.. good quality batteries.  They were the newest batteries in the RV and the first to be repalced... I also had a couple of maintenance free's already owned,  they lasted much longer, and the OEM Flooded wet cells made it 9 years. Their replacements look like they may last longer.

 

[twscfltx]

Should I worry that if the Optima goes bad that it could damage the other batteries or equipment?

What battery do you recommend for a fourth? 2x Deka AGM, 1x Optima blue top, and 1x ...?