Combining Dissimilar Battery ah?

[greenrvgreen]

I know there are a few people here with real-world experience with this, so I hope they chime in!

QUESTION: With a small 12v AGM battery (33ah), I have the choice of paralleling a slightly larger battery (35ah), a much larger battery (42 ah), or a somewhat smaller battery (28 ah). All are 12v. Is there any reason to prefer one combo over the other? The charger appears to be ~6a.

Thanks!

[pianotuna]

Hi,

Bubble a Gel cell and the capacity will be permanently reduced.

[full_mosey]

pianotuna wrote:
Hi John,

No, equalizing is not about stirring. It is about bringing "weak" cells up in capacity. Unfortunately that means the "strong" cells do get overcharged some what.

...

I understand that as well. Gel and AGM do need THAT definition of equalization as their cells will not always be evenly charged. The 'good' cells will be overcharged.

Stirring is not something you want to do with AGM and Gel.

I had a hard time understanding the use of bubbling and equalization in the same sentence. I see now that you are using the slight bubbling as a signal that you ARE slightly overcharging in order to de-sulphate all cells.

In Lifeline AGM terminology, you would be conditioning per their paragraph 5.5, where they advocate a 15.5V charge, with temp-comp, for 8 hrs. They even have a temp-comp table in the paragraph. I suspect the temp-comped 15.5V accomplishes a controlled overcharge and de-sulphates much like your slight bubbling process.

HTH;
John

[MEXICOWANDERER]

Again, good facts get diluted and become useless...

It is common, for batteries NOT CONNECTED ALL THE TIME TO A MAINTENANCE CHARGER to bubble freely when being recharged at a RATIONALLY FAST generator powered recharge or fast charge.

VOLTAGE! VOLTAGE! VOLTAGE!

Charge batteries at a constant 14.8 volts at 20C until they BEGIN to bubble freely then reduce charge level to 14.0

They will gradually stop bubbling the longer they are maintained at 14.0 volts.

When specific gravity tops out at 14.0 volts, reduce to appropriate float voltage.

Shore power versus generator

Like comparing Maxim's of Paris to Hospital Food

If you confuse either of the two, god help you.

[pianotuna]

Hi John,

No, equalizing is not about stirring. It is about bringing "weak" cells up in capacity. Unfortunately that means the "strong" cells do get overcharged some what.

It one really wanted to, it would be possible to charge only the weak cells in a flooded battery--but usually it is not bothered with.

[down_home]

Same type of lead acid battery or different spec lead acid battery, the oldest battery or weakest battery discharges at a faster rate and will bring the healthiest or strongest battery down. The battery bank will go to zero, so to speak quicker. This is the reason for battery equalizing circuits on Mh built in chargers, which also serves for desulfation. I caan't say about the new Lifeline etc deep cycle batteries. Where I got this information, was from Factory rep some time ago.

[BFL13]

full_mosey wrote:

pianotuna wrote:
Hi John,

I believe the most often quoted specification is one bubble per second breaking the surface of the electrolyte, for equalization.

I understand that!

Where I am not clear is how as little as one bubble per second has any relevance to charging as opposed to stirring.

Equalization is stirring, right?

My belief is that a temp-comp charger will not bubble your battery.

HTH;
John

Confusion here between de-stratification (stirring) and de-sulfation (charging)

All temp comp does is adjust the voltage to the temp. The voltage used remains the same "value" "Gassing voltage" for wets is 14.x so your temp comp will just make that voltage different. ( The temp may may also make the battery's gassing voltage different, so chicken and egg thing going?)

A way to de-stratify is to stir by charging at high voltage and cause bubbles which do the stirring. (Another way is physical agitation) However that stirring is not the objective of charging, but only a by-product. The bubbles can be the objective if the batteries are charged but stratified.

The charging desulfation is to return the sulfate to solution which takes higher voltage to keep the current flowing (the current does the charging) It happens that you get bubbles at those voltages, but getting bubbles is not the aim, getting more current is the aim.

So you get past "gassing voltage" but the bubbles are just an indicator you are up where you need to be for charging (your volt meter would tell you that too)

There is no "fine tuning" needed with Wets to get the most current with no bubbles yet (bubble threshold? 🙂 ) AFAIK AGMs have more of a fine tuning charging voltage restriction spec so the gas can combine ok without out-gassing.

[full_mosey]

pianotuna wrote:
Hi John,

I believe the most often quoted specification is one bubble per second breaking the surface of the electrolyte, for equalization.

I understand that!

Where I am not clear is how as little as one bubble per second has any relevance to charging as opposed to stirring.

Equalization is stirring, right?

My belief is that a temp-comp charger will not bubble your battery.

HTH;
John

[pianotuna]

Hi John,

I believe the most often quoted specification is one bubble per second breaking the surface of the electrolyte, for equalization.

[greenrvgreen]

Picked up a Renogy 30a PWM solar charger for $30. Good call, BFL!

AC charger is a whopping 5a, it appears, so down the road I'll be looking for a 15a-20a external charger for use with my 500w gennie. Problem is they're no longer cheap--everything I've seen is at least $100 or so.

Main box is a Goal Zero Yeti 400 charger/inverter/power supply, which has an internal 33ah battery, and a handy chaining port, which can also be used to apply an external charge, in addition to chaining more batts.

All this gear is for use when tenting, which I've started doing again. I couldn't resist the gigantic semi dome-style tents (REI Kingdom 6), or the absurdly plush triple-height inflatable queen-sized beds. So in addition to all that stuff, I went out and bought new/smaller versions of all the electricals in my TT.

BTW, I"ve already taken my Tailgater+sat reciever out and run it from solar, watching the Tour de France while camped on the side of a mountain.

FWIW, I have to keep reminding myself that an Igloo cooler with ice works perfectly, and that I don't need to run out and buy one of those stupidly expensive Engel or ARB fridges.

[full_mosey]

I'm having a problem believing that gassing of any type is required for charging wets, Gels or AGMs. I have always read that bubbles were caused by electrolysis and represent an over charge Voltage.

Outside of stirring a wet, what advantage is there to using extra energy to force bubbling? A bubble in a Gel or AGM will be trapped in place. How can the area of the plate under the bubble be receiving any charge?

HTH;
John

[wa8yxm]

Gassing and out-gassing are two different things HTH John.
Gassing is normal part of lead acid battery recharging, On SEALED Valve Regulated Lead Acid Batteries there is chemistry inside the battery that tends to encourage the gasses to re-combine into water.. Thus the battery does not "OUT-GAS".

Out gassing (Where the battery VENTS those gasses to the outside is not normal on AGM or other maintenance free types. at least not in any quantity. Some out-gassing MAY occure but it should be minimal. Which is why you can use AGM's indoors safely I might add.. The path is convoluted enough that acid fumes are normally held in even if Oxygen and Hydrogen are not. And very little H2 and O2 escape as well.

Maintenance free are not quite as good at this containment, and thus should be installed outdoors away from delicate electronics

Flooded wet cells do not even attempt to contain it and thus MUST be outdoors out of contact with delicate electronics or in a box vented to the outside

[full_mosey]

Gassing is neither normal, required, nor necessary with AGMs. It is the improper application of excessive volts that force current and causing the unwanted gassing that may cause the problems.

I use temp-comp chargers with my AGMs because I prefer unattended charging with an actual cable-on-the-post RTS sensor. If the AGM battery is getting warm enough for gassing, the RTS tells the charger to roll back the Volts. This in turn, automatically lowers the current that is heating the battery. I believe this is optimal.

I have measured AGM temps from 6F to 110F and the Volts were automatically adjusted with the RTS cables.

HTH;
John

[MEXICOWANDERER]

Same type and age of battery?

KUMBAYA

[westend]

With batteries, more is better.
FWIW, I have three AGM telecom batteries in parallel, one 42AH and two 26 AH. They are treated similar to other batteries I'm using, solar does most of the charging.