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**skipmac** · 25-07-2018, 11:14

Quote:

Originally Posted by Cadence

If you say so?.

Based on personal experience. Years ago had a boat and the only easy option to add house battery capacity was to mix 2Ds and 4Ds. Ran that way for years with no problems and batteries last 5-6 years with only a non smart alternator for charging. Didn't even have a charger that would equalize.

Know of several others that did similar and got by just fine. So opinion and speculation in my book is trumped by real world experience. Hard math and calculations don't hurt either.

Cadence · 25-07-2018, 12:41

Quote:

Originally Posted by skipmac

Based on personal experience. Years ago had a boat and the only easy option to add house battery capacity was to mix 2Ds and 4Ds. Ran that way for years with no problems and batteries last 5-6 years with only a non smart alternator for charging. Didn't even have a charger that would equalize.

Know of several others that did similar and got by just fine. So opinion and speculation in my book is trumped by real world experience. Hard math and calculations don't hurt either.

Sorry, I think I misinterpreted your post. I think we are on the same page. I said it was an extreme. I just cannot see advocating mixing sizes and capacities.

**skipmac** · 25-07-2018, 12:56

Quote:

Originally Posted by Cadence

Sorry, I think I misinterpreted your post. I think we are on the same page. I said it was an extreme. I just cannot see advocating mixing sizes and capacities.

I think we probably are. Mixing battery sizes, ages and types is not optimal but when circumstances, situations or budget aren't ideal then some sub optimal solutions are acceptable.

I would NOT mix different battery types or chemistries except in an urgent need. I would only mix old and new batteries if the old were not very old and were verified good. Mixing different sized but not too different I believe is the least objectionable of the options.

Cadence · 25-07-2018, 13:24

Quote:

Originally Posted by skipmac

I think we probably are. Mixing battery sizes, ages and types is not optimal but when circumstances, situations or budget aren't ideal then some sub optimal solutions are acceptable.

I would NOT mix different battery types or chemistries except in an urgent need. I would only mix old and new batteries if the old were not very old and were verified good. Mixing different sized but not too different I believe is the least objectionable of the options.

Yep, If I had my druthers {sp?) I'd replace everything at one time, size and date of mfg.. Often not financially practical. Repurposing good batteries might make sense? Another thought with new replacement batteries is that they leave the supplier topped off.

LongRange · 26-07-2018, 02:45

Quote:

Originally Posted by Cadence

I think people should parallel mix lawn mower or motorcycle batteries with 8Ds. The chemistry is the same but see how the miniscule plate size works for them. Probably work fine till the plates on the smaller one are eaten up. Hopefully only reducing it's life not shorting out the depleted plates thus shorting out the 8D. I know, a bit over stated, give it some thought.

Why would the plates on the smaller one get eaten up?

hzcruiser · 26-07-2018, 04:46

Quote:

Originally Posted by LongRange

Why would the plates on the smaller one get eaten up?

They won't and he's already back-paddling...
The smaller batt will get slightly overcharged only because it takes longer until the bus voltage comes to the float (or absorption) setting due to the bigger batt taking the current (and keeping the voltage lower than if you only had the small batt attached).

You cannot "pump" current into a batt without raising the voltage significantly, only this correlation is, admittedly, not so easy to grasp. And by that I don't mean to belittle anyone or calling anyone stupid, it really is not that obvious and the old way of thinking about water pressure in a hose doesn't always explain every phenomenon with electrosity correctly.

Having said that, fluid dynamics might actually be just as hard to grasp...

Cadence · 26-07-2018, 09:47

Quote:

Originally Posted by LongRange

Why would the plates on the smaller one get eaten up?

Because of trying to suck the same amperage from far less squire area of the plates.

john61ct · 26-07-2018, 10:44

Many lead chemistries are damaged by overcharging, keeping absorb volts high after endAmps acceptance has been reached. FLA not as much, long as you keep water topped up.

And all are damaged by habitually being undercharged, dropping to Float before that point.

LongRange · 26-07-2018, 14:11

Quote:

Originally Posted by Cadence

Because of trying to suck the same amperage from far less squire area of the plates.

It is not the same amperage. When hooked up in parallel, each battery supplies amps proportional to its internal resistance:

1/Rtot = 1/R1 + 1/R2 + 1/R3 + ... + 1/Rn

An analogy is that every person pushing a broken-down car pushes only as hard as they are able. If a five year old joins a teenager and a grown man in pushing, the five year old is in no more danger of hurting themselves than the other two.

For batteries in series, the current flowing through each battery would indeed be equal, and then it might become dangerous to mix different capacities of batteries, depending on the details:

Rtot = R1 + R2 + R3 + ... + Rn

Now the adult has a rope tied around their waist, which is in turn tied around the teen's waist, then around the five year old, and lastly a front hook on the car... and everyone is pulling as hard as they can. The force generated by the adult could indeed harm the other two.

Cadence · 26-07-2018, 14:51

Quote:

Originally Posted by LongRange

It is not the same amperage. When hooked up in parallel, each battery supplies amps proportional to its internal resistance:

1/Rtot = 1/R1 + 1/R2 + 1/R3 + ... + 1/Rn

An analogy is that every person pushing a broken-down car pushes only as hard as they are able. If a five year old joins a teenager and a grown man in pushing, the five year old is in no more danger of hurting themselves than the other two.

For batteries in series, the current flowing through each battery would indeed be equal, and then it might become dangerous to mix different capacities of batteries, depending on the details:

Rtot = R1 + R2 + R3 + ... + Rn

Now the adult has a rope tied around their waist, which is in turn tied around the teen's waist, then around the five year old, and lastly a front hook on the car... and everyone is pulling as hard as they can. The force generated by the adult could indeed harm the other two.

If you say so.