AGM charging: battery-to-battery, shore charger, struggling on best approach

[dwh]

I've been told that the batteries will not get to 100% charge without many hours on a charger (which in my case will never happen) and will sulfate and die rather quickly. I'm skeptical though, since the way I'd be using them is typical of an offgrid solar house. Thoughts?

Don't be sceptical, it's certainly true. A very high percentage of off-grid solar do it yourselfers end up destroying their first set of batteries until they learn that chronic undercharging is a tool of the devil.



And for vehicle starting use, I'll second the 2x12v rather than 2x6v.

 

[rruff]

Probably easier to wire up with two 12v batteries in parallel. This would allow you the option to install an isolator should you wish.

If I didn't want to isolate is there a reason? I've read that batteries in parallel will kill each other if one of the cells is bad.

Thinking about what dwh said regarding starting, would max amps be less with the 6v in series?

 

[DiploStrat]

If I didn't want to isolate is there a reason? I've read that batteries in parallel will kill each other if one of the cells is bad.

Thinking about what dwh said regarding starting, would max amps be less with the 6v in series?

Dude, all of this is discussed, in painful detail, here: https://diplostrat.org/documents/

There are only two reasons to isolate two batteries:

-- The total capacity of the bank is too small and you intend to pound the second battery so badly the if the batteries were not isolated, you would kill your starter battery. Think about it, you don't have an isolated battery to run your radio for 15 minutes while someone runs into the store.

-- You want to run two different types/sizes of batteries, for example, a starter battery and a deep cycle battery.

Why do you expect a cell to be bad? And if you have a bad cell, it doesn't matter if the batteries are in series or parallel a bad cell is going to kill the battery. Remember, there are no 12v batteries or 6v batteries, only 2v cells boxed either three or six to a box. Once you understand that, it all gets easy. Thus it doesn't matter whether you use a pair of 12v batteries in parallel or a pair of 6v batteries in series - amps is amps. It may be easier to manage a battery bank made of several small batteries vs. one big case, but that is a different issue.

 

[rruff]

Remember, there are no 12v batteries or 6v batteries, only 2v cells boxed either three or six to a box. Once you understand that, it all gets easy. Thus it doesn't matter whether you use a pair of 12v batteries in parallel or a pair of 6v batteries in series - amps is amps.

It seems different to me. With 6v x2 I have 6 cells total which are independent. With 12v I have too sets of 6 cells which have to balance each other.

 

[dwh]

It seems different to me. With 6v x2 I have 6 cells total which are independent. With 12v I have too sets of 6 cells which have to balance each other.

As long as the two twelves are same type, size, age, etc., if they are permanently tied into a bank, then they will act like one big battery - because that's what they will be. (Same with two sixes.) They don't "balance each other" - they are always balanced. (Same with two sixes.)

And sure, if a cell takes a crap in one battery it won't be healthy for the other battery. (Same with two sixes.) But that doesn't happen often. That's not a normal occurance, that's a catastrophic failure event.

The difference is, if a cell takes a crap with two twelves, you unhook one battery and keep going. If a cell takes a crap with two sixes, you're dead in the water.

 

[rruff]

As long as the two twelves are same type, size, age, etc., if they are permanently tied into a bank, then they will act like one big battery - because that's what they will be. (Same with two sixes.) They don't "balance each other" - they are always balanced. (Same with two sixes.)

What I mean is that manufacturing variations will always result in two batteries being not quite identical. If the batteries are in parallel they are forced to run at identical voltage in spite of this. Batteries in series are not. Maybe it isn't a big deal but it's been mentioned as something that prematurely ages batteries.

I do like the idea of having one good battery get me home, though!

 

[DiploStrat]

Details, details.

What I mean is that manufacturing variations will always result in two batteries being not quite identical. If the batteries are in parallel they are forced to run at identical voltage in spite of this. Batteries in series are not. Maybe it isn't a big deal but it's been mentioned as something that prematurely ages batteries.

Doesn't work that way. With a lead acid battery, a group of cells, whether in series or parallel, will tend to move to an average voltage. Example, connect two batteries, one at 14v and the other at 13v, together and they will move to an average voltage. Connect them to a 14v charge source and they will, eventually, reach the same voltage as the charge source.

Lose a cell and the entire battery is dead. It is theoretically possible that you might be able to tell that one of two batteries in parallel had lost a cell early enough to separate the two batteries, but unlikely. Especially with sealed batteries where you cannot test the specific gravity of a single cell. The most likely scenario is that you will notice declining performance and when that performance gets to be bad enough, you will have to replace all of the batteries.

Remember any "12v" or "6v" "battery" that you buy is simply a series of batteries (or cells) tied together. In lead acid, the individual batteries are nominally 2v each.

I do like the idea of having one good battery get me home, though!

As noted, this is really only an issue if your battery bank is too small and you are going to abuse it. Properly sized, it will always have adequate reserves to get you home.

 

[dwh]

What I mean is that manufacturing variations will always result in two batteries being not quite identical.

"Cells". Diplostrat keeps making this point - it's nothing but a bunch of 2v cells.

With flooded batteries there is a maintenance procedure call equalization. What gets equalized? The cells.

That which gets out of balance and leads to premature failure is not "batteries" - it's "cells".

If the batteries are in parallel they are forced to run at identical voltage in spite of this. Batteries in series are not.

Sorry, but no. It doesn't matter how you rig the bank - all the cells are "forced to run at identical voltage".

Two 6v batteries in series is six 2v cells in series. A single 12v battery is six 2v cells in series - no difference.

Two 12v batteries in parallel is actually "two series strings of two volt cells, with the series strings paralleled". If you were setting up a 12v battery bank for an RV or cabin, using four 6v batteries, you would have exactly the same thing - "two series strings of two volt cells, with the series strings paralleled".

No matter if the cells are in series or parallel or series/parallel - if any one or more *cells* gets out of whack, it's going to make all the other cells work harder leading to premature failure.

This is why batteries permanently connected into a full-time bank - whether series or parallel doesn't matter - MUST be same size, type, age, etc. Because once connected, it's just one big battery made up of a bunch of cells.

And yes, no two cells are perfectly identical, even fresh from the factory, so you do your best to buy them as closely matched as you can, and do your best to keep them that way for as long as you can.


But if you're truly serious about it, you buy flooded so you can EQ the cells.