Same Chemistry, Same Age, Different AH

[Thin Line]

My main batteries on bank one are 2 370ah 6 volts that are less than a month old. My bank 2 batteries decided to go the other day and I am going to replace them, however don't have room for any more of the big 370's. I can however put in 4 6v 215ah batteries for my new second bank.

My question is, can I put these on combine and use them as one big bank until I want to separate them? I know never to mix battery types, but these are the same type, chemistry, voltage etc, just different AH.

Thank you!

[colemj]

Yes, no problem at all. The batteries will contribute the same. When your bank is half full (for example) all batteries will be at the same voltage and same state of discharge regardless of their individual capacities.

Mark

[Thin Line]

Colemj,

Thank you for the response.. I've heard both answers and last night I got told by someone that does it for a living that it is a mistake. They said charging they would be fine, however when under load and not charging it would cause an imbalance where one would always be trying to charge the other..

Your answer makes sense, was just hoping more people would chime in.

[transmitterdan]

Kainoa,

You will be ok paralleling batteries of same chemistry and age. The Ah rating of the banks does not have to be the same.

[colemj]

Originally Posted by Kainoa

last night I got told by someone that does it for a living that it is a mistake. They said charging they would be fine, however when under load and not charging it would cause an imbalance where one would always be trying to charge the other..

Never use this person for work on your boat. And tell your friends.

I think the marine industry must lead all others for being able to hang out a shingle with no real understanding of what they do. I'm willing to venture that 90% of all advice I have ever received from "marine professionals" has been wrong.

Mark

[Thin Line]

All I needed to hear, thank you guys!!!!

[noelex 77]

Originally Posted by Kainoa

I've heard both answers and last night I got told by someone that does it for a living that it is a mistake.

Even professionals get it wrong sometimes.

[smac999]

if they were a few years old then I wouldn't do it. but brand new should be fine.

[d design]

Its not ideal but it should be ok

[noelex 77]

Two separate house banks are worth considering for a cruising boat. This gives a lot of versatility.

When adding batteries in a different location this can sometimes be easier than wiring them as one big bank (the cables will be back to the battery switch rather than to the original battery bank). It does mean the expense of an extra battery switch.

The advantage of two house banks is redundancy and batteries of different ages and chemistries can be utilised when necessary.

[ramblinrod]

I have never understood the concept of separate house banks.


For a given load, you are discharging the connected bank more than you would if all batteries are connected as one bank.


As a result, you are needlessly shortening your batteries life expectancy.


example:


If you have two 200 A-hr banks, and draw 100 A-hrs/day, you are discharging one bank to 50% SOC. If you remember to switch over religiously, each day you would alternate 50% discharge depth every second day.


Conversely, if they were all in a single bank, you would only be discharging 75% / day, every day. The batteries life expectancy is much higher.


No matter what scenario, if a cell goes bad, that battery has to be disconnected from the circuit immediately.


If you have two separate banks, now the bank with one battery disconnected will be completely discharged in a day. What's the solution? Combine the banks.

[Dockhead]

Originally Posted by ramblinrod

I have never understood the concept of separate house banks.


For a given load, you are discharging the connected bank more than you would if all batteries are connected as one bank.


As a result, you are needlessly shortening your batteries life expectancy.


example:


If you have two 200 A-hr banks, and draw 100 A-hrs/day, you are discharging one bank to 50% SOC. If you remember to switch over religiously, each day you would alternate 50% discharge depth every second day.


Conversely, if they were all in a single bank, you would only be discharging 75% / day, every day. The batteries life expectancy is much higher.


No matter what scenario, if a cell goes bad, that battery has to be disconnected from the circuit immediately.


If you have two separate banks, now the bank with one battery disconnected will be completely discharged in a day. What's the solution? Combine the banks.

With a given quantity of house batts, it is much more efficient to use them in one big bank, than two small ones, exactly for this reason.

The advantages of separate banks as explained by Noelex do exist, but they pale in comparison to this disadvantage, unless you have space and load carrying capacity for a whole lot of batteries -- enough so that either separate bank easily carries your load with a shallow discharge -- say between 65% and 90% SOC if you're using a generator (if you have a lot of solar and regularly get over 90% SOC, then more). And if you have this many batts, then you're carrying around a load which is slowing you down; either that or you have exceedingly modest power needs.

But if having smaller separate banks forces you to discharge more deeply -- well, look at the tables showing battery life in cycles versus depth of discharge and decide yourself. You can switch back and forth during single duty cycle to mitigate this, but do you need the hassle?

The last reason NOT to separate your house batts into separate banks is the ugly old Mr. Peukert. Any given load becomes double, as a % of your bank capacity, if you separate your bank into two halves. This will mean that certain loads or combinations of loads will consume more than their nominal amount of power and reduce the battery SOC more than the nominal amount of amp/hours represented by that load. Whether this is significant or not again depends on the size of the two banks compared to the load, but for most cruisers, especially those using an inverter, it will be very significant.

For most cruisers this is a very bad idea. The exception would be someone with very modest power demands and/or a lot of batteries on board, and especially someone (like Noelex?) with so much solar power that the batteries never get worked hard. These things apply to fairly few of us, I think.


I think Nigel Calder says more or less this same thing in his book; I would certainly read that from cover to cover before making any big changes to your electrical system.

[Dockhead]

Originally Posted by Thin Line

My main batteries on bank one are 2 370ah 6 volts that are less than a month old. My bank 2 batteries decided to go the other day and I am going to replace them, however don't have room for any more of the big 370's. I can however put in 4 6v 215ah batteries for my new second bank.

My question is, can I put these on combine and use them as one big bank until I want to separate them? I know never to mix battery types, but these are the same type, chemistry, voltage etc, just different AH.

Thank you!

What Nigel Calder says about this is the following:

"In both series and parallel connections, all batteries should be the same make, age, and type. In a series installation, they also have to be the same size (Ah capacity), but this is not necessary in a parallel installation."

So your installation should be ok from this point of view.

[sailinglegend]

Originally Posted by colemj

Never use this person for work on your boat. And tell your friends.

I think the marine industry must lead all others for being able to hang out a shingle with no real understanding of what they do. I'm willing to venture that 90% of all advice I have ever received from "marine professionals" has been wrong.

Mark

Marc, you may not be aware that a derogatory response violently disagreeing with your old posting above has been quite rightly deleted by the moderator. I responded on your behalf agreeing with you but my post has been deleted because it quoted a "deleted" post. I am re-posting part of my post which I have a copy of - I hope this is not deleted again. I'm glad to see the moderators "moderating" at last!

My response to the deleted post.

Marc is 100% right on this occasion. Lifeline did extensive tests to prove that batteries of different Ah capacities can be mixed in a house bank, as long as they are the same type and the same age.

The reason I came here years ago, and now post here, was also because it seemed to me that 90% of people in the Marine Industry did give bad advice.

I was an Electrical Engineer who knew nothing about boats, but it was very quickly apparent that those who said they new about boats knew nothing about Electrical Engineering.

That is why these Forums "can" be so valuable, but "Reader Beware" and wey up the often confusing advice.