Upgrading 12 volt battery banks

[mirage]

I have two 12v battery banks on my boat, one below deck and the other above. They are each charged by a separate solar arrays and Midnight Classic 150's. The busses for each bank are connected by 10 foot cables. My plans are to upgrade to AGM batteries. Does it matter if I use a larger group batteries for one bank than the other? For instance if I use group 27 batteries in the bank below deck and group 4D batteries in the bank above deck.   

[mike90045]

The chargers won't care, but the health of batteries  WILL be an issue.   Large banks of parallel batteries are subject to slight charge imbalance from the resistance of the cables and connections.  With flooded batteries, the internal battery resistance helps swamp that out a bit, but with AGM batteries, their resistance is quite low, and the cables & connectors predominate, causing imbalance.
The Smartguage site has a nice writeup on how this happens:
http://www.smartgauge.co.uk/batt_con.html
you can help some with wiring on the Diagonal.

Mixing types/sizes in parallel will not help solve the issue.    if you can build the banks up from 2V, 4V or 6V batteries to avoid paralleling 12V batteries, that would be a better solution.   Of course, the 2 & 4 v cell sizes are less popular and have lower mass production (higher cost per watt hr)

[mirage]

I read the smart gauge article. Good info. Thanks. Right now, the banks I have are wired as two batteries each feeding into a common bus for the respective bank. They aren't diagonally wired so they are out of balance. I will fix that. Since there are never any more than two batteries in parallel leading to the bus, would there still be a benefit to using 6 or 4 volt batteries in series? The banks I'm using are fairly large, 5 and 9 batteries, so I could tweak the configuration to accommodate series connections without too much trouble.

[Vic]

I have two 12v battery banks on my boat, one below deck and the other above. They are each charged by a separate solar arrays and Midnight Classic 150's. The busses for each bank are connected by 10 foot cables. My plans are to upgrade to AGM batteries. Does it matter if I use a larger group batteries for one bank than the other? For instance if I use group 27 batteries in the bank below deck and group 4D batteries in the bank above deck.

Hello mirage,

It is not quite clear the configuration of your system ...

So,  you have two battery banks each charged from its own PV array,  with separate CCs,  but they are connected together by those 10-foot cables ??

If the above is correct,  then,  it would be a very bad idea to connect two battery banks together,  but each bank having different Capacity,  especially if they are AGM batteries.  Generally,  the bank with the largest Capacity would hog much of the charge current,  as it would generally have lower impedance,  and the smaller bank could be chronically under-charged.  And,  a 10-foot interconnecting cable is a bit long,  in my book.

Perhaps,  this is not your question,  and I have it wrong.
FWIW,  Vic

[mirage]

I've attached a drawing of the existing battery banks. I could probably make the two larger banks closer if not exactly the same capacity. The busses are about 10 feet apart and there isn't much I can do about that. The smaller bank forward is for a windlass motor. It uses a large power load for a short period of time, so there has to be a source of power close. This is the way it was configured when I bought it a few years ago. I'm wondering if I can improve it as I upgrade the batteries.

[tecnodave]

Mirage,


AGM's in this configuration would be a disaster, two in parallel is difficult, 14 in parallel is asking for lots of trouble.   At this point I think you need to really gather as much information on batteries that you can get. I basically NEVER use banks in parallel. I do have multiple banks of large capacity 6 volt batteries but almost never parallel them with few exceptions such as drawing 2.5 kW for an hour, but even then I open the Marinco  battery select switches that I have installed on each controller, inverter, and charger as soon as th heavy loads are done.

Every installation I have seen that has three or more strings has failed in short time, including a Shchneider dual 8 kW inverter feed by 3 strings of Trogan L-16 Solar batteries on a 48 volt system
These batteries are totally unbalanced in less than 1 year. I replaced the entire set with one string of 2 volt Trogan L-16's @1150 amp hour, two years later, perfect balance....all cells are same specific gravity.......I did sell them continued battery maintaince as part of the deal to assure the installation will not fail

In all fairness I must admit that my battery of choice is flooded lead acid, large format....not group 24 automotive batteries.........car batteries.....and any battery that has a CCA (cold cranking amps) is a motor starting battery......NOT...suitable for solar or long term storage... My bias goes way back to an apprenticeship at Alaska Husky Battery in Fairbanks, Alaska in 1965.

Hope that this will help.....read others comments on parallel batteries

td