Advice on caravan-park-independent energy?
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 Motorhome Facts -> Tech / Mech Chat
Author: DABurleigh, 

Location: Basingstoke

 PostPosted: Thu Feb 18, 2010 6:40 pm    Post subject:  
For a given battery capacity in a 12V system, battery life is extended by ensuring no parallel connections, but having the (only) charge and discharge current pass through all lead-acid cells. As soon as you parallel batteries up, then the currents are split according to the effective internal resistances of the batteries, and these can vary slightly according to type, design, age, other history of the batteries. When this happens, DIFFERENT charge/ discharge currents flow through each battery, curtailing the life of them both. Hence the received wisdom of not adding a new battery to an old one, but putting 2 new identical batteries in together.

So, for a given total capacity (total weight) at 12V, two 6V batteries are better than 2 12V batteries, and each should be just as manageable size/weight wise.

For 4 12V batteries, rather than splitting the current 4 ways, adding more chances for mismatches, in principle splitting it just two ways, with 2 6V in series and EACH of these pairs in parallel, is better.

In practice, the pricing due to supply and demand usually means that in Blighty, large 6V batteries are more expensive that 12V batteries. In the US, there is a larger market for 6V batteries. Most RVs use 6V batteries I believe.

Dave
Author: coppo,  PostPosted: Thu Feb 18, 2010 7:05 pm    Post subject:  
DABurleigh wrote:
For a given battery capacity in a 12V system, battery life is extended by ensuring no parallel connections, but having the (only) charge and discharge current pass through all lead-acid cells. As soon as you parallel batteries up, then the currents are split according to the effective internal resistances of the batteries, and these can vary slightly according to type, design, age, other history of the batteries. When this happens, DIFFERENT charge/ discharge currents flow through each battery, curtailing the life of them both. Hence the received wisdom of not adding a new battery to an old one, but putting 2 new identical batteries in together.

So, for a given total capacity (total weight) at 12V, two 6V batteries are better than 2 12V batteries, and each should be just as manageable size/weight wise.

For 4 12V batteries, rather than splitting the current 4 ways, adding more chances for mismatches, in principle splitting it just two ways, with 2 6V in series and EACH of these pairs in parallel, is better.

In practice, the pricing due to supply and demand usually means that in Blighty, large 6V batteries are more expensive that 12V batteries. In the US, there is a larger market for 6V batteries. Most RVs use 6V batteries I believe.

Dave


Thanks, i,ve read about 10 times, think i,m understanding a little more.

Paul.
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