T&T: Battery Connections

[Candy Chapman and Gary Bell]

Pascal replied to Tim's question:

<snip>... connect all 
the POS, connect all the NEG, then tap the POS at one end, the NEG at the 
other one.

supposed to balance the load on the batteries a little better than taping 
POS and NEG on the same end/battery


Mr. Science chimes in:

Absolutely.  However small, there is a real resistance to the battery 
cables and connections involved.  During the usual discharges involving 
a house bank, they are insignificant, but during charging, with much 
higher currents involved, those little short cables and big husky 
connectors -- with poor design and a little corrosion -- can provide 
significant voltage drops.  With the paralleled batteries connected to 
the system at the same battery the two batteries potentially 'see' the 
charger alternatively with and without that resistance, and take a 
slightly different charge, setting up a voltage difference between the 
paralleled batteries.  So long as the resistance difference is small and 
the resistance doesn't change, nothing happens.  Should one of these 
connectors happen to corrode significantly, the voltage differences 
could become large, and significant currents -- or the potential for 
great current surges could develop.  Batteries charging at different 
rates would likely be at different temperatures, and recalling that as 
the temperature of a battery increases its internal resistance 
diminishes, the warmer battery will draw more of the charge current, 
further reducing it's internal resistance, with the attendant (small but 
still real) risk of thermal runaway with a meltdown, fire or even a 
hydrogen explosion.  I don't want to Hindenberg my boat!   

Lessons:  Strap together your parallel battery bank with the system 
(house load and charger) cables connected to OPPOSITE corners of the 
bank, so each battery (or series pair) 'sees' the same resistance and 
the same charge current.  Keep all the cables between the various 
batteries in the bank the same size/length, and use identical, new, 
connectors.   Be diligent about inspection of the battery banks, not 
only testing 'gravity and topping up cells as appropriate, but paying 
special attention to periodically removing every battery post connection 
and cleaning both the battery post and the connector with one of the 
simple battery connection gizmos from the auto parts stores.  Only use a 
charger with temperature sensors on the battery bank. 

In fact, if you can make a house bank in series -- out of  huge 2 volt 
cells in series, like Rolls systems; or at least series parallel -- say  
four  golf cart  batteries with series pairs in parallel you will dodge 
the risk of a single element in a parallel array shorting out the whole 
disastrous mess.  A final downside of parallel elements is that if one 
degrades or even fails (say by sulphation or an 'open' cell, as opposed 
to the shorted cell or thermal runaway mentioned before) the output 
voltage remains the same, or nearly so, and the amperage available drops 
off.  You would only notice that your house bank provided virtually all 
of it's ititial voltage but had lost most of it's capacity.