Making up a 24v battery bank question.

Re: Making up a 24v battery bank question.

TucsonAZ wrote: »

With the 8 batteries versus the 4 I would gain a lot of amp hours as a clear advantage but what is the down side to going this route?

Parallel batteries are not best design. Short discussion here: http://www.wind-sun.com/ForumVB/showthread.php?14674

Two 48 volt strings at the same voltage do not necessarily receive the same current. I'm not saying that parallel batteries cannot work (in fact, they do), but as the cells age and diverge in their characteristics the charge imbalance will grow. One of your 48 cells is already the weakest (even when they are brand new... they can't all be above average). That cell will set the limit for the whole bank.

At first it is difficult to notice a problem because the good string may mask the weak string. While this masking occurs, you are doing real damage to the stronger string. If you have a DC clamp ammeter you can see if the battery strings are receiving equal current. As the divergence progresses you may find that after charging, one string is actually discharging into the other string. As I mentioned, the entire bank is only as good as its weakest cell. The unequal charging in a parallel system just makes things worse.

So yes, parallel batteries can function well for awhile, but they are very likely to have a shorter lifespan than a single string. Things get really bad when one cell shorts out... that string will take most of the charging current and can go into thermal runaway.

btw, even if you construct a battery bank with impossibly perfect wiring, and even if your battery cells are impossibly identical, you still have a system that is, at best, in an unstable equilibrium. If one string is a bit warmer than the others (inevitable) it will take more of the charging current which makes it warmer which makes it take more of the charging current which makes it warmer which makes it take more of the charging current...

The more cells you have the more likely you are to have an outlier weak cell. In a system with two parallel strings, you have almost twice the chance of having an outlier weak cell. Every connection, every crimp in your battery bank wiring is a potential point of failure. Why double the potential points of failure?

btw, all the wiring problems of parallel batteries are worse with AGMs because of their lower internal resistance.

--vtMaps

Re: Making up a 24v battery bank question.

TucsonAZ wrote: »

Sooooo, go with the 500ah so that I'm only dealing with two parallel strings instead of the 4 I would be dealing with in the 800ah bank?

All the Batteries we use have parallel plates and parallel cells. When you construct a parallel bank of Batteries you must use proper design, cables and connections. All battery banks require proper maintenance of the connections.

AGM batteries are a shot in the dark at best, These batteries sound more like automotive batteries than true deep cycle. That would concern me more than configuration of the bank. It is very hard to judge the health of a sealed Battery.