Solar panels not keeping batteries charged up. Need assistance to diagnose.

very roughly the back of the napkin version of the calculation is

a fixed amp draw for a given amount of time gives you the AH rating of the battery. its terminal voltage is determined ahead and the test stops when it hits that value

if the battery is rated for a 5 amp draw (just to make this example easy) for 8 hours untill the terminal disconnect voltage its a 40AH battery.

in reality you only usually get to about 80% of that with a dummy load like lights with a fridge the batteries get to rest and cycle and you will get closer to say a 90% discharge (see that little kick up at the end of spudboys graphs when the voltage rises?) if you let the battery sit for half an hour you can then use the amps in the battery until the voltage drops down again until it doesnt come back up over the disconnect voltage but this is torturing the battery. back to the math.

so with my example 40 AH battery and a 5 amp load I want to hit a little over 6 hours worth of draw (6.4 or near enough 6 hours 20 minutes lets call it 6)

so my expected 100% value for my example batteries as per the way spudboy has conducted the test is 6 hours. If he batteries only get to say 5 and a half hours then they are down 10% of their life.

this is the complicated version of it and if I did it right if you click it it will lead you to the wiki.


https://www.aulro.com/afvb/images/im...016/05/272.jpg

if you understand all of that you just unlocked the secret that lets drivesafes traxide units white man magic amps out of nowhere on your batteries.

on edit.

Spudboy is disconnecting his batteries at a higher than the nominal "flat" voltage, at a guess maybe 90 minutes (on the 5 amp chart) worth based on his graphs and when I would disconnect probably closer to 2.5 if he wanted to push. In doing this he is protecting and extending the life of the batteries.

I think hes lost a little more than 10% because his post charge voltage is a little lower than I would have expected which mean sulphation is starting to occur. If I had to project a life of the battery till they drop to 50% useful life I'd say maybe 2 more years.

the voltage difference between the posts will be because of lead oxide if you want to prove that measure the resistance between the stainless terminal and the lead one. I've also seen them where a fracture has developed in the mount inside the battery going to one of the terminals.

Quote:

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Originally Posted by Ferret

Can you explain how that calculation is done please.

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Hi Ferret, I state ROUGH because there are a number of factors that come into play when measuring varying DC voltages.

The current load would most likely have reduced as the battery's terminal voltage dropped.

How much the load reduced may vary from next to nothing to a noticeable difference.

So as above, the calculation was a rough one, but it still gives a good comparative idea of the overall capacity of the batteries.

Using the data David posted up, and based on amperes used over a given time, with a 75Ah battery, you would have expected to get close to 9 hours use by the time the battery voltage reached that same 11.95v or approximately 55% of a 75Ah battery in top condition.

As both Dave and I have pointed out, there is likely to be some reduction it total capacity, due to the effects of some sulphation taking place.

As posted earlier, a number of conditioning chargers may revers some or all of the sulphating, but at a capacity lose of only around 10%, David, I sure as hell would not be replacing those batteries just yet.

Ahhh - here's something interesting :eek:

I mentioned above that I'm getting slightly different voltage readings, depending upon if I use the threaded posts to measure the voltage, or the big round terminals.

So, I went out and bought a Battery Analyser to see what readings I'd get, and the results are striking. Top row of photos is the threaded post reading. Bottom row is the round automotive posts. All readings taken within 30 seconds of each other:
https://www.aulro.com/afvb/images/im...016/05/260.jpg

What do you think is going on there? You'd think they'd be welded together to each other under the plastic, and therefore give identical readings.

Battery 3 has readings where one says replace the battery, and the other says it just needs a recharge!

Hi again David and an interesting reply.

When you use the battery terminal posts ( those used for connecting battery clamps ), you are getting exactly what you need, to be able to see if the battery is in good condition.

The threaded studs are NOT intended for use for supplying motor cranking high currents, but are solely intended for low to medium RV current use.

So your readings taken from the threaded studs are pretty well irrelevant.

The three bottom readings clearly indicate all three batteries are fine.

A point of clarity, CCA readings have nothing to do with either SoC or a battery's remain/available Ah capacity.

If a battery is in good condition, it should be able to deliver it's rated CCA when discharged as low as 40% of it's original capacity.

Confusing but the fact is CCA and Ah are unrelated and you can not use one to determine the other.

You can have a battery with a very large Ah but a low CCA rating, or like with Optima batteries, you have small Ah capacity but very high CCA ratings

the lead in the posts can oxidise and pick up contaminates. if you scrub the dark lead off of the battery post and then retry it I bet you get better readings.

you could also have had some monkey whack the lead post and thats caused it to fracture away internally. These internal fractures then build up corrosion and become internal resistances that mess with the readings on those battery testers.

The threaded posts are stainless steel, so no problems with oxidising there.

I've bought 6 of these
https://www.aulro.com/afvb/images/im...2016/05/50.jpg

So I'm now running everything through the big round posts, not the threaded posts.

The way I have always treated a deep cycle battery for 12V auxiliary power is that the threaded posts are for power out, the big round posts are for power in.