Two identical RV batteries. They have been unhooked from the RV 'converter' (smart trickle charger) for 5 days. Checking with a hydrometer shows that all cells are 75% charged. (12 cells, two batteries). HOWEVER one of the batteries reads 12.35 volts while the other reads 8.6 volts. Why this discrepancy when the hydrometer readings are identical? Obviously, I will replace the low voltage battery, but, I want to learn why the hydrometer readings are in discrepancy with the voltage readings. BTW, checked the voltmeter against my truck battery (everyday use). It reads
Two identical RV batteries. They have been unhooked from the RV 'converter' (smart trickle charger) for 5 days. Checking with a hydrometer shows that all cells are 75% charged. (12 cells, two batteries). HOWEVER one of the batteries reads 12.35 volts while the other reads 8.6 volts. Why this discrepancy when the hydrometer readings are identical? Obviously, I will replace the low voltage battery, but, I want to learn why the hydrometer readings are in discrepancy with the voltage readings. BTW, checked the voltmeter against my truck battery (everyday use). It reads
Two identical RV batteries. They have been unhooked from the RV 'converter' (smart trickle charger) for 5 days. Checking with a hydrometer shows that all cells are 75% charged. (12 cells, two batteries). HOWEVER one of the batteries reads 12.35 volts while the other reads 8.6 volts. Why this discrepancy when the hydrometer readings are identical? Obviously, I will replace the low voltage battery, but, I want to learn why the hydrometer readings are in discrepancy with the voltage readings. BTW, checked the voltmeter against my truck battery (everyday use). It reads
hydrometer checks the density of the acid. it doesn't check how many cells are actually working. you have some dead cells in the battery. you need to replace it.
One cell is shorted (or more likely 2). If these batteries are in parallel, it would be extremely foolish not to replace both, with edentical batteries, of the same age, and preferably from the same production batch.
PLEASE check out replacing with two 6v. batteries. I did on mine, and it was one of the best things I ever did on my MH. Costs the same, and will give you many more hours of boondocking battery power. Made an incredible difference in mine, but my 12 v.'s were ready to take a dump anyway, so going to the stupefying performance of the 2/6v. setup was light years ahead. Don't take my word for it, ask around.
After reading these posts I went hunting for a web page I had read which explained a certain 'effect' which gave parallel batteries more capacity than serial. I didn't find it though. However I did find this one which was pretty good at explaining the differences. They do mention parallel having more capacity, all things being equal. Anyway choosing parallel or serial would depends on different factors, which they discuss.
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Maybe you guys got better results with serial because the battery technology was better than the batteries you replaced. Just a thought.
Nope. I'll educate you if you are teachable. tho batteries of THE SAME CAPACITY in parallel add their capacities together. The voltage stays the same.
Tho batteries in series, the capacity stays the same, and the voltage adds. That's the simple explanation.
However, two batteries in parallel not only have voltage and capacity, they have resistance and a few other esoteric qualities. If batteries that are not matched are connected i parrallel, they can FIGHT each other, and both capacity and lifespan can be severely diminished. If their resistances are different, one gets more charge than the other, and one provides more power than the other. Their charge levels become more sisparate with time and charge cycles.
Now, take 2 batteries od half the voltage, and double the capacity, which are roughly the same size as those you had connected in parallel, and connect them in series. Whatever current flows through one CELL of that combined battery pack flows through all the rest. In both directions. All cells reach very close to the same level of discharge - and take the same level of charge. No fighting between shared current paths.
Also, six volt batteries of 400 ah capacity, for example, are MUCH more sturdily built, on the whole, than 12 volt batteries of 200 ah capacity. The plates and intercell connectors are, by nececity and design, more robust - so they last longer.
When you connect 2 batteries of 3 cells each together, you still have a single battery - of 6 cells, in a split case. When you put 2 batteries of 6 cells together in parallel you still have 2 batteries of 6 cells each.
The only battery technology that works RELATIVELY well with parallelled cells is lithium iodide or lithium polymer. And they require an intelligent charging controller.
I did expect to be flamed, I didn't expect to be insulted.
Not entirely true. The capacity in parallel is 2.14 times, not 2 times. It's based on the Peukert effect. See
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and scroll down to Implications for increasing the RV battery bank
Agreed.
Sounds resonable. I'm assuming both batteries would be bought at the same time and of the same manufacturer/model. That should reduce the mismatch.
Agreed.
A lot of what you say was brought up in the original URL that I posted. They took an unbiased view of the comparisons and left it up to the reader to decided what is best for their application.
No insult intended. Just that I've been a teacher in a past life - Those who choose not to read/listen are unteachable. Those who are intelectually challenged, yet wish to learn are teachable.
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