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OT: Keeping stationary diesel engine batteries in parallel charged

Getting OT here but mismatched batteries in series are all well and good until one dies and the other keeps passing current through it. Though maybe it's not a concern further killing a dead battery.

If you find it necessary to split the batteries just put a battery disconnect switch in the jumper between the battery positives.

I would have the opposite concern - a dead/deadish battery will kill the good one. Simplest example would be the cell short-circuit scenario.

So normally for a 24v configuration bulk charge voltage would be around 29.2 volts, that is 2.43v per cell. If one shorts, that means 2.65v per cell will be pushed, or nearly 16v for the healthy battery.
 
First-off, it is just plain FOOLISH to mess with FAILED-cell batteries.

My post wasn't a suggestion for the OP or anyone to "mess with FAILED-cell batteries." I was illustrating why it may be a bad idea to have a suspicious battery charging in series with a good one.

With that said, I rebuild forklift batteries from time to time :P
 
Thanks for the replies. Yes neihlo they are truly in parallel. No offense to DaveWitts but what he suggests will not make any difference at all. They are in parallel. It doesn't make any difference where the negative or the positive goes from the charging circuit unless I misunderstood what he is saying.The fact that one battery does in fact not charge even though they are in parallel is the crux of the problem. Yes, I agree that it is due to resistance....


You understand Dave's method perfectly, yes the batteries are absolutely in parallel, and and you may very well be right that the problem is unwanted resistance. I'm not sure about that, because there are a couple other things that haven't been checked out yet, but I tend to agree with you on that.

There's always resistance in the battery connectors (cables?) and connections, even though it may be small. The present connection method guarantees that the "off" battery will see a lower charging voltage than the "on" battery. If the "diagonal" connection is used for charging, both batteries will see the same voltage. It will be lower than optimal because of connector resistance, but it will be the same for both batteries. BTW, that connection is SOP for lead acid battery banks.

If it's true, as you say, that one battery does not charge when coupled to the other battery, but does when connected directly to the charger, your battery cable/connectors have a LOT of resistance in them.

FWIW, voltage checks are a quick, easy way to check battery health. Equivalent to specific gravity tests and a lot easier. They're not definitive, only energy in/energy out comparisons over time are, but voltage checks will identify the majority of battery problems.

What I'd do: Charge the battery fully, independently of the other battery. Disconnect it from the charger, let it sit for a day. (I think someone else here mentioned that.) Check overall voltage, then check each cell voltage individually - take the caps off, take readings directly off the plates. They should all be very nearly the same. A shorted or partially shorted cell will be obvious. Same checks the next day, and the next will also be useful.
 
I assume that the battery charger is only hooked to one battery. The fix is to hook the positive clamp of the battery charger to the positive terminal on one battery, then hook the negative side to the second battery.

Hooking up this way forces the charger to charge both batteries.

WHAT? Edwin Dirnbeck
 
See the sketch below. What Dave means is charge the battery set/bank at points A and D, instead of A and B.

View attachment 342163

See my post above for an explanation as to why.

That makes more sense for drawing power from the batteries to make sure they both contribute to the high amp load equally. At the low currents used in charging the resistance is negligible, and if it isn't then you have bigger issues.
 
Again, thanks for the replies. Neilho....the only time it would make any difference where you connect the charger in a parallel circuit would be if you had a bad connector. All my connections are good. I'm going to go by the theory that the "weaker" battery is below the voltage of the new battery and the charger never sees it. I have a newer battery and I will throw that on there to see if they both maintain a charge. Thanks again.
 
.......................

If there is one cell in the old battery that is much lower capacity (often because it has cracked plates) then the battery will charge up very quickly, and seem good. But when discharged, it will rapidly drop by a cell voltage (about 2.2V) and that will drag the voltage down for both. The specific gravity will be bad on that cell, just high enough to register a fairly normal voltage.

....................

Actually it is even worse.....

After the low capacity cell is discharged, the voltage does first drop by 2.2V, but then drops by about the same amount more, because it starts getting charged in reverse by the current from the good cells.
 
Wet cell batteries are initially constructed in SERIES configuration. FOR a REASON! ;-)

If you can dedicate the automatic charger to the gen set, just ditch the two battery system and put ONE good one in it's place. Hook up the Charger and smile as you walk away.

FIXED!
 
Just curious but what make/model genset needs 2 batteries in parallel to crank?


If I have it correctly, those gen sets are manufactured by the False Economy Corporation of America.

It's a Green initiative start up that proclaims that disposal of "tired" lead acid batteries is going to cause Global implosion and corrosion of electrical potentials ;-)
 
Just don't use old and new in parallel. Problem solved. Charging separately will do absolutely nothing.

If the older battery has a bad cell, particularly one that is not shorted, but "leaky", that cell will typically charge up quickly and look fine. When both are in parallel, the good one will keep "feeding the leak", and the voltage will look OK. The charger will see that the battery is at voltage, and won't keep charging.

But the bad cell will discharge quickly and that battery will drop in voltage if used by itself. If used with a good battery in parallel, it just will not be contributing

If you have a cell that seems to use more water than any of the others, that cell is likely bad.

Again, check the gravity. Bad cells are low. Bad/used/old batteries are low.


NO, batteries do not act like a simple resistor.

NO, you do not have to use separate chargers if the batteries are all matched... which is why you don't want to combine old with new batteries.... they will not be matched.

The old ones will drag down the new ones, and get the new ones quickly sulfated to the same point the old ones are. It does not matter if you use one or many chargers, as soon as you connect the batteries in parallel, the "dragging down" and sulfating will start.
No, batteries do not act as simple resistor, way too many things going on.

However, the intent of the statement is in general dealing with them, keep it simple.

When charging it draws current like a resistor and if one has a battery PLANT, consider a 10000 Amp hour 48 volt plant made with multiple parallel strings of 2 volt jars it can be cumbersome.

Treating them as resistors makes life very simple.

By converting thought to resistors and ignoring EVERYTHING ELSE we then can use SIMPLE OHMS LAW.

If all batteries are same model number we can assume they should be SAME.....WHAT...THE.....?

RESISTANCE!

When charging or providing power they have an internal resistance that limits powerful since there's resistance we can use ohms law when analyzing them.

Theabove battery strings have 24 jars or cells in each in SERIES and OHMS law states all series connected things shall valve same CURRENT.

So by measuring the voltage across each cell (can do this on any battery that has lids) the voltage across each should be same, but sometimes it is not.

Why?

As the cell degrades the internal resistance changes.

This different value resistance results in a different voltage drop as all cells must have same current in series.

I the same plant of 5 strings of 24 cells I can find the string containing the bad cell in short time under load and to the cell shortly after with simple DVM.

Does not matter what is going on inside, do not care.

The effect of that shows up as a difference in measurement from other cells in same test, one needs to know what to look for and how to interpret their measurements.

Once we find the bad cell we could do other testing to see why it is bad, we usually do not care unless we suspect a system issue that caused it.

Sent from my SM-G781V using Tapatalk
 
That makes more sense for drawing power from the batteries to make sure they both contribute to the high amp load equally. At the low currents used in charging the resistance is negligible, and if it isn't then you have bigger issues.

Dunno how low an amperage his charger finishes at, but if there's 1 ohm resistance in each connector, at 2 amps charger current, that's a 2V drop. That's significant.


...the only time it would make any difference where you connect the charger in a parallel circuit would be if you had a bad connector.... .

Correction: the only time it would make any difference would be if you have a connector with resistance. That assertion could be tested by checking for voltage drops while charging, change the charger connections and test for a difference.

...All my connections are good..

Have you tested them? And what value qualifies as "good"?

...I'm going to go by the theory that the "weaker" battery is below the voltage of the new battery and the charger never sees it.

To clarify, I think you mean that the "weaker" battery voltage is below that of the newer battery and that charging voltage at the "weaker" battery is lower than that at the newer battery. So low that the "weaker" battery does not charge. If that's a correct interpretation of what you wrote, I agree, that's a very reasonable theory.

It's easy enough to test. Set up the charger, set for as high an amperage as is safe and measure charging voltage at the "weaker" battery.

Frankly, the assumptions/assertions made here conflict. They are: the connections/connectors are good, the older battery has been load tested and is good, the charger is good, a changed charger connection will make no difference. Yet.... the older battery is not being charged. (Therefore, the battery is evil? :D Just kidding)

At least one of these assumptions is false, and the only way to find out which one is false is to test.
 
Even simpler.... keep an eye on the cell specific gravity. There is not much that can go wrong with a battery that SG does not show.
 
Dunno how low an amperage his charger finishes at, but if there's 1 ohm resistance in each connector, at 2 amps charger current, that's a 2V drop. That's significant.

If you have an ohm then you don't have two batteries in parallel. The 6 amps a battery could contribute is meaningless to a starting generator.
 
I agree.

IF the load test was valid and the charger works, something else is wrong. We just don't know what. Lots of theories and speculation here, all good as starting points, but only one test so far.
 
I would re-wire using forklift type connectors and use a separate charger for each. Use the same connectors on the harness that parallels them and connects to the generator.

There are numerous methods to desulfate a weak battery. One that sometimes gives modest success is to connect an old time simple charger (transformer/rectifier/current meter) periodically. These will overcharge a good battery if left on too long but can also equalize weak cells on a sulfated battery. Just make sure to monitor the process rather than just leave it.

It one of my "off-duty" pursuits we use multiple automotive batteries and each battery always gets its own charger.
 








 
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