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Author Topic:   Unequal Discharge of Batteries in 24-volt Set-up
OldKenT posted 06-25-2015 09:40 PM ET (US)   Profile for OldKenT   Send Email to OldKenT  
I have two 12-Volt batteries connected in series to power a 24-Volt trolling motor on my [Boston Whaler boat]. Since buying them two years ago, the difference in voltage of the two batteries has consistently been less than 0.2-Volts, both after use and after charging. After using the trolling motor for about eight hours over the last two days and before recharging, I found that the voltage on battery #1 had dropped to 10.40-Volts, with battery #2 at 12.32-Volts. I measured this three times to be certain. After my last recharge four days ago, battery #1 was at 12.87-Volts and #2 was at 13.06-Volts. What could cause the unequal discharge? The batteries supply power only to the trolling motor. The trolling motor operated properly for the entire time. I have maintained a record of the post-use voltage and post-recharge voltage for each battery since I have owned them. The batteries were purchased together and have the same manufacture date. The setup is #1 Pos to circuit breaker to motor; #1 Neg to #2 Pos; #2 Neg to motor.

Any ideas as to what might cause this?

contender posted 06-25-2015 10:17 PM ET (US)     Profile for contender  Send Email to contender     
I think you have one cell going bad, luck of the draw. Just because you purchased two bateries on the same day from the same company does not mean a thing, one could last a year longer than the other. It just the way things are
OldKenT posted 06-26-2015 07:06 AM ET (US)     Profile for OldKenT  Send Email to OldKenT     
Thanks, contender. Do you know how to check for failure of one cell? These have six cells each. Some additional info may help: I recharged overnight and, after being disconnected from the charger for 25-minutes, battery #1 now tests at 13.04 and #2 at 13.16, and across the batteries tests at 26.1-Volts. If the past holds true, these rates should stabilize at about 12.85 and 13.00 over the next hour or so. I use a MinnKota 230PC charger to charge both batteries at the same time. It signals some battery problems, such as failure to take a charge, but it did not identify any problem this time. These are Sears Platinum Marine Group 31M Absorbed Glass Mat batteries.
Jefecinco posted 06-26-2015 09:35 AM ET (US)     Profile for Jefecinco  Send Email to Jefecinco     
Those are excellent batteries with a lot of capacity. They also have long warranties.

The batteries are sealed and therefore the specific gravity of the cells can't be checked. I don't know of any way to test the batteries other than a load test which you have, in effect, performed.

It's just possible you may have a high resistance connection in your set up. I would disconnect every connection, clean it and reconnect. If any connection seemed suspicious I would retest the system. If not I would take both batteries to Sears for load testing and if a defective battery is found have it replaced, hopefully under warranty.

Those are about the heaviest batteries I've ever installed. I hope you don't have to take them to Sears. If they can be load tested in the boat it may be much easier to take the boat to their store.


OldKenT posted 06-26-2015 12:46 PM ET (US)     Profile for OldKenT  Send Email to OldKenT     
Thanks for the suggestion, Butch. I also don't know how else to test them. Does the fact that the "bad" battery took a full charge suggest that it doesn't have a bad cell? Online research suggests that each of the 6 cells in a 12V battery will charge to about 2.1V when in good condition, so could it charge to 13V if a cell is bad?
You are right, these are excellent batteries - so strong that the only time I have gotten a reading below 12.70 volts after hours of use is when I have been trolling against strong currents or high winds at moderately high speeds for a few hours, as I did this time. They are very heavy - about 80 lbs. each, and I still have a year plus on the warranty.
weekendwarrior posted 06-26-2015 03:42 PM ET (US)     Profile for weekendwarrior  Send Email to weekendwarrior     
If the batteries are charged in parallel, then probably one battery is failing before the other as mentioned above. If they're charged in series by a 24v charger, then they might just be unbalanced. In this case charge them in parallel (separately) to see if that resolves the issue.
OldKenT posted 06-26-2015 04:12 PM ET (US)     Profile for OldKenT  Send Email to OldKenT     
Hi weekendwarrior. The batteries are set up in series to power a 24v trolling motor, but the Minn Kota charger I use has 2 banks, so each battery is recharged separately from the other, but at the same time. Each bank allows a selection for the type of battery (flooded acid, AGM, gel, etc.) and each bank has an indicator for the state of the charge on that particular battery - 25%, 50%, 75%, 100%, and maintenance mode. I always recharge to 100% promptly after use, as is recommended for AGM batteries, and I allow the recharging to continue until both batteries have reached 100% for at least a few hours.
jimh posted 06-26-2015 09:03 PM ET (US)     Profile for jimh  Send Email to jimh     
Since you charge them separately, the two batteries may end up with unequal charges. When batteries are in series, the internal resistance of the batteries affects how the voltage is divided between them. Apparently, one battery has more stored energy than the other. The battery with less stored energy depletes its charge faster.
OldKenT posted 06-27-2015 08:09 AM ET (US)     Profile for OldKenT  Send Email to OldKenT     
Hi jimh. Thanks for your comments, and what you say about the battery with less stored energy makes sense to me. But as I know little about batteries and even less about batteries in series, I have a couple of questions about your comments.
When you wrote "The battery with less stored energy depletes its charge faster", did you mean at a faster rate of discharge, or simply that its charge is depleted sooner than the other battery because it had stored less energy, or both?

My uninformed expectation is that the rate of discharge is the same for each of the batteries when connected in series - i.e., that the energy required to operate the TM is drawn equally from each of the batteries. But this would mean that the battery with the lesser voltage would have to deliver higher amps, wouldn't it?

Also, if #1 has a bad cell, would it test at 12.85 volts after charging? My expectation is that it would not, and that this means it does not have a bad cell.

Thanks for your input.

jimh posted 06-27-2015 09:03 AM ET (US)     Profile for jimh  Send Email to jimh     
When the batteries are connected in series, the current being provided to the load comes equally from both batteries. The rate of discharge should be the same. The battery terminal voltage is an indicator of the state of charge. Battery #1 has less stored energy, so it runs out of energy sooner.

The battery with the lower terminal voltage typically has less charge. However, this is not always true. The internal resistance of the battery is also a factor. But internal resistance is very hard to measure. A specialized instrument is needed, typically, to measure internal resistance of a lead-acid battery.

What seems to be happening:

--the battery called #1 does not get as much charge from your charging system, indicated by its lower terminal voltage after charging;

--as a result, when the load is applied, battery #1 run out of stored energy (lower terminal voltage) sooner than battery #2;

Try swapping the position of the batteries in the series circuit. Keep the charger leads as they were, so that, in the new positions, the batteries will be charged by a different charging circuit. See if that affects the outcome. Perhaps the charging circuit of your charger is not delivering full charge or full voltage to one battery. Or, perhaps that battery just needs to be charged longer to get to full-charge state. Or perhaps the battery #1 has just started to fail sooner than #2. In a series connected battery circuit, both batteries must be very similar in their characteristics if they are to equally share a load.

jimh posted 06-27-2015 09:17 AM ET (US)     Profile for jimh  Send Email to jimh     
A lead-acid cell creates a voltage of about 2.1-Volts when charged. If you had a shorted cell, there would be only five cells. The terminal voltage would be

5 x 2.1-Volts = 10.5

A normal six-cell battery would have a full charge terminal voltage of

6 x 2.1 = 12.6-Volts

During charging, it is possible to pull the terminal voltage of a cell battery to be 2.5-Volts or more, thus you might see around

5 x 2.5 = 12.5-Volts

during charging of the five-cell battery. Once the external charging voltage was removed and the battery was allowed to reach its resting voltage, the voltage would return to about 10.5-Volts.

Jefecinco posted 06-27-2015 04:59 PM ET (US)     Profile for Jefecinco  Send Email to Jefecinco     
It appears the likelihood of a bad cell in one of Ken's batteries is remote.

There is however a good possibility that one of the batteries in Ken's set up is considerably older than the other due to the possibility that one was older stock than the other when both were purchased, presumably as a pair. I don't know if that could lead to one battery discharging more quickly than the other.

If that's a possibility I would have the batteries load tested if they are in warranty.


OldKenT posted 06-27-2015 05:15 PM ET (US)     Profile for OldKenT  Send Email to OldKenT     
jimh, I am going to try your suggestion of switching the position of the batteries in the series but keeping the charger leads as they are, but it will take a couple of days to test the results and I will report on it then.

Today, I again used the TM but only for about 2 and 1/2 hours. The before use (after full charge via the Minn Kota charger) voltages were: #1 - 12.85 volts, and #2 - 13.04 volts. After use, they were: #1 - 10.70 volts, and #2 - 12.40 volts.
So, I have a couple of comments:

First, I took Butch's suggestion and took #1 to Sears for a load test this afternoon. They say it tests fine! To me, it is questionable whether they tested only for charge or for load as the rep would only say "it took a full charge" and "it is ok" when I asked did they do a load test.

Second, the TM itself has an indicator for remaining battery power measured in quartiles - 100%, 75%, 50%, and 25%. Never during the previous two years has the TM indicator gone below 75%, and it usually remains at 100% even after several hours of use. In the last two uses, including today's 2+ hours, it has gone to 25%, which to me reinforces the voltage test and suggests that the failure of #1 is clearly the issue.

Lastly, this TM has 80 lbs of thrust and Minn Kota recommended it for a different boat which weighed 2300+ lbs. I am now using it on a 1988 13 foot super sport with a total weight, including original 40 hp Johnson, of about 600 lbs, and additional TM and 2 battery setup of another 225 lbs. Clearly, these batteries should power this TM for days, like they used to.

Thanks greatly for your help on this.

OldKenT posted 06-27-2015 05:35 PM ET (US)     Profile for OldKenT  Send Email to OldKenT     
Hi Butch, they both have manufacture dates of June 2013 and I purchased them that July. I am going to try Jim's suggestion of switching their positions in the series to see if that makes a difference. If the result is the same, I will try another Sears store for another load test.

Since batteries are not precision instruments, it seems likely to me that a 24 volt setup will almost always have two batteries with slightly different full charge capacities, so I did not think the difference in full charge of .1 to .2 volts was significant. Maybe I underestimated that, but it has been the case since day one with these two batteries. And until these last two uses, it never seemed to make a difference. After use, they were always within .1 to .2 volts of each other. Unless Jim's suggestion proves the answer, something has happened to #1 and I don't think Sears' load test has found it.

jimh posted 06-27-2015 11:59 PM ET (US)     Profile for jimh  Send Email to jimh     
My recommendation to swap the battery wiring so the charger circuits switch to different batteries was made in order to test the charger. The cause of this problem could be from a charger output circuit that is not providing proper charge.
jimh posted 07-02-2015 10:22 AM ET (US)     Profile for jimh  Send Email to jimh     
Two batteries of the same make and model, purchased at the same time, and used always in the same service, ought to age in a similar manner, but, unfortunately, this is not always the case. Some years ago I used to manage two large uninterruptible power supplies that each had 40 12-volt batteries in a series string. These batteries were continually monitored for their internal resistance by a sophisticated monitoring system.

Over about three years of service, it was typical to see that about 25-percent of the batteries (in this case, ten out of forty) would begin to show increasing internal resistance. This was interpreted as a loss of capacity in those cells. It was also seen that as early as two years of service would produce a few cells, perhaps one or two, whose internal resistance would begin to increase significantly compared to the rest of the string. There was never any correlation between where in the string of 40 cells the cell showing increased resistance would be located. It could be cell number one or cell number 40, or any cell in between. It was quite random. This was my experience in tracking two sets of 40 cell UPSs over three sets of batteries, that is, a total of 240 batteries in service over about ten years.

Back to KEN's situation with two cells. After two years of service, I would not be surprised that one of the batteries seems to have more capacity than the other. A variation like that in the properties of a battery seems to be the nature of lead-acid storage batteries that are mass produced. I have read anecdotal reports of very large flooded-cell lead-acid batteries used in static power service, such as the batteries used to generate the -48-Volts of the telephone line current in the USA, having extremely long service lives. Similarly, the battery cells used in diesel electric submarines might have much longer service lives. But two, random, off the shelf, not-especially-expensive consumer-grade lead-acid batteries might have divergent characteristics after two years of service.

n1ywb posted 07-09-2015 11:14 AM ET (US)     Profile for n1ywb  Send Email to n1ywb     
These batteries may simply need an "equalization charge". This is a standard maintenance procedure for lead acid batteries. We had to do this pretty regularly on our solar electric race car that had 18 6v trojans.

The trouble here is that they are AGM batteries; equalization charging is essentially a forced overcharge which typically boils off some electrolyte which may damage AGM batteries.

This AGM manufacturer offers some advice on equalization

some more info here which is claimed to come from lifeline's web site although I can't find it

Here's a thread about it on another forum

jimh posted 07-10-2015 10:52 AM ET (US)     Profile for jimh  Send Email to jimh     
The simplest explanation for the unequal storage capacity between the two batteries is unequal charging of them. As I mentioned earlier, the unequal charging could be due to a defect in the charging system, and I recommended investigating that. I'd rule out the charging system as a cause of the unequal storage capacity before going on to other causes.
OldKenT posted 08-17-2015 01:52 PM ET (US)     Profile for OldKenT  Send Email to OldKenT     
I did [swap the battery wiring so the charger circuits switch to different batteries]--what Jim suggested. The batteries continued to charge as they had previously, with the same one charging to about 0.2-Volts less than the other. However, on a few short runs on the trolling motor, no significant difference in discharge of the batteries occurred, and the difference in voltage after use remained about 0.2-Volts.

Today, however, the ring terminal on the negative end of the connector between battery #1 and battery #2 separated from the cable that connects the negative pole of one battery to the positive pole on the other. I was not applying excessive stress on the ring terminal, so this suggests that the connection had probably loosened over time. Perhaps this is just a coincidental fact and has nothing to do with the excessive discharge of battery #1.

The setup was #1 positive terminal to circuit breaker to motor; #1 negative terminal to #2 positive terminal; #2 negative terminal to motor. So, if a complete break in the circuit had been caused by the bad connection of #1 to #2, the motor should have stopped working. If this was simply a weak or intermittent or perhaps shorting connection, could that have been the cause?

Over the next few days the trolling motor will get some good workouts, so I should be able to provide reliable charged and discharged voltage readings for each battery. The connection now is #2 positive terminal to motor; #2 negative terminal to #1 positive terminal; #1 negative terminal to motor.

Thanks for all the input.

Don SSDD posted 08-18-2015 04:15 AM ET (US)     Profile for Don SSDD    
I would say the [loose battery cable to ring terminal connection] could have been your problem, but you will only know when you continue your checks with the [bad connection] fixed. If you continue to have the same problem, you have one battery that is weaker than the other. Keep watching [the charged and discharged voltages of the two batteries] and see if [the difference in the terminal voltage between the two batteries] gets worse, and replace [the battery whose terminal voltage is the lowest].--Don
jimh posted 08-18-2015 09:02 AM ET (US)     Profile for jimh  Send Email to jimh     
It is hard to assess how a loose connection in the battery cable between the two batteries connected in series would affect their discharge and charging characteristics, but such a bad connection could not have been helping you get the best return of stored electrical energy from the two batteries.

If you have repaired the bad connection, you need to continue to test the batteries for charging and discharging characteristics.

Hoosier posted 08-18-2015 09:06 AM ET (US)     Profile for Hoosier  Send Email to Hoosier     
One thing my solar energy guy told me is that screw type battery terminals will self loosen because of the thermal cycling during charging. Always have a lockwasher on that type of terminal. Do not use a locking nut with a nylon insert, it could melt. My off-grid power plant has twenty 6-Volt golf cart batteries. Last year the terminals melted off two of them because the leads got loose. When you add the vibration and motion in a boat there are even more ways for the leads to work loose.

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