Battery Bank Dying

Greetings from Cape Verde!
We had a fast sail from the Canaries and are now getting ready to cross to Barbados.

On the trip here the four year old AGM battery bank started giving low voltage alarms. We were running the generator 3 times per night for an hour at a time. The solar kept up during the day. The SOC never went below 84%. The batteries are 12 volt, 135 amp hour AGM’s. Bought in France by my cousin. Eight batteries wired in series parallel to give a 540 ah bank at 24 volts.

Fully charged this morning all batteries were fine. After running the bank down to 87% (shore power and solar disconnected) one battery of each pair shows extremely low voltage. Pair 1, 8.7 and 12.92 v. Pair 2, 8.76and 12.94 v. Pair 3, 12.8 and 8.9. Pair 4, 9.24 and12.48v. Unable to post a photo.

My solution is to take out the bad batteries and combine the remaining good ones for lower overall capacity but much better (I think) longevity. Buying new is not an option before we leave.

Between solar during the day and the generator if needed I think we will be fine.

What say you?

Thanks,
Rob

The first thing I would do is try to recover each of the low batteries with a stand alone 12 V charger to see if they are salvageable. Also check with the manuf to see if the batteries can be equalized. Few AGMs can, most cannot.
Then you have to find and correct the cause of this failure or the remainders will suffer a similar fate.
My SWAG, charging at too high of voltage.

Thanks Archie, it’s a 24 volt boat with matching charger. They have never been equalized. Wonder if it is worth the risk. Power comes in and out of the center of the bank. I think it should be from either end. That is the way I have done it before.

Any suggestions on what to look for, what the problem might be? We have checked the wiring carefully. At a loss right now.
Rob

I would first do as you suggest. Remove the bad batteries and run on the good ones.


Then you can try things like equalizing charges on them in your spare time, and while they are otherwise disconnected from the boat. But they are probably toast.

Except for #7/#8 pair there is a definite pattern consistent with your comment that you are charging from the middle of the bank. It appears (except #7/#8) that the battery closest to the charging source is absorbing the charging current.

I agree with attempting to recover the low voltage batteries one at a time with a charging absorption voltage a bit higher than normal. Maybe +0.3VDC so 14.6VDC normal would be 14.9VDC. Monitor the battery temperature during the overcharge and secure the charging source if the battery start getting hot. Warm is ok, hot is not.

Rewire the bank with the positive charging and load off of one end of the battery and the negative to the vessel ground off the other end so that all batteries take part in discharging and in charging. Method 2 in https://tinyurl.com/nflwy3

In general, AGMs are not tolerant of being left in a discharged state so the four may be ruined.

I agree that you don’t want to go in and out from the center. You want the positive off one end and the negative off the other end.

Ok, here is a diagram of the installation. The heavy ink is positive and lighter negative. To me, this seems like a center connection. I have been told it is not. Please educate me and if possible add a diagram.
Thanks for your help.
Rob

Never mind all that - what boat are you on, not yours?

With my Irish cousin on an Amel Super Maramu. Great boat, ketch rig.

Rob

This is how I would wire it.

Thanks Dave. It is my understanding that this is the preferred way to wire a battery bank but I don’t know how to explain why it is better than the diagram in my previous post. Can you help me with that?
Thanks,
Rob

Datenight said:

With my Irish cousin on an Amel Super Maramu. Great boat, ketch rig.

Rob

Click to expand...

Careful of the Irish - where is he from?

The resistance in the cabling, although small is enough that it will make some of the batteries invisible to the bank and they might as well not be there. The way I drew it with the positive on one end and the negative on the other end the charge goes through all the batteries because it has to to get to the negative. Same when the bank is discharging. When the charge and draw is from the center the outer batteries disappear.

SmartGauge Electronics - Interconnecting multiple batteries to form one larger bank

Read the above link. I ran into this years ago and found I had done this,
ACCIDENTALLY. I used #3.

I used buss bars for the connections with NO connections on the actual battery posts except for each lead to the busses. THat way as much as possible each battery charge/draw is as equal as possible.


Take a look at diagram #3 as the best. #2 is darn good.

It looks like yours is set up like #3 providing each of the positive leads are the same length as each other and each of the negative leads are the same length as each other. Note the - & the + leads do not have to be the same length though, just each type the same length. Yes, it is picky but it helps to maintain load/charge balance between battery sets.

.However do the leads from each 24V set run to a heavy buss bar? If not then I wonder if your connections are more along the line of #1, just taken off in the middle instead of one end.

The other caveat is all the leads must be the same wire type AND size.

Of course the connections and the crimps can also play into this. If not all are clean and tight and well done then resistance can play into an imbalance of not just the recharging but also the draws.

I don't have AGM, but rather Gels, and my system is 12V so not entirely comparable. When new and after an initial charge after installation I checked the draw for even current between batteries. I do the check the draw every once in a while to see if over time the draw remains even. If not then a problem may be developing. I don't expect perfectly even but within a couple amp on a total 30 amp load.


OOPS! Yeah, yeah. I goofed up the battery symbol for a ground symbol. Ignore and note the + and the - indications.

Datenight said:

Ok, here is a diagram of the installation. The heavy ink is positive and lighter negative. To me, this seems like a center connection. I have been told it is not. Please educate me and if possible add a diagram.
Thanks for your help.
Rob

Click to expand...

This is ideal, provided the cables are the same length.

menzies said:

Careful of the Irish - where is he from?

Click to expand...

Wexford! One of many first cousins! My mother immigrated here in the early 50’s after winning a local football pool.

Rob

Thanks Twist and Clect. Looks like I was wrong this connection is good. Will check cable lengths. The cables all go to the same post as in and out.

Just wish we could figure out why one of each pair is bad. Eamonn (owner) is a software engineer, Eric an EE. My degree is Music. Oh well

Rob

It’s probably just a case of chronic operation at a partial state of charge. The fact that it’s in such bad shape and is only being noticed now suggests a certain lack of attention.

You have an electrical engineer on board?

menzies said:

You have an electrical engineer on board?

Click to expand...

We do.

Twisted, Eamonn is quite meticulous and on top of the batteries. Hard to believe it is lack of attention. Nonetheless one battery in each pair is bad. We are currently (!) taking out the bad ones and combining the good ones.
Rob

I've several electrical engineer friends who are a bit baffled by marine electrical systems. Low voltage DC, Chargers, grounding, GFICs, neutrals etc are a science unto themselves.

Two years ago I enjoyed listening to electrical contractors disagreeing on new US dock wiring codes. Kinda like Dems Vs Republicans.

Are you drawing 12v from any of that 24v bank? Tempting to do, but causes charging imbalance like seen here.

I agree with Comodave. You have been wired incorrectly. You have only been drawing and charging from the batteries with the shortest cables. You need to draw from opposite ends as in his diagram. Done it this way for some 20+ years on 3 boats and had no problems. I currently have 10 xL16's giving over 2000amps. Runs all the boat with 2x325 Panels and midnite solar controller.

Ski in NC said:

Are you drawing 12v from any of that 24v bank? Tempting to do, but causes charging imbalance like seen here.

Click to expand...

12 volt comes from a converter so should be no problem.
Rob

Martin J said:

I agree with Comodave. You have been wired incorrectly. You have only been drawing and charging from the batteries with the shortest cables. You need to draw from opposite ends as in his diagram. Done it this way for some 20+ years on 3 boats and had no problems. I currently have 10 xL16's giving over 2000amps. Runs all the boat with 2x325 Panels and midnite solar controller.

Click to expand...

We have the same solar and controller. I thought they were wired incorrectly also but looking at information posted earlier what we have may be ok. Wire looks to be very close to the same length.

Rob

I have seen batt banks screwed up because the set installer chose SS washers and nuts .

Copper washers and nuts are preferred. Any big electrical supply house will have them..

Do not bother with the dead bats (except as cores) , they stay dead forever.

@FF #26:
SS nuts, lock washers and washers are not inherently unsatisfactory in this application. The nut provides clamping force between the ring terminal (lug) and the battery terminal. From a practical standpoint, no current passes through the nut. The washer and lock washer must be on the top of the battery lug (ring terminal) so that they are not in the current path.

It is extremely suspicious that one battery in each bank is being drawn down so low. The charging/grounding diagram you have drawn is not inherently bad and I don't believe it could cause the severe problem you are seeing.

Ken

I like Ski’s theory that their might be 12V loads being drawn off the 24V bank by tapping between the batteries. Easy to check and determine one way or another.

twistedtree said:

I like Ski’s theory that their might be 12V loads being drawn off the 24V bank by tapping between the batteries. Easy to check and determine one way or another.

Click to expand...

Exactly.