Strange asymmetrical charge among linked batteries?

[tmoandj]

Hi,
I'm charging two 210 Ah AGM batteries. They're my house bank and normally they are wired together (+to+ & -to-). However, when I separate them I find that one is nearly 100% soc and the other is deeply discharged. They are identical batteries, installed at the same time and always treated the same. They are discharged now because the solar power just hasn't kept up with the demands of our first year of full time living aboard. I find it difficult to understand why one battery would discharge preferentially over the other! Is this unusual? Am I missing something?
Thanks everyone,

[sailormed]

how is your charging circuit wired to the 2 batteries? the best way is to use the + of one battery and the - of the other. This way the charging current has to go through all the cells of bothe batteries. Maybe this is causing the situation you found.

[Pete7]

Originally Posted by sailormed

how is your charging circuit wired to the 2 batteries? the best way is to use the + of one battery and the - of the other. This way the charging current has to go through all the cells of bothe batteries. Maybe this is causing the situation you found.

Wouldn't that produce 24v?

Tj, I think you have at least one knackered battery and possibly two if you haven't been able to keep up a good charging regime. Try charging them both up separately and then leaving them for 24 hours to see if they hold a charge.

Pete

[smattie]

Originally Posted by sailormed

how is your charging circuit wired to the 2 batteries? the best way is to use the + of one battery and the - of the other. This way the charging current has to go through all the cells of bothe batteries. Maybe this is causing the situation you found.

100% correct. 14V charge going to BOTH batteries. Good observation

[smattie]

Originally Posted by sailormed

how is your charging circuit wired to the 2 batteries? the best way is to use the + of one battery and the - of the other. This way the charging current has to go through all the cells of bothe batteries. Maybe this is causing the situation you found.

100% correct. 14V charge going to BOTH batteries. Good observation

[Suijin]

Originally Posted by Pete7

Wouldn't that produce 24v?

I think you misread what he's saying. He's talking about connecting positive to battery #1 and negative to battery #2. If that's not what the OP has done, and one is entirely "downstream", it's going to likely be damaged over time.

Or, one could have simply failed. It happens.

[btrayfors]

Originally Posted by smattie

100% correct. 14V charge going to BOTH batteries. Good observation

Correct, but not relevant in this case.

If, indeed, the batteries are paralleled (positive to positive and negative to negative) and one of the batteries is deeply discharged compared to the other, then he has a bad battery. Period. No matter how the charging leads are connected (both to one battery or split as would be proper procedure).

And, of course, they both may be bad. We simply don't have enough information.

Bill

[tmoandj]

Hi everyone, sorry I was unclear. The batteries were connected in parallel. The charging leads were both on one battery. In fact, they were on the battery that remains fully charged. If the discharge battery were wrecked wouldn't I expect to see power being drawn from the good one and some commensurate change in the soc of the good one?

[btrayfors]

Originally Posted by tmoandj

Hi everyone, sorry I was unclear. The batteries were connected in parallel. The charging leads were both on one battery. In fact, they were on the battery that remains fully charged. If the discharge battery were wrecked wouldn't I expect to see power being drawn from the good one and some commensurate change in the soc of the good one?

How are you estimating/measuring "fully charged"??? By voltage? How measured and when?

If two batteries are connected in parallel while charging, yes, it's possible that the battery to which BOTH charging cables are connected will receive slightly more charge than the other one. HOWEVER, there won't be a great difference in the charge levels between the two, assuming that the paralleled connections are clean and tight.

Further, the ability of a battery to "hold a charge" does NOT relate directly to the health of the battery. Heavily sulfated batteries which have, e.g., only 25% capacity left may well look the same in terms of "holding a charge" as will fresh full-capacity batteries.

Bill

[john61ct]

Yes, the wrong wiring pattern can kill a bank over time, imbalances between them in charging /discharging V and/or A. Read this closely and understand both the details and the underlying logic:
http://www.smartgauge.co.uk/batt_con.html

​When combining series or parallel batteries, it is a good idea to health-check each one independently as a maintenance routine, maybe several times a year if they are expensive.

[Dadrock33]

Way to go, John61ct! That is an excellent piece and one that I have copied many years ago for just such an occasion. It, quite literally, contains almost all the information that you need to know regarding connecting batteries. It isn't totally complete but it sure is the way to get started.


To the OP (original poster) tmoandj - others have already said it but here it is: you've got one bad battery. Sometimes the cells go bad, sometimes it's the way it was manufactured, sometimes it's the way you use or charge it . . . . doesn't matter now, one of them is done. Personally (I'm assuming these are 12V via your discussion), I would replace both of them and maybe even wire up the remaining "good" one to have a 3 batt bank for your house items. Depending on location issues, I'd use a 2' batt cable from each positive post and connect them together at one point to your cable going to the house bank switch (believe it was item #3 in the Smartgauge document) and then do the same for the negative posts. . . lots less stress and strain and I think it's actually a bit easier than other methods. . . . but, keep reading and learning, that's what we're here for . . . that and the barley pops!

[Martkimwat]

You could try swapping the batteries around and see if the problem persists or the problem arises in what was the good battery, then you will know what the problem is. I think Sailormed was on the right track, have the positive and negative leads going to separate batteries and then the link to each other after that, that way they should both receive the same level of charge. So, (+) goes to batt 1 then batt 2 and (-) goes to batt 2 then 1. make sure you use the same size cable for all the connections and that they are suitably sized for the expected load and total cable length.

[hamburking]

I had a battery finally go dead after many years of faithful service. I replaced it with a brand new, name brand battery...and I still had low volts even after tons of charging...so I began examining my charging system and other wiring, looking for the problem. Spent A LOT of time on it. Long story short....new battery was bad right from the start. All my efforts wasted. The store gave me a replacement, grudgingly, they assumed I must have done something to toast it. New battery worked great. A lesson in frustration.

It could be that your second battery had been bad for a long time...and killing the good battery too.

After that, I separated my batteries, and each with its own charger. I can combine with a selector switch when needed. At least this way if one battery goes bad, I still have the other.

[Mr O]

To properly test a batteries capacity, you will need to do a load test. The tester is attached to the terminals of each battery and a large load is applied. Voltage is monitored during the process which takes about 10 seconds. A heavily sulfate battery or a bad cell will cause an immediate voltage drop well below 12v.

A load tester is a very handy tool and not expensive.

[hamburking]

Originally Posted by Martkimwat

You could try swapping the batteries around and see if the problem persists

I did this once, and it worked!

I had just bought a paceship 29 in Toronto, and had stopped in newcastle on the delivery home. Even though the charger had been plugged in all night, the batteries were still flat in the morning. I swapped batteries (via the cables, did not move the actual batteries), disconnected everything else on the batteries, and suddenly I had volts and was on my way.

After much examination later on, I found a mystery wire connected directly to the bad battery, and the bad battery was draining the good one. I disconnected the mystery wire and replaced the bad battery, and had no more problems.

Much later, after selling the boat, I get a call from the new owner...the batteries are dead (including my almost brand new one). It seems their mechanic did them a favour and reconnected the mystery wire (seemed to think it was vital), and the batteries failed shortly after.