Strange asymmetrical charge among linked batteries?

[donradcliffe]

The 'wrong' battery wiring did not kill your batteries. I don't buy into the smartguage writeup, because the loads on house batteries are more like C/40, not the C or C/2 they used in their analysis. In your case, the smartguage analysis would predict failure of the wrong battery.

Don't give us the SOC--outside a laboratory, no one can accurately know the SOC. Give us the voltage of each battery, and any amps in/out associated with that measurement.

Buy or borrow a clamp-on DC ammeter( $50 at Sears). With that instrument you can determine the amps going in/out of each battery without disconnecting them.

[skipmac]

Originally Posted by 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:
SmartGauge Electronics - Interconnecting multiple batteries to form one larger bank

​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.

I subscribed to this theory for years as it seemed to make sense, intuitively. However in a discussion on this topic a couple of years ago an electrical engineer that designs and manufactures devices for marine electrical and charging systems joined in and disputed the necessity of drawing off opposite ends of a bank. I was initially skeptical but she added the data and calculations to back up her claim. I followed the calculations and at the end had to agree, assuming:

1. All battery connections are clean and tight and all cables are properly sized.

2. You are talking a moderate current draw system. If you're frequently running an inverter or something that draws 100 amps or similar for more than a fraction of a minute then there may be some benefit to opposite end connections.

Bottom line, house battery banks that on a typical boat are usually supplying 5-10 amps max, with a properly installed house bank, moderate loads will draw quite equally from all batteries in the system and any small imbalances will equalize over a fairly short time when the bank is idle. Same for charging.

I do still wire my house bank to draw off opposite ends because it's convenient. However I am quite certain that drawing power from just one battery's + and - terminals that is parallel connected to several other batteries is not going to kill the far batteries in the chain.

By the way, no matter how the bank is wired, undersized cables and bad connections can kill the batteries.

[ramblinrod]

Originally Posted by 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,

If the batteries are connected in parallel, connections and batteries are sound, with no load or charging source, they will equalize.

The posts about battery wiring to opposite ends vs one end being the cause is total hog wash (especially for only 2 batteries in parallel), if the connections are good.

(Just because it's found on the internet doesn't mean it's true.)

Most Likely Possibilities:

1. Shorted cell. At rest, it would read about 11 Vdc when "fully charged".

2. High Z connection. Either loose connection or corrosion between cable and terminal or terminal and battery post is impeding current flow to the low battery.

My recommendation is:


1. Reconnect the batteries.
2. Remove all loads.
3. Apply shore power charger for 8 hours.
4. Remove charger for 4 hours, (or apply a ~5A load for 15 minutes).
5. Measure voltage on the battery posts (not the terminal connections) at steps 3, 4, and 5.

If they are different, you have a bad connection.

If they are identical, disconnect the batteries and measure at the posts again, if one is low, that battery likely has a shorted cell.

[varkpilot]

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?

Have you checked battery connections? Corroded leads could do what you're seeing. Only takes a minute to pull the leads off and clean them. Might save you the cost of a new battery or two.

[Uplander]

Apparently with deep cycle batteries you should fully charge the battery over 24 hours and then discharge using a resistance such as a headlight bulb. Measure the time of discharge. The load test is for engine batteries or so my guru says.

[Dave Lochner]

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?

This is the root of the issue. The second battery never saw the charge. As was mentioned earlier, the charging currents need to flow through both batteries. In a parallel setup one battery will have all the positive charges (alternator, solar, charger) attached to it and the other battery will the negative connected.

The common negative that goes to the engine block or ground bus bar should not be attached to the battery that has the main positive wire going to the switches and panels.

[ramblinrod]

Originally Posted by Uplander

Apparently with deep cycle batteries you should fully charge the battery over 24 hours and then discharge using a resistance such as a headlight bulb. Measure the time of discharge. The load test is for engine batteries or so my guru says.

I'm going to challenge your guru...sorta.

He is alluding to a proper capacity test is better than a load test.

No argument.

The first test you describe is a crude capacity test, and yes, this is very good to confirm the overall "health" of the batteries.

There's a lot more to a proper capacity test than described.

The proper test takes 20 hours (unless one has the battery capacity specs for shorter duration).

The second test you describe is a 100 A load test.

If one puts a 100 A load on a fully charged battery for 20 seconds, and it drops down significantly and does not recover quickly, very close from whence it came, you've most definitely got a bad battery.

20 seconds vs 20 hours.

Guess which one my customers tend to be willing to pay for? ;-)

For someone who lives aboard, every night becomes a pseudo capacity test.

If you are fully charged before sunset and by sunrise, with all loads off (especially fridge) for 15 minutes, your house bank voltage is now always 12.2, progressively from the 12.5 Vdc it used to be shortly after the the batteries were new, it's time to replace the batteries.

[john61ct]

Do NOT use the remaining good battery in your new bank, they should all be as close to exactly the same from start to finish.

Use the singleton as a separate starter, sell it, whatever.

Originally Posted by hamburking

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.

New batteries, at least lead ones, should be "broken in" just like a new engine. Most chemistries spec the first 10-20 cycles I believe, check with the maker's tech support.

Obviously done for each cell as a standalone - this will also detect any lemons.

And after that point, other than maintenance, testing, balancing etc, they should never be used apart from the bank.

A large bank will ALWAYS last for MUCH longer than if you kept the component cells in use as separate batteries.

quoting Maine Sail:

"Shallower discharges lead to longer bank life and with a bigger bank you'll be drawing it down less as a % of discharge and the bank will last longer. Not only that you can actually get more Ah's out of a larger bank when compared with the same load than you can on a smaller bank due to Peukert effect."


Some chemistries the difference is 9000 cycles (decades) when the DoD depletion rate is kept to 10%, down to 300 cycles or less if discharged down below 50%. These are lab tests, getting back up to true 100% between discharges, probably overnight. Real world results are always much less.

[smattie]

A shorted cell is relatively easy to find as the voltage drops by 2 volts per cell (assuming a 12V battery)

[john61ct]

I never said this problem was caused by mis-wiring, just countering the previous statement that the wiring problem doesn't ever matter much.


The State of Charge is very important, and easily determined. Cheapest is a hydrometer, which is one major reason not to spend more on AGM unless you have to mount your bank sideways.

Chris Gibson's SmartGauge (now Merlin/Balmar in NA) is also very accurate, and gets more so over time monitoring amps in and out, as well as changes in internal resistance.

He has forgotten more low-voltage DC and battery science than I bet everyone in this thread's ever learned, does lots of consult work for very high-end customers including the military.

So feel free to be skeptical of his generously published findings, but I'd suggest you look for hard research to back up any contradictory advice, for my noob level I'll just take it as gospel for now.

And if anyone knows of a user-friendly shunt-based coulomb counter battery SoC monitor that works nearly as well as the Smartgauge, I'd really like to hear about it.

I've heard of auto-recalibrating ones in the $$ LiFePO4 world, but not for lead. And never one that keeps on top of the (sometimes rapid) "walk-down" over time in bank AH capacity.

[skipmac]

For another take on wiring battery banks here is a link to one discussion on this. Note the comments by forum member Andina Marie http://www.cruisersforum.com/forums/...ml#post1859930

A search on old discussions will turn up more.

Here's a link to the company Yandina Marine Electronics

[StuM]

Originally Posted by donradcliffe

The 'wrong' battery wiring did not kill your batteries. I don't buy into the smartguage writeup, because the loads on house batteries are more like C/40, not the C or C/2 they used in their analysis. In your case, the smartguage analysis would predict failure of the wrong battery.

Don't give us the SOC--outside a laboratory, no one can accurately know the SOC. Give us the voltage of each battery, and any amps in/out associated with that measurement.

Buy or borrow a clamp-on DC ammeter( $50 at Sears). With that instrument you can determine the amps going in/out of each battery without disconnecting them.

This! One bad connection between the two batteries can do it in this situation.

[ramblinrod]

Originally Posted by john61ct

I never said this problem was caused by mis-wiring, just countering the previous statement that the wiring problem doesn't ever matter much.


The State of Charge is very important, and easily determined. Cheapest is a hydrometer, which is one major reason not to spend more on AGM unless you have to mount your bank sideways.

Chris Gibson's SmartGauge (now Merlin/Balmar in NA) is also very accurate, and gets more so over time monitoring amps in and out, as well as changes in internal resistance.

He has forgotten more low-voltage DC and battery science than I bet everyone in this thread's ever learned, does lots of consult work for very high-end customers including the military.

So feel free to be skeptical of his generously published findings, but I'd suggest you look for hard research to back up any contradictory advice, for my noob level I'll just take it as gospel for now.

And if anyone knows of a user-friendly shunt-based coulomb counter battery SoC monitor that works nearly as well as the Smartgauge, I'd really like to hear about it.

I've heard of auto-recalibrating ones in the $$ LiFePO4 world, but not for lead. And never one that keeps on top of the (sometimes rapid) "walk-down" over time in bank AH capacity.

The Smartgauge is not so smart when a charge source is applied to the battery monitored. The readings cannot be relied on.

[john61ct]

My understanding is the margin of error goes to 5% rather than 2-3%.

If that's a problem, could A use switches to "put that bank to rest" for an hour or two - either at a naturally quiet time, or make it a good scheduled maintenance routine, good time to test the integrity of your "emergency" backup arrangements.

or B maybe calibrate with a hydrometer if you have that FLA luxury, and

C what is better? Not being snarky, would honestly welcome tested alternatives. . .

[ramblinrod]

Originally Posted by john61ct

My understanding is the margin of error goes to 5% rather than 2-3%.

If that's a problem, could A use switches to "put that bank to rest" for an hour or two - either at a naturally quiet time, or make it a good scheduled maintenance routine, good time to test the integrity of your "emergency" backup arrangements.

or B maybe calibrate with a hydrometer if you have that FLA luxury, and

C what is better? Not being snarky, would honestly welcome tested alternatives. . .

More like, 25%. Way off. Not usable.

If one has a solar and/or wind charging system, not cool.

For the better part of waking hours, the display is more or less meaningless.

If one has to "put the bank to rest" to get reliable readings, it would be just as useful to just use a voltmeter.

While a coloumb counter has to be set up properly, synchronized (with a full charge) frequently, and recalibrated occasionally, the readings are at least repeatable day to day.

I'm not saying a Smartgauge is no good at all, it just isn't the end all be all to battery monitoring that some imply.