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Old 17-10-2014, 00:47   #1
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Balance charging of 6v flooded batteries in 24v bank

I am building up a new bank of 6v golf cart batteries (US Battery 2200xc) with 8 batteries comprised of 2 24v strings in parallel.

I am initially charging all of the batteries in parallel with a small bench supply and watching the specific gravity of the cells. They were wildly different to start with but are all now between 1.265 and 1.270.

The question I have is with regard to ending the charge and making up the series / parallel strings.

Should I stop the charge and immediately pull the paralleled batteries apart or let them set for 24 - 48 hours in parallel prior to pulling them apart and making up the 24V bank.

Unfortunately the supply I have available for 6v charging is only capable of 5A so I have been charging 4 batteries in parallel and then moving on to the other 4.

Some specific gravities started as low at 1.230 but all are now up in between 1.265 and 1.270 (probably identical within measurement error at this point).

I tried pulling a bank of 4 apart after charging to an equalization voltage, the result was 3 of the four batteries are within 0.01v of each other when charging and identical with discharging. The fourth battery is about 0.07v higher than the other three. Specific gravity of all of the cells is very close to each other although one cell in the high battery is 0.005 higher specific gravity than the rest (1.275 rather than 1.270).

Thus the question of if parallel charging flooded lead acid batteries prior to making up a series / parallel set should they be left connected and allowed to sit for 24 to 48 hours or immediately put into the series configuration after charging?

Thanks
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Old 17-10-2014, 02:31   #2
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Re: Balance charging of 6v flooded batteries in 24v bank

The all sound a nice SG I would not be to bothered about the pulling them apart/ reconfiguring them too much, What I would suggest is when you get them to their final configuration, Give them a a normal charge untill all SG's stop rising, Then an EQ untill SG's stop rising, Record SG's for later reference, Batteries take a while to "run in" So I would suggest another EQ after about 70 Cycles to get full capacity,
With multi strings you do get temperature & Charge in-balance, So every 3 ish months (or shorter if you have lots of spare time) It is a good idea to check your SG's, If any of the cells has drifted more than about 25 points from the others/reference records, It's worth an EQ

Tim
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Old 17-10-2014, 12:37   #3
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Re: Balance charging of 6v flooded batteries in 24v bank

Prior to parallel charging and balancing I was seeing 6v batteries which were higher than the average by 0.15V and some lower than the average by 0.1 v (a total of a 0.25V swing highest voltage battery to lowest voltage battery) and was diverging with equalization charging.

The specific gravity was all over the map. So started this process.

The question (from a "best practices" perspective) is it better to pull them apart right after completing an equalization charge (all in parallel at about 7.6V), at the end of an "absorption" charge voltage (all in parallel at about 7.2v), held at float for a period of time (all in parallel at about 6.7v), or let them all settle in parallel for a couple of days without any charge applied.

Probably any of the above would be just fine at this point but I am curious if there is a preferred answer?

Thanks
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Old 18-10-2014, 16:05   #4
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Re: Balance charging of 6v flooded batteries in 24v bank

I think you are overthinking things.. hook them up as 24v. connect 24v charger. replace in ~8 years. nobody does what you are talking about. sounds like a lot of work to get an extra couple months outa them.
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Old 18-10-2014, 21:11   #5
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Re: Balance charging of 6v flooded batteries in 24v bank

Perhaps in many cases. However, I was seeing diverging voltages across the bank which got worse with each cycle, it was quite disturbing. (about 5 cycles into the bank use)

Each string has midpoint monitoring and I was seeing the balance go outside of 2% top to bottom half of each string. 2% is about the point where the voltage difference can cook the hot cell.

Interestingly, with all of the work that is going into Lithium banks, Victron has recently released Flooded Lead Acid active balencers. Unfortunately they are only available for 12V batteries for 24V and 48V banks. Neat device and it would be possible to use with 6V batteries but not be able to automatically balance between the 2 batteries making up a 12V half string in the bank.

Definitely thinking that if I put the batteries which tend to be higher voltage (out of the series set of 4) in the top half of the series and the lower voltage batteries into the bottom half of the series I would get the benefit. They are inexpensive devices which basically turn on during the high voltage part of the charge cycle and shunt a small amount of current around either the top half or the bottom half of the string to reduce the charge rate slightly (about 0.7A max, appears to have some step resistor which adjusts the amperage to balance the charge voltage).

Even if it does not significantly change the battery life, it would significantly reduce the water consumption of the cells with higher voltage across them in the string which would be nice.

At any rate, can't imagine that working to balance the bank is any worst than a waste of time, probably can't hurt the batteries
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Old 28-07-2015, 21:08   #6
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Re: Balance charging of 6v flooded batteries in 24v bank

Still having fun with this bank of batteries.

I have about a season of cycles on them (perhaps 25 cycles to 50%). During discharge the voltages across the batteries are absolutely identical.

It is interesting that one of the batteries is at a higher value during float charging (perhaps 0.06V higher on a 6V battery) than the rest of the bank. This does mean that the rest of the batteries in that string have slightly lower voltage.

As an experiment I have tried discharging that 6V battery with a 4 ohm resistor (about 1.5A) for 20 minutes at a time. I do this with the charger turned off.

After pulling about 1.5% of charge (about an hour and a half if memory serves) in 10 to 20 minute sessions and then running a cycle or two that battery is now about 0.02V higher than the other batteries.

It is easy to keep an eye on these batteries as each string has it's own BVM 702 and I am using midpoint monitoring.

I do see various batteries having different voltages relative to the bank during late absorption and float which settle out. When the charger goes to "storage" mode the values are even closer.

Probably just general variation in the batteries, nothing to worry about but fun to try and see how closely I can keep them in balance.

Any thoughts on what is "normally" seen in a bank of this type?

Hydrometer readings across the bank are very flat at this point.
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Old 30-07-2015, 09:51   #7
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Re: Balance charging of 6v flooded batteries in 24v bank

The Trollbridge24 charges a 24 volt bank by putting the 2 12 volt batteries in parallel for charging or any time the 24 volt load is not applied. When a load is applied it automatically switches them in series. This is not as good a balance as putting all 4 6 volt batteries in parallel and charging however it is a major improvement over charging in series at 24 volts. It also has the bonus of charge-on-the-run from your 12 volt alternator.
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Old 07-10-2015, 21:27   #8
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Re: Balance charging of 6v flooded batteries in 24v bank

In my case it is the 12v loads which are the odd man out. Only the generator start battery is 12v the rest of the boat is 24v with a few loads supplied by DC to DC converters to drop 24 to 12v
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