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Old 15-03-2010, 21:45   #1
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Equalizing Batteries - Getting Strange Amp Acceptance Levels

I wondered if anyone could explain this to me...
I equalize my 700a 24v batterys at 31.5v
When i do this they accept around 35 amps and this never gets less, actually after a while it even starts to go up, so the acceptance heads towards 40 amps. At this point I normally stop the process.

Normally when charging at 29.6v the acceptance decreases with time as you would expect.

So why when equalizing would it appear to increase with time???

Thanks John
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Old 15-03-2010, 22:22   #2
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Hi John,

That doesn't make sense to me either. I think you would need to check those readings with a couple of good multimeters: one clamp-on to check the current and a second one to check terminal voltage. The internal resistance should go up so at same voltage current should go down.
But I also think 35A is a bit much for a 700Ah bank... so I am thinking that may be it isn't fully charged yet when you start equalize. In that case, voltage can still go up and that would explain more current.
If you have a way to keep the batteries at a float voltage around 26.4V for a couple of days, I would do that and try again.

cheers,
Nick.
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Old 15-03-2010, 22:37   #3
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Thanks Nick for the reply.
Ive got a victron battery monitor so I generally trust what it says. I had the batteries on shore power for about 4 days before starting the equalization. So this means they have been through a few 29.6v acceptance cycles and 26.4 volt float cycles(every 24hrs the charger does 4 hrs at 29.6v). Also before starting the equalization I put them back into 29.6v acceptance for 4 hours.
They have always shown this behaviour during equalization.

Another point, during 29.6v charging the batteries are never even warm. During equalization they are warm.

Could it be that i need to equalize for longer... generally I do it for 5 hours which is around about the time I see the acceptance start to increase towards 40 amps.
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Old 15-03-2010, 23:11   #4
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Hmmm okay... I trust the Victron monitor pretty well..

Okay, next question is the batteries: what are they and do you have access to the electrolyte? Can you measure specific gravity? Because that is really the only way to tell...

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Nick.
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Old 15-03-2010, 23:36   #5
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The batteries are 2v forktruck traction batteries. I did not check the gravity since Christmas. But back then it was around 1.28 which they have always been. I look after them, always left fully charged. Single point watering system.
Good fast charge at anchor with 150a of ac charging. propshaft alternator when sailing. 170a of dc charging under engine. So they never have long periods left undercharged.
I have equalized the batteries 3 times in 2009, never ever been below 50%. Generallly the batteries work fine, so its not that I have any problems with them.
Its just that I dont understand their reaction when being equalized.
Regards john
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Old 16-03-2010, 08:12   #6
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Those are great batteries. Next time you equalize, you should measure specific gravity (SG) before equalizing. Write it all down and determine the min. max and average values. After two hours of equalizing, measure each cell again (don't stop equalizing). Do the same math and check if the avg SG goes up and if the difference between min. and max SG gets less. After 4 hours do it again. If there is still improvement with the 4h measurements, continue to 5 hours and if it improves again, to 6 hours. I would not continue after that.

If you do not see the difference between min. and max SG become less but you do see the avg. SG improving this means that the cells don't want to equalize. You should do a capacity test to find out what's left in them.
If you don't see the avg. SG improving the batteries aren't sulfated. This is good when the value of the SG shows a full charge after correction for temperature. Good SG probes have a thermometer to make this one easy.

Save the measurements for next time to find a trend. If the time beyond 4 hours doesn't give you improvements you can limit to 4 hours again.

cheers,
Nick.
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Old 16-03-2010, 10:18   #7
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Some questions

Hi John,

I remember on my last boat, using a Cursing Equipment 3 step regulator, the condition charge was a constant current charge (set by user depending on bank Ahr size). It would charge until the battery voltage reached a certain level and then level off and continue the charge for some time interval. It takes a quite a while. So the condition charge is constant current, with the voltage rising past the normal gassing voltage, and held there for some time. Hence why you don’t do it too often. I think the charging current was pretty small, maybe 5% of capacity. I used Hydro caps to prevent water loss.

If the charge controller is not holding the current constant, I think this is wrong and it is not working properly.

Also, it seems to me if you are hooking up to shore power, have other charging sources that top off the batteries, and are not cycling the batteries down very much, I’d extend the time between conditioning, or even forgo it. If on the other hand you are working the batteries between the 50% and 80% level of their capacity, then conditioning more often makes sense.

Conditioning, as you mention, helps to remove sulfates and keeps them from becoming hard and impossible to remove from the plate surface. Since getting the batteries up to 100% charge is a time consuming process, most boats operate in the 50 – 80% range and condition now and then. But if you are charging on shore power, or aren’t cycling the batteries much, you don’t need to do it as often.

Always, the best way to test charge state is by checking the specific gravity of each cell. That will show up any cells that are lagging behind and that might become a problem. It is better to allow the bank to rest a while, to get a more accurate reading. If you still have doubts you can load test the battery bank. An old tired bank will fail a load test, but may have SG readings that look fine. I haven’t done a load test in years. As I remember you do this by putting a 20% of capacity load on the battery bank, and monitoring voltage levels over a period of time. I don’t remember exactly how to do it, but I’m sure you can look around for the info.

I’m currently building a new boat and it will have a 24V 700Ahr bank. The boat is a 60’ cutter. I’ll have many ways to charge the battery bank. I’m really interested in you’re prop shaft system. I have started to set this up myself. Any data I can get would be really useful. What is the length of your boat, type, the size of your prop, and what PMA are you using? You said you get about 300-prop shaft RPM sailing around 5knts? How well has the system worked? Is your gearbox a mechanical one? Has it gotten hot while freewheeling? Really like to know more about this elusive subject.

Where do you sail out of?

Cheers,
Tim
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Old 16-03-2010, 13:50   #8
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Hi Tim,

I haven't heard that term condition charge before. Many chargers with an equalize feature will abort this when the current goes up too high, but allow variations.

Note that you should remove your hydrocaps during equalize because they can get too hot and even melt (I did test that multiple times ;-)

ciao!
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Old 16-03-2010, 19:12   #9
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I agree with Nick in #6 except on the termination criteria. It is not uncommon to not get SG convergence for 8 or 10 hours on a large (>1000) amp-hr bank. You should terminate the equalization if the temperature of the batteries exceeds the manufacturer's specs. Otherwise, terminate when two successive hourly SG readings show no change and the difference in SG between the cells in one battery are within a very few points of each other.

The caps should be off and it is wise to keep a paper towel over the fill ports to capture the sulfuric acid fumes and moisture that will boil off.

Hope this helps.
Charlie
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Old 16-03-2010, 21:32   #10
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Thumbs up Looks correct

Hi Nick and John,

Yea, they say that you should remove Hydrocaps to equalize, or they may get so hot as to melt. I lived aboard with a 720 Ahr bank and never removed them once. I had the Rolls batteries for 15 years and they were still going strong when I sold the boat. The Hydrocaps would get quite warm, but they seemed to handle it. Maybe on a much larger bank it could be an issue. Depends on how much hydrogen they are converting to water I guess. There are newer designs that are less effective, but still convert about 30% of the gas back to water. These you can keep on without issue. Goggle “Water Miser Vent Caps” and you will find them.

Conditioning or equalizing is used by different manufactures meaning the same thing. I went and looked over my old set up notes and looked over a few specs on regulators that have this feature. It is supposed to be done at a constant current of not more than 5% of the bank capacity. So John, your system is right at this, at 35 amps. Some recommend 3 to 4%, so you may want to adjust this down a bit. The regulator should continue holding this constant current and the voltage should rise to something like 32.4 on a 24V bank. This should be measured at the battery terminals. Now to know when to stop this process you need to look as SG of each cell. Once the readings stop rising, hopefully reaching around 1.265 SG, or the readings remain unchanged for three hours, you then should stop the process. The batteries will gas vigorously. Good ventilation is a must.

So your system is working just as it should. I would drop the equalizing current down a bit to 4%, which would be 28 amps. You should be able to adjust this on your regulator. This will mean they will gas a bit less during the process, but it may take longer.

The equalize process takes a long time. Something like three to five hours was what I saw on my Rolls batteries. This was a 720 Ahr 12V bank.

The tricky part is figuring out how often to do this. It depends on how you are cycling your bank, like how much capacity you remove, what other charging systems you have, if you are topping up the bank with shore power, solar and such. The best situation is to have lots of charging options and the bank not cycling much, staying topped off frequently from the aux charging sources. If you are using no more than down to 60%, the number of cycles you will get out of deep cycles goes up and this is about ideal. See battery manufacture web sites for this information. So how often you condition really depends on how much time the bank spends between full charges. If you have solar, wind, shore, or other charging systems frequently topping up the bank, you can decrease from once a month to once in three or four months for the conditioning/equalizing.

You have to remember though, that overcharging is just as bad as not fully charging. It causes galvanic activity that attacks the positive plates causing deterioration.

Undercharging can allow the lead sulfate to harden to a crystal form, which means it will not easily convert back to the sponge lead that hold the many electrons ready for the next cycle. If you do everything right, you cannot beat the longevity of deep cycle traction type batteries. They can last easily 15 years and more.

If you think about the way traction batteries are use in something like a small fork lift, they get discharged all day and are then recharged overnight, or just left plugged in when not being used. These don’t get equalized and don’t need to, because they are frequently fully charged by being on a charger for long periods. So the lead sulfate does not get a chance to harden forming lead sulfate crystals. It just gets converted back to sponge lead ready for the next cycle. Boats suffer because they don’t often have a charging device that is left on for the hours it takes to reach deep into the battery to fully charge the battery. You would not want to run your boat engine for six hours to put back a very small amount of capacity. It’s just not efficient. So condition or equalizing is the solution. The picture changes somewhat with the addition of wind, water, and solar charging.

With flooded batts, you have to be a bit of a careful caretaker and get to know your batteries. You need tools, voltmeters, hydrometers, and to concern yourself with careful installations and things like voltage drops. If you do, they are still the best in my opinion.

If you want to check the banks current health, do a capacity test. For a C20 rated battery, you do a capacity test by starting with a fully charged battery and put a constant steady load on it of 1/20 of its rated capacity. That’s 35 amps for your bank. Note the time you start the test. Continue the test until the voltage falls to 21.0V for a 24V bank. Now multiply the number of hours times the rate of discharge to get the capacity. If the number is not above 80% of the rated capacity it may be time to replace the batteries. This test is more valuable if performed when you fist get the batteries and once per year thereafter, so you can look for trends. Also, you want to make sure the batteries were meeting their stated capacity when you first install them.

A great book with lots of information on this subject, as well as others, is Nigel Calder’s Boatowner’s Mechanical and Electrical Manual. I love Nigel’s work. Another favorite author who writes about boats it David Gerr. For batteries you can also talk to the manufactures. They are very helpful.

Cheers,
Tim
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Old 18-03-2010, 21:10   #11
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Thanks again for all the replys.
I am going to buy a temperature compensated sg probe and test further. I wont be doing it for another 3 months though as i dont like to do it too often.
John
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