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Forum Index : Electronics : Deep-cycle flooded battery problem
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| davef Guru  Joined: 14/05/2006 Location: New ZealandPosts: 499 |
I recently acquired 8 used 6V deep-cycle flooded golf-kart batteries. They do not seem to have anywhere near their rated capacity. Some observations during testing: - under solar charge the batteries will get to 28.0V at which the charge current drops to about 1A, indicating to me that they are floating and I hear them gassing - as soon as the charging current drops to zero the battery terminal voltage drops to about 24.0Volts. On another system (not deep-cycle) normal drop would be to about 26Volts. - S.G. reading at this point is 1.20 to 1.22, which is the border between Fair and Discharged on my hydrometer. These 6V batteries are within +/-50mV of each other. The other battery system (not deep-cycle) gives S.G. readings between 1.25 and 1.26 (bottom end of Good). Should not the S.G. of a new deep-cycle system be the same as a new non-deep-cycle system? If it should be similar is there anything I can do with this batteries? |
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philb Regular Member  Joined: 05/07/2008 Location: United StatesPosts: 96 |
I have a 24 volt system. All are golf cart batteries. I purchased them 4 at a time, with the same manufacturing date. Mine are in series to make 24 volts, then the strings are ran in parallel. When I get a new string, I usually charge that string separately until it equals the original string voltage.This also tends to prevent sparks when strings are connected together. My complete strings did not hold a charge of 25.5 volts just by using a charge controller because I didn't have the charge controller's voltage set high enough. So I changed that value to 29.0 volts and raised the absorb time. The more battery strings, the more absorb time. If you can equalize the batteries with your charge controller,that will help. I took the charge controller out of the circuit and connected an AC line directly from my generator to the outback inverter AC in port. That really got the batteries bubbling. Once I charged the batteries with that, the battery voltage stayed at 25.5 after charging and has remained that way ever since. Also the individual battery voltages do not vary more than .10 volts from one to the next. The exception is when the individual battery voltage does drop 0.2 volts, that battery needs water. I always keep the water level the same in all the batteries and never add water until the batteries have reached float voltage. I use all the batteries as one big unit. Mine has 24 six volt batteries. I don't set up a tap when I need to run 12 volts for an application and another battery for a 6 volt app. That will cause battery string unbalancing. Then I spend time and lots of headaches re-balancing the system. I take individual voltage readings every month and fill the batteries. The specific gravity of each cell is a bit harder to check and I'm lazy so I check that every 3 months. The SG moves very little after you equalize and get all the batteries to act as one large cell. My theory is that when the golf cart batteries are used for what they are made for, golf carts, they battery acid gets stirred up on a regular basis. When they are sitting in the floor of my shop they are not. So the acid stagnates. A deep cycle battery and non-deep cycle battery have different charging rates. The automotive batteries seem to cherge faster because the batteries are smaller, IMHO. That's my take on keeping the batteries healthy. philb |
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| davef Guru Joined: 14/05/2006 Location: New ZealandPosts: 499 |
Philb, Thank you for your comments. The solar controller is a Tracer MPPT 20A which is suppose to have the following settings: Equalisation Voltage 29.6V Boost Voltage 29.2V Float Voltage 27.6V Boost Return Voltage 26.4V As I measured 28.0V it looks like I may have "caught" the batteries in the float state, but maybe I need to see if they actually reach the Boost and Equalisation stages. Shouldn't operating in the Boost phase help to stir the electrolyte up? Have you any experience with adding "after-market" products like "Battery Equalizer"? What S.G. readings do you normally get? Cheers, Dave |
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Gizmo Admin Group  Joined: 05/06/2004 Location: AustraliaPosts: 5078 |
Each day I boost my batteries at 60 volts for a couple of minutes, then go through a long absorb cycle at around 56 to 58 volts for a couple of hours, then float at around 54 for the rest of the day. Its a 48 volt system as you probably guessed. I would definitely be checking the charging cycle is reaching the boost voltage. If the solar panels are not supply enough watt hours to reach the boost voltage, then your battery bank will never fully charge. This is a problem with miss matched battery banks and solar arrays. A big battery bank could end up with less capacity than a small battery bank, simply because its not completing a charging cycle. Glenn The best time to plant a tree was twenty years ago, the second best time is right now. JAQ |
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| davef Guru Joined: 14/05/2006 Location: New ZealandPosts: 499 |
Glenn, Thanks for your comments. I have "400 Watts" of PV and about 250-300Amp/Hours at 24Volts of battery capacity left. We haven't had a lot of sun since I put the system in, so I will do some more tests. Cheers, Dave |
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yahoo2 Guru Joined: 05/04/2011 Location: AustraliaPosts: 1166 |
Hi dave, golf cart batteries are usually at the higher end of the SG range when they are new. anything from 12.65 right up to 13.0 backup batteries from phone exchanges and the like are the ones with the lowest SG's around the 12.25 -12.4 range. the boost phase is just another trendy term for absorption, that is just a part of the regular battery charge cycle. If you can trigger the equalization stage and make sure it gets up to voltage before it switches off and check your water levels and SG after. if the batteries are crook it will be using a fair bit of water in some cells. if you haven't used a lot of water and the SG readings are coming up don't be frightened to give them a couple more on some sunny days over the next few weeks. if they wont come up to at least 12.3 they are tired or sulphated (or both). If the weather is against you then give them a tickle with a generator and charger first thing in the morning, that gives the controller more time to equalise charge. I'm confused, no wait... maybe I'm not... |
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| davef Guru Joined: 14/05/2006 Location: New ZealandPosts: 499 |
yahoo2, Thanks for your comments. We have had a few nice sunny days recently and today the batteries were gassing merrily and reached just over 29.5Volts. At which time the Tracer said the batteries were overcharged and the inverter said "overload"! Had a switch on a 1KW heater to trick both units into thinking everything was normal. I have put a datalogger across the battery bank, so I will have some info to digest when I go back in 2-3 days. S.G. is still sitting around 1.20 I will talk to the previous owner as he was taking S.G. readings and then go back to the distributor to find out what S.G. value they were when new. Looking more and more like they are just tired batteries :( Dave |
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| yahoo2 Guru Joined: 05/04/2011 Location: AustraliaPosts: 1166 |
don't worry about putting any load on them, just charge them during the day and rest them at night. If they have been sitting around uncharged for a while, they will need 2-3weeks to show some improvement in the SG and dissolve some sulphate. The resting volts should be up to 25 -25.2volts in the early evening if it is holding some charge. Just keep an eye on the water levels. I have had batteries use no water for a week then 2 litres in a day as they start to come good. as far as expected capacity goes ????? if they are 215aH rated new, that's roughly 3.5Kw of safely usable power overnight. Would be nice to get 2 Kw out of them at 50% discharge. I'm confused, no wait... maybe I'm not... |
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| davef Guru Joined: 14/05/2006 Location: New ZealandPosts: 499 |
yahoo2, After 3 days of reaching about 29Volts for a good percentage of the day I decided to do a load test (before reading your post!) Using a commercial current shunt and a DVM I verified a 1KW load at 24Volts. It took 3 1/2 hours to get the batteries down to 21Volts, at which point the inverter shut down. The batteries are specified for capacity at 1.75V/cell, which is 21V for 12 cells. The two strings, when new, should be good for 24V at 440Amp/H, ie a 1KW load (44Amps). That works out to be 10.5KW/H. My genset has a 12V output, so it looks like I will just have to fast-track the microhydro installation. That will kick the batteries off to a running start first thing in the morning! Cheers, Dave |
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| yahoo2 Guru Joined: 05/04/2011 Location: AustraliaPosts: 1166 |
rough guesstimate that equates to about 1.9 Kw over 12 hours finishing at 50% charge. That a bit over half their new capacity. Once you replace the 3.3 kw that has been removed you will only use 100- 160 watts/hour to keep some charging pressure on them provided you are not drawing power off. If the specific gravity and voltages come up over the next couple of weeks, you should gain 10 - 25% more capacity. I'm confused, no wait... maybe I'm not... |
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| davef Guru Joined: 14/05/2006 Location: New ZealandPosts: 499 |
The discharge curve from about 15 minutes into the test until the last 15 minutes was a pretty constant slope. I think 1 or 2 KW of discharge per day would be more then enough. It would be nice to only use the top half of the charge. That would make me happier as I paid about 1/3rd of the new price for them! How do you work this figure out? I will now focus on the self-discharge rate as it does seem high. At the moment with about of 6 hours of sun, it appears I can only just maintain the charge and draw off a relatively small amount for my use. Another issue that will be sorted with 150-300Watts/H going in from the microhydro. |
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| yahoo2 Guru Joined: 05/04/2011 Location: AustraliaPosts: 1166 |
most battery companies provide discharge curve graphs for a 20 hour discharge at a set temperature and start at 100% capacity. I have roughly mentally adjusted that to fit your load test. if the batteries have been sitting around and losing charge for a while, the self discharge you are seeing could be sulphate dissolving back into fluid. Like I say, its tough to rejuvenate a battery in winter if you are using power from it overnight. if the voltages will come up and sit at the proper levels after a couple of weeks charging and resting overnight, then you have a chance. if not, its a downward spiral. I am surprised that the load test didn't finish the batteries off completely, that's a lot of stress on batteries in poor condition. I'm confused, no wait... maybe I'm not... |
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| davef Guru Joined: 14/05/2006 Location: New ZealandPosts: 499 |
Thanks for the explanation. I realised after posting that I had in effect tested the batteries at more like the 5Hr rate, which would have been 2 * 195AH or 9.4KW/H, rather than the 20hour rate. So, the 3.5KW/H I got was a solid 33% of their original capacity. We haven't moved into the house yet, so they are only getting small, infrequent use. Which is just as well as we are not getting much sun. I'll not do any more load testing until summer. Thanks, Dave |
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