Fixing degrading battery bank cells

RoySalisbury · January 2012

Right now I have 4 ea 6v, 420ah Trojans in series. I plan on adding two more strings and putting the 3 in parallel very soon. My plan was to take the 2 new strings and replace the exisitng one, and then work on getting the current string "revitialized" before adding it back into the bank. Plus this would allow the new batteries to get a couple dozen charge/discharge cycles into them.

The bank I have now is showing a cell or two starting to degrade. How should I go about getting trying to get all the cells the same. I have done about 5 hours of equalization on the bank this last weekend, but it did not seem to do much. Do I just need to equalize longer? How long is too long? What else can I try?

Roy

mike95490 · January 2012

Re: Fixing degrading battery bank cells

One cell, I'd call defective, several cells, the bank is likely due for replacement, if several hours of EQ has not helped. How old is it ??

mike95490 · January 2012

Re: Fixing degrading battery bank cells

Parallel strings, while viable, are not recomended, look into larger capacity 2V cells, that can be a single series string at the needed amps.

Cariboocoot · January 2012

Re: Fixing degrading battery bank cells

How much degradation? A per cell SG difference of 0.010 or more is cause for equalization. If the EQ does not improve things or the cells are off by 0.050 or more there's not much chance of it getting better. You really can not revitalize old or abused batteries. The fact that they can be recharged at all to begin with is something of a chemical reaction miracle.

On the whole it is a bad idea to mix old and new batteries. The new ones won't add any life to the old, but the old ones will suck life out of the new.

RoySalisbury · January 2012

Re: Fixing degrading battery bank cells

mike90045 wrote: »

One cell, I'd call defective, several cells, the bank is likely due for replacement, if several hours of EQ has not helped. How old is it ??

The bank is only about 2 years old. It charges and holds the charge fine, but the SG in one or two cells is off by about .03 or so from the others.

Roy

RoySalisbury · January 2012

Re: Fixing degrading battery bank cells

Cariboocoot wrote: »

How much degradation? A per cell SG difference of 0.010 or more is cause for equalization. If the EQ does not improve things or the cells are off by 0.050 or more there's not much chance of it getting better. You really can not revitalize old or abused batteries. The fact that they can be recharged at all to begin with is something of a chemical reaction miracle.

On the whole it is a bad idea to mix old and new batteries. The new ones won't add any life to the old, but the old ones will suck life out of the new.

We are only talking about .03 difference between one of the cells in a 6v battery ... Its hard to tell exactly with the SG tester I have. Basically there are 3 color sections on the tester (green, white, red). Most of the batteries are about 2 ticks into the green, and one is about 2 ticks into the white (real scientific I know).

Roy

Cariboocoot · January 2012

Re: Fixing degrading battery bank cells

RoySalisbury wrote: »

We are only talking about .03 difference between one of the cells in a 6v battery ... Its hard to tell exactly with the SG tester I have. Basically there are 3 color sections on the tester (green, white, red). Most of the batteries are about 2 ticks into the green, and one is about 2 ticks into the white (real scientific I know).

Roy

My eyesight isn't any better than that.

The point is that equalization should bring the lower cells up. If it doesn't, nothing will. This is exactly the kind of battery you do not want to mix with new ones. If the SG readings were a bit low (like 1.255) but consistent you'd be better off.

BTW it's okay to equalize these at 31 Volts (24 Volt system). Do it for one hour at a time, let them rest, recheck the SG, then try again. It's a bad idea to put the higher EQ Voltage to batteries for several hours straight as it creates heat which can do more damage.

RoySalisbury · January 2012

Re: Fixing degrading battery bank cells

Cariboocoot wrote: »

My eyesight isn't any better than that.

The point is that equalization should bring the lower cells up. If it doesn't, nothing will. This is exactly the kind of battery you do not want to mix with new ones. If the SG readings were a bit low (like 1.255) but consistent you'd be better off.

BTW it's okay to equalize these at 31 Volts (24 Volt system). Do it for one hour at a time, let them rest, recheck the SG, then try again. It's a bad idea to put the higher EQ Voltage to batteries for several hours straight as it creates heat which can do more damage.

I basically set the equalization for 2 hours and checked the SG each hour. After teh first 2 hour run I let it rest for about 20 min, and then did another 2 hours. The last run was basically becuse the sun went down and I did not want to start the generator (the equalization at 31v only needed about 14 amps so the panels were more than enough).

I have read elsewhere that some people say that you may need to equalize for 12 hours or more to get the readings to come into sync ...

Roy

RoySalisbury · January 2012

Re: Fixing degrading battery bank cells

mike90045 wrote: »

Parallel strings, while viable, are not recomended, look into larger capacity 2V cells, that can be a single series string at the needed amps.

Unfortunatly, that is just not very realastic unless you have lots of $$$ .. and if I had that, I would just pay the power company to run me a power line the 5 miles that I needed. Parallel strings are really the only option until batteries are cheaper and can be easily replaced when you need to increase the capacity of the bank.

Roy

Cariboocoot · January 2012

Re: Fixing degrading battery bank cells

In my experience a battery that need 12 hours of EQ to show any improvement is what is known as "scrap metal".

There is a problem with read SG "on the fly", so to speak. The electrolyte may not be remixed properly and the bubbles may lift the hydrometer float. If you're aware of this then you can get a feel for the process and determine if any improvement is occurring or if you're just wasting time.

Cariboocoot · January 2012

Re: Fixing degrading battery bank cells

Almost forgot ...

You're talking about upping the bank capacity by 3X from 450 to 1350 Amp hours. Assuming you have enough panel to supply 45 Amps to the existing set, you'd have to triple that (and add charge controller) as well. You can get away with doubling a battery bank using existing panels but that is only if you really do have 10% or better charge rate available and will be able to manage with this dropping to the 5% recommended minimum.

Darn doesn't it all cost money!

RoySalisbury · January 2012

Re: Fixing degrading battery bank cells

Cariboocoot wrote: »

In my experience a battery that need 12 hours of EQ to show any improvement is what is known as "scrap metal".

There is a problem with read SG "on the fly", so to speak. The electrolyte may not be remixed properly and the bubbles may lift the hydrometer float. If you're aware of this then you can get a feel for the process and determine if any improvement is occurring or if you're just wasting time.

I don't want to imply that after 5 hours there was no improvement.. just that it was not completly fixed. And I agree, after 12 hours if I had seen no improvement I would assume that they were pretty much scrap. However, my batteries still charge, and hold the charge, so I think they are still in good shape. I just want to make sure they are in the best shape they can be.

After my last equalize I made sure to get all the water levels of all the cells back up to "manufacture full level" and will let them go through the normal cycles for a week. Then I will check the SG levels again. So it sounds like I am doing all I can right now.

Roy

mike95490 · January 2012

Re: Fixing degrading battery bank cells

RoySalisbury wrote: »

.... Parallel strings are really the only option until batteries are cheaper and can be easily replaced when you need to increase the capacity of the bank....

Parallel strings are also MUCH more difficult to diagnose when sick, and have more cells to open and refill, and more interconnects and fuses.

If you are needing to increase bank capacity, don't fall into the old theory, of large battery bank, and small trickle charger, all that does is sulfate the bank while it's waiting to get charged !

there may be some merit to looking at a SINGLE string, and working it hard, and planning on early replacement vs a large bank, light workout, and the batteries die of old age before they wear out.

RoySalisbury · January 2012

Re: Fixing degrading battery bank cells

Cariboocoot wrote: »

Almost forgot ...

You're talking about upping the bank capacity by 3X from 450 to 1350 Amp hours. Assuming you have enough panel to supply 45 Amps to the existing set, you'd have to triple that (and add charge controller) as well. You can get away with doubling a battery bank using existing panels but that is only if you really do have 10% or better charge rate available and will be able to manage with this dropping to the 5% recommended minimum.

Darn doesn't it all cost money!

I think this is what may have been part of the issue before. When I started I only had 3 - 210 watt panels. So basically ~26 amps .. around 6% peak charge amps. I now have 6 panels so around 52 amps peak. Problem is, the math never works out. I need 130 watt draw (as of today) for the inverter, so ~3200 watt/day. For 3 days of no sun, your looking at 9360, and no letting the bank get below 1/3 (I know, people say 1/2, but I want things to last a bit longer), I need ~28,000 watt bank. The panels are sized close enough for the load, but not for the bank. and if I add more panels to size it to the bank, then I have WAY more panels than I need and will never get to peak amps for the charge.

Roy

Cariboocoot · January 2012

Re: Fixing degrading battery bank cells

RoySalisbury wrote: »

I think this is what may have been part of the issue before. When I started I only had 3 - 210 watt panels. So basically ~26 amps .. around 6% peak charge amps. I now have 6 panels so around 52 amps peak. Problem is, the math never works out. I need 130 watt draw (as of today) for the inverter, so ~3200 watt/day. For 3 days of no sun, your looking at 9360, and no letting the bank get below 1/3 (I know, people say 1/2, but I want things to last a bit longer), I need ~28,000 watt bank. The panels are sized close enough for the load, but not for the bank. and if I add more panels to size it to the bank, then I have WAY more panels than I need and will never get to peak amps for the charge.

Roy

It's like chasing your own tail, isn't it?
The trick is to plan the system the right way: power needs determine battery bank size which determines array size.
In this case:
3200 Watt hours on a 24 Volt system is 134 Amp hours. If you use a 25% DOD plan that is a 536 Amp hour battery bank. That gives you two days (50% DOD) before you have to start the generator.
536 Amp hours wants a peak current potential of 53.6 Amps @ 24 Volts or 1286 Watts. Factor in the typical efficiency derating and you have a 1670 Watt array.

That's the ball park figures, and it depends on the Watt hours being right (too easy to get that wrong which causes the rest of the calculations to collapse).

Then you have to start applying the practical, such as batteries not coming in any size you want. You can go down if you can shave some power needs. But that isn't likely so it's safer to go up. You might pick some 6V 305 Amp hour Crown batteries for example, and get 610 Amp hours @ 24 Volts (eight batteries, two parallel strings of four).

Then you have to adjust the array size to suit, now aiming at 1900 Watts. This is maxing out a 60 Amp controller too, and you probably won't find panels that will add up to exactly 1900 Watts. Once more you have to decide if you can leave off a bit of charge potential or go a bit higher to meet what's available. Since the sun does not shine brightly all the time going bigger is again usually the better choice. High ambient temperatures, long wire runs, higher elevation, local weather expectations - all affect the performance and need to be taken into account when selecting components.

The other thing to check is that the array is capable of supplying the daily Watt hour needs. If the peak potential charge current is in line and you're going for 25% DOD this usually follows. Using the famous Icarus formula:
1670 Watts * 4 hours equivalent good sun * 50% over-all efficiency = 3340 Watt hours, which covers the 3200 Watt hours expected loads.

Sorry for this no doubt redundant explanation. I did it mainly because other people might be reading this looking for answers. You're right that the numbers never work out to ideal. There are always variations in loads, sunlight, et cetera. On a best day you'll have "potential harvest" (batteries full, sun is still shining) that you can't make use of without load shifting. Everything ends up being a compromise.

RoySalisbury · January 2012

Re: Fixing degrading battery bank cells

Cariboocoot wrote: »

It's like chasing your own tail, isn't it?
The trick is to plan the system the right way: power needs determine battery bank size which determines array size.
In this case:
3200 Watt hours on a 24 Volt system is 134 Amp hours. If you use a 25% DOD plan that is a 536 Amp hour battery bank. That gives you two days (50% DOD) before you have to start the generator.
536 Amp hours wants a peak current potential of 53.6 Amps @ 24 Volts or 1286 Watts. Factor in the typical efficiency derating and you have a 1670 Watt array.

Yes.. and then hope you never increase your power needs. I started this system with only needing about 50 watt of power (about 1200 per day) .. it worked out until I started using more power.. I'm now up to ~130 watt draw and expect to go to 175 by the end of the year. Already got a new generator (6kw LP with a 250 gallon tank), but I need the battery bank to operate as if it was not there (generatror is mainly for larger loads that I don't want on the bank or for emergency).

Perhaps I can look into getting a device that will allow me to charge a secondary bank from the same charge controller (I have seen it, just cant remember the name). Basically it will charge the second bank once the first one is done. I'd have two inverters, but I already have a 250 watt inverter that I can use. That would allow me to use the older bank seperate from the new one, but only one set of panels and charge controller. Just cant see scrapping this bank when its still has a few years in it, but not good enough to put along side a new set of batteries.

Roy

Fixing degrading battery bank cells

Comments

Categories