This HAS to be idiotic...but please explain why it is!
openplanet
Registered Users Posts: 33 ✭✭
OK, I'm still trying to figure out a way to upgrade my existing, healthy, 2 year old, series string of 4 Rolls S530 6 volt batteries from 24 to 48 volts by adding 4 new ones to the string...WITHOUT violating the apparently sacred rule that one Shalt Not Mix Old And New Batteries.
Here is the idea, which I presume is completely idiotic...but I'd really like someone who knows what they're talking about to try and explain to me precisely WHY!
*Keep my existing array, controller, and 4 battery, 24 volt string.
*Set up a second array with its own controller, charging a second series string of 4 NEW batteries.
*Put those two strings in series...and there's your 48 volts, with old and new batteries being charged independently.
Yes, there will be times when one controller is in bulk while the other might be in absorb, or one in absorb while the other is in float. Why does that matter? End to end it's a 48 volt string, and a 48 volt inverter wouldn't know or care about the details.
I CAN see that there would be challenges getting each (Midnite Classic) controller to have its own WhizBangJr. for determining endamps. But beyond that...???
So again, I know this must be lame-brained. I'm just interested in hearing any clear explanations of WHY.
Thanks.
Here is the idea, which I presume is completely idiotic...but I'd really like someone who knows what they're talking about to try and explain to me precisely WHY!
*Keep my existing array, controller, and 4 battery, 24 volt string.
*Set up a second array with its own controller, charging a second series string of 4 NEW batteries.
*Put those two strings in series...and there's your 48 volts, with old and new batteries being charged independently.
Yes, there will be times when one controller is in bulk while the other might be in absorb, or one in absorb while the other is in float. Why does that matter? End to end it's a 48 volt string, and a 48 volt inverter wouldn't know or care about the details.
I CAN see that there would be challenges getting each (Midnite Classic) controller to have its own WhizBangJr. for determining endamps. But beyond that...???
So again, I know this must be lame-brained. I'm just interested in hearing any clear explanations of WHY.
Thanks.
0
Comments
If it were my battery bank, I would perform a capacity test on the existing batteries to be sure that they really are fully healthy.
Certainly, people will come along and say how bad the idea is based on something they read somewhere. I do not claim to be a world class expert on the subject, but I can offer opinions based on what I have seen or done. I design and sell AGM battery systems for a living, so I am opinionated. There are some very talented battery engineers who agree with this.
Just be aware that your battery bank will only be as strong as the weakest battery in the string.
Marc
Are you going to keep the systems separate with diodes so you don't get flow between the two banks?
As soon as they are connected in series won't there be a gateway for current to flow between banks?
People do mix old and new batteries together--And get useful life out of them--It is just a recommendation (at least on my part) that you try to avoid it--If practical.
If you need to mix old and new batteries--I would probably suggest paralleling into 2x24 volt strings rather than two 24 volt stings in series... But I am not sure that would never use an "old/good" lead acid string in series with a "new/good" lead acid string (same type of battery--model/mfg ideally).
Older lead acid batteries tend to have high self discharge. So when you put old+new in series--You have to "over charge" the new battery to make up for the self discharge of the old battery. More equalization of the new cells will use more water, erode plates, cause more positive grid corrosion.
How bad that will be--I am not sure I can tell you. Will the 2 year and new batteries "fail" (age/cycle out) in 6 years instead of 8? I cannot tell you.
Would you be happy if your bank lasted 6 years (instead of 8-2 for old bank) and 8 years for new bank?
When your bank gets ~6 years old--And one or two cells start to die every few months--When will you pull the plug on the whole bank and replace it?
There is probably enough variability in any lead acid battery bank that, in the end, you may have troubles isolating failures in 4-6 years as being caused by mixed age of batteries--Or just the luck of the draw (battery mfg) and local factors (how you charge the bank, how you cycle the bank, any mistakes you may have made, etc.).
If you need the extra capacity, install the extra charging (solar panels+charge controllers as needed) and cannot find anyone that needs your 24 volt bank for a reasonable price--Then build out your bank (24v or 48v) and just keep a good eye on specific gravity, battery voltages, etc.
Your other option--Install the 2x solar array + controllers, and keep your current bank--And just cycle it more heavily until it wears out. The replace with new bank of your needs. You may need some more genset time during bad weather--But those costs are probably comparable or less than replacing your old bank now. You have good batteries and you can deeper cycle them--As long as they get recharged relatively quickly (i.e., don't take to 25% state of charge and let sit for a week until the sun returns--that will probably sulfate them--Get them recharging the next morning with solar and/or genset--Don't need to get to 100% but get 10-30% minimum state of charge back in the battery the next day). One of the alternative methods of running a battery bank is to cycle from 50-80% state of charge--The batteries do not sulfate or show "harm" from this cycling. And get to >90% State of charge every 7 days or so.
For me, I suggest looking at the risk/rewards of decisions. Is the risk of old bank+new panels+genset run time better or worse than 1/2 old + 1/2 new bank, or all new bank (and more panels as needed).
I do not think there is a good black and white answer here.
-Bill
Are either of the Neg or Pos 24 Volt output wires of either charge controller internally bonded to ground?
That could cause an issue.
-Bill
Forget diodes.
|| Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
|| VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A
solar: http://tinyurl.com/LMR-Solar
gen: http://tinyurl.com/LMR-Lister ,
2.1 Kw Suntech 175 mono, Classic 200, Trace SW 4024 ( 15 years old but brand new out of sealed factory box Jan. 2015), Bogart Tri-metric, 700 ah @24 volt AGM battery bank. Plenty of Baja Sea of Cortez sunshine.
Not to mention that there is no flow path here such that diodes are needed.
If " I know this must be lame-brained." and you are off grid, and want to find the reason why, it is because a battery failure is always something we try and put off as long as possible. This is because we never want to lose power and when the bank starts acting funny it is our warning. You would be starting out with the warning. It also hurts my head to think about people making this more difficult because of money. If you can't afford it you really should not be doing this.
http://members.sti.net/offgridsolar/
E-mail offgridsolar@sti.net
Vocations make money and avocations are where it goes.
The diodes comment was to make that point, you evidently didn't understand it and took it literally
Let me make it clearer where the electricity will flow between the two banks...
My circuit clearly shows how to connect two Battery Chargers to a set of batteries that are connected in series - which is the very point of this thread. And yes I did read what you wrote and you stated "DIODES". And yet your diagram has no diodes - LOL. No, I did not forget the "other half of the circuit" because it is obvious to everyone, where the 48 Volt Inverter would need to be connected (which is not part of this discussion). Your diagram does not explain anything and it specifically does not explain why you claim "diodes" are needed. Your circuit diagram is just one battery, a bulb and some wire. LOL
But the moment you install any required grounds, it shorts out one bank.
|| Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
|| VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A
solar: http://tinyurl.com/LMR-Solar
gen: http://tinyurl.com/LMR-Lister ,
And BB mentioned in message #7 that the "upper" charging circuit needs floating outputs - both (+) and (-) need to be fused
Remember:
It's chapter 1 volume 1 of basic electronics concepts. Adding more to the circuit does not alter the flow.
|| Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
|| VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A
solar: http://tinyurl.com/LMR-Solar
gen: http://tinyurl.com/LMR-Lister ,
If I had to make the best of what I have--I would suggest adding the second new string of 24 volts in parallel. No diodes. Upgrade amoutn of solar panels+charge controllers+backup genset+battery chargers to support the larger battery bank and loads.
Get/use a good hydrometer to monitor the state of charge for each cell (if flooded cell lead acid battery). Get a DC Current Clamp meter to measure the charging/discharging current between old and new string (you want each bank to handle its portion of charging and discharging current--They will not be identical, but they should be reasonably close). Watch battery/cell temperature. Monitor voltage across each battery. Log specific gravity and battery voltages and watch for things that change--Investigate why changed (i.e., dirty/corroded wiring or connections, failing cell, etc.).
-Bill
http://members.sti.net/offgridsolar/
E-mail offgridsolar@sti.net