Can I wire a battery bank for 12V, 24V, and 48V?

[nbritton]

Is there a way I can wire a battery bank of twenty 12 volt batteries in such a way as to get one 48 volt rail, two 24 volt rails, and four 12 volt rails?

I'd like to get a 48 volt charge controller, but my two inverters are 24 volt. Then I also have some 12 volt gear. 

[BradC]

As long as your consumers don't share a common earth, then yes you can.

20 12V batteries is in the order of 240V, so it's a mystery as to what you are doing though.

I'll give you an example of why it might not be a great idea though :

When we got our boat, it had 2 24v banks comprised of 4 6V batteries. The 12V supply for the boats electronics was tapped from half way down the bank. What this did was make sure that the bottom half of each bank discharged faster and deeper than the top half of the banks. When charged with a 24V charger the batteries never properly charged or equalized, and so we used to go through a set of batteries every year or two. Batteries that don't last their rated lifespan generally don't die, they are killed.

By using a 24V to 12V dc to dc converter and removing the mid-bank taps, we've gone from 1-2 yearly battery replacements to 5-7 years. Battery strings don't like uneven charge/discharge, and using a 48V charger to charge banks you are going to pull 24V & 12V from is likely to cause you a world of hurt.

[Jeroen3]

You shouldn't create a three terminal battery pack.
Unbalanced (dis)charging causes rapid wear, or even fire, depending on the battery chemistry.

This obviously isn't the case if you use a balancing controller. But something tells me you do not.

[nbritton]

You shouldn't create a three terminal battery pack.
Unbalanced (dis)charging causes rapid wear, or even fire, depending on the battery chemistry.

This obviously isn't the case if you use a balancing controller. But something tells me you do not.

No I don't have anything. I'm just experimenting at this point, I found two UPS extended battery cabinets in the trash. Each pack has ten Yuasa NPX-25 batteries, which are rated at 12V 5Ah. The ten batteries in the pack are split into two 60V rails with five batteries wired in series. The rails can be jumpered together for 120V. What about staying at 60 volts, and then converting it down to 24V with DC to DC converters? I would have to tear apart the cabinet trays if I wanted to rewire them for any other voltage.

http://www.yuasabatteries.com/pdfs/NPX_25_DataSheet.pdf

[BradC]

No I don't have anything. I'm just experimenting at this point, I found two UPS extended battery cabinets in the trash. Each pack has ten Yuasa NPX-25 batteries, which are rated at 12V 5Ah.

Before you do anything at all get 2 or 3 of those batteries and do a capacity test. UPS batteries are rarely discarded unless they are toast, and by toast I mean down to 1/5 of their rated capacity (if that).

If you have CSB's like your picture, check the serial number. The first 2 digits is the year. If they are older than about 12 or 13 you'll want to be checking them out very closely before investing anything at all. Those CSB units used in UPS's are bloody good batteries, but unless they've lived an exceedingly easy life they are pretty dead at the 5 year design life. I just picked up 12 17AH units from a UPS replacement. They are 2012 manuf, installed in 2013 and still have rated capacity, *but* those UPS were super high grade liquid cooled MGE units, highly under-loaded and would have spent about a total of 30 minutes on battery in the 5 years they were installed. They also lived in a controlled environment where they sat at a solid 22 degrees C for their entire life. Most UPS batteries don't get treated so gently. I have some 17AH's here that I installed new in 2011 (dated 2010) and they are down to about 4AH each now. Nice UPS, but nothing like the easy controlled life of the ones I just picked up.

SLA's will die very, very quickly when treated to an uneven discharge/charge regime. Flooded survive better because they tolerate some level of abuse if you keep them topped up. SLA's just die. Technically these are VRLA, but "same, same but different".

[nbritton]

No I don't have anything. I'm just experimenting at this point, I found two UPS extended battery cabinets in the trash. Each pack has ten Yuasa NPX-25 batteries, which are rated at 12V 5Ah.

Before you do anything at all get 2 or 3 of those batteries and do a capacity test. UPS batteries are rarely discarded unless they are toast, and by toast I mean down to 1/5 of their rated capacity (if that).

If you have CSB's like your picture, check the serial number. The first 2 digits is the year. If they are older than about 12 or 13 you'll want to be checking them out very closely before investing anything at all. Those CSB units used in UPS's are bloody good batteries, but unless they've lived an exceedingly easy life they are pretty dead at the 5 year design life. I just picked up 12 17AH units from a UPS replacement. They are 2012 manuf, installed in 2013 and still have rated capacity, *but* those UPS were super high grade liquid cooled MGE units, highly under-loaded and would have spent about a total of 30 minutes on battery in the 5 years they were installed. They also lived in a controlled environment where they sat at a solid 22 degrees C for their entire life. Most UPS batteries don't get treated so gently. I have some 17AH's here that I installed new in 2011 (dated 2010) and they are down to about 4AH each now. Nice UPS, but nothing like the easy controlled life of the ones I just picked up.

SLA's will die very, very quickly when treated to an uneven discharge/charge regime. Flooded survive better because they tolerate some level of abuse if you keep them topped up. SLA's just die. Technically these are VRLA, but "same, same but different".

I just remembered that I'm under a NDA, so I can't explain the back story as to how they ended up in the trash pile. However, you should assume they are new units.

[BradC]

I just remembered that I'm under a NDA, so I can't explain the back story as to how they ended up in the trash pile. However, you should assume they are new units.

Well that's a whole other ball of wax then.

So, let's assume you *are* going to use them. Yes you can use a DC-to-DC converter to derive your voltages. Float the batteries at the minimum recommended float voltage (refer to the data sheet) and make sure the charger is properly temperature compensated. The data sheet will also have projected design life vs temperature, and design life vs depth of discharge charts, so you can assess how much damage you'll do in what amount of time. The manufacturers data sheets are pretty accurate on the whole. Cool batteries last longer.

APC commonly run batteries in a series-parallel configuration to keep the voltage as low as possible. There's a nice whitepaper floating around somewhere with more detail on that. The lowest practical number of cells in a string results in the longest battery life as they tend to be easier to equalize. So I'd be stripping those packs out and re-wiring them for whatever voltage you actually need and just parallel up as many batteries as you have to get the capacity you want.

Make sure to put fuses in. You'd be astonished how quickly a 7.2AH CSB UPS battery can melt the shaft of a screwdriver your wife accidentally knocks across the terminals.

[NJM]

Hi everyone i have a question for you. Can I hook up one tracer 4215BN CC  40amps on a 6x 320w  solar panels?. I have a plan to mount 6X 320w panels in parallel, then i think Ill use 3 charge controllers feeding 3 different battery bank 12  24  48v with 3 inverters and then put all the inverters and the charge controllers in automation. When one bank is charged, the other starts charging. But the problem if i will have a problem using the charge controllers with the higher amperage on input. Thank to all in advance. Njm