# Wiring battery bank for multiple DC voltages?

Registered Users Posts: 3
Can a single battery bank be wired to energize multiple grids at different voltages? Each grid would be wired to all batteries to distribute wear on plates uniformly, just varying in series / parallel, resulting in multiple voltage and amp hr grids.

Are there easier/less costly ways to have a multiple voltage DC grid?

Thank you

• Banned Posts: 17,615 ✭✭
Re: Wiring battery bank for multiple DC voltages?

Welcome to the forum.

The answer is "yes, but no". As in technically it can be done but it's not a good idea to do so.
The reason being tapping, say, 12 Volts off "one half" of a 24 Volt system inevitably means the two halves will have different degrees of discharge due to the unequal loading. as such they will not recharge equally as one battery bank. So the batteries that have to supply greater load will die sooner as they will not be fully recharged yet will be more deeply discharged.

There are ways around this, depending on exactly what the circumstances are. The most common is a DC to DC converter like this: http://www.solar-electric.com/12to24or24to.html

What exactly are you trying to do?
• Solar Expert Posts: 3,123 ✭✭✭✭
Re: Wiring battery bank for multiple DC voltages?
avanosch wrote: »
Can a single battery bank be wired to energize multiple grids at different voltages? Each grid would be wired to all batteries to distribute wear on plates uniformly, just varying in series / parallel, resulting in multiple voltage and amp hr grids.

Are there easier/less costly ways to have a multiple voltage DC grid?

Thank you

From the way that you describe it, I get the impression that you are talking about having one set of batteries, with, say, two sets of interconnecting cables, one that puts them in series and the other that puts them in parallel, and then connecting both sets of cables at the same time to get two different voltages. Electricity does not work that way because any two points connected by a wire will be at exactly the same voltage regardless of what other wires you add to the mix. All the setup that I think you are describing would do is short out all of the batteries.

If what you really mean is that you will periodically change the wiring to try to equalize the load on the each battery, then Coot's list of drawbacks and potential workarounds comes into play.
SMA SB 3000, old BP panels.
• Registered Users Posts: 3
Re: Wiring battery bank for multiple DC voltages?
Welcome to the forum.

The answer is "yes, but no". As in technically it can be done but it's not a good idea to do so.
The reason being tapping, say, 12 Volts off "one half" of a 24 Volt system inevitably means the two halves will have different degrees of discharge due to the unequal loading. as such they will not recharge equally as one battery bank. So the batteries that have to supply greater load will die sooner as they will not be fully recharged yet will be more deeply discharged.

There are ways around this, depending on exactly what the circumstances are. The most common is a DC to DC converter like this: http://www.solar-electric.com/12to24or24to.html

What exactly are you trying to do?

Thank you both for the reply.

As an example, (with tall battery terminal posts) could 4 12v 200ah batteries be wired to produce "grids" of 12v w/ 800ah, 24v w/ 400ah, and 48v w/ 200ah?

I tried to negate uneven discharge/ plate wear by utilizing all batteries in each voltage setup.

Maybe the highest voltage would propagate through everything?

I am trying to think through a 1-2k sqft off-grid home and workshop with ~5k w of solar to a [tbd] battery bank where different dc voltages would be needed for LED lights inside and out, water pump, peltier coolers (heats/ cools for fridge, climate control, atmospheric water generation) and home automation sensors, servos and blDC motors. There would also be 120v and 240v AC through 2 inverters off of the 48v battery "grid" if possible.
• Banned Posts: 17,615 ✭✭
Re: Wiring battery bank for multiple DC voltages?

There is no problem with wiring 2, 6, 12, or 24 Volt batteries in different configurations to increase the Voltage and/or Amp hour capacity.

There is a problem with wiring them for higher Voltage and then "tapping" the battery bank at some point to draw of at a lower Voltage for different loads.

Very often we get asked about this. Usually it is wanting 24 Volts for an inverter but also wanting 12 Volts from the same batteries to supply some 12 VDC loads. The problem is that one half of the battery bank is supplying power for both the 24 Volt loads and the 12 Volt loads while the other half is supplying power only for the 24 Volt loads. The result is a discharge imbalance which can not be corrected by charging both halves as a 24 Volt bank. It can be compensated for by charging as two 12 Volt halves, but there are problems with that as well. Not the least of which is that even if you do get the wiring right one half of the battery bank will "age" faster due to the higher load demands placed on it.

When designing an off-grid system, ignore the size of the place. Do not think in terms of "5kW of solar". Start with the loads. How big will they be at any given time? This will determine how large an inverter is needed. How many Watt hours will be used daily? That will determine how big the battery bank should be and what system Voltage to use.

As for needing lower Voltage for 12 VDC loads it is best to make an entirely separate system. This can either be completely independent of the inverter system, or can be charged from it either using an AC charger (not very efficient but inexpensive) from the inverter or using an MPPT controller from the larger battery bank (more efficient and more expensive).

Any time you have a higher Voltage battery bank and tap one or two batteries to feed lower Voltage loads you will have an imbalance in the batteries with resulting problems. How large the imbalance and how severe the problems will be directly proportional to how large the lower Voltage loads are. In other words using 12 Volts from a 48 Volt bank to supply a couple of Watt hours every other day would probably go unnoticed, but drawing a lot of Watt hours daily will cause problems very soon.
• Registered Users Posts: 3
Re: Wiring battery bank for multiple DC voltages?
There is no problem with wiring 2, 6, 12, or 24 Volt batteries in different configurations to increase the Voltage and/or Amp hour capacity.

There is a problem with wiring them for higher Voltage and then "tapping" the battery bank at some point to draw of at a lower Voltage for different loads.

Very often we get asked about this. Usually it is wanting 24 Volts for an inverter but also wanting 12 Volts from the same batteries to supply some 12 VDC loads. The problem is that one half of the battery bank is supplying power for both the 24 Volt loads and the 12 Volt loads while the other half is supplying power only for the 24 Volt loads. The result is a discharge imbalance which can not be corrected by charging both halves as a 24 Volt bank. It can be compensated for by charging as two 12 Volt halves, but there are problems with that as well. Not the least of which is that even if you do get the wiring right one half of the battery bank will "age" faster due to the higher load demands placed on it.

When designing an off-grid system, ignore the size of the place. Do not think in terms of "5kW of solar". Start with the loads. How big will they be at any given time? This will determine how large an inverter is needed. How many Watt hours will be used daily? That will determine how big the battery bank should be and what system Voltage to use.

As for needing lower Voltage for 12 VDC loads it is best to make an entirely separate system. This can either be completely independent of the inverter system, or can be charged from it either using an AC charger (not very efficient but inexpensive) from the inverter or using an MPPT controller from the larger battery bank (more efficient and more expensive).

Any time you have a higher Voltage battery bank and tap one or two batteries to feed lower Voltage loads you will have an imbalance in the batteries with resulting problems. How large the imbalance and how severe the problems will be directly proportional to how large the lower Voltage loads are. In other words using 12 Volts from a 48 Volt bank to supply a couple of Watt hours every other day would probably go unnoticed, but drawing a lot of Watt hours daily will cause problems very soon.

Got it. I drew out a wiring diagram and it does not look like the original concept would work as desired. I think I would need the batteries to each have multiple + and - terminals that are isolated from current backflow to compose the multiple voltage grids.
Re: Wiring battery bank for multiple DC voltages?

Unless you have "native" 12/24/48 volt loads--I would consider using AC inverter(s) to power your stuff. You may have ~85% efficiency--But compared with all of the other issues, the fact that you can use 120 VAC (or 230 VAC) to run the power to various uses--It is probably more cost effective and easier to maintain.

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Solar Expert Posts: 1,973 ✭✭✭
Re: Wiring battery bank for multiple DC voltages?
avanosch wrote: »

As an example, (with tall battery terminal posts) could 4 12v 200ah batteries be wired to produce "grids" of 12v w/ 800ah, 24v w/ 400ah, and 48v w/ 200ah?
No. You cannot wire batteries simultaneously both in series and parallel.
• Solar Expert Posts: 630 ✭✭
Re: Wiring battery bank for multiple DC voltages?

What about a voltage divider network? Used in small electronics all the time to tap of of 12vdc supply, how difficult would it be to scale up to solar?
Re: Wiring battery bank for multiple DC voltages?

A typical DC Voltage divider would be two resistors in series, pulling the voltage from the mid-point. The resistors would dissipate power, and they only accurately divide the voltage if the current drawn at the center point is a small fraction of the total current flow through the resistor.

The only way, that I know of, that you can draw both 12 and 24 volt power from a battery bank would be to use a DC to DC converter that can balance the voltage between the whole 24 volt bank and the 12 volt section. The DC to DC converter can recharge the 12 volt battery from the other 12 volt battery section--And since the DC to DC converter is bi-directional, a charging source on the 12 volt battery can recharge the other 12 volt section too.

Commonly used on RV's where there is a 24 volt battery to start the large diesel and run the vehicle electrical system, and still have 12 volts for the house loads.

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Registered Users Posts: 1
Re: Wiring battery bank for multiple DC voltages?

I have pondered this, while considering charging a 8 x 6v 12ah bank wired in 1s8p configuration at 6v in parallel, and discharging at 24v in a 4s2p config. Or if you want 48v 8s1p, 12v 2s4p. Or vice versa. The idea is for a backup dc source to a atx dc powered ps for an itx media server, and several tb harddrives attached as well. I use a y-pwr converter atm, that switches instantly to the highest voltage input, but its only rated for 10a at each input, and 20a output if the voltage sources are the same and its just paralleling two identical dc source voltages. With an a10 & asrock fm2a85itx, overclocked a bit my full load draw is 150w as measured with a killawatt. Thats the safe limit for my meanwell 16v 10a supply. I have a 24v 7a supply i want to upgrade to so i can draw that wattage safely without overloading the 150w supply, or the y-pwr by pulling more than 10a from one input. I'd rather not buy another y-pwr device (\$20 ea) and run it in parallel, since i have a ton of hardware lying around and a power supply i could use to charge and discharge the battery better by changing from at 1s8p, and discharging at 4s2p. Since i need the extra wattage, and have a 10a cieling. The y-pwr can take up to 30v input iirc, and the m4 atx supply is the same iirc.

What i want to do, is wire up the 24v supply i have to the y-pwr, and to a 6v charger. Charge the bank, and power the atx at the same time, and use a # of fused solid state relays (fast switching, hopefully fast enough or i'll add some caps to give it a bit of a buffer to provide uninterrupted transfer to battery power). Then have those relays when powered run a 6v parallel wiring config, and when the ac adapter turns off in the event of power failure, the relays pop over to 24v in microseconds or less, and i can draw more than ~130w off the battery bank (10a input limit on the y-pwr ideal diode input switcher).

Basically have a relay bank, fused of course, perhaps even diodes too, that is triggered to switch the bank from 8 x 6v parallel charging, to a 24v series/parallel discharge, at ac power loss.

Possible?

I dont see any feasable way to do multi voltage draw off a bank in tandem, but you could to that for two voltages, or depending on how much hardware you want to buy, more. Just might be alot of relays, fuses, diodes, and you can only use one at a time with manual switching on/off of certain relays.

Ideally, i would think parallel charging a bank at the lowest possible voltage, individual cells, and discharging in series at the highest voltage (less amperage draw) is the best way to do it. Then just fuse those 5,12,24,36v up to 10a regulators you can get on ebay to a output strip with on/off switches and go from there. Only way i see multi voltage loads being powered off a bank. It can only be wired at one voltage at a time, but you can switch that voltage manually with relays protected with fuses and diodes, that changes the wiring config to more/less series/parallel oriented, or higher voltage and low amp draw, or low voltage and high amp draw. The only advantage i see for lower voltages is for charging, since paralleled would keep the 8 x 6v bank more balanced.
• Solar Expert Posts: 1,973 ✭✭✭
Re: Wiring battery bank for multiple DC voltages?
eleseur wrote: »

Ideally, i would think parallel charging a bank at the lowest possible voltage, individual cells ... is the best way to do it.
I don't think so. In series charging, all the batteries get the same amount of current, while in parallel charging, even seemingly minuscule differences in cable resistance and battery internal resistance will result in uneven/unequal charging, which influences battery internal resistance, which exacerbates the uneven charging, etc. It's a positive feedback loop with negative results.
• Solar Expert Posts: 92 ✭✭
Re: Wiring battery bank for multiple DC voltages?
BB. wrote: »
Unless you have "native" 12/24/48 volt loads--I would consider using AC inverter(s) to power your stuff. You may have ~85% efficiency--But compared with all of the other issues, the fact that you can use 120 VAC (or 230 VAC) to run the power to various uses--It is probably more cost effective and easier to maintain.

-Bill

Keep It Simple Stupid or Murphy will show up!
• Registered Users Posts: 2
Not to change the subject but my issue is kind of simular. I have a 36v folklift battery that I have a 30amp charge controller for. I want to charge 36v and have 3 12v inverters off the three 12v cells. Is this possible without hurting the battery. I can charge at 30amp 12v but 30amp 36v will utilize my power better.
• Solar Expert Posts: 402 ✭✭
I'm kinda stepping into this a little late, but I'll throw my 2c into this from our experience here.  We've spent quite a few years building up our existing system, and in the process of this we've inadvertently reached a point where we have a mixture of DC voltages (12v, 24v, and 48v) in our system.  However, we don't typically have mixed voltages on a single system.  Only in a few situations do we have a need for both 12v and 24v, or inversely 24v and 48v off the same battery bank.  After much tinkering our solution ended up being as simple as adding a step down transformer to the lower voltage side.  Yeah, some will complain that it costs you too much energy considering the conversion losses, and I would agree.  But when you absolutely need a device, but you can't get it in the higher voltages, or in rare cases in the lower voltages, rather than risking your charge controller, and more importantly your batteries, it's better to just use a transformer to step the power up or down as needed.

It's still better to match all your devices to the voltage of your system.  But I understand that you can't always do that.  So in the end a small transformer is your best solution, at least that we've found, to resolve that issue.  And given that solar and wind are free energy, but your batteries and charge controller are not, it's better to spend your free pennies than your expensive ones, so to speak.  Purists might disagree, but it's worked for us.  Just be sure if you do take that route that the transformer you use is properly sized for what your load demand is or you'll lose tons of energy as waste heat.  In fact, if you need to, it may not hurt to oversize it quite a bit, because the less heat you generate, the less loss you'll have.
• Solar Expert Posts: 8,970 ✭✭✭✭✭
Whoa !!   you can't use a transformer to change DC voltages, you use a DC-DC Convertor.  It "acts" much the same overall, but if you are buying a part, you select a DC-DC convertor for the voltage and amps you need.
http://www.digikey.com/product-search/en?keywords=DC-DC
DC DC Converters (236142 items)   Power Supplies - Board Mount DC DC Converters Power Supplies - External/Internal (Off-Board) (1806 items)

Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
|| 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 ,

• Solar Expert Posts: 402 ✭✭
No, we have an actual block transformer.  You might call it a dc to dc converter, but we just call it a transformer.  It looks kinda like the transformers you see inside microwave ovens.  Kinda like this: http://p.globalsources.com/IMAGES/PDT/B1020327143/AC-DC-Power-Transformer.jpg  It's specifically built to step dc power up or down depending on which way you need it to go.  Works great for us.  Yours seems to be more electronically based which we don't have.  Even so it does the job and the losses it generates are minimal.

• Solar Expert Posts: 3,487 ✭✭✭✭✭

Not to change the subject but my issue is kind of simular. I have a 36v folklift battery that I have a 30amp charge controller for. I want to charge 36v and have 3 12v inverters off the three 12v cells. Is this possible without hurting the battery. I can charge at 30amp 12v but 30amp 36v will utilize my power better.

If you are suggesting that you would put an inverter across each group of 6 cells, 12V, and charge the 36Vbattery, the answer in my opinion  is no. The reason being is that you would be loading groups of cells within a battery, bringing some to lower capacity than others and result would failure of the battery. There is probably a long winded explanation but from experience and all I have read, it will not work, it would be the same as center tapping a 24V to achieve 12V.

1500W, 6× Schutten 250W Poly panels , Schneider MPPT 60 150 CC, Schneider SW 2524 inverter, 400Ah LFP 24V nominal battery bank
Second system 1890W  3 × 300W No name brand poly, 3×330 Sunsolar Poly panels, Morningstar TS 60 PWM controller, no name 2000W inverter 400Ah FLA 24V nominal used for water pumping and day time air conditioning.
5Kw Yanmar clone single cylinder air cooled diesel generator for rare emergency charging and welding.
edited November 2018 #19
Steven,

Do you have a link to the actual "block transformer" you guys use on DC circuits?

As far as I know, all transformers (traditional types) need a changing voltage so that the changing magnetic field can couple energy from the input coil to the output coil.

There is always some sort of circuit in the input to a "true" transformer to "chop" the DC voltage into something the AC transformer can step up/down/etc. Searching for "radio vibrator + transformer" will give you (by the way, dropping "radio" from the search term is not work safe ):

DC to DC converter/switching power supply:

https://www.maximintegrated.com/en/app-notes/index.mvp/id/2031

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Solar Expert Posts: 8,970 ✭✭✭✭✭
PLEASE stop calling a DC-DC converter a transf%@&amp!     It is an abomination to allow someone to think that such a thing exists and the marketing wiener that started calling it that should get the PT Barnum award this year.
Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
|| 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 ,

• Solar Expert Posts: 3,487 ✭✭✭✭✭
If Seven Lake has a ( blocking transformer ??) Perhaps  he could post a picture of the one he actually has, the microwave transformer pictures look like a simple AC transformer, can't work on DC, never has, never will, there has to be switching, which would make it a DC-DC converter, agree with Mike 95490 on that.
1500W, 6× Schutten 250W Poly panels , Schneider MPPT 60 150 CC, Schneider SW 2524 inverter, 400Ah LFP 24V nominal battery bank
Second system 1890W  3 × 300W No name brand poly, 3×330 Sunsolar Poly panels, Morningstar TS 60 PWM controller, no name 2000W inverter 400Ah FLA 24V nominal used for water pumping and day time air conditioning.
5Kw Yanmar clone single cylinder air cooled diesel generator for rare emergency charging and welding.