Battery System Voltages and equivalent power

Because this is one of the issues that's asked about repeatedly (in different ways) I thought I'd just outline the basics here for reference.

For comparative purposes, this example uses eight "golf cart" batteries of 220 Amp hours and 6 Volts, or 1320 Watt hours (DC) each. Times eight that is a battery bank capacity of 10,560 Watt hours, up to 50% of which would be "usable" for 5,280wh.

We will also assume a 10% charge rate (the array sizes would be equal), 1200 Watt load, and standardization of other factors (including ignoring losses) because the point here is to relate the differences between the three normal system Voltages: 12, 24, and 48.

First, the 12 Volt system:
This would be four parallel strings of two batteries in series. That's 880 Amp hours @ 12 Volts.
Charge rate: 88 Amps, necessitating either a MidNite Classic controller or two other types of MPPT controllers to handle the current or at least 2 PWM controllers without the efficiency of going MPPT if the voltages are right for it.
The wiring should have positive and negative bus bars, individual battery string fuses, and twelve interconnecting wires.
The 1200 Watt load will draw 100 Amps.
Problems: keeping current flow even through all batteries, handling the charging current, many connections (points of failure).

Second, the 24 Volt system:
This would be two parallel strings of four batteries in series. That's 440 Amp hours @ 24 Volts.
Charge rate: 44 Amps, which can be handled by one charge controller of many different types/brands.
The wiring can be done with the "diagonal" system. Individual string fuses can be used, but not as critical as with the 12 Volt set-up. Only eight interconnecting wires.
The 1200 Watt load will draw 50 Amps.
Problems: most of the problems of the 12 Volt configuration are eliminated. The issue of keeping current flow even is still there, but greatly reduced. Far fewer connections.

Third, the 48 Volt system:
This would be one string of all eight batteries connected in series. That's 220 Amp hours @ 48 Volts.
Charge rate: 22 Amps. Easily handled by any number of different charge controllers.
No wiring difficulties; no need for individual string fuses. Only seven interconnecting wires.
The 1200 Watt load will draw 25 Amps.
Problems: minimal. Current flow will be as even through all batteries as possible. Fewest connections.

The point here is to demonstrate that the higher system Voltage eliminates many of the problems that come from a need to store and handle larger amounts of power (all the battery banks in the example have the same equivalent power capacity).

Comments

  • CariboocootCariboocoot Posts: 17,615Banned ✭✭
    Re: Battery System Voltages and equivalent power

    Second Part: Two sixes or two twelves?

    Another Frequently Asked Question is whether it is better to use two 6 Volt batteries in series or two 12 Volt batteries in parallel on a 12 Volt system. Since this is related to the previous post (and can be derived from it if you analyze long enough) I've included it here.

    For this example we will use two different "theoretical" batteries, as we only need to compare the configuration differences.

    Two 6 Volt 200 Amp hour batteries in series:
    200 Amp hours @ 12 Volts.
    One battery interconnection.
    Even current flow through both batteries.
    Six cells to check.

    Two 12 Volt 100 Amp hour batteries in parallel:
    200 Amp hours @ 12 Volts.
    Two battery interconnections.
    Possible uneven current flow through both batteries, can be reduced by using proper wiring technique.
    Twelve cells to check.

    Of course the "cells to check" is dependent on them being flooded cell not AGM type, but it needs to be mentioned. Likewise using the diagonal wiring method (Smart Gauge method #2 http://www.smartgauge.co.uk/batt_con.html) virtually eliminates current sharing problems. But you can still see the difference between the two configurations.
  • raydiasraydias Posts: 68Solar Expert ✭✭✭✭
    Re: Battery System Voltages and equivalent power

    Hello Cariboocoot

    Could you add information on proper charge rates. Example if you have a 10 amps (12v) coming from your charge controller you wont get very far if you have a 500 amp (12v) battery bank. What's the proper % of the battery bank in charge capacity to properly charge a set size bank.

    thanks
  • CariboocootCariboocoot Posts: 17,615Banned ✭✭
    Re: Battery System Voltages and equivalent power
    raydias wrote: »
    Hello Cariboocoot

    Could you add information on proper charge rates. Example if you have a 10 amps (12v) coming from your charge controller you wont get very far if you have a 500 amp (12v) battery bank. What's the proper % of the battery bank in charge capacity to properly charge a set size bank.

    thanks

    The actual rate depends on the manufacturer's recommendations for the particular battery involved. For instance; as a rule flooded cell types have lower current limits than AGM's. But within any given battery type there are ranges and limits. Some of it has to do with the amount of time involved; a higher rate for a short time on occasion will not be as damaging as a consistently high (or low) charge rate.

    For more information on battery charging, check the deep cycle battery FAQ's here: http://www.windsun.com/Batteries/Battery_FAQ.htm
  • CariboocootCariboocoot Posts: 17,615Banned ✭✭
    Re: Battery System Voltages and equivalent power

    An additional note on choosing system Voltage.
    The largest available charge controllers at this time can handle 80 Amps output at a given Voltage. If you consider the rule-of-thumb 10% peak current and 25% DOD that would limit the battery bank for a single controller to 800 Amp hours. Thus we get stored power of:

    12 Volts @ 800 Amp hours = 2.4 kW hours DC @ 25% DOD
    24 Volts @ 800 Amp hours = 4.8 kW hours DC @ 25% DOD
    48 Volts @ 800 Amp hours = 9.6 kW hours DC @ 25% DOD

    Above those numbers and you are needing more than one charge controller.
  • waynefromnscanadawaynefromnscanada Posts: 3,009Solar Expert ✭✭✭✭
    Re: Battery System Voltages and equivalent power

    And adding to all this is the hindsight folly of starting out with a small 12 volt system, not realizing how it would grow in future years, and end up painted into the 12 volt corner with no way out unless one has a good credit rating - - not to mention the extra $ poured into the 12 volt system (extra controllers, heavy cables etc) to make it work. Oh that I could have seen the future. Yes, my system works and works well, but it would have been far, far better in all ways had I gone 24 volts from the start.
    Just didn't have a clue where my addiction would lead. Or even that I would get addicted. Some learn early, others learn late.
  • nielniel Posts: 10,311Solar Expert ✭✭✭✭
    Re: Battery System Voltages and equivalent power

    so carry this further as many don't realize their inverter voltage is holding them back more than the inverters ability to handle large wattages like 4kw or 6kw. the inverter industry generally follows a rule of thumb of about 12v being inputted for roughly every kw of inverter power handling. this rule of thumb doesn't work very well for us as it makes it difficult to plan for any real autonomy from ones battery bank.

    for example a 1kw 12v inverter can generally have up to 2 parallel banks of batteries. ok it may be at its best for common batteries to have 2 banks of 2 6v l16s in series for roughly 700ah-800ah total. 1/2 of it is useful at 350ah-400ah or 4200wh to 4800wh. over the course of 24hrs this translates to 175w to 200w consistently used. design for 2 or 3 days and forget about it as you are now restricted on an hourly basis to 1/2 to 1/3 of that consistent power amount drawn. who needs a 1kw inverter to consistently deliver 60w-70w, it's dumb. note here that a 300w morningstar inverter can handle the continuous power needed here.

    you might say so what as that's a 12v inverter, but this is paralleled up to the 4kw 48v inverters. you have 4x the voltage now with 4x the inverter power output. load capabilities go 4x what i stated as well due to 4x the battery capacity. autonomy still sucks. smaller more common battery banks are even worse and going to the 2v l16 is no answer as they are paralleling 3 cells within each 2v battery so you are seeing your max here with 6v l16s.

    answer that everybody knows and has asked for is higher inverter voltages from the industry. only common inverter company that has a few higher voltage models we've been dealing with here that i know of off hand is exeltech. a 66v model and a 108v model. the series strings will be much larger and afford more operating time. now maybe there are other batteries that can up your ah capacity, but these are often uncommon or specialty batteries that are extra costly in many cases.

    this is why many have hounded morningstar for a 24v model inverter even though it's only a 300w inverter. and i asked midnite too and was pooh poohed by them as they said they won't design any for higher voltages and being told to go buy an exeltech. the inverter industry needs to change that unspoken rule of thumb as we need to be able to design our systems with more battery power capacities than you allow for.
  • CariboocootCariboocoot Posts: 17,615Banned ✭✭
    Re: Battery System Voltages and equivalent power

    Yes: addressing the issue of the increased efficiency for the same amount of power at higher Voltage, less current is also lacking. Current creates heat and that heat is power lost; it is not doing any useful work. So if the power required is delivered as more Voltage and less current it becomes more efficient.

    This is true both of in-use power (Watts) and stored power (Watt hours) just as Niel said.
  • nielniel Posts: 10,311Solar Expert ✭✭✭✭
    Re: Battery System Voltages and equivalent power

    i think you may have missed my point. if say a string of batteries were allowed to go to 72v instead of 48v like the inverter industry likes we can have more batteries in series for each battery bank. max of 2 banks remember. this equates to more power available.

    with the l16 examples i was citing and battery bank with 2 banks of 6v l16 batteries at 400ah it looks like this;
    48v x 400ah x2 banks = 38,400wh with 19,200wh useable.
    72v x 400ah x2 banks = 57,600wh with 28,800wh useable.

    the 2nd can deliver more watts over a day than the 1st one and gives better autonomy. the classic can already accommodate such a battery bank so that aspect is covered and you are right as it will be efficient, but more importantly it would allow the same 4kw capacity inverter to have operations for longer periods of time with more battery power. also the cc will be able to handle more in pv before a 2nd cc is needed.
  • CariboocootCariboocoot Posts: 17,615Banned ✭✭
    Re: Battery System Voltages and equivalent power

    Niel;

    I understand it. I was just adding to it. Your 72 Volt inverter is an extension of the improvement a 48 Volt unit enjoys over 24 Volts.

    Quite some time ago (years in fact) I advocated 120 VDC inverters. Ten times as good as 12 Volt. :D
  • ThomThom Posts: 178Solar Expert ✭✭✭
    Re: Battery System Voltages and equivalent power

    I just got rid of a 120v dc bank. It was installed in 1982 . 20 golf cart batteries. It was connected to a windmill. Glad to see it go.
    Installed 280w panel , Kid controller,2 golf cart batteries ,300w inverter .

    Thom
    Off grid since 1984. 430w of panel, 300w suresine , 4 gc batteries 12v system, Rogue mpt3024 charge controller , air breeze windmill, Mikita 2400w generator
  • nielniel Posts: 10,311Solar Expert ✭✭✭✭
    Re: Battery System Voltages and equivalent power
    Thom wrote: »
    I just got rid of a 120v dc bank. It was installed in 1982 . 20 golf cart batteries. It was connected to a windmill. Glad to see it go.
    Installed 280w panel , Kid controller,2 golf cart batteries ,300w inverter .

    Thom

    i guess you would be glad to see it go as your needs are small indicated by the solar you got + inverter wattage size. my point was one many do share though as inverter wattage size can be misleading as to one's long term needs. if you needed say 200w constant draw on average over 24hrs you would have need of 4,800 watt hours. on a 12v system this is 400ah and the batteries would need to be sized at 800ah to not exceed 50%. remember the inverter could be run at 300w too upping those insane ah requirements. this is why many are asking for a higher voltage model in order to cut the ah requirement. that ah requirement means too many paralleled batteries or an excessively expensive oddball battery setup just to run the inverter at a consistent 200w at 24hrs. now if one's needs are higher or longer autonomies needed then you can see the dilemma developing with too many ah needing to be paralleled which is too difficult to do with most batteries.

    in the example of that 300w inverter drawing 200w on average per hour over 24hrs if one had a 24v model it would've represented a 200ah draw over that 24hr period and a battery bank of 400ah for the 50% dod buffer which is in fact very manageable. this could be 2 banks of 4 gc batteries in series per string for a total of 8 gc batteries.

    now if they could only make batteries more reasonably priced? different subject matter.
  • cryosphearcryosphear Posts: 6Registered Users ✭✭
    Hi. I hope someone can shed some light as I'm a total noob with regards to my solar system. I have a 10kva 48v inverter which is hooked up to 8x 12v batteries. I included a photo. Now what I want to do is hook up 4 more of the same batteries. When I add 2, system functions but doesn't charge, when I add 4, system either trips or shows Low Battery when switching on. Batteries are connected in parallel.
  • cryosphearcryosphear Posts: 6Registered Users ✭✭
  • BB.BB. Posts: 27,444Super Moderators admin
    What are the batteries themselves? 12 volts? xxx Amp*Hours? Gel/AGM?

    Basically, each string of batteries needs to "add up" to your battery bus voltage.. 48 volts in the case of your inverter (and chargers)?
    • 48 volts / 12 volt batteries = 4x batteries in series (12 volt batteries)
    And then a second string would be 2 parallel strings of 4x 12 volt series batteries. So, your battery bank would either be 4, 8, 12, etc. batteries in series parallel.

    That is the basics, there are more issues to discuss.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • EstragonEstragon Posts: 2,327Registered Users ✭✭✭✭
    Hard to say for sure from the photo, but it looks to me like the batteries are some sort of sealed type (AGM or gel?), and at least some appear to be wired in series (+ive to -ive). The wire and connections look undersized to me for a 10kva inverter, and I don't see any string fusing (needed with >2 strings).

    As Bill said, there's no good way to add 2 x 12v batteries to a 48v system, and more issues to discuss. I'd recommend not adding to the bank until these are addressed.
    Off-grid.  
    Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
    Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter
  • cow_ranchercow_rancher Posts: 86Solar Expert ✭✭✭
    From the picture, it appears to be two banks of 12 volt batteries, six in each back for a 72 volt battery bank.

    The jumpers are way too long, probably too small, and should never cross over one another.

    Rancher
  • cryosphearcryosphear Posts: 6Registered Users ✭✭
    Morning all. Yes, it is 12x 12v sealed lead batteries. csb cs3 HRL 12390w. 12v 390w/Cell/1.67V/15min. Pic also inverter
  • cryosphearcryosphear Posts: 6Registered Users ✭✭
    I have a total of 16 batteries. 8 is currently connected. I don't have to connect all 16 but have found 8 is not enough when it's a rainy day. The guys who installed it methinks are not up to scratch with how to do it correctly. Side job of theirs.
  • PhotowhitPhotowhit Posts: 4,544Solar Expert ✭✭✭✭
    I like to take things back to basics so perhaps you can have a greater understanding.

    All lead acid batteries are about 2 volts.
    A 12 volt "battery" is actually a string of 6 - 2 volt cells

    So what you have before adding anything, if setup correctly is a 48 volt battery bank made up of 2 strings of 24 cells.

    Now the cells may be already in a string that is being called a 12 volt battery, but your goal is creating a 3rd string to add to the 2 strings you already have setup.
     
    More than 2 strings becomes tricky, You want to have equal resistance through each string. The easiest way is to have each string connected with the same length and gauge(thickness) of wire. and attach each to a single post or Bus bar and draw your energy from the busbar. Like this but with only 3 strings.



    Looks like you have added batteries to strings that were already at the correct voltage and created a problem.


    It might be worth looking at how you are charging and what the problem you are having is. It might be several things having nothing to do with the battery banks capacity. Could be under charging, or too small wiring for the load, it could be that you have a bad battery in one of the strings...

    Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Prosine 1800 and Exeltech 1100, ForkLift battery. Off grid for @13 of last 14 years. 1000 watts being added to current CC, @2700 watts to be added with an additional CC.
  • EstragonEstragon Posts: 2,327Registered Users ✭✭✭✭
    It may be useful to check the voltage of each battery individually. With no loads or charging sources for several hours, a fully charged 12v battery should have a resting voltage of ~12.8v. If any are found to be more than ~0.1v lower, they should be charged individually and left to rest (or better yet, load tested).

    As well as the proper wiring Photowhit explained, it can help to start out with the batteries in a balanced state of charge and health.
    Off-grid.  
    Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
    Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter
  • bill von novakbill von novak Posts: 731Solar Expert ✭✭✭✭
    I have a total of 16 batteries. 8 is currently connected. I don't have to connect all 16 but have found 8 is not enough when it's a rainy day. The guys who installed it methinks are not up to scratch with how to do it correctly. Side job of theirs.
    Connect all 16.  You do not do a battery any good by leaving it disconnected, unless it's left on a float charger/maintainer.  And as others have mentioned, shorten the cabling, go to equal length connections and go to a heavier gauge.
  • cryosphearcryosphear Posts: 6Registered Users ✭✭
    Thank you all! I figured out from your posts what I did wrong. I connected 2 batteries to the string of 4 batteries in series so more than 48v. I parallel connected them now I have 3x strings of 4 batteries in series. And went for heavier gauge wiring. You all deserve a beer if you partake!
  • littleharbor2littleharbor2 Posts: 915Solar Expert ✭✭✭✭
    When the time comes to replace your batteries you might want to look into Some 6 volt L-16 sized batteries. You can replace all those batteries with one string of 8 batteries. Much better and simpler configuration.  Sure to outlast this multi string and cable menagerie. 

    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.

  • PhotowhitPhotowhit Posts: 4,544Solar Expert ✭✭✭✭
    When the time comes to replace your batteries you might want to look into Some 6 volt L-16 sized batteries. You can replace all those batteries with one string of 8 batteries. Much better and simpler configuration.  Sure to outlast this multi string and cable menagerie. 
    These are a different creature, designed to live in float and be discharged over a very short period. I suspect for short power outages. My guess is he's getting these used from a telecommunication industry?

    They are 72 lb lead acid batteries, and while they don't give a c/20 discharge capacity. their capacity for 15 minutes is 34 amps!
    http://www.csb-battery.com/english/01_product/02_detail.php?fid=8&pid=38

    I too would suggest a better type of battery for his use, but would imagine he is getting these for free?
    These are designed for UPS systems. If he's paying for them, he's not getting the right type of battery IMHO.
    Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Prosine 1800 and Exeltech 1100, ForkLift battery. Off grid for @13 of last 14 years. 1000 watts being added to current CC, @2700 watts to be added with an additional CC.
  • cryosphearcryosphear Posts: 6Registered Users ✭✭
    I use it to power my cozy 78 square meter log cabin. Gas stove and geyser. LED lights. But have dishwasher and washing machine that I only operate during the day (8x 325w Canadian solar panels). I paid roughly $90 per battery. When time comes to replace them I would be financially able to buy very decent batteries with a better setup than I currently have. Money is a bit tight atm so hopefully they will last at least 2 years. Will be finished with my studies in the middle of next year (South Africa) shortage of teachers here so will get a job fast.
  • Raj174Raj174 Posts: 589Solar Expert ✭✭✭✭
    edited June 6 #27
    So as others mentioned, these are UPS batteries. The deeper you cycle them the shorter their lives will be. I would recommend not taking them below 80% state of charge, maybe 49.8 volts under load. And as bill von novak suggested, add the the last four batteries to the bank. This will help avert deep cycling. Good luck with them!

    Rick
    12 x 300W Renogy PV, MNE175DR-TR epanel modified, MN Classic 150, Outback Radian GS4048A, Mate3, 54.4V 207AH HI Power LiFePO4 no BMS, 4000W gen.
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