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 Banned Posts: 17,615 ✭✭
    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 Solar Expert Posts: 68 ✭✭✭✭
    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 Banned Posts: 17,615 ✭✭
    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 Banned Posts: 17,615 ✭✭
    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 Solar Expert Posts: 3,009 ✭✭✭✭
    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 Solar Expert Posts: 10,311 ✭✭✭✭
    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 Banned Posts: 17,615 ✭✭
    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 Solar Expert Posts: 10,311 ✭✭✭✭
    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 Banned Posts: 17,615 ✭✭
    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 Solar Expert Posts: 169 ✭✭✭
    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 Solar Expert Posts: 10,311 ✭✭✭✭
    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.
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