# Another Charge controller choice thread

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## Comments

292✭✭Books incoming, but I have a couple of real basic questions.

If I have a total of eight,6V, 225ah batteries that are running in a 48v configuration, that would be a series connection and the total Ah would be 8*225 for a total of 1800ah. Is this correct?

If I have four 12V, 225Ah running in a 48V config that would be 900Ah?

Or does the amp hour not change when in series?

33,327adminNo--When you connect batteries in series, their voltage adds, but the AH Capacity remains the same:

- 8 batteries * 6 volts = 48 volts
- 1 series path for current through a 225 AH Battery = 225 AH @ 48 volts

No, again, You are adding voltage with series connections ONLY. The current is not adding as the "same electron" travels through the entire path of 48 volt batteries.Think of each battery as a 12 PSI "pump" with a certain capacity of 225 Gallons per Hour.

When you put the 4x pumps in series (outlet to inlet, etc.)--You add the pressure to 48 PSI--but the ability to flow water is still 225 GPH.

Correct--With one set of batteries, you EITHER add voltage (when connected in series), OR Add current (when connected in parallel).

So, as the example above--take those 4x 12 PSI @ 225 GPH pumps and connect them in parallel (outlet to outlet, etc.)--The maximum pressure is still 12 PSI--but now the output is 4x 225 GPH or 900 GPH total flow from the four parallel pumps.

Note, we are still following the conservation of energy... Back to 8x 6v @ 225 AH batteries in a parallel or series connection:

- Work = Power * Time (hours in our case) = Volts * Amps * Hours
- Work = 8x batteries * 225 AH (parallel connection) * 6 volts = 10,800 Watt*Hours
- Work = 8x batteries * 6 volts (series connection) * 225 AH = 10,800 Watt*Hours

So, with that sized battery bank, if you had a 150 watt light bulb (or desktop computer, motor, etc.) load:[1,800 AH at 6 volts][225 AH at 48 volts]- 10,800 Watt*Hours / 150 Watt Load = 72 hours (very approximately)

Note that battery equations are not as exact as I showed above... The amount of "apparent stored power" depends on lots of things (temperature, how much current you pull, battery type, battery construction, age, etc.).For for a starting point, that is how the math works.

-Bill

292✭✭deleted prior quote so I wouldn't screw myself up later.

292✭✭When this thread is finished I will take all the basics and put it into one post for other noobs.

33,327adminPerfect.

You are just shuffling the batteries around and will end up with the same amount of "work" at the end... Either at 48 volts @ 225 AH or 6 volts @ 1,800 AH (or something in-between like 12 volt or 24 volt battery banks).

Higher voltage battery banks use less current for the same amount of power--So, they are typically easier and less costly to wire.

Plus placing a lot of batteries in parallel has its own issues of current sharing and trying to monitor for weak/dead cells, corroded connections, extra fuses/breakers, etc.

-Bill

PS: Sorry, there were a few stupid typos scattered about in post #34 -- Fixed most of the ones I could find. Tbbssbss--Post #33 is my stupid post--Not 34.

292✭✭Sticking with my back-to-basics thinking here.

If I have four, 12v 225ah, in series, that gets me 48v at 225ah. If I were to add four more of the same, and I wired them in parallel with the existing ones I would increase my AH to 450.and it would have to be the same number of batteries because only adding one would only get me a 25% increase in AH? Losses in wiring excluded.

Is that correct?

post 34 was mine, lol, I went looking for what I did wrong.

33,327adminYes.

In practice, no, that is not the reason...

if you have 4x12 volt batteries--that sets 48 volts as your bank voltage.

If you have one more 12 volt battery---You could add to the 4x in series already and it would "work fine". But, your new battery bank voltage is now 60 VDC... A very uncommon voltage for Off Grid Folks (I have seen specs. for 60 VDC telephone switch gear).

The only way you could parallel a 5th 12 volt battery would be to put in in parallel with one of the other four batteries (+ to +, - to - minus).

That would double the AH capacity of that one battery in the series string of "12 volt batteries)... But since the other three batteries are still the original AH rating--they can not make use of the extra AH of the one extra 12 volt battery.

If you tried connecting your 12 volt battery directly to the 48 volt battery bank--bad things would happen. The 48 volt battery bank will try to charge your 12 volt battery to 48 volts--Very large current flow (into the 12 volt battery) and your 12 volt battery would probably try to melt down/catch fire (depending on the size of the battery bank and wiring).

I am very sorry--Even my clarifications tonight are full of mistakes.

-Bill "Time to go to bed" B.

292✭✭what does c/8 mean? What does C/2.5 mean? Why do you use 1/8 continuous current as part of the equation?

33,327adminC is the battery's capacity in Amps*Hours... We use the 20 Hour rate as being close to what most people with off-grid power needs use as a discharge rate...

C @ 20 Hour Rate means that you can discharge a 225 Amp*Hour battery with a fixed 22.5 amp load and it will take 20 hours to discharge the battery bank.

That 225 Amp*Hour battery capacity will "measure" larger if you discharge at the 100 Hour Rate... Or be less than 225 AH if discharge at a 5 Hour rate (batteries have some internal resistance and chemical reaction issues--At the higher discharge rates, the batteries are less efficient as the discharge, or charge).

Here is a very nice set of web pages that go into the whole issue of batteries, capacities, and such:

Smart Gauge Technical Pages

Regarding C/8, etc... Some rules of thumb for generic batteries (flooded cell storage batteries) are that we take the rated battery capacity (20 Hour Rate) and then take a fraction of that amount for rules about charging and discharging. Here are the ones I use:

- C/20 (5% of battery Amp*Hour capacity) is the minimum recommended rate of charge for an Off-Grid system. Also, roughly the average rate of discharge used by the mythical "average" off-grid application).
- C/10 (10%) roughly the maximum continuous rate of discharge--battery will last ~5 hours at that load to 50% maximum discharge--try not to go below 50% State of Charge for day to day operations. An never below 20% State of Charge or you may damage your battery bank. Also 10% is the minimum/optimum rated charging current for some brands of storage batteries (tall cased storage batteries may mix electrolyte better with 10% rate of charge).
- C/8 (12.5 or 13%) usually the maximum continuous rated discharge current for standard storage batteries. Continuous discharge rates in excess of C/8 may overheat the battery bank (flooded cell type).
- C/2.5 (40%) maximum recommend surge current. Surge current in excess C/2.5 may collapse the battery output voltage (especially as the battery is discharged).

So, the C/8 I used in the inverter example was a "sane" amount of current draw from a flooded cell battery bank of XXX Amp*Hour capacity at YY volts.For example, a 6 volt 225 volt flooded cell golf cart battery. Put 4 of them in series for a 24 volt 225 AH battery bank.

The maximum continuous inverter output to design for would be ( Capacity/8 ):

- 24 volts * 225 Amp * 1/8 * 0.85 inverter efficiency = 574 watts

And the maximum heavy starting surge/short term power would be C/2.4):- 24 volts * 225 Amp * 1/2.5 * 0.85 inverter efficiency = 1,836 watts

Make sense?The above are just numbers out of a hat--we generally work the design process "backwards" from your loads--define the battery bank, inverter sizing, solar array sizing, etc. using these various rules of thumb to make for the "quick and dirty" calculations.

Getting closer than 10-20% with more accurate numbers is a lot more work and of dubious value... Just too many variables for 2% calculations and even if we did get that accurate--changes in weather, temperature, battery lots/age/temperature/etc. all make those 2% calculations pretty much 10-20% accurate anyway.

I try to error on the conservative side so your system will, at least, meet your needs.

-Bill

292✭✭I just re-read this thread. It makes so much more sense now.

8✭It'll make even more sense once you start putting things together

I had a total brain fart when it came to the battery CHARGE voltages mentioned above. I bought 6 - 200 watt panels and said to myself "1200 watts is pretty good, my Xantrex mppt will handle 60 amp at 150Volts, I'm stylin". After a day of wondering why the power being produced never went over 860 watts, I looked at the display of 14.4V... and gave myself a palm slap to the forehead! DUH!

14.4V x 60Amax = 864 watts

This is why the $1/watt thin film doesn't seem like it will work for me, being off grid. The voltages are so high you can't parallel more than 3 strings. And the current is so low it would take 60! panels to get 60 amps, unless I'm misinterpreting the specs, which is highly possible.

17,615✭✭✭Yeah the input/output confusion!

It'll handle 60 Amps - at 12 Volts, 24 Volts, or 48 Volts

output. That's about 720 Watts, 1440 Watts, or 2880 Watts respectively.Then you have all those "nuance" (or perhaps "nuisance") factors: 12 Volts charges at 14.2, panels don't actually put out their nameplate rating, efficiency losses through wire and equipment, bleah!

With 1200 Watts behind a "12 Volt" system you should be able to "max out" a 60 Amp controller.