2V or 12V dilemma

[islandnorm]

I was hoping for some guidance with respect to batteries.

I've been running 12V to my 2800W inverter but it's time to change the batteries. Winter temps can dip to -25C.

I originally had my heart set on 8 X 12V X 200 AH @ 20HR (MK 8A4D AGM batteries) at 1.75 volts per cell

My local supplier is recommending 6 X 2V X 500 AH AGM batteries with the following specs:

2 volts DC 500 Ah @ 20 hour rate 26.2 Ah continuous for 20 hours to 1.85 volts @ 20 - 25 C
@ 10 hour rate 49.4 Ah continuous for 10 hours to 1.85 volts @ 20 - 25 C
@ 8 hour rate 59.3 Ah continuous for 8 hours to 1.85 volts @ 20 - 25 C

So I don't mind accepting a little less total AH with 2V batteries and I think I can probably get more cycles out of them but I'm not convinced with the final comparison I was given from the supplier.

Their final numbers (to compary apples to apples, so they say) are as follows:

6 X 2V will give me 480 minutes at 60A (bringing the 2V cell down to 1.85 VPC)
8 X 12V will give me 520 minutes at 65A (bringing the 12V batteries down to 1.75 VPC)

Is he right? Based on the information I was given, I still feel the 8 X 12V cells will give me way more power (3X)! Any insight into this would be great.

Thanks

IslandNorm

[Naptown]

If you continue at running that system a 12 V the 2 volt batteries in series will be a longer lived bank
The problem is running batteries in parallel is difficult to even charge/discharge to be even across the batteries. Series will last longer.
At that size though you should consider a switch to 24V. You will be reducing the AH draw on the batteries and will get more out of them. (Peukerts law is in force here)

[Sunking]

The very last thing you want to do is parallel 8 - 12 volt batteries, to achieve a 12 volt 1600 AH. It is even sillier to run at 12 volts 1600 AH.

Your dealer is on the right track but both of you are way off.

• With 8 -- 12 volt 200 AH batteries in parallel = 12 volt @1600 AH
• With 6 - 2 volt 500 AH gives you 12 volt @ 500 AH

Those two setups are not even remotely close to having the same capacity. 1600 AH has over 300% more capacity than 500 AH

The point your sales man was making is valid, you want only 1-string of batteries. So if you really want 12 volts @ 1600 AH you would buy 6 - 2 volt 1600 AH cells.

FWIW doing this is foolish at 12 volts as this screams for a 48 volt battery @ 400 AH

[islandnorm]

Thanks for the advice. All good info.

I've decided to go 24V, which means new inverter (Magnum MA4024PAE from the MA2812 currently) and possibly upgrage my panels from 2x135W Sharp to 2x270W. I guess I'll need to change the controller as well. Any recommendation would be great keeping in mind I deal with snow and cold during the winter months.

Now for my battery decision....am I OK with the 8 X 12V in a 24V configuration? It's out of the question to buy 12 X 2V for me. I was thinking MK 8A4DLTP-DEKA 12V 198Ah AGM battery. Suggestions? I would like about 800 Ah at 24V while considering the overall life expectancy of the batteries.

Thanks

IslandNorm

[Mike90250]

You can lead a horse to water, but you cannot make it drink.

Please review the adivce sunking gave you, before you purchase anything.

[Sunking]

I've decided to go 24V, which means new inverter (Magnum MA4024PAE from the MA2812 currently)

Well that is a decent start, but with what follows still screams 48 volt battery.

and possibly upgrage my panels from 2x135W Sharp to 2x270W.

Now for my battery decision....am I OK with the 8 X 12V in a 24V configuration? It's out of the question to buy 12 X 2V for me. I was thinking MK 8A4DLTP-DEKA 12V 198Ah AGM battery. Suggestions?

OK Norm here is where you are really making poor decisions. I am not trying to pick on you, I am trying to keep you from making a huge mistake. You are making two huge mistakes

First as I stated earlier you only want to make 1 string of batteries, and avoid parallel strings if at all possible. It is always possible to avoid parallel strings. That is what your salesman was trying to tell and warn you about. You are asking for big trouble. I will not go into all the technical details but rather tell you what you need to know. If you go with multiple parallel strings like you want, you will be replacing then in a year or two. Is that what you want to do. Do it correctly and they will last 5 to 7 years. Your choice.

If you need 24 volt @ 800 AH, then buy 2 or 4 volt cells @ 800 AH. So for example if using 2 volt 800 AH cells you would need 12 of them wired in series for 24 volts.

Now let's move on to your other huge problem, solar panel wattage. So you want to use 2 Sharp 270 watt panels which is a total of 540 watts. That is only enough wattage to handle a about a 24 volt 250 AH using a MPPT charge controller or 24 volt @ 180 AH using a PWM controller. You are planning on 24 volts @ 800 AH. It is not going to work and you will destroy the batteries. With a 24 volt 800 AH battery you need a minimum panel wattage of 1600 watts using a 80 amp MPPT Controller, or 2400 watts with a PWM Controller. I advise you either up the panel wattage, or lower the battery capacity, because what you are planning is a disaster.

Ball is in your court, choose wisely and good luck.

[islandnorm]

Thanks for all your help. I'll tweek my power requirement estimates and add more panels.

IslandNorm

[islandnorm]

OK so I've run some numbers.

Solar Insolation 5.7 kWh/m2/day

Load with MPPT = 2600 Wh X 1.6 = 4160 Wh

Panels required 728 W (3x240W Sharp planned)

I have a 5000 W generator I will add a GenStart so my batteries don't go lose more than 30% of their charge.

So can I go with a 24V or 48V inverter? From the Solar Off-Grid Battery Design discussion I don't seem to have enough panel wattage but I can use the generator to top up the batteries when necessary. I would like to make use of 8 x 12V 200Ah batteries at my disposal and I'm certainly understanding the pitfalls...if I go 24V that means 4 x 24V arrays in parallel...for 48V it's 2 x 48V arrays in parallel (but that's better). Any suggestions?

[Sunking]

OK so I've run some numbers.

Solar Insolation 5.7 kWh/m2/day

Excuse me but there is no way on earth your get 5.7 sun hours in Ontario Canada. For off grid battery systems you design for worse case months which for you is November/December of 2.04 Sun Hours.

If you use 5.7 you will be dark with no power and dead batteries from August to March. Or you could run your generator every day from late August to March.

[islandnorm]

I'm not there Nov/Dec/Jan so the 5.7 is right. You can check the results for North Bay Ontario with 1-axis tracking.

[islandnorm]

Here are the numbers for my position:

"Station Identification"
"City:","North"
"State:","ON"
"Lat (deg N):", 46.35
"Long (deg W):", 79.43
"Elev (m): ", 358
"Weather Data:","CWEC"

"PV System Specifications"
"DC Rating:"," 4.0 kW"
"DC to AC Derate Factor:"," 0.770"
"AC Rating:"," 3.1 kW"
"Array Type: 1-Axis Tracking"
"Array Tilt:"," 46.3"
"Array Azimuth:","180.0"

"Energy Specifications"
"Cost of Electricity:"," 0.1 CanB/kWh"

"Results"
"Month", "Solar Radiation (kWh/m^2/day)", "AC Energy (kWh)", "Energy Value (CanB)"
1, 3.98, 416, 0.36
2, 5.94, 542, 0.47
3, 7.37, 720, 0.62
4, 7.06, 653, 0.56
5, 7.70, 705, 0.61
6, 6.85, 593, 0.51
7, 7.75, 691, 0.60
8, 6.81, 604, 0.52
9, 5.63, 496, 0.43
10, 4.50, 418, 0.36
11, 2.31, 210, 0.18
12, 2.59, 263, 0.23
"Year", 5.71, 6313, 5.44

[Sunking]

OK now go back and use 50% efficiency. You used 77% for a grid tied system

[russ]

OK now go back and use 50% efficiency. You used 77% for a grid tied system

In PV Watts you have to change the derate factor to 50 or 55% for an off grid system.

[islandnorm]

Sunking...I got the hint and went for the 48V (8X6V @ 400Ah in series) solution. I added a 4048 Magnum inverter. I also upped my panels to 4X240W Sharps from the measly 2X130W panels I had.
thanks for your input
IslandNorm

[Sunking]

Norm you are welcome but you still might have a problem on your hands.

A 960 watt solar array at best will generate 20 amps of charge current which is only a C/20 charge rate on a 400 AH battery. Are these AGM or flooded batteries?