2V or 12V dilemma

[islandnorm]

AGM batteries

[islandnorm]

I guess that means it will take 20 hours at 20 A to charge the batteries. With the way I use my cabin (3 days on 4 days off) and a GenStart for my generator, I should be able to manage. Does the charge rate change if I go to a 24V system and 800Ah? As long as I'm able to pump whatever sun power I can into the system without "hurting" the batteries, I'm ok. I've added some monitoring products to make sure I keep the batteries up.

[Sunking]

Does the charge rate change if I go to a 24V system and 800Ah? As long as I'm able to pump whatever sun power I can into the system without "hurting" the batteries, I'm ok. I've added some monitoring products to make sure I keep the batteries up.

Yest but it changes everything.

a 24 volt 800 AH battery has a total capacity of 19.2 Kwh of which only 50% can be used. A 48 volt 800 AH has twice the capacity of 38.4 Kwh with only 50% available. So if you go to 24 @ 800 you loose 50% of your capacity but fix the charge problems.

You are going about this the wrong way. A off-Grid Battery system is designed around how many watt hours you use in a day, and your location. If done that way everything falls into place unless you live in a place like Seattle WS where the Sun Shine is rare as seeing Baptist Preacher there.

For example let's ay you need 5 Kwh per day, and live in Kansas City where the December/January Sun Hours = 3.2 Sun Hours, and you will use efficient MPPT Charge Controllers to minimize cost.

Right away we can calculate the battery capacity in Watt Hours then solve for Amp Hours later after we select the voltage. Based on the maximum discharge rate of 20%/day we just multiply the Daily WH x 5, so 5 x 5 Kwh = 25 Kwh battery capacity.

For the panel wattage we need to make a adjustment to account for over all losses using a MPPT system of 66% efficiency. 5000 wh / .66 = 7575 Watt Hours. Round that up to 8000 wh per day to make up for 1 cloudy day recovery. So to find the panel wattage take the 8000 wh / 3.2 Sun Hours = 2500 watts of solar panel power is needed.

OK now that we know the panel wattage needed we can select the battery voltage. At 2500 watts requires 48 volt battery system. So th erequired battery AMP Hours is 25,000 wh / 48 volts = 520 Amp Hours @ 48 volts.

Last step is to find the right MPPT charge controller. The maximum amperage for the panels is Panel Wattage / Battery voltage = 2500 watts / 48 volts = 52 amps. So a 60 Amp MPPT charge controller is needed.

Now noticed what happened automatically. We need a 520 AH battery and the charge current will be 52 amps or C/10 charge rate. C/10 is the perfect charge rate for flooded lead acid and AGM batteries. The range for FLA is C/12 to C/8 so C/10 is the perfect charge rate. AGM is C/12 to as high as C/4.

Now look at what you have. C/20. This means one of two things.

1. You do not have enough panel wattage
2. Your batteries are over sized.

What it really means is your system is not designed to do what it is intended to do. Make sense?

[islandnorm]

Sunking...I feel enlightened!

So for my system:

Required 2600Ah per day x 5 = 13000Wh

MPPT fudge factor 2600Ah/.66 = 3939Ah or 4200Ah for additional fudge

I'm going to use a sun hours of 4 since I use zero power Nov/Dec/Jan.

4200/4h = 1050W of panels which brings me to 24V system

13000Wh/24V = 542Ah of batteries needed at 24V

1050W/24V=43.75A for the controller

542Ah/43.75A = 12.4 h or C12 which is ok for charging AGM

OMG....I think I've finally clued-in!

Let me know if my numbers make sense.

IslandNorm

[islandnorm]

now I guess this would mean that 4 x 6V at around 500Ah at 24V would suffice. Please confirm.

[Sunking]

Required 2600Ah per day x 5 = 13000Wh

MPPT fudge factor 2600Ah/.66 = 3939Ah or 4200Ah for additional fudge

I assume you mean watt hours. Do not confuse Watt Hours with Amp Hours as they are not the same thing.

[Sunking]

now I guess this would mean that 4 x 6V at around 500Ah at 24V would suffice. Please confirm.

Yeah Norm you are on the right track now. Only problem I see is I doubt you are going to find 6 volt 500 AH AGM batteries. 2 and 4 volt units yes, 6 volt perhaps, but I do not recall seeing any.

[islandnorm]

Would 4 x 12 V @ 260Ah be an acceptable compromise in a serial/parallel configuration? I was thinking S12-290AGM. 2V is cost prohibitive. I haven't found the 4V 500Ah. The 6V I found was only 415Ah.

[Sunking]

Would 4 x 12 V @ 260Ah be an acceptable compromise in a serial/parallel configuration? I was thinking S12-290AGM. 2V is cost prohibitive. I haven't found the 4V 500Ah. The 6V I found was only 415Ah.

Would not be my preferred choice, can you can make it work.

[Mike90250]

All this work to stay for some reason at 12/24V. Batteries in parallel is a BAD plan. Consider again, upping the voltage to 48V, use 220Ah golf cart batteries (dirt cheap, and nearly everyone destroys the first battery bank) in series, and keep shopping around for the erplacement bank.

[Sunking]

. Batteries in parallel is a BAD plan..

I know Mike but I am getting tired of repeating myself. Folks just seem stuck in a 12 volt world and cannot think outside the 12 volt box.

Norm when it comes to batteries pricing is basically on a Watt Hour capacity, which is what you designed for. A good 5 year battery regardless of the voltage is going to cost around $140/Kwh.

A 12 volt 800 AH battery has the exact same capacity as a 48 volt 200 AH capacity battery. They should both cost roughly the same. The difference is with 12 volt you would have to use 4-200 AH batteries in parallel, or the exact same 4 batteries in series.

[Dagr8one]

Need a little help here

8kw system
8000 watt inverter 24v
800 watts of solar panels
500 Ah 24 v Battery bank?
40 amp mppt charge controller?


Loads
Washer 1150 (2hrs maximums a day)
Fridge 840 (intermittent all day)
Television 240 (5hrs continuos)
A/C 2300 (10hrs continuos)
Water pump 2900 (10 secs) 30 times a day

I am in the Bahamas so I have about ten hours of sun in the winter and thirteen hours of sun in the Sumer
Is this an adequate system??

[Naptown]

Ah no
Not by a long shot

[Dagr8one]

Can you elaborate??

[SunEagle]

Can you elaborate??

First off your system is not rated 8kw. You only have 800 watts of panels not 8000 panel wattage.

Simple answer as to why it won't work is that your total daily electric load in watt hours is very big. With that 500 Ah battery and 800 watts of solar panels you may be able to just run your tv for those 5 hours. To run anything else will require probably 25 times the amount of panel wattage and battery storage you currently have.