how to set up my bank to desired ah's

[captianjuan]

I want to build a 12v, 1200ah bank with the possibility to expand. should I use three 400ah cells or four, 300ah cells? could you tell me the pros cons of these set ups. im new to all this and appreciate any info you may have.
thank again.

[boatpoker]

Originally Posted by captianjuan

I want to build a 12v, 1200ah bank with the possibility to expand. should I use three 400ah cells or four, 300ah cells? could you tell me the pros cons of these set ups. im new to all this and appreciate any info you may have.
thank again.

How are you going to charge this 1200ah bank ?
What battery chemistry are you planning ?

[captianjuan]

LiFePO batteries. ill be charging them with solar, ill get a diesel gen before I depart, maybe wind, not sure about that yet.

[boatpoker]

Originally Posted by captianjuan

LiFePO batteries. ill be charging them with solar, ill get a diesel gen before I depart, maybe wind, not sure about that yet.

Have you calculated the size/number of panels you'll require to charge a bank that size ?

[captianjuan]

im going to buy the best solar panels I can and supplement with the generator if need be until I work out exactly what I need, and if I need more ill get more. but right now im focusing on battery bank....

[boatpoker]

Originally Posted by captianjuan

im going to buy the best solar panels I can and supplement with the generator if need be until I work out exactly what I need, and if I need more ill get more. but right now im focusing on battery bank....

it doesn't work that way.
You need to plan an entire system including all methods of charging
and do a power audit. Otherwise you can't possibly know how many panels
to buy or their output and if they will even fit on your boat.

[captianjuan]

I don't see why I cant design a system that will allow me dynamic movement within the capabilities of that system. for instance, if I buy say, the Victron Quattro, I should be able to upgrade my panels or add more so as to increase my charging capability. or maybe I could add say another 400ah battery to my bank to increase my power. or I could buy more effecicient panels, or I could build a big ass davits on my stern beam on my boat is 14 feet. I think Ill have a good amount of room for panels. what if I want to upgrade to induction stove. this all seems practical to me, but as I said, I am still in the learning phase.

[Paul Elliott]

Originally Posted by boatpoker

Have you calculated the size/number of panels you'll require to charge a bank that size ?

Is there a minimum charge rate for Lithium batteries?

Yes, of course with a low charge rate it's going to take a long time to recharge a big battery bank, but as long as average charging power is greater than average load power the battery will eventually reach full-charge, won't it?

I have a big bank of AGMs (900Ah / 12V) and a relatively small solar array (300W). It takes days for the panels to recharge the deeply-discharged bank -- perhaps a week if the bank is near 50%. But this setup lets me go for several days of heavy power-use before I need to run the engine for recharging. And if I turn off enough gear I can sail forever and the panels keep up.

So unless I'm wrong about recharging Lithium, captainjuan should have a lot of flexibility in choosing his solar configuration. He just has to be aware of how the charging sources, loads, and bank capacity interact.

[boatpoker]

Originally Posted by Paul Elliott

He just has to be aware of how the charging sources, loads, and bank capacity interact.

I think that was my point. He just does not want to hear it.

[PaulCrawhorn]

4*300Ah is better than 3*400Ah

The balancing act is between how much you consume per 24 hours on average, vs how much you can put back into the bank per 24 hours on average.

You can have as large a bank as you like, so long as you reduce your consumption on cloudy days if solar-only

Having a larger bank just gives you a bigger buffer for the "averaging" to catch up. With a high amp alternator setup, you can refill to some extent every time you need to do some motoring.

If you also have power generation on demand, then you don't need as big a bank, with LFP your limit is just how much fuel you're willing to burn. But then there is no harm having "too big" a bank, it's just money.

Adding solar then becomes optional, simply reduces your dependence on fuel sources, no need to ruin your boat's lines or windage.

[HeywoodJ]

Here’s the thing, cells, in LiFePO4 refer to a 3.2 (nominal) volt battery. 3 of those 400Ah cells, or 4 300Ah cells, configured to give you a 1200Ah battery will give a 1200Ah battery at 3.2V. If you want a 12V (nominal) bank, as stated in the OP, you will need 12 x 400Ah cells (a 4S3P configuration) or 16 x 300Ah cells (a 4S4P configuration).

OTOH, if you are talking pre-configured “drop-in” 12V batteries that already have 4 cells in series inside, then, to me, the decision would come down to the limitations of the internal BMS. If the 300Ah and 400Ah have the same current limit at the BMS I would go with 4x300Ah to get the higher total current limit.

[PaulCrawhorn]

Many drop-ins specify a limit for how many you can parallel.

Using bare cells and choosing your own BMS will usually mean a better result at lower cost.

[captianjuan]

oh ok. I thought that when I wire up the cells since they would be wiring in series to increase voltage from int 3.2v individual and using 4 cells to achieve 12v that the amps stayed the same. as I said im still learning about this but I was thinking that when we wire in series the voltage increases, and in parallel the amps increase. that's how I always figured it with full batteries. never did cells before.

[StuM]

Originally Posted by captianjuan

oh ok. I thought that when I wire up the cells since they would be wiring in series to increase voltage from int 3.2v individual and using 4 cells to achieve 12v that the amps stayed the same. as I said im still learning about this but I was thinking that when we wire in series the voltage increases, and in parallel the amps increase. that's how I always figured it with full batteries. never did cells before.

Firstly Amps are not the relevant units when talking about battery capacity, you need to be talking about Amp hours. Amps and Watts are measures of power (i.e. rate of current flow), they have little relevance when talking about the quantity of energy stored/deliverable.

And importantly Amp hours are not absolute values- they depend on both Wattage and Voltage.
Watt hours are an absolute amount of energy.


Watt (hour)s = Amp (hour)s x Volts
Amp (hour)s = Watt (hour)s / Volts

The best way to understand series v parallel is to convert your battery capacity (Amp hours and Volts) to Watt hours.

If you are talking about connecting four 3.2 V 300 Ah cells then:
Converting to Watt hours: each cell is 960 Wh (3.2 * 300)
Combining four of them you get a total of 3840 Wh (4 x 960) regardless of whether you connect them in series or in parallel!

If you combine them in parallel, they are still 3.2 V so you get
3840/3.2 = 1200 Ah @ 3.2 V
If you combine then in series, they are 12.8 V so you get
3840/12.8 = 300 Ah @ 12.8 V

However: if you are combining three 3.2V 400AH:
Combining in parallel , you still get 1200 Ah @ 3.2 V
BUT
Combining them in series you get 3840/9.6 = 400 Ah @9.6 V
====================
To get a 1200AH @ 12.8V bank you need at total of 15,360 Wh
which is 16 x 300AH @ 3.2V cells or 12 x 400AH @ 3.2v cells

[motion30]

Originally Posted by boatpoker

it doesn't work that way.
You need to plan an entire system including all methods of charging
and do a power audit. Otherwise you can't possibly know how many panels
to buy or their output and if they will even fit on your boat.

I think an energy audit is a waste of time . how do you calculate the difference between July and December in your energy audit? How do you calculate for several cloudy days in a row? The truth of the matter is 99% of the time you will run out of real estate for solar panels Before you have enough Power from solar .What is the harm of having too much solar capacity?