House Bank for Dummies 101

[koliver]

Correct me if I am wrong but when I connect a battery in series (two 6V batteries), I only double the volts but the amps stay the same.

Think the deliverable energy would be DOUBLE if you connect the two batteries in series or parallel, but voltage would double if in series, stay the same if in parallel.

The math is like this:

W=VxA, so your pair of 6V batteries at 225 Amps each, will produce

(6+6 in series=)12 x 225 = 2700 Watts.
(in parallel) 6 x (225+225 =)450 = 2700 Watts

 

[contingensea]

Thoughts

Here’s a link to a post I offered back in April 2021 that you may find helpful.

https://www.trawlerforum.com/forums/showpost.php?p=1000823&postcount=18

The LifePO4 articles can be found within the >Electrical Topics >Battery Topics section on Jim’s web site.

Confession time.. On the food chain of knowledge I am the single cell amoeba when it comes to my knowledge of electricity. Everyone knows more than me but what I do know is

• electricity doesn't smell,
• can't taste it,
• can't see it
• but if you TOUCH it, it hurts.

Correct me if I am wrong but when I connect a battery in series (two 6V batteries), I only double the volts but the amps stay the same. Each of my 6V batteries provides 225 amh thus when connected in series, I now have 12V with 225 amh. When I connect the 12V string in parallel to another 12V string, my understanding is I have doubled my amps but the voltage remains the same. Am I thinking right?

With this said, I am needing to replace my house bank which consists of 8 Six Volt Crown lead acid golf cart batteries connected both in series and parallel to provide a 12 volt 900amh system. Which my understanding is only 450amh is usable since you never want to drag your batteries below that magical 50% mark.

I have read all the threads, watched Youtube videos about AGMs or LiFEPO4s and they are making my head spin. I have tried to understand the nuisances of each system regarding their respective pros and cons. I'm afraid if I tip my head, all the BBs will roll out and I lose what I have learned!!

The LiFEPO4s

• Pro... will last longer than I will
• Con... dang expensive

So... here is my question.

There are some LiFEPO4s that advertise 12V and 208amh and I have the real estate in my engine room to easily put 4 batteries and maybe more. If I go with 4 batteries this should give me a little north of 800amh which from what I can glean from advertisements, all is usable.

• Is this something I should consider? Our lifestyle is to anchor out as much as possible.
• Am I going to need to reconfigure my charging system (we have an Outback Inverter)?
• Will my alternators on the Cat 3208s be enough to recharge the LiFEPO4s?
• Can I someday add a solar system to this configuration?

I appreciate any advice or counsel.

Thanks,

Rusty Bliss
M/V Kristine
DeFever 53 POC
Iuka, MS

 

[danderer]

I have seen several cautions about putting them in your engine room causing a reduced service life. I am interested in the actual data that supports this?

This suggests there is an impact though I'd personally want to see more data:

The Degradation Behavior of LiFePO4/C Batteries during Long-Term Calendar Aging

 

[twistedtree]

The math is like this:



W=VxA, so your pair of 6V batteries at 225 Amps each, will produce



(6+6 in series=)12 x 225 = 2700 Watts.

(in parallel) 6 x (225+225 =)450 = 2700 Watts

You are mixing up units here. The battery spec says it can store 225Ah, which means it can theoretically deliver 225A for a duration of 1hr. Or any other combination of current and time that equals 225. That’s where the Ah unit comes from. It’s Amps multiplied by time.

The formula for the energy stored in a battery is Wh = V * Ah. So the battery can store 2700 Wh.

 

[Bacchus]

The battery spec says it can store 225Ah, which means it can theoretically deliver 225A for a duration of 1hr.
Or any other combination of current and time that equals 225.
That’s where the Ah unit comes from. It’s Amps multiplied by time.
.

Conceptually you are correct but the details are where it varies.
Actually to be correct an AH spec should state the rate of draw. Usually AH specs are for the 20 hr rate. (At which point it reaches an unusable V... I think something like 10.5-11V?)

Lower draws will result in a higher AH and higher amp draws a lower AH for the same batty... its not linear.

 

[SteveK]

For some reason worthy of another thread the battery gods decided to rate batteries on what amps at a average rate they can handle over 20 hours.
So a 225Ah battery / 20 = 11.25.amps per hour. so if it can do 11.25 amps for 20 hours it is rated a 225Ah battery.
However I think 225 amps for one hour is also correct.

 

[johnrupp]

Peukert’s Law is applied here. The higher the rate of discharge the less available capacity of the battery. The lower the rate of discharge increases the available capacity of the same battery.

If you look at a spec sheet for a deep cycle battery it reflects this by showing the 5 hour, 10 hour, 20 hour etc capacity of the battery.

https://en.m.wikipedia.org/wiki/Peukert's_law

John

 

[twistedtree]

Conceptually you are correct but the details are where it varies.
Actually to be correct an AH spec should state the rate of draw. Usually AH specs are for the 20 hr rate. (At which point it reaches an unusable V... I think something like 10.5-11V?)

Lower draws will result in a higher AH and higher amp draws a lower AH for the same batty... its not linear.

Agreed, and thanks for adding. This is what I meant by the "theoretical" capacity.

 

[Hippocampus]

Agree with OC on this one. For new construction Li makes perfect sense. For a refit unless you expect to own the boat for a real longtime firefly maybe your best choice. Get the usable amps. Don’t need to reconstruct your entire system. Save a bit of weight. So look at firefly as the midway point between AGM and lithium. Agree lithium is the best current choice as regards technology but firefly maybe the best practical choice for most boaters. Still no good choices for batteries. Better is always 5 years away. Rather deal with India than China as well.

 

[SteveK]

house batteries 660Ah.
5 days ago I turned off Float and lowered recharge from 12.5 to 12. The only power draw is the 12V fridge @ 3A, inverter is off.
The battery level quickly dropped to 12.8 (from float 13.8) and has declined to present 12.5. There is a roller coaster along this line as fridge comes on and turns off.
I can see the battery recovery mentioned from 12.57 drop to 12.51 then up to 12.55 after fridge cycle. recovery is a little less each time.

This is more exciting than watching paint dry.