House Bank for Dummies 101

[oceancrosser]

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

 

[Solly]

No practical experience here but I would think the charging system would need upgraded too if you go with the LiFEPO 4s. At least some kind of smart charging controller.

Electricity will make motors AND your head spin.

 

[READY]

Rusty,
I did almost this same process. Feel free to pM me and we can get on the phone. Too complex to cover in a thread but definitely something you can accomplish. Your general thinking is spot on so you are at least a multicellular creature!

 

[Bacchus]

Rusty
Your summary of series / parallel connections is correct.
I have frequently taken exception to the myth (my opinion) regarding the magical 50% DOD that many repeat. I have not seen any batty mfg data or DOD / SOC vs cycles that supports this. In fact, Trojan states that a 80% DOD is a practical "limit" that provides some cushion against batty damage. Battery University has a similar position stating a 60% DOD is a very conservative estimate.

I will let others comment on LiFePo as I'm not interested enough to seriously consider them. I would fully explore the implications for alternator and shore charging of both house (LiFePo) and start (FLA or AGM). I don't believe LiFePo are suggested for starting due to potential problems with the alternator(s).

 

[firehoser75]

Rusty,
First off, I am not a marine electrician, but just a boat owner who has done some research into boat power systems. I am also, not very familiar with LiFePo, but it is my understanding that changes to the charging system may be needed (equipment changes not just settings) depending on what you have now! So, if you want to go that route, I strongly suggest hiring a qualified marine electrician to help you out.
As stated by others, your comments regarding the batteries are correct.

My free advice (worth every penny you paid ), would be to stay with what you have now (just replace the batteries) unless you are experiencing issues with your system. Issues could include, excessive corrosion in the battery storage area (engine room?), short battery life, not enough capacity for your power usage, etc.

Also depending on your "budget" your costs could vary greatly. For example, best bang for the buck would be flooded lead acid 6V golf carts from Sam's Club or Costco, followed by better quality FLA's like Trojan, followed by AGM , etc. Also, just replacing like for like will probably not require any rewiring, changing of settings, etc.

For most battery types, operating regularly at a partial state of charge (in other words not getting back to a true full 100% recharge regularly) will (greatly) shorten battery life and performance. Stock engine alternators (internally regulated) will not adequately recharge a house bank, especially a large one like 900 AH. They are not designed nor set up to do so. They are designed and setup to "top up" the start battery (like for a car or truck) and that is about it (a bit of a simplification but not far off) and are usually wired so that they sense the start battery not the house. Therefore, relying on the stock alternator for house bank charging, as most boats come wired, will not be adequate. When at anchor, long generator run times are required to bring deep cycle batteries back to a full 100% charge (read like 5+ hours), with the last hours putting very little back in and not being very efficient. A properly sized solar system in conjunction with generator run time will achieve the return to 100% which will optimize battery life. If being able to regularly get back to 100% is possible with your system (either now or after modification - like adding solar), then spending more on better batteries like AGM or premium FLA could be worth the extra expense. Otherwise you will just be paying for "premium" but basically "murdering" them by a poor charging regime.
Other than just replacing what you have now and "carrying on" as is, consulting a marine electrician about your system, how you use it, and getting advice for improvements may be a very good idea? JMHO. Good luck.

 

[Comodave]

I put Lithium batteries in our last boat. They are amazing but they come with a lot of restrictions. They have a BMS, Battery Management System, that will shut the batteries down if they get too hot or too cold. The heat shutdown, if I remember correctly, was 111 degrees. So the engine room mounting was out. I mounted them in the salon. The charging really needs to be from an external programmable regulator. If you aren’t good at electrical work it is best left to a qualified marine electrician. In your case I would go with the 6 volt GC batteries. It sounds like you have a handle on that.

 

[O C Diver]

I was in a similar boat to you. Had 8 Trojan T-105s in series and parallel to equal about 880 AH. Never drained mine below 75% and rarely below 80%, so useable was 180 AH.

I elected to replace them with 4 Firefly G31 12 volt batteries wired in parallel. The 4 have a rated capacity of 464 AH. They can be drawn down to 20% with a 1,000 cycle life. I chose to only draw them down to 50% which should yield a 3,600 cycle life. This yields 230 AHs.

https://fireflyenergy.com/images/pdf/OASIS-MCF-G31-Brochure.pdf

Probably worthy of note:
Other than standard engine alternators, you made no mention of external multistage regulators to control the recharging from the alternators. Standard alternator regulators don't properly recharge deep cycle batteries and probably contributed (along with the 50% depth of discharge) to an early demise. Whether going with open leadacid, AGM, Lithium or any other flavor of deep cycle, proper recharging is life or death, for the batteries.

Ted

 

[ranger58sb]

Each of my 6V batteries provides 225 amh thus when connected in series, I now have 12V with 225 amh.

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.

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.

Can't speak much to LiFePO4, but... the words Battery Management System (BMS) play heavily in there somewhere. Need to study on that, too.

Your V and Ah math is correct.

If you're just trying to add capacity, there are taller version of your Crown batteries that could add Ah in the same footprint... if you have sufficient height. Look at L16s or similar, up to about 300 Ah per battery (or battery pair, at 12V) I think

If you're also trying to reduce service work, AGMs do that pretty well... and Lifeline makes 6V AGMs of varying capacity.

Yes, you'll want to match charger capabilities to batteries.

-Chris

 

[TBill36]

Interesting discussion as I have a similar thread going with a slight twist. The other responders know a lot more than these are just some thoughts that I'm going through with my system.

You didn't mension how old you current bank is. I more than 4 or 5 years, you have a pretty good set up. I would also think with that large a bank you must have an externally regulated alternator or a standard internally regulated one would overheat trying to charge a depleted bank of your size. So if you have old batteries and haven't trashed your alternator then you have a pretty good system as is and the least expensive route is to replace with what you have.

The next step is AGMs that will reduce maintenance and possibly let you charge a little faster.

If you have an externally regulated alternator you may be able to program it for LiFePo batteries which will make the leap to them moderately more expensive than AGMs. There are other considerations to address too such as the BMS of the LiFePo shutting them off which would fry your alternator unless the external regulator can accomodate that.

 

[RedRascal]

One thing I look at is how many times I plan to cycle the batteries deep. For example if you plan on 30 nights a year on the boat where the batteries will be discharged to down to 30% that may be okay. You may be able to get 250-300 cycles out of lead acid batteries doing that which works out to about 8+ years of life. You have to think if you are still going to have the boat that long. If you cruise fulltime and live on the hook then that scenario would definitely favor a different battery chemistry than lead acid.

 

[READY]

The big advantages that to me make lithium the winner if you spend much time away from the dock.
1)ability to discharge deeply with minimal impact on battery life. (Maybe reduce from 5k cycles to 2.5k)
2)BMS makes it difficult to kill them via overcharge or discharge
3) at least 5x more cycles so you don't even think about using batteries vs cranking up the gemerator..
4) much lighter yet still much more usable power
5) maybe the best benefit is how quickly you can recharge to 100% off your genset. In my situation we typically will run all day off the batteries then turn. On the genset for cooking dinner and hot water. Well before bedtime (normally 2-3 hours max) the lithium are back to 95-100%
When I did the math the cost to switch vs the benefits and life expectancy pushed me to lithium. Plus I have enough things to track and manage and knowing I could harm lead batteries just by a deep discharge was a risk I was happy to eliminate.

Victron has a wealth of information on their site.

 

[koliver]

Rusty
Sounds like you have a decent amount of house capacity. If you don't think so is that due to excessive loads that could be reduced? Like, eg; Refrigeration. Is yours ac only, AC/DC or DC only? Are you running your inverter all the time while away from the dock? Is your fridge an old Norcold?
For less $$ than upgrading both batteries and charging system you could refit the fridge with DC only and then have excess battery capacity with your present system.

 

[Wdeertz]

I recently installed a 600Ahr @24v LFP system and am loving the performance. As others have stated it’s not a simple drop in replacement as you need to consider your charging modes both from AC and the alternator, and all electrical connections. As an example on my boat the builder put a switch to parallel the house bank and starter bank. This isn’t a problem when both banks are AGM but with a LFP house bank and AGM starter bank the amp surge if this switch is fliPped when the battery voltages are significantly different could produce a lot of heat. LFP batteries have the capacity to dump a lot of energy quickly since there is little internal battery resistance. In this example the BMS might step in and shut down the batteries but better to avoid.

If I can give one piece of advice it’s don’t put LFP batteries in an engine compartment, it will likely lead to premature failure. Feel free to reach out by PM if you’d liKe to discuss details.

 

[DennisB1]

I have two 3208’s. The stock internally regulated alternators did not do a good job charging. The presto lite alternators regulated to 13.8 volts. The 50 amp output occurred in the low 12 volt range. Rod Collins provided a single belt 100 amp with an external mounted smart regulator that fit where orginial was mounted. It charges the engine start battery and has transfer switch to charge the house bank if required. The other 3208 has two belt 160 amp externally regulated unit. This one charges 5 215 Northstar AGM’s. A magnum inverter maintains on shore power or generator. Our all electric boat requires about 40 minutes morning and evening run time for batteries, cooking and refrigeration

 

[oceancrosser]

Hello,

I am just returning to the house and thanks so much for all the valuable information and guidance.

There is a LOT of good advice and a ton of information to digest so it may be a day before I can make a coherent reply.

Just an FYI, I have a separate start bank and house bank with the biggest users of the house bank being the full sized refrigerator and the 110 outlets.

Thanks again for all the help.

Rusty

 

[ranger58sb]

I have frequently taken exception to the myth (my opinion) regarding the magical 50% DOD that many repeat. I have not seen any batty mfg data or DOD / SOC vs cycles that supports this. In fact, Trojan states that a 80% DOD is a practical "limit" that provides some cushion against batty damage. Battery University has a similar position stating a 60% DOD is a very conservative estimate.

FWIW, Lifeline includes an "Expected Life Cycles" graph in their Technical Manual For Lifeline Batteries.

It's a curve, so my eyeballing isn't exact...

But it looks like about 5000 cycles at 10% DoD, 1000 or so cycles at 50%, 550 cycles at 80%, and 375-ish cycles at 100%.

-Chris

 

[Bacchus]

FWIW, Lifeline includes an "Expected Life Cycles" graph in their Technical Manual For Lifeline Batteries.



It's a curve, so my eyeballing isn't exact...



But it looks like about 5000 cycles at 10% DoD, 1000 or so cycles at 50%, 550 cycles at 80%, and 375-ish cycles at 100%.



-Chris

Chris
Not sure what your point or conclusion is and didn't do the math for your #s.
If you use batty spec AH X DOD% X #cycles for life you get AHs delivered over batty life. When I have done the math for other mfg curves the result is very close (~+/-10%) to the same.
Many people equate cycles to life but it's not that simple...
For a simplistic example say your batty runs down to 40% DOD each day and you recharge daily you are at one point on the curve and get AH delivered over batty life.
Now if you recharge every other day your DOD% is 80% and the # cycles used is 1/2 the above case. I will bet the total AH delivered over batty life is fairly close.
Yes you get (roughly) half as many cycles but they each provide twice as many AHs
EDIT...
So doing the math for your #s
Assume a 200 AH bank
200 AH X 50% DOD X 1000 cycles = 100,000 AHs over its life

200 AH X 80% DOD X 550 cycles = 88,000 AHs

So going from 50% to 80% DOD provides 12% less AH over the batty life
Not hardly the " destruction" that many claim when cautioning folks to "never" exceed 50% DOD!
You either get MANY SMALLER AH cycles or fewer HIGH AH cycles but the total energy delivered is fairly constant (+/- ~10%)

 

[Iggy]

I have done a lot of research on this, but no expert. But I will not get into DOD and such since its been discussed.

First off, you want to meet ABYC standards. If something happens, you want the Ins. Co. to pay off the claim. Two very important ones is that the BMS must warn the Capt before the BMS shuts down and there must be a 2nd source of power if that happens.

This spring once I get my boat into its slip, is to do the upgrade from AGM to Li. I am picking a battery with Bluetooth for many reasons. The app will be able to warn me if the BMS is shutting down and the condition of each battery.

The last owner upgraded the Alt to 120amps, but I added a temp sensor just in case it starts overheating. A $30 upgrade, no biggie! I am going to disconnect the Voltage sensitive relay (VSR) between the house and starting bank. So the Alt will only charge the AGM starting bank.

This solves two the problems. No need to upgrade the Alt and if the Li bank was on the Alt side on and the BMSs were to shut down, it would burn out the Alt. Than add a DC to DC 60amp charger to recharge the house bank.

The current onboard charger will only handle the house bank since it must be setup for Li. I will install a small 20amp charger for the AGM starting bank.

As to a 2nd source, turn the house battery switch to OFF and than turn the combine switch to ON. So the Li bank is total disconnected and the boat is running off the starting bank.

Please, if I am missing something, say something.

 

[friz]

....... Standard alternator regulators don't properly recharge deep cycle batteries and probably contributed (along with the 50% depth of discharge) to an early demise.....Ted

Well that caught my eye. If you would, purely looking for elaboration/explanation of that statement. The part about standard alternator regulators.

 

[ranger58sb]

Chris
Not sure what your point or conclusion is and didn't do the math for your #s.

Didn't mean to imply a specific point or conclusion. And haven't ever done much of the math. I just read Lifeline's life cycle predictions off their charts.

-Chris

 

[firehoser75]

Standard internally regulated alternators that are often OEM installed on boat engines, are really automotive or light truck style alternators. They are not "heavy duty", nor are they designed to run at high output for long periods of time without overheating resulting in possible failure. They are designed to "replenish" the small amount of energy that is used to start the engine from the start battery as well as keep up with the various small loads associated with running the engine. Usually these alternators are originally setup to "sense" the start battery (as the controlling voltage) and will therefore reduce charge (amps) quite quickly (as the start battery is usually mostly fully charged). Even if connected to the house bank as well, like with an ACR, these alternators will not be capable of putting out a high output for long periods of time that a discharged (possibly large) house bank would require and their internal regulators would not allow for long, high output run times. (For example, running at full output for several hours). These alternators can be altered to use "external regulation" and can be wired directly to the house bank. As long as temperature sensing is also used, they can then be used (still in a limited manner) to recharge the house bank with "improved" results (from the way boats are "normally" setup).

To obtain good results using an alternator to recharge the house bank, you would best be served to install a properly designed high output alternator with external (user adjustable) regulation, including both battery and alternator temperature sensing (compensation), with the alternator connected directly to the house bank, and use another system to "recharge" the start battery (eg. echo charge, separate standard alternator, etc.) as one example.
Also realize, that to fully (to 100%) recharge a flooded lead acid (and many other chemistrys) battery requires a long time, often 5+ hours with a controlled, diminishing charge. Therefore, the only practical ways to achieve this full recharge are long run times (either generator or engine with a good alternator (most boats don't have this)), connecting to shore power, or solar.

 

[READY]

Engine room temps and lithium batteries

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?

It is not clear to me that is really much of a factor but I am curious what temps others have in their engine rooms? From the data it seems that the low temp concern (if charging or discharging Lithium) would be a more significant consideration but that is why they build BMS to stop charging close to freezing. Fortunately here in San Diego not a problem on either extreme. Now they have added heaters so that should mitigate the charging concern in colder climates as well.

Here are the specs from the Lifeline AGM tech manual:
I could not cut and paste from their document so I had to retype it but the data is correct:
Para 4.5 Temperature Range
Storage (when fully charged): -67F (-55C) to 122F (50 C)
Operating -40F (-40C) to 160F (71 C)

Compared to the specs for the Ampere Time Lithium bats:
Charge: 0℃ to 50℃; Discharge: -20℃ to 60℃

I don't see most engine rooms exceeding the range of -20C or 60C when operating and if so I would think there would many other components that might be at risk. So it temperature really a concern? If you mount them elsewhere you better plan on some very heavy and expensive cables. For me I would try to keep the runs short which in my case dictates placing them in the engine room where the main DC distribution panels and battery switches are located.

 

[CPD]

Oceancrosser - as you state, lots of good advice already. I made the change from Golf Cart to LiFePO4 2 years ago, performed the design and installation myself, and am still tinkering to get it all working the way I expect. It was/is a big job. There are so many variables that everyone's particular circumstances are fairly unique, but if you are going to entertain a significant effort to switch to Lithiums, then your present problems and issues you wish to resolve should be significant to warrant the effort and expense.
If you anchor out a lot, then Solar should be in your plans regardless. Solar is now compatible with legacy or new Lithium battery technologies, so proceeding with a Solar installation will not conflict with a later decision to go Lithium. If Solar acceptably resolves your issues with a Golf Cart House Bank, then you might not need to take the plunge into Lithiums.
Your Questions:
1. Perhaps consider installing Solar first, then if you still have House Bank capacity/recharging issues, then certainly Lithiums are a viable solution.
2. Yes, as many have commented, you are going to have to reassess your overall charging system (alternators, regulators, Engine Start Battery charging, fusing sizes, wire gauges, etc).
3. Alternators need to output an acceptable charging voltage (about 14.5 VDC for Lithiums), the more current the better, and should cease charging once Lithiums are charged rather than output a continuous Float Change. Hence the need for a smart/external Regulator.
4. Solar - Yes; MPPT Charge Controllers are programmed to be compatible with Lead Acid and Lithium technologies.

 

[Simi 60]

The LiFEPO4s

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

So... here is my question.

Can be if buying already assembled drop ins
If prepared to do a small amount of research and assembly, can actually work out cheaper
For us, considerably cheaper on a usable AH comparison

We have been enjoying 840ah @ 24v LFP for 5 weeks now
And today will be the first day on water with them so will be keen to see our existing 2.5kw of solar smashing in the amps all the way untill 100%

Boat has no alternators.
Start battery charge comes from a 24v Victron blue smart charger.

 

[TopDog]

Chris
Not sure what your point or conclusion is and didn't do the math for your #s.
If you use batty spec AH X DOD% X #cycles for life you get AHs delivered over batty life. When I have done the math for other mfg curves the result is very close (~+/-10%) to the same.
Many people equate cycles to life but it's not that simple...
For a simplistic example say your batty runs down to 40% DOD each day and you recharge daily you are at one point on the curve and get AH delivered over batty life.
Now if you recharge every other day your DOD% is 80% and the # cycles used is 1/2 the above case. I will bet the total AH delivered over batty life is fairly close.
Yes you get (roughly) half as many cycles but they each provide twice as many AHs
EDIT...
So doing the math for your #s
Assume a 200 AH bank
200 AH X 50% DOD X 1000 cycles = 100,000 AHs over its life

200 AH X 80% DOD X 550 cycles = 88,000 AHs

So going from 50% to 80% DOD provides 12% less AH over the batty life
Not hardly the " destruction" that many claim when cautioning folks to "never" exceed 50% DOD!
You either get MANY SMALLER AH cycles or fewer HIGH AH cycles but the total energy delivered is fairly constant (+/- ~10%)

Using this math How do I determine how many Ah I need to run my boat for 8-10 hours. Can I measure the amp draw and calculate how many AH I will use in an hour?

 

[Mischief Managed]

if your current golf cart batteries and battery charging system work, and you just want more capacity, add a 48V/100AH LFP bank somewhere cool in the boat and use it to charge the 12V bank through two or three 100/50 12V MPPT controllers in parallel. You'll need a 48V charger for the LFP bank, but it only needs to run on shore power or generator power, so there's no need to upgrade or modify your alternators. If you add solar, it should connect directly to the 48V LFP bank only.



Set up this way, the boat will run on the LFP bank the vast majority of the time and the GC batteries will only start discharging when the LFP bank is depleted. This will typically leave the GC batteries fully charged and they will last forever like that.



This would be the least expensive and simplest way to add capacity and LFP technology because you don't have to replace any of the hardware you already own, you are just adding more battery, a charger or two, and some MPPT controllers. Since the bank will be 48V, the ampacity of the wiring can be 25% of what it would be for 12V too, so wiring runs will be cheaper and easier.

 

[READY]

Mischief Managed: that is an I interesting approach. The tradeoff or downside would be no charging of the LFP via the engine which may or may not matter depending on how they use the boat. I did something not too dissimilar by keeping my AGM as the start battery and using a victron buck boost DC to DC to step up the voltage and also current limit to protect my alternator. But the buck boost does add a significant cost. The benefit is reduced genset run time if running the engine daily moving the boat. But I can see the economy of your solution and you would still get the benefit of extra batt capacity and fast charging from the generator to the LFP. Clever idea!

 

[Mischief Managed]

Mischief Managed: that is an I interesting approach. The tradeoff or downside would be no charging of the LFP via the engine which may or may not matter depending on how they use the boat. I did something not too dissimilar by keeping my AGM as the start battery and using a victron buck boost DC to DC to step up the voltage and also current limit to protect my alternator. But the buck boost does add a significant cost. The benefit is reduced genset run time if running the engine daily moving the boat. But I can see the economy of your solution and you would still get the benefit of extra batt capacity and fast charging from the generator to the LFP. Clever idea!

Thanks! Wish I could take full credit, the concept is partially something that Nigel Calder showed me...



One thing I forgot to add, under normal circumstances, the lead acid batteries would always be fully charged, so one could typically run the 48V charger(s) off the 12V inverter while the main engines are running. Is it as efficient as a 48V alternator? no, but because of the incredibly good charging efficiency of LFPs, it would still be more efficient than charging lead acid batteries directly, and there's no extra cost since you need the 48V charger(s) anyway. Additionally, one could easily use more than one 48V charger and use the flexibility of that configuration to limit the charging current on the alternators while still allowing really fast 48V charging on the genset or shore power. For example:



Let's say you have two 48V 15 amp chargers; each would require about 65 amps at 12V, if powered by the inverter. If you don't have 65 amps X 2 available from the main engine alternators, only power up one charger off the inverter while the engines are running. If you know you only have 50 amps at 12V available when the engines are running, make sure at least one 48V charger is rated for 10 amps or less, etc. If you know you are going to be idling the main engines a lot (thus ,charging current is very limited) or the 12V batteries need a charge, shut the 48V charger(s) off until there's enough excess amperage to run it/them.


FWIW, adding a LFP bank this way is the ultimate way to make use of solar, IMO. LFPs and solar panels could not be better suited for each other on a boat.

 

[Bacchus]

Using this math How do I determine how many Ah I need to run my boat for 8-10 hours. Can I measure the amp draw and calculate how many AH I will use in an hour?

That math is for batty capacity over its life and has nothing to do with usage / demand.
If just taking A readings you would need to take readings over an extended time to get a good average. There are recording devices that will record usage for AC circuits but not sure about inexpensive DC ones. There are tables of amp draw for various appliances and you could confirm your specifics by turning on/off various devices & appliances. Then you will need to monitor, at least the high amp draws (e.g. refrigeration) to estimate the duty cycle.
What some do if interested in upgrading house batty system is to start by installing a shunt based batty monitoring system which will capture & record all DC draws over an extended time to get an accurate usage. If going that route it's important to not have Alt or shore charging active as the monitor captures the net in/out amps... you want to know total draw to properly size a batty bank.

 

[billdomb]

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.