House Bank Sizing Question

[R lucky]

Hi All, it's time to replace (and up-size) my house bank. I have my amp draw computation per 24-hours, but what I need to know is what formula to use so that I'm left with say, 60% state of charge at the end of that 24-hour cycle.

Thanks for all help

 

[Bacchus]

So from a pure mathematical point of view - if you have a draw of Y AH and you want to know the bank size that this represents 40% of (to leave 60% SOC).
Y (AH Used) / 0.4 = Bank Size Or in other words the bank will need to be 2.5 X the avg 24 hr draw.
60% is pretty conservative and would more than allow for a slightly higher than avg 24 hr draw and a 10% degradation of bank capacity over time.

From a summary I put together (attached)
My Conclusion: A Max of 75% DOD is a very practical limit for battery bank design purposes for both FLA & AGM batteries.

Note: To reinforce the above I quote the Trojan Battery Co User Guide
https://www.trojanbattery.com/pdf/TrojanBattery_UsersGuide.pdf
Maximizing the Performance of Your Trojan Battery
1. Follow all the procedures in this User’s Guide for proper installation, maintenance and storage.
2. Do not discharge your battery to more than 80% depth of discharge. This safety factor will eliminate the chance of over-discharging and damaging your battery

 

[O C Diver]

Take your consumption, divide it by 4, and multiply that number by 10.

Ted

 

[Lou_tribal]

Take your consumption, divide it by 4, and multiply that number by 10.

Ted

Then multiply by 2 for redundancy

L

 

[High Wire]

The smaller of:
1) the most capacity you can afford or
2) the biggest you can fit

The theory being that lead acid batteries live longer if the depth of discharge is shallower. There is data that shows dod of 50-60% gives best total amp-hours per battery lifetime but there is no guarantee.
100% SOC = 0% DOD or full charge.

 

[SteveK]

You need 6 or 8 batteries, the 6 volt golf cart type if you are like me without a generator and wanting to be at anchor for 3-5 days without engines re charging.

 

[FF]

The usual power HOG in DC consumption is the fridge.

A different system could extend your batt life by more than 2x.

Eutetic works , with an engine drive system our 90/90 cruises 4 days before needing a 2 hour run.

Propane is even better , no electric and a single bottle is about a month of silent operation.

Off the shelf boat or RV- DC units are easy to install, but not great to cruise with , if you like to anchor with silence.

 

[MYTraveler]

My Conclusion: [/B]A Max of 75% DOD is a very practical limit for battery bank design purposes for both FLA & AGM batteries.

Note: To reinforce the above I quote the Trojan Battery Co User Guide

So, the math for that would be daily usage / .75 (=4/3 X daily usage). But, that assumes a full recharge. Since the rate of charge slows radically as the battery approached full charge, it may make more sense to plan to charge only to 85%, in which case only 60% of the batteries' capacity is usable (the bottom 25% and the top 15% will not be used). The math for that is daily usage / .60 (= 5/3 X daily usage).

The problem that that approach is that it ignores that your batteries will lose capacity over time. To account for that, size your batteries to give you 2 times your daily discharge.

 

[Shrew]

So, the math for that would be

daily usage / .75 (=4/3 X daily usage).

From a mathematical notation perspective (order of operations), this calculation doesn't make sense.

Could you provide a practical example??

 

[rslifkin]

The usual power HOG in DC consumption is the fridge.

A different system could extend your batt life by more than 2x.

Eutetic works , with an engine drive system our 90/90 cruises 4 days before needing a 2 hour run.

Propane is even better , no electric and a single bottle is about a month of silent operation.

Off the shelf boat or RV- DC units are easy to install, but not great to cruise with , if you like to anchor with silence.

If solar is out of the question, I agree. But if there's space for a couple of solar panels, it's not hard to keep up with the consumption of a fridge.

 

[MYTraveler]

From a mathematical notation perspective (order of operations), this calculation doesn't make sense.

Could you provide a practical example??

There is only one operation, so any confusion can't be based on order of operations. But here you go. If the daily usage is 75 amp hours, and the intention is to start with a 100% SOC and run it down to 25%, thereby using 75% of the total capacity, the formula says 75 amp hours / .75 = 100 amp hours total capacity. And, we can test the accuracy of that be easily recognizing that if we take 75 amp hours out of a 100 amp hour battery, we will have 25 amp hours left.

 

[Panacea123]

Hi All, it's time to replace (and up-size) my house bank. I have my amp draw computation per 24-hours, but what I need to know is what formula to use so that I'm left with say, 60% state of charge at the end of that 24-hour cycle.

Thanks for all help

More importantly, how are you going to recharge?
You have to come up to 90-100% soc which can take up to 7 hours!
No matter what size your bank, if it not charged to 100% every few days your bank will fail prematurely.

Ideally daily amp draw x 4= bank size
FLA batteries can only be charged so fast, it takes time.
AGM and others can be charged faster. (And discharged more deeply)

Charger should be 15% of bank size 500ah bank = 75 amp charger. To give proper 3 stage charging (bulk, absorption and float)
Bigger charger doesn’t mean faster charge (can’t cheat time) FLA bank.

If you don’t have access to shore power you’ll be running gen set ALOT (and most of the time not properly loaded)
Solution is properly sized bank, charger, and solar panels.

My setup is 950ah deep cycle
130 amp smart charger
780 watts solar.
Most days (overnite I’ll use 150-225ah) yes I have ice maker, lights, fridge,TV, etc.
Run Gen 1 hour in am (Takes care of bulk and goes into absorbtion )
Solar takes care of the rest.

The less the discharge (I shoot for 25%) the longer the life of the bank, but it still must be charged for it to last. (Look up Trojan Batteries website)

I once trashed a 500ah bank in 3 months at anchor with a 100 amp charger, because of not enough charging time.

Long rant, some will disagree (and probably much smarter than I am)
But this works for me!

5yrs same bank no deterioration so far

 

[motion30]

This is why solar is such a great thing. my solar production covers all of my loads in the daytime and fully recharges the batteries to a a state of Float most everyday. This may be a big factor in picking the size of your battery Bank

 

[mvweebles]

Is this a trick question? Why isn't this a simple math question?

If 60% is the desired end state, then take the 24 hour consumption and divide by 40%. Examples: If 24 hour consumption is 300ah, then battery bank needs to be 750 ah capacity (300 / 0.4 = 750). If 24 hour consumption is 400 ah, then battery bank needs to be 1000ah (400 / 0.4 = 1000). BTW - this is more or less what Ted/OCDiver said.

I'll assume that if OP wants input on how to reduce the energy budget or how to replenish the consumed energy, he'll ask. But simple sizing with no other inputs such as solar or generator appears to be a simple answer.

What am I missing?

Peter

 

[Shrew]

There is only one operation, so any confusion can't be based on order of operations. But here you go. If the daily usage is 75 amp hours, and the intention is to start with a 100% SOC and run it down to 25%, thereby using 75% of the total capacity, the formula says 75 amp hours / .75 = 100 amp hours total capacity. And, we can test the accuracy of that be easily recognizing that if we take 75 amp hours out of a 100 amp hour battery, we will have 25 amp hours left.

That makes more sense. I see it now, though it was notated oddly in your first post. Based on this theory, it would be:

Capacity = Draw / % of Depth

That is provided you know the estimated daily amp draw, then you can calculate the projected bank size.

I honestly feel that drawing down 75% of the bank is way too low for a daily basis. From everything I've read, the less you draw into your capacity in a given charge/draw cycle the longer the battery life. This means that battery banks need to be much larger than initially anticipated.

The question becomes where is the breakeven point where the cost of the bank exceeds the savings over time. In other words If I buy x times as many batteries will I get x times as much time out of those batteries?

From what I've been reading (citations required) battery longevity is better achieved if you never drop more than 25% into the bank capacity.

Capacity = Draw / .25

So If I burn 100ah per day, I should be looking at a 400ah bank

 

[DavidM]

It often works this way: someone asks a provocative or even a plain vanilla question like this one, which elicits lots of answers and some questions. Two days later we still haven't heard from the OP.

Why do we bother?

David

 

[MYTraveler]

T
I honestly feel that drawing down 75% of the bank is way too low for a daily basis. From everything I've read, the less you draw into your capacity in a given charge/draw cycle the longer the battery life. This means that battery banks need to be much larger than initially anticipated.

The question becomes where is the breakeven point where the cost of the bank exceeds the savings over time. In other words If I buy x times as many batteries will I get x times as much time out of those batteries?

From what I've been reading (citations required) battery longevity is better achieved if you never drop more than 25% into the bank capacity.

If you use only 25% of capacity, your battery bank will be 3 times larger than if you use 75%. Ignoring installation costs, time value of money, additional fuel costs to lug around the extra batteries, etc., they would have to last 3 times longer. Based on my personal experience, they won't. I was surprised to see that the battery mfg cited above (Trojan?), said 80% was OK. Personally, I regard 75% used (25% remaining) as the point at which, like a fuel tank, it is empty. But I rarely run to empty, so I tend to recharge at around 35% remaining capacity. I typically get 5 years from my house bank (but don't use them every day or even every week -- 3 days a month is probably my average).

 

[mvweebles]

It often works this way: someone asks a provocative or even a plain vanilla question like this one, which elicits lots of answers and some questions. Two days later we still haven't heard from the OP.

Why do we bother?

David

Operative phrase being "it often works this way" Not always, but often.

Remember the experiment where they took two groups of pigeons? Both sets could peck a target and get food. After a while, they abruptly turned-off the food for Group #1. Didn't take long before Group #1 stopped pecking.

For Group #2, they gradually/randomly turned-off food - sometimes the birds would peck and they'd get food; sometimes they'd peck and get no food. Eventually, food was turned-off entirely yet they kept pecking long after the food was turned off.

Many OP's disappear, but not all. Welcome to Group #2.

Peter

 

[bgillroy]

R Lucky. Starting point is a load survey spreadsheet. There are examples and methods to create. Once you have lined out your personal power consumption accurately, the solution can be addressed next. Don't buy anything til you get that survey done.

 

[FF]

"This is why solar is such a great thing. my solar production covers all of my loads in the daytime and fully recharges the batteries to a a state of Float most everyday."


Great till it rains for a few days , then its noisemaker time for a partial recharge or being stuck at a dock.

 

[Mako]

2.Do not discharge your battery to more than 80% depth of discharge. This safety factor will eliminate the chance of over-discharging and damaging your battery

Boy I hope no one actually follows this manufacturer’s advice. So in essence you buy 5 expensive batteries but only use one!

 

[rslifkin]

Boy I hope no one actually follows this manufacturer’s advice. So in essence you buy 5 expensive batteries but only use one!

80% depth of discharge would be drawing them down to 20% left, so like buying 5 and using 4.

 

[Bacchus]

Boy I hope no one actually follows this manufacturer’s advice. So in essence you buy 5 expensive batteries but only use one!

80% depth of discharge would be drawing them down to 20% left, so like buying 5 and using 4.

Exactly - Big difference between SOC and DOD

The point being made was that even respected batty mfg do not subscribe to the myth that discharging below 50% SOC somehow "Damage" you batteries and shorten it's life.

 

[Mako]

Okay perhaps I misread that. I assumed it stated do not draw down below 80% (depth of charge). Actually I would agree then, because a heavy duty industrial grade FLA battery should be able to draw down to 80% and still maintain a useful number of life cycles. For example, the big 2V batteries at my friend's ranch were rated about 1500 cycles at that DoD. That's probably ten years life for the average boater with some solar panels up top.

 

[FF]

The better bat mfg usually have charts that compare depth of discharge with the number of times it can be done .

50% is commonly selected as a better compromise than deeper discharge.

This concept is ONLY for deep cycle batts , start batts have a very short life if discharged too deeply.

 

[rslifkin]

Planning for 50 percent also leaves more reserve in case of cloudy weather or anything else that could lead to either more power draw or less charging input on a given day.

 

[Bacchus]

The better bat mfg usually have charts that compare depth of discharge with the number of times it can be done .

50% is commonly selected as a better compromise than deeper discharge.

This concept is ONLY for deep cycle batts , start batts have a very short life if discharged too deeply.

Here is a typical DOD chart that I have seen. I have never seen a Mfg DOD Chrt that illustrates the common 50% DOD "Limit" causing much shortened life when exceeded (much steeper slope beyon 50% DOD). My understanding is it applies to SLI (starting, lighting, ignition) battys but I haven't seen a mfg publish a chart that supports this... Has anyone?


From: https://www.mpoweruk.com/leadacid.htm#deep

Batteries (Starting Lighting and Ignition)

This is the typical automotive battery application. Automotive batteries are designed to be fully charged when starting the car; after starting the vehicle, the lost charge, typically 2% to 5% of the charge, is replaced by the alternator and the battery remains fully charged. These batteries are not designed to be discharged below 50% Depth of Discharge (DOD) and discharging below these levels can damage the plates and shorten battery life.
Note: This is not in reference to the chart below.