Marine batteries for backup power.

[lkruper]

I currently have a manual transfer switch and a 2000/2200 inverter generator for emergency power for my grid-connected cabin. I do not need batteries, but I want batteries.

My design specifications are to have enough power to keep my TV, satellite receiver, DSL modem, laptop computer and security cameras on over night so as not to need to listen to the noise of the generator. I would also likely use the battery initially if power goes out and when we are not sure when power will be restored. These devices add up to 113 watts if they are all on at once and a total of 138 AH @ 12v for one "overnight" time-period.

If the power outage lasts more than one day, I will use my generator to power during the day and charge the batteries for the next quiet period. I am not planning on powering my two refrigerators (95w 3.2 cu ft, 700w Energy efficient) with batteries but will power them doing the day while running on generator. Does it seem reasonable that 12 hours of power every day is sufficient to keep refrigerators cold long term (ie 1-2 weeks)?


My current implementation plan uses two Costco group 27DC Interstate Batteries at $79 each + $15 core to provide 120 AH at 50% DOD. I calculate that these will run my loads for 12.7 hours at 50% DOD. My spreadsheet has grown unmanageable... I need to refactor it. A sanity check would be appreciated.

So that's the background and here are my thought processes. Please critique.

I want to keep the cost down for my first set of batteries.
I plan to configure these two batteries at 24v to allow for future expansion when I replace the batteries.

The choice of batteries is based on price and the fact that they are primarily for backup purposes. I have not been able to find consistent statistics on the number of cycles for marine batteries but have seen some mention of something like 150 - 200 cycles. Is this reasonable? I think that this number of cycles for a 3-5 year period is more than I need unless civilization as we know it collapses.

Am I viewing Marine (hybrid) batteries realistically? I don't need more cycles but also don't want to pay for ups or telecom batteries because of the expense.

Also, I am making the following assumption. I am going to start with an inexpensive charger, the $99 Stanley golf cart charger widely available for about $99. It is 6/12/24 at 20 amps and 12 amps for 48. It does not have temperature compensation. My batteries will be stored in the basement of my cabin which has large boulders on two sides. In the summertime it is always cool, almost like a root cellar and in the winter it should rarely freeze. Does this sound like a good environment for batteries that are in float most of the time? I have a 3.5 amp NOCO charger that I plan to use to float these two batteries after they are charged with the Stanley.

Finally, I plan to use the 150w Cotek 24v inverter. My total loads all at one time without surge is 113 watts. My loads are all electronic communications equipment that probably all have internal DC power supplies. Is 150w pushing it on the low end?


Lots of questions in bold. I did search the archives and the internet and found bits and pieces of information. I have learned a lot here in a few months but the more I know the more I realize I have to learn.

[Sunking]

A Marine Battery is a Hybrid Battery. That means is tries to be both a Starting and Deep Cycle battery. Hybrid batteries have a place, a niche place like AGM. They can deliver higher discharge currents without excessive voltage sag up to around C/4. The trade-off is cycle life being about 1/3 to 1/2 that of a true Deep Cycle battery. On the Cranking side they cannot deliver the very high starting currents of a SLI battery.

You also need to quit working with AH. No one gives a damn about AH because it is a meaningless number, and introduces errors. You want to work in Watt Hours. Let me say that again WATT HOURS. Did I fail to mention WATT HOURS. That you have not done yet and if you do not, you will fail.

So how many hours are you going to run 120 watts? 1 hour, 5 hours, 12 hours, what?????????. Lets say 10 hours and you want to discharge to 50%.

113 watts x 10 hours = 1130 WATT HOURS. That means you need a battery capacity a minimum 2260 WATT HOURS.

So what Amp Hours is that? Beats the crap out of me. I cannot tell that until I know the nominal battery voltage and WATT HOURS.

2260 wh / 12 volts = 188 AH
2260 wh / 24 volts = 94 AH
2260 wh / 48 volts = 47 AH
2260 wh / 96 volts = 24 AH
2260 wh / 120 volts = 19 AH.

So you tell us what you come up with. I will tell you if you got it right or not. But you will not get an answer from me. That is your job. All I will do is teach you how. But as of now you are dead in the water without WATT HOURS. Did I mention WATT HOURS?

[lkruper]

A Marine Battery is a Hybrid Battery. That means is tries to be both a Starting and Deep Cycle battery. Hybrid batteries have a place, a niche place like AGM. They can deliver higher discharge currents without excessive voltage sag up to around C/4. The trade-off is cycle life being about 1/3 to 1/2 that of a true Deep Cycle battery. On the Cranking side they cannot deliver the very high starting currents of a SLI battery.

You also need to quit working with AH. No one gives a damn about AH because it is a meaningless number, and introduces errors. You want to work in Watt Hours. Let me say that again WATT HOURS. Did I fail to mention WATT HOURS. That you have not done yet and if you do not, you will fail.

So how many hours are you going to run 120 watts? 1 hour, 5 hours, 12 hours, what?????????. Lets say 10 hours and you want to discharge to 50%.

113 watts x 10 hours = 1130 WATT HOURS. That means you need a battery capacity a minimum 2260 WATT HOURS.

So what Amp Hours is that? Beats the crap out of me. I cannot tell that until I know the nominal battery voltage and WATT HOURS.

2260 wh / 12 volts = 188 AH
2260 wh / 24 volts = 94 AH
2260 wh / 48 volts = 47 AH
2260 wh / 96 volts = 24 AH
2260 wh / 120 volts = 19 AH.

So you tell us what you come up with. I will tell you if you got it right or not. But you will not get an answer from me. That is your job. All I will do is teach you how. But as of now you are dead in the water without WATT HOURS. Did I mention WATT HOURS?

Sunking, you are right about one thing. The hard part about tracing through my spreadsheet is the conversion between AH and WH. However, the two group 28 Marine batteries which are sold as 120 AH @ 12v give me:

240 ah @ 12 volts which matches the 188 AH nicely or
120 ah @ 24 which works for the 94 AH above.

My choice is to put them in series for 24 volts to provide 2880 Watt Hours in 10 hours.

I do need to start thinking in Watt Hours. I believe I got off on the wrong foot because batteries are sold in AH.

[NorthRick]

You should consider the golf cart batteries at Costco instead of the marine ones. They are around the same price but will last longer. The biggest issue would be that they are 6v, so you would need 4 to get a 24v system.

[lkruper]

You should consider the golf cart batteries at Costco instead of the marine ones. They are around the same price but will last longer. The biggest issue would be that they are 6v, so you would need 4 to get a 24v system.

Yes, I have my eye on them as well. That speaks to one of the questions I have regarding hybrid batteries. What do you mean by last longer? More cycles or age? I don't need more cycles. If they last 2X longer and cost 2X more for 4 batteries instead of 2 Marine batteries, that is a wash.

The golf cart batteries were my first plan, then I saw that I could get started with much less of an investment and a smaller installation with two marine batteries.

The question as to the usefulness of Marine batteries for what I am trying to do cuts to the heart of my post.

[NorthRick]

With a 113w load a 12v system would be fine, so just two batteries would work. They can cycle more times than the marine ones. As for a time limit on batteries that are lightly cycled I don't know how that compares.

What I do know is that I have Costco golf cart batteries in our weekend cabin. They see 20 to 25 weekends a year of use and have been doing the job for 5.5 years so far. The way they get used, they spend most of their time in float and occasionally get drawn down past 50%.

[lkruper]

With a 113w load a 12v system would be fine, so just two batteries would work. They can cycle more times than the marine ones. As for a time limit on batteries that are lightly cycled I don't know how that compares.

What I do know is that I have Costco golf cart batteries in our weekend cabin. They see 20 to 25 weekends a year of use and have been doing the job for 5.5 years so far. The way they get used, they spend most of their time in float and occasionally get drawn down past 50%.

There are only three types of flooded lead acid batteries, starting, deep cycle and hybrid. Marine batteries and golf cart batteries are both hybrids.

The Trojan 105 re data sheet gives a cycle life curve. The t105 does not because it is a hybrid. Sunking said that hybrids give 1/2 to 1/3 the cycles of deep cycle batteries. If I was off grid there is no doubt I would buy Trojan 105 re over 105.

Your recommendation to use two golf cart batteries is a good one. However if I expand my system it will be to 24v and probably 4 golf cart batteries. Then I must re-purpose the inverter and buy a 24v one.


I have a feeling that cycle life is directly proportional to the amount of lead in the battery. 2 6v golf cart batteries weigh more than 1 12v marine battery and should have 2x the cycles but also 2x the cost. At least that is my current working theory until someone teaches me differently. So is it really true that two golf cart batteries will have more cycles than two marine batteries?

I would like to know the answer to those questions but for me what I really want to know is if there is a better cost/performance than marine for my low cycle requirements!

[Sunking]

Hybrid batteries have many Marketing names, like Marine, Golf Cart, RV, Trolling, Leisure and the list goes on.

The Trojan T-105 is a good example of a high end battery. There are two, T-105 and T-105RE. The T-105 does have discharge curves, just gotta know where to look. T-105 is Signature Line and RE is Premium Line. Industrial is Top Dog in Trojan line. . The main difference you will find are two fold. The RE is 5 pounds heavier with lead, and Carbon-Lead Alloy.

The T-105 is a 3-4 year battery, the T-105RE is a 5-7 year battery. You can see this reflected in WARRANTY and Cycle Life. The RE is about 15% higher in cost and well worth it considering it should last twice as long.

Bottom line you get what you pay for.

[inetdog]

The T-105 is a 3-4 year battery, the T-105RE is a 5-7 year battery. You can see this reflected in WARRANTY and Cycle Life. The RE is about 15% higher in cost and well worth it considering it should last twice as long.

But not necessarily the best choice for a first system, since new owners tend to do other things to murder their first battery bank in only a couple of years.
"Are you feeling lucky?"

[Sunking]

Sunking, you are right about one thing. The hard part about tracing through my spreadsheet is the conversion between AH and WH.

AH is just an end result. WATT HOURS is a constant and equal at any voltage. 100 wh can be at 10 or 100 volts. Last thing you need to know is Amp Hours and is just a resultant product of Watt Hours and Battery Voltage.

If you work in AH, can be done, but opens you up to all kinds of conversion errors and jus tmakes it more complicated then need be. . You eliminate that possibility using Watt Hours. Last step is finding Amp Hours.

I am a simple man and KISS it. Keep It Simple Stupid.

[Sunking]

But not necessarily the best choice for a first system, since new owners tend to do other things to murder their first battery bank in only a couple of years.
"Are you feeling lucky?"

True for most folks. But some people are just smart. They know to listen to those who have gone before them, and learn from others mistakes and does not repeat them. The majority prefer to do it their way, the HARD WAY and can only learn by loosing their money and that is an excellent teacher as well, just not too smart.

I gotta feeling Lurker is on the smarter side and doing homework. Wished more would do that.

[lkruper]

Hybrid batteries have many Marketing names, like Marine, Golf Cart, RV, Trolling, Leisure and the list goes on.

The Trojan T-105 is a good example of a high end battery. There are two, T-105 and T-105RE. The T-105 does have discharge curves, just gotta know where to look. T-105 is Signature Line and RE is Premium Line. Industrial is Top Dog in Trojan line. . The main difference you will find are two fold. The RE is 5 pounds heavier with lead, and Carbon-Lead Alloy.

The T-105 is a 3-4 year battery, the T-105RE is a 5-7 year battery. You can see this reflected in WARRANTY and Cycle Life. The RE is about 15% higher in cost and well worth it considering it should last twice as long.

Bottom line you get what you pay for.

Thanks for that link to the comparison of cycles including the t105 signature line. That was very informative. When you say last twice as long that is based on frequent cycles, correct? What about a backup battery on float most of the time?

[Sunking]

Thanks for that link to the comparison of cycles including the t105 signature line. That was very informative. When you say last twice as long that is based on frequent cycles, correct? What about a backup battery on float most of the time?

My comments are general and broad. Just remember battery manufactures are not created equal. In general terms a Deep Cycle will last about twice as long as Hybrid. But you cannot compare say a Wally World Golf Cart battery with Trojan RE as you are now comparing a 1-2 year battery to a 5 to 7 year battery. The take away is if you compare Acme Battery Company Hybrid and Deep Cycle battery line, the Deep Cycle is going to last roughly twice as long, bu thas some current limitations.

As to daily cycle vs Emergency or infrequent cycle you are now working on Calendar Life. Batteries have both Cycle and Calendar life, which ever comes first. Cycle every day, and much shorter than Calander life. With Emergency Stand-By use, you can use a smaller battery and discharge to 50 to 80% DOD. But there is a catch with the smaller battery. You are much more likely to exceed the maximum Discharge Rate on a smaller battery. In which case you are now looking for Hybrid or AGM. Like I have always said Hybrids have a place, but a niche place.

You are on the right path, keep it up. Its YOUR MONEY. Spend it wisely. Don't be like the 95% of those who come here after the fact wondering what went wrong.

[lkruper]

With a 113w load a 12v system would be fine, so just two batteries would work. They can cycle more times than the marine ones. As for a time limit on batteries that are lightly cycled I don't know how that compares.

What I do know is that I have Costco golf cart batteries in our weekend cabin. They see 20 to 25 weekends a year of use and have been doing the job for 5.5 years so far. The way they get used, they spend most of their time in float and occasionally get drawn down past 50%.

From the link Sunking provided (http://www.trojanbattery.com/markets...ble-energy-re/) you will get 600 cycles if you don't abuse the batteries and 1200 if you baby them.

At 25 2-day weekends per year you have 7 more years life in those batteries and 18 more years with good care.

Good choice!

[PNjunction]

My design specifications are to have enough power to keep my TV, satellite receiver, DSL modem, laptop computer and security cameras on over night so as not to need to listen to the noise of the generator. I would also likely use the battery initially if power goes out and when we are not sure when power will be restored. These devices add up to 113 watts if they are all on at once and a total of 138 AH @ 12v for one "overnight" time-period.

Tip: If you didn't actually measure the wattage of the TV, just know that the sticker is a worst-case scenario of full backlighting. If you are just going by the sticker, that's great for your calculations since it is unlikely you are using 100% full backlighting in the first place leading to a larger power hedge. I'm talking the typical lcd tv.

In normal use, you can save a LOT of power by running a lower backlight. Do NOT confuse this with "brightness". In my case, I reduced the power draw to HALF of the sticker rating by reducing the backlight (again, NOT brightness) to a very low level.

You may want to experiment and see just what your level of tolerable backlight reduction is. How low can you go before it becomes unusable? Saving up to 25% or more of the sticker value should be very easy. You may not want to use a low level backlight all the time under normal grid-power, but when the juice goes out and the batteries kick in, just remember to reduce the backlight as much as comfortable. When testing, give yourself some time to get adjusted to it before making a decision as the abrupt change can sway your decision too soon. When you see the power savings, that might be the arbiter instead!

A kill-a-watt ac meter or clamp-on dc ammeter at the battery terminals will reflect just how fast that power requirement goes down with even moderate reductions in backlighting.

I have a 3.5 amp NOCO charger that I plan to use to float these two batteries after they are charged with the Stanley.

The Stanley may actually do a better job if it offers a standard 13.6v float. The NOCO doesn't really do an active float, but "float monitoring", designed for guarding against parasitic loads and long-term self discharge. Even though it has a "trickle" stage, (version 1's, version 2 have a different optimization technique) there is not much trickling at all taking place. Thus, your batteries are just going to self-discharge until they reach a trigger point or a high parasitic load occurs. This may not be desirable if you desire true active float instead.