Marine batteries for backup power.

[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!

[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]

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]

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]

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.

[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]

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.