Battery Shopping and Capacity Confusion

[visi0n]

My current House battery bank is comprised of these wet cells

The SRM-24 from this link:
Interstate Batteries

It lists capacity in that chart as

Hrs. at Ampere Load
5,16.4
15, 4.6
25, 2.3

I'm wanting to go to AGMs and possibly size up a tad.
Looking at group 24s in agm chemistry, they seem to have a slightly lower capacity

I was thinking Optima batteries
The d34m fits the group 24 size I currently have :
http://www.optimabatteries.com/_medi...specs/D34M.pdf

They list the capacity at 55ah at a c/20 rate

Seems wierd the newer batteries (seem to) have a slightly lower capacity?

[btrayfors]

Apples and oranges.

The SRM-24 isn't a true deep-cycle battery. The giveaway is that Interstate doesn't list the AH capacity of this battery....only the CCA/MCA, and hours at 3 load levels.

So, how did you determine that "the newer batteries (seem to) have a slightly lower capacity" ?

Bill

[visi0n]

My math for that determination, which admittedly probably isn't complete, was to the the 5 hr rate listed (16.4) and multiply by 4 to get a close 20 hr rate (65ish) for my old cells.

Many of the group 24s I've seen and the optima that fits a group 24 footprint advertises 55 aH c/20

By the way. Am I correct in that a 55 ah 20/c advertised battery will theoretically do 110 hours at 10 amps, 220 at 5 amps, etc or am I mistaken?

[Maine Sail]

Originally Posted by visi0n

\

By the way. Am I correct in that a 55 ah 20/c advertised battery will theoretically do 110 hours at 10 amps, 220 at 5 amps, etc or am I mistaken?

Yes, you are mistaken.

Amp Hour Rating:
The measurement of a batteries capacity to deliver a specific current for a given period of time.

Normally rated at a 20 hour Ah rating (industry standard for deep cycle)

So a 100 Ah battery can be discharged at 5 amps for 20 hours before falling below 10.5 volts.

To determine your batteries "rated" amp draw divide the amp hour rating by 20 to determine the 20 hour draw/support that the battery can handle to meet the "20 hour rating".

In Practice:

Battery "A" has a 20 hour amp hour rating of 150, dividing by 20 = 7.5. This battery can support a 7.5 amp load for 20 hours before dropping to 10.5 volts/dead.

A battery with a 60 Ah rating will carry a 3 amp load for 20 hours before dropping to 10.5 volts.

You want deep cycle batteries not dual purpose or starting. Many of the Sam's Club locations are now selling the East-Penn / Deka line and they are the same exact batteries West Marine sells only for about half the price.

A general rule is to not buy batteries that do not list the 20 hour rate. Reserve Capacity or reserve minutes is a different rating and one usually reserved for starting or dual purpose batteries.

Some private labeled batteries, such as the East Penn/Deka batts NAPA & Sam's Club sell do have Ah ratings but NAPA & Sam's don't print it on the battery.

How do you plan to use the AGM batteries and why do you want to switch from wet to AGM?

[visi0n]

Thank you for the great info.

So, given this math...

The 55 Amp Hour Group 24 compatible optima battery (Advertised C/20 of 55)
... will run 11 amps for 5 hours before dying?

With only room for 5 group 24's in the quarter birth compartment, it looks like the most time I can get at 11 amps on my house side would be 3 of these batteries for 15 hours? Sounds like we need to just bring an ice chest.

We've been just daySailing throughout the summer but with fall around the corner we'd like to do some anchor-out weekend trips. Concerned I don't have the juice to handle the comforts of the cabin, though.

My Link 2000 shows 9-11 amps being consumed when the cabin fans are on, a few cabin lights, the frigoboat fridge, VHF, Radio, Radar and the iPad charging off the DC port on the companionway entrance.

My start bank has two batteries... Any good reason to not go down to 1 battery for start and 4 for the house? (Then just switch over if I get in trouble with the start battery)
All of these AGM deep cycles seem to have enough CCA to start my atomic 4.



Previous owner bought the same batteries for both banks.. My house bank and start bank are the same SRM-24's. Each bank with two of them. The start bank is pretty solid, for the moment -- holds a charge. I'm guessing having been kept charge through the alternator, they are fine.

My house bank, however-- won't hold more than 11.6 volts and never get into the chargers' maintenance mode. Cells are all equal height when I check them..

I'm guessing, like you said and like I've been reading -- the house bank batteries weren't really designed for this and are degraded considering the charge/recharge cycles being put on them.

I wanted to go to AGM's to resolve the future issue on the house bank, get to a lower maintenance battery and maybe be able to use the flexible mounting to squeeze in 1 or two more batteries on the house side for larger capacity to help run the boat's DC powered fridge that averages 5-6 amp draw and we never use on weekend trips for fear of drain..

BTW: My local Sams carries "Energizer" AGM's.. any indication as to who really makes those and if they are solid or not?

[mitiempo]

If you have room for four 6 volt golf cart type of batteries they will provide more bang for the buck as well as being a lot less finicky of charging than AGM batteries. A lot less expensive as well. 4 will give you about 450 AH, half of which is usable. They are a bit taller than group 24 so you will need the height.

[visi0n]

Can I have three or four 12v batteries in a single bank?

If so, I may just isolate the single 12v for start purposes and get 4 batteries for the house bank. That, plus a Honda 2000i and some conservation should put us weekend-capable with amenities, I'm thinking.

[mitiempo]

You can use as many 12 volt batteries in parallel as you can fit. But the 6 volt golf cart batteries are more durable and will withstand more cycles because of their thicker plates. The most durable flooded batteries that are affordable.

[Maine Sail]

Originally Posted by visi0n

Thank you for the great info.

So, given this math...

The 55 Amp Hour Group 24 compatible optima battery (Advertised C/20 of 55)
... will run 11 amps for 5 hours before dying?

Not so simple. The rated 20 hour draw on a 55Ah battery is 2.75A and this battery should support that load for 20 hours before hitting 10.5 volts.

Anything above a 2.75A draw and you'll get LESS Ah's out of the battery due to a guy named Peukert. If your draw is below 2.75A you'll get slightly more Ah's due to Peukerts law.

The big problem is that you do not want to draw any battery or bank regularly below 50% of capacity or short life will ensue.

So a 400Ah bank quickly becomes a 200 Ah bank, at 50%, and then if charging off an alternator you'll only have about 120 Ah's usable because you'll only usually ever get back to about 80% of capacity using an alternator.

Originally Posted by visi0n

With only room for 5 group 24's in the quarter birth compartment, it looks like the most time I can get at 11 amps on my house side would be 3 of these batteries for 15 hours? Sounds like we need to just bring an ice chest.

We've been just daySailing throughout the summer but with fall around the corner we'd like to do some anchor-out weekend trips. Concerned I don't have the juice to handle the comforts of the cabin, though.

My Link 2000 shows 9-11 amps being consumed when the cabin fans are on, a few cabin lights, the frigoboat fridge, VHF, Radio, Radar and the iPad charging off the DC port on the companionway entrance.

My start bank has two batteries... Any good reason to not go down to 1 battery for start and 4 for the house? (Then just switch over if I get in trouble with the start battery)
All of these AGM deep cycles seem to have enough CCA to start my atomic 4.



Previous owner bought the same batteries for both banks.. My house bank and start bank are the same SRM-24's. Each bank with two of them. The start bank is pretty solid, for the moment -- holds a charge. I'm guessing having been kept charge through the alternator, they are fine.

My house bank, however-- won't hold more than 11.6 volts and never get into the chargers' maintenance mode. Cells are all equal height when I check them..

I'm guessing, like you said and like I've been reading -- the house bank batteries weren't really designed for this and are degraded considering the charge/recharge cycles being put on them.

I wanted to go to AGM's to resolve the future issue on the house bank, get to a lower maintenance battery and maybe be able to use the flexible mounting to squeeze in 1 or two more batteries on the house side for larger capacity to help run the boat's DC powered fridge that averages 5-6 amp draw and we never use on weekend trips for fear of drain..

BTW: My local Sams carries "Energizer" AGM's.. any indication as to who really makes those and if they are solid or not?

You would be best to see if 6V GC2 size batteries will fit and then focus on energy conservation. More efficient fans, LED lighting etc. etc... An 11A draw per hour is a lot of energy..

You would also be well served to look at the consumption on your LINK as a daily number not a "snap shot"..

[PorFin]

Vision,

Before you decide the wet cell v. AGM v. gel question, I think your first step is to do an accurate measurement of what your capacity requirements are. Since you've already got a battery monitor (the Link 2000) it will be fairly simple. Measure the power consumption of each load individually, then write it down.

My Link 2000 shows 9-11 amps being consumed when the cabin fans are on, a few cabin lights, the frigoboat fridge, VHF, Radio, Radar and the iPad charging off the DC port on the companionway entrance.

Remember than not all loads will be constant drains on your batteries. Of all of those you've listed above, only the lights and the fans will be constant loads (and they should be quite small). The VHF uses less power receiving than when transmitting, the fridge will cycle on and off (the frequency of which will depend on how well insulated your cold box is, the ambient temp, how much stuff is in the fridge, etc). The radar's going to be a power hog, but again how much of one will depend on how often you use it.

If you haven't yet, I'd also recommend you get (or borrow) a decent book on boat electrics. When we decided to buy our first "big boat", I bought Nigel Calder's Boatowner's Mechanical and Electrical Manual: How to Maintain, Repair, and Improve Your Boat's Essential Systems -- I consider this to be some of the best money spent on the boat so far. Charlie Wing's Boatowner's Illustrated Electrical Handbook is also a great reference.