Maximum House Battery Amp Hours Question

[GJarrett]

The Sundancer I bought last year has two deep cycle batteries for the house bank but they are smaller batteries (either 24 or 27 series?) and run down in the middle of the night if we run lights, all fans, and the refrigerator. I've already gone LED on lights and made other mods to reduce electrical draw; I suspect the main amp draw is the frig. Even setting its thermostat up, it still cycles on during the night and pulls the batteries down. The Admiral insists that we keep the frig running for our weekend excursions. We don't have A/C so the fans are essential. So I have to figure out how to keep everything running while we're on the hook.

Batteries seem to be marketed with the goal to confuse the consumer and I'm no electrical expert, so I've tried to simplify my understanding by concentrating on amp hours. As I understand it, I think I need to look for a dual battery bank that supplies me with the most amp hours I can get to run the house bank on the boat so I don't have to get up in the middle of the night to start the engines and recharge batteries. That is my goal. I've been shopping for 29 or 31 series 12v deep cycle batteries and the best seem to be rated at about 115 amp hours or so. However, I have learned from my local RV dealer that they get maximum duty from 6v batteries - their Interstate Battery 6v model (the same basic size as a 29 or 31 series 12v) is rated at 235 amp hours.

That brings my question.... which I assume is elementary electronics to the experts (but I am not one) so here goes.... is an amp hour an amp hour no matter the voltage?

As I understand it, I can buy a couple of 12v 31 series deep cycle batteries with 115 amp hour ratings and wire them parallel to hope for a 230 amp hour performance from the batteries to power my house bank for weekends on the hook. However, I also have the option of buying a couple of 6v RV batteries with a 235 amp hour rating. If I connect them in series to generate the 12v needed for my battery bank, will those two batteries still only supply the 235 amp hours of performance of a single battery, or will the two of them double up and provide 470 amp hours at 12v? If that is true I'll get the 6v batteries.

Or, for the experts, is there something I'm getting wrong and not understanding, and need to go another direction?

 

[MonacoMike]

Far from an expert, but this is my understanding: The 6v batteries will only provide the 470 amp hours at the 6v level. If you parallel them to 12v you will get 235 amp hours at 12v.

We always turn off the refrigerator overnight when on the hook and not running the genie. Just wait until you are done getting in and out of it, let it cool back down and turn it off overnight. The keep the admiral happy get a cheap indoor/outdoor wireless thermometer to keep an eye on the internal refrigerator temperature.

MM

 

[GJarrett]

That's what I'm afraid of but was hoping for otherwise - the 6v batteries are about the same size but I know there's no free lunch. If I can't get double amp hours out of them there's no real reason to convert.

Waiting on an electrician to confirm for sure..... 235 amp hours or 470 amp hours at 12v?

 

[Alegria]

Have you load of tested the batteries to make sure they are taking a charge properly? How old are the batteries? Also when I was looking at fans I found the Marine fans used less amps then the 12V ones you get in auto stores. A whole lot less. A Group 27 battery should be able to handle your load overnight. Add up the amps you are pulling or just turn on all your loads and read the DC amp meter in your panel if you have one time hours gives you how long your battery will hold. Can you use battery fans that run off D cells and save your batteries? I guess you are running your frig all day and night that plus your other lands may be too much.

Sent from my Galaxy S3

 

[Westie]

2 6 volt batteries at 235 amp hours each can either be connected

in series to give 12 volts at 235 amp hours
or
in parallel to give 6 volts at 470 amp hours.

I have two 12 volt 115 amp hour batteries wired in parallel for my house battery and can be on the hook for a day and a half before I have to charge them. This is with the fridge on the whole time, the vacuflush, lights (including anchor light on over night) and the inverter for the coffee maker in the morning.
Are you sure that your batteries are fully charged before you start?

Your fridge should draw 4 to 5 amps when running so even if it was running continuously it would use 120 amp hours over a 24 hour period. This would be about all the usable energy from a 230 amp hour battery pack. The duty cycle of your fridge should be on the order of 20 to 25 % so the fridge alone should last a few days with the batteries.

You either have a large current draw that you don't know about, have bad batteries or are starting with them not fully charged.

 

[GJarrett]

Thanks for the info. I bought the boat last year. A couple of months ago I removed the batteries and had them checked (they're 3 yrs old) and they checked okay but when I looked at the readout I noticed that the test showed good for CCA which doesn't really apply here, and I've learned that a newly charged battery can throw some sort of false-positive test. I wondered at the time if they could test good for CCA but since I had spent all summer last year completely draining them (way down past 50%) several times, the deep cycle performance was shot even while CCA showed good. I'm leaning toward thinking that I simply need new ones and any 29 or 31 series will take care of my problem.

Now that a couple of you have provided info on how yours lasts and the current draw that the frig should be using, I realize that one battery should do all I need overnight; two for sure should be giving me no problems at all. The engine alternator fully charges them before we shut down and anchor, so if two new batteries don't solve my problem then you're right; I have a large current draw that I'll have to track down and fix.

Thanks again for the replies.

 

[MonacoMike]

You really should consider AGM'S as they can be discharged to a lower level without damaging the batteries.

MM

 

[kaz911]

Buy a cheap DC Amp Clamp meter. That can measure consumption where ever you clamp it to wire. So start from batteries and move on to fuse boxes. They can be had for < $100 and some have a Multimeter built in as well - which is essential for debugging ANY power issues on a boat.

But for a good battery test - fully charge your batteries - disconnect them from ANY load. If they show LESS than 12.7 volt after 1-2 hours they are not in peak condition.

And as MM says - get AGM batteries. If you are looking to upgrade - get the LifeLine 125 Ah G31 batteries. They offer the most Ah/lbs and have a 5 year prorated warranty. They are dedicated MARINE batteries. You do need to upgrade your battery tray's though.

 

[GJarrett]

Thanks again. I just looked at those LifeLine batteries and they are very impressive but HOLY COW a couple of them would cost $800+!

 

[kaz911]

Thanks again. I just looked at those LifeLine batteries and they are very impressive but HOLY COW a couple of them would cost $800+!

On my old sail-boat I put in 2 - and they are still going strong after almost 8 years without any form of A/C or forced ventilation in Tropical+ heat.

And you could manage with smaller - as Lifelines does not complain as much as LA batteries about discharge levels. I always calculate Ah/lbs, number of factory specified charge cycles for 70, 50 and 30% discharge. Then you get a good indication potential battery life. Lifeline is an offshoot of Concorde batteries for Airplanes. I think you can find the 100 Ah GPL-31T for around $270-300 if you shop around.

But read their specs / battery manual - then try to find the same kind of detailed specs from ANY other suppliers of batteries. I have not found any yet. And @ 50% discharge - they should last for 1000 cycles... That is a lot of drain. Most batteries last for about 500 cycles @ 50%

 

[M99]

Here's where your confusion comes in. It starts in that your appliances, lights, etc, all need a certain number of watts of energy per hour to function. Volts x Amps equal watts. So a 50 watt light bulb, just as an example, will draw 4.16 amps per hour on a 12V circuit. Most of your appliances will probably be designed to work on 12V and will draw a certain number of amps per hour at that voltage. As the voltage drops, they will either draw more amps to get the watts they need, or stop working as a protection from electrical damage to them. Running batteries in Series increases the voltage, not the amperage available. Running batteries in Parallel, increase the amperage but not the voltage. There is no logical reason to run 6V batteries in a 12V system.

Battery CCA ratings are meaningless and just marketing gimmicks. What you care about is the total AMPS, meaning Amp Hours at their rated voltage that they store is.

 

[Creekwood]

The reason for using 6v golf cart batteries as a house bank are as follows:
1- they are designed to run down very low and recharge over a high number of cycles. They last longer if that's the way you use them.

2- a large battery bank of 2 x high capacity 6v can be lifted by one person into place into the back of a bilge. Can't say that for an 8D with similar capacity.
3- ifor the dollar I don't think you can do better $ per amp hour by trying to find good deep cycle 12v batteries.