Help! Dual batt advice!

[JoY]

Hi all. Currently in the market for a good dual batt setup for my d40. Have done some research and am lookin at an AGM optima deep cycle yellow top batt with isolator. Basically only need it to run a few LEDs but primarily for my 60l techni ice fridge/freezer. I think the biggest batt I can get in the yellow top is a 75ah. Apparently my fridge only draws 1.69ah in freezer mode on lowest degree setting.
So few questions.

1) roughly how long can I expect to run my fridge for without charging battery through alternator on car?
2) assuming I will only be going out a few times a year at a max stay of a week, is the 75ah sufficient for my needs or should I go bigger?
3) I've heard deep cycle batts don't like being discharged to quickly, I will be fitting a sub and 2 amps shortly, is it ok to run amps off aux batt or to save it should i go to main batt?
4) what size cabling is recommended for a setup like this. I will be running a redarc isolator. If I go to big of a cable wil it give me dramas or will I be better off. Don't mind coughing up a bit extra to go bigger.
And lastly I've heard mixed reviews on the earth cable. Is it better to run from main batt or is it fine to earth aux batt to chassis.

Hope it's not to long of a story but just want to get setup right. Fairly new to this forum and from what I've read there are a lot of cluey people on here.

Thanks in advance to all

 

[sparra04]

my thoughts
1; very hard to awnser,what load will be on fridge?
2; I have 2x 55ah optimas,i can sit for 3 days without solar,5 days with solar,running a 60 ltr fridge
3; A little bit of draw and charge is good for your battery so i would say yes
4; where is battery being mounted? 6b&s is usially good enough,remember to fuse at both ends
4a; a GOOD chassis earth is fine, i personally run earths back to battery

i would reccomend you ditch the redarc isolator and get a dc/dc charger instead
you will get a more complete charge and look after the battery properly
ctec and redarc sell good units
goodluck
cheers

 

[Old.Tony]

Can't disagree with Sparra.

At 1.7A your fridge ought to consume somewhere around 20A per day. The D31A (75Ah Optima yellow top) can safely be discharged to 20% of its capacity, so there's two answers for "how long will it last".

If you charge using an isolator, you'll get about 75% charge, or around 56Ah remaining. You can safely discharge to 20% (15Ah remaining) so you have about 41Ah - that's 2 days of operation.

If you charge using the DC-DC charger suggested by Sparra, you'll have 100% of the battery's capacity (75Ah) or 60Ah available - that's 3 days of operation.

Any lead-acid battery enjoys a full charge. At partial charge levels they start forming sulphate crystals - the lower the charge level, the faster the formation. These eventually kill the batteries. Spiral wound AGMs do resist this quite well - but an equalisation charge (with a mains charger) every couple of months is needed to slow the degradation of the cells.

As for earthing, the thing to remember is that EVERY join in any system (plumbing, electrical etc) is a potential weak point. If you put a single cable that runs from the main battery to the rear, you have just TWO points of potential failure. If you pick up from the chassis, you have at least 4 - and possibly 6 (battery-chassis, chassis-tub, tub-charger).

 

[JoY]

Thanks boys. Opened up a few options for me. Gonna get the ctek dc dc charger. I'll just run earth from batt to batt. Hey tony, Do you think it'll be alright to power amps from aux batt? And as for the ctek, is it basically like running an isolator but the aux batt has the ability to charge up to 100% instead of 75% through alternator? Im assuming the aux batt will h ave to isolate it self from main batt still right?

 

[sparra04]

your charger will be looking for a set voltage from your main batt(14.something)
when it sees this voltage(when alt is charging main batt) it will then start charging aux battery
when charger sees a drop in voltage(eng off) it will stop charging aux battery
the 2 batteries are never joined
hope this helps

 

[happy john]

Have to agree with sparra use it at home first then when in the bush you are prepared running 2 120 ah i still idle for 10 mins even though the solar will top it up with 2 engels plus the van the main thin is don't let it get flat.

 

[Old.Tony]

Thanks boys. Opened up a few options for me. Gonna get the ctek dc dc charger. I'll just run earth from batt to batt. Hey tony, Do you think it'll be alright to power amps from aux batt? And as for the ctek, is it basically like running an isolator but the aux batt has the ability to charge up to 100% instead of 75% through alternator? Im assuming the aux batt will h ave to isolate it self from main batt still right?

Do you mean "power amps" as in "supply electrical power to a sound amplifier" ?

If so, you can do it - you can put ANY load on the Optima yellow tops (including your starter motor - the D31A turns 900 amps of cranking power), but the more you use, the shorter its duration.

 

[JoY]

Awesome. Thanks again sparra and tony for the great advice. One last question if I may, at what voltage would the battery be 20%? I'm assuming somewhere around the 10 volt mark? If the yellow tops are fine to drain to 20% safely I might fit a digi volt meter so I know not to drain battery below the safe level!

 

[JoY]

Oh and also at what rate should I be charging the aux battery? I would assume a 5ah dc to dc charger would be safe? I've heard charging to fast can boil battery and shorten life.

Thanks again fellas ur knowledge is priceless

 

[crushu]

Why wouldn't you use a N70ZZ deep cycle battery? $150 gets you one with about 100AH...

 

[Old.Tony]

Technically 11.5V is considered about 20%.

Unfortunately Waeco fridges trigger at 11.75V, on the far end of the wire (and there's always a voltage drop), so realistically you might not get it down that low. I changed the power cables in my caravan from 6sqmm to 4ga (14sqmm) so that over the 6m of cable, the voltage drop was reduced from 1.1V to 0.3V.

So if you have 0.3V loss over your cable, and the fridge triggers at 11.75V, you need your batteries at 12.05V or better - and that's slightly over 50% of charge.

The key here is to minimise cable lengths, use good quality, heavy multistand cable to minimise loss, and again try to reduce the number of joins. Ideally, you want:

Battery->Fuse->---heavy---cable--->switch-->fridge

How to test for voltage drop?
This one's easy. Plug the fridge in and turn it on. The compressor will start. Now, before the compressor stops, use a multimeter and measure the voltage on both ends of the cable.

It's important to have some load drawing power through the cable, and better still to use the intended load.

 

[JoY]

Oh ok i see. My fridge actually has a 3 way switch to choose when u want it to kick in n out.
1 off 10.7 on 11.6
2 off 11.6 on 12.5
3 off 10.4 on 11.3
So I guess the third switch is best as it allows me to have fridge on at a lower voltage. But on the other hand the batt may drain below safe level voltage!

 

[Old.Tony]

Oh and also at what rate should I be charging the aux battery? I would assume a 5ah dc to dc charger would be safe? I've heard charging to fast can boil battery and shorten life.

Thanks again fellas ur knowledge is priceless

Charge rates depend on the battery. Spiral wound AGM batteries like the Optima D31A that you are looking at are nearly bulletproof and charging them at C/2 is possible. "C/2" means "Capacity divided by 2" - so in that battery, you can charge it at 37A per hour without harm. You can charge them at lower rates without harm.

Gel batteries generally don't like charge rates higher than C/10 - so a 50A gel shouldn't be charge at much more than 5 amps per hour. Flooded cell deep cycle batteries (look like car batteries) shouldn't be charged much faster than C/5 or C/4.

Personally I use an inverter + mains charger. The fridges switch over to the 240V (because I hook that up to the inverter too) and the battery becomes unloaded - allowing the charger to manage the battery perfectly. It's much more predictable than the varying charge rates you'll get off your alternator and at a specific designed voltage. It's also not as susceptible to voltage drop, because the inverter will raise the AC to 240V regardless of the input being 12.2V or 13V or 14V.

What causes a battery to fizz is also voltage. Gel batteries are really sensitive to it and thus make poor deep cycle companions unless you're careful. Fizzing is just hydrogen bubbles in the battery acid - but put bubbles into gel and they stay, they don't rise up and pop. Charging gel batteries at anything over 14.1V can shorten their lifespan - it's best to use a gel-specific charger for them, but don't use the gel-specific charger on an "ordinary" battery - it will struggle to fully charge it.

That's a lot of info there - I might have to put some of this guff into a FAQ one day.

 

[Old.Tony]

Why wouldn't you use a N70ZZ deep cycle battery? $150 gets you one with about 100AH...

If money's a concern then this is a perfectly reasonable idea.

In the longer term though, the cheap DC battery will start to degrade significantly in performance after about 2-3 years. The AGM will be going strong for 8-10 years and can handle more abuse.

I've had it suggested to me that I can drain my D31A to 0% charge - using all 75Ah of its capacity - without really hurting it. Now I'm a little bit skeptical about that, because it's a lead acid battery and still has both lead and acid in it just like the others. It's structurally superior to normal flat-plate batteries - I still won't take it that low, but knowing that I've got a much stronger, more reliable battery in there does give me a lot more confidence in my power system.

 

[Old.Tony]

Oh ok i see. My fridge actually has a 3 way switch to choose when u want it to kick in n out.
1 off 10.7 on 11.6
2 off 11.6 on 12.5
3 off 10.4 on 11.3
So I guess the third switch is best as it allows me to have fridge on at a lower voltage. But on the other hand the batt may drain below safe level voltage!

It depends on the voltage drop across the cable. If you can minimise that, try it on setting 2 to begin. If the fridge starts to power-cycle a lot, then choose the next setting. Letting the battery drop too low can be bad for the battery, although the spiral-wound AGMs are hard to kill!

 

[JoY]

That sounds great. There is a lot of info up there and all of it very useful. Hope I'm not being a pain in the ass, but if I got a mains charger running through a inverter, what would the inverter and charger draw in terms of amp hours? The 25ah main charger says a draw of 2.9 rms at full chargin rate?

 

[Old.Tony]

The charger itself will draw a bit of power - let's say an amp - and the inverter will use a little too (my 300W/100W Jaycar [Powertech] inverters draw 0.5A on their own). That means you need to be able to deliver about 27A to the inverter - around 350W. Call it 30A off the battery to be safe (and use a 400W inverter to be safe - actually I'd probably look at a 600W).

I have a little doubt about the alternator's ability to supply that power constantly. When it can't, it will just draw off the SLI (starter) battery, but pulling 25A off that while you're doing a lot of idling is not going to do any of the batteries any good.

My preference would be to look somewhere around the 7-10A charge rate, which might take a while longer to charge the battery, but it won't suck the life out of my vehicle's charging system.

In my car, I have a 6A charger in the tub, a 7A charger for my AGM in the caravan, and a 1.6A charger in the caravan (for the gel battery that runs the lights/pumps).

 

[JoY]

Hmmm, man there's so much to consider. I think I'm more confused now then when started. Lol.

To keep things simple, I think it might be worth getting the dc to dc charger. If I need fridge while car is running I can run it off main batt. When car is off run fridge on aux batt. That way when aux is chargin the fridge will draw power from car alternator. Sounds good to me, but what do I know =)

 

[sparra04]

Hmmm, man there's so much to consider. I think I'm more confused now then when started. Lol.

To keep things simple, I think it might be worth getting the dc to dc charger. If I need fridge while car is running I can run it off main batt. When car is off run fridge on aux batt. That way when aux is chargin the fridge will draw power from car alternator. Sounds good to me, but what do I know =)

theres no need to swap power sources for fridge
keep your fridge on aux battery regardless
if you invest in a good setup it will be fine

 

[JoY]

theres no need to swap power sources for fridge
keep your fridge on aux battery regardless
if you invest in a good setup it will be fine

That would make it even easier. But if I have 2 amps running from aux drawing power, will it damage charger? Fridge will only be used when camping but amps will be full time.