Confused about dual battery setup????

Submitted: Monday, Oct 04, 2004 at 23:52

ThreadID: 16792 Views:26428 Replies:8 FollowUps:9

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troy35

Well I have read many posts on this forum about dual battery setup and must admit I am very confused about all the conflicting information. Some of you say get a deep cycle for your second battery whilst some of you say deep cycle are no good unless you can charge them quickly and suggest an AGM instead. I had some major problems over the weekend with my current setup and need to fix it. Heres what happened...
I have a standard starting battery 720cca as batt 1 and the same type of battery but only a 550cca as batt 2. We left Friday morning and arrived at our destination just after lunch. We were staying in the one spot for four days so I brought along my generator to charge the batterys and run a fan at night (We are in the tropics) and a flouro light. The gennie is a Robin with a 500watt output both ac and dc at the same time.
I switched to battery 1 as it is bigger and my car will start on the second battery anyway. Oh and just to add my fridge is a Waeco 80 litre.

Anyway first night no problems, I ran the gennie at night for the fan until the fuel ran out, which kept the kids cooler but also charged my battery via the dc output.
On the second day I tried to start my car with batt 1 but no go so I cranked up the gennie again and had it running for about 12 hours. By morning though the error light on the fridge was on so I switched to batt 2 , started the car and switched back to batt 1. This gave it a decent charge after a few hours running and I also used the gennie at night again. Next day dead flat again. A mate who was with us suggested I switch to batt2 and could start my car with his power pack when it was time to go so I did that.
When it was time to go I jump started the car but it would cough and splutter under load which we later realized it was due to lack of power for the electrical fuel pump. Both batterys were dead flat and for some reason the alternator wasn't putting out a charge. I ended up driving home on my mates spare battery with no aircon in the 40C heat. I only just made it back to Broome with the car coughing and spluttering as we pulled up the driveway and the volt meter reading 6 volts.

Okay so question 1 is what could have stuffed my alternator? My mate said that switching from batt 2 to batt 1 (via col herse switch) whilst the engine is running is not a good idea. However I have been told this is okay if you do it quickly otherwise it would defeat the purpose of having a switch.

Question 2 is what type of batterys should I use for my main and aux batterys? My mate has a AC delco Voyager marine battery as his main which is both a deep cycle and a starting battery. His aux is a deep cycle. Whilst he also had to run his Robin gennie many hours each day his fridge managed to keep running overnight and he never ended up with a dead battery.

Oh and question 3 is can I plug my 240volt Mobitronic rectifier into my gennie to run my Waeco? My mate does the same thing with his engel when his gennie is running so his battery is getting a good charge without the fridge sucking out the juice at the same time. However I have been told that you shouldn't run the rectifier through a gennie as the power supply isn't regulated. Even better still can I plug the rectifier into the gennie and then connect the fridge to the rectifier and also connect the battery up to the 12 volt supply to charge the battery? Is this what the 12 volt supply on the mobitronic is for??

Sorry about the long post but I need to fix this problem.

Thanks in advance,

Troy.

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Reply By: Wok - Tuesday, Oct 05, 2004 at 05:50

Tuesday, Oct 05, 2004 at 05:50

Hi Troy35.......sad trip

>
Only your sparky could answer that .

>
That depends on your camping style & your battery drain
1. If you travel at least 6hrs or so everyday = deep cycle (Ah rating at least 2 times your battery usage)
2. If you travel a couple of hrs per day = AGM or Cranking (Ah rating at least 2 times your battery usage)
3. If camping for days = AGM

IMHO there is a POSSIBILITY of alternator failure with recharging flattened (20%)AGM because of heat from high current involved; only long term field testing will establish this.

>
The Mobitronic is not just a rectifier, it has a AC voltage stepdown circuit as well.Its stepdown circuit is SWITCHMODE. IMHO NO Switchmode should be used with Modified Sine Wave sources ( it gets hot!)
Regulation of the source is irrelevant ( there are SW supplies that will run happily from 90VAC to 260VAC, just depends on the designer)

>
IFF your genie is Pure Sine Wave output.

>
Yes, if the source is Mains/Clean sine wave genie eg: Honda eU10i

Hope this helps

AnswerID: 78863

Reply By: Member Eric - Tuesday, Oct 05, 2004 at 07:10

Tuesday, Oct 05, 2004 at 07:10

Well I got sick of all this dual battery noncence . So I connected 2 crank batteries in paralel, One icolator for nights at camp . Problem solved . Has been like this for over 18 months . I get great winching power , also I will run fridge /lights stereo , dvd player . Never had a start issue . If I have my jumper pack in the back , I wont even bother isolating one of the batteries at night

AnswerID: 78864

Follow Up By: Crackles - Wednesday, Oct 06, 2004 at 00:02

Wednesday, Oct 06, 2004 at 00:02

Nice simple system Eric. Why most people fit complicated electronic isolators has got me beat. I've gone one step further than you fitting 2 115 amp hour deep cycles in parrallel as a trial. 4 days in up to 35 degree heat running fridge & lights & still it started. Like your's never had a start issue, 20 months & going strong.
Cheers Craig.....................

FollowupID: 338435

Reply By: Wok - Tuesday, Oct 05, 2004 at 08:49

Tuesday, Oct 05, 2004 at 08:49

Just to clarify my last statement;

The Mobitronics CANNOT be used to charge the battery, but you can run the Mobitronics in parallel with the DC output to the battery from a clean AC source.
Seriously, you need to measure the DC voltage from the genie to check if it is enough to fully charge your battery of choice (eg 14.1V for wet cell Pb-Pb). The genies I have seen give out 13.8V.

AnswerID: 78875

Follow Up By: troy35 - Tuesday, Oct 05, 2004 at 10:50

Tuesday, Oct 05, 2004 at 10:50

Wok,

So can I use the mobitronics on the gennie 240 vlt supply to run the fridge whilst also charging the battery to give the battery a break??
The gennie from memory does output around 13.8volts. Is this enough? As I mentioned further down in this thread my mates setup works fine and he never seems to have a problem with keeping his battery charged from his gennie. The only thing different he does is run his fridge from his gennie while charging his battery, but he has an engel so doesn't use a mobitronics.

What exactly is the + and - terminals on the mobitronics for? I thought it was too charge the battery? Could you explain that again for me as I am a little confused.

Thanks again.

FollowupID: 338310

Follow Up By: Wok - Tuesday, Oct 05, 2004 at 12:59

Tuesday, Oct 05, 2004 at 12:59

Troy,

The battery should be charged via a direct connection from the genie DC out.
The Mobitronics is intended to be used in a caravan park situation where powered sites are available.It is not necessary to use the Mobitronics when you already have DC available from your genie.
The genie is running the fridge & charging the batttery simultaneously when connected in parallel to the DC output of your genie.

What is the model of your Mobitronics? Hope I am not creating more confusion!

FollowupID: 338345

Follow Up By: troy35 - Tuesday, Oct 05, 2004 at 13:07

Tuesday, Oct 05, 2004 at 13:07

Wok,

No I am not confused and understand what you mean. A caravan park would have clean power so you can charge your battery and run your fridge at the same time. The mobitronics is an MPS-50A.
The reason why I ask about running the fridge through the mobi is that my mate who does this with his engel says that his battery gets charged quicker because the battery doesn't get drained by the fridge which is running on 240v from the gennie.
Then again someone else told me that the gennie outputs 8 amps and the Waeco only draws around 4 amps when running which isnt constant anyway so the gennie has more than enough power to charge the battery whilst the fridge is still running.
Would you agree with this?????

FollowupID: 338347

Follow Up By: Wok - Tuesday, Oct 05, 2004 at 16:46

Tuesday, Oct 05, 2004 at 16:46

Troy,

I believe that a battery is necessary when the primary source is a battery-charger (at least with a Trailblaza). The start current is about 14A in my case...maybe your Waeco is different? Your handbook should give you a recommended method.
Whether there is enough current depends on your fridge's dutycycle at the time eg: If dutycycle is 50%; Over 10hrs running the genie;total Ah is 10X8=80Ah; fridge takes 10X0.5X4=20Ah Balance 60Ah goes to battery, the actual value is lower because 13.8V won't charge the battery to 100%...assume 80%...then battery gets 48Ah.
You need to do measurements to get an objective answer...guessing is no good!
Maybe your mates Engel has a Linear powersupply to convert AC to DC (THE method to use with unpredictable AC sources!)
There is nothing stopping you building a Linear PS....thats another story!

FollowupID: 338385

Reply By: Willem - Tuesday, Oct 05, 2004 at 08:50

Tuesday, Oct 05, 2004 at 08:50

I have two 700cca cranking batteries wired up in parallel through a Rotronics MH10 battery isolator. I have recently transferred this system to my newer vehicle. I did all my own wiring and followed given instructions. I run the winch off the cranking battery and fridge, lights, laptop, GPS etc off the auxilliary battery. The system works well and I have had no problems whatsoever.

I have very little knowledge of electrics but what I have found in the past is that when charging you car battery with a gennie just let the gennie run for 5 minutes and then try cranking the engine. Then you have to take the car for a drive to get the battery charged up. You may have spiked(whatever that means) your alternator when charging through the gennie whilst running other appliances. Who knows? Maybe na sparky on this forum could help you.

AnswerID: 78876

Reply By: Peter McGuckian - Tuesday, Oct 05, 2004 at 09:01

Tuesday, Oct 05, 2004 at 09:01

First question - what vehicle?

I have a patrol and use a Waeco. I have the Piranha system fitted and it works very well. Use 2 cranking batteries - xide etxremes. It is important to watch voltage levels and if the 2nd battery level drops below 12 then run the car for an hour or so. This charges the battery back to full. I also use a solar panel to run the fridge (via the battery) when we are camped.

Hope this helps.

Peter

Peter

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AnswerID: 78877

Follow Up By: troy35 - Tuesday, Oct 05, 2004 at 10:59

Tuesday, Oct 05, 2004 at 10:59

Peter,

My vehicle is a GQ Patrol. I ran the gennie to charge the battery when it got to low but after running the gennie for about 5 hours I still couldn't start the car so switched over to the 2nd battery, started the car and then switched back to batt 1 to start charging it up. 2 hours later it was charged enough to re start the car. However by the next morning (and this is even with the gennie charging the battery up again for another 5 hours at night while I ran the fan), the car wouldn't start.

FollowupID: 338312

Reply By: MrBitchi - Tuesday, Oct 05, 2004 at 09:11

Tuesday, Oct 05, 2004 at 09:11

In a modern vehicle with a computer I personally would never use a mechanical switch as voltage spikes can kill the computer.

Put in a Readarc Isolator (or similar). Run all the accesories off the second battery. When it goes flat, disconect it from the vehicle and recharge it with the genny. Using an electronic isolator will prevent electrical spikes from damaging the alternator or the vehicle computer. This setup can also be wired to use the second battery for jump starting the vehicle if the main battery goes flat, simply by wiring in a switch.

I use a Trojan deep cycle. A deep cycle battery will survive complete discharge better than a normal battery but will take longer to charge.

I just plug my Waeco into the second battery. (I don't have a genny). Haven't had a flat battery yet, but I tend to go driving each day, even when camped in one spot for long periods.

John

AnswerID: 78881

Reply By: tonysmc - Tuesday, Oct 05, 2004 at 10:22

Tuesday, Oct 05, 2004 at 10:22

Giday Troy, I would first check if the robin does put out ac and dc at the same time! All gennies I know of will only do one at a time. This would explain why you could run the fans all night but the battery didn't charge. What I always do is carry a seperate battery charger and plug it into the 240 volts while the genny is going. Also when you charge off the genny,I believe it puts out about 8 amps and have been told that unless you disconnect the battery it can stuff the diodes in the altenator.A stand alone charger only puts out 2amps with 4 amps max which is ok. With an aux battery and a isolator everything should be disconnected from the system anyhow and wouldn't be a problem, but charging the main can be. I know I will get some abuse for this, but get 2 batteries the same size and both starting batteries. The reason is deep cycle (wet cell) take a long time to charge up. I know you can buy those flash deep cycle but how can you justify the expense when a starting battery of the same amps does the same job. As with all Aux batteries I would put in a low volt cut out switch which cuts off the power when it gets down. Hope this helps some. Cheers

AnswerID: 78900

Follow Up By: troy35 - Tuesday, Oct 05, 2004 at 10:43

Tuesday, Oct 05, 2004 at 10:43

Tonysmc,

Yes the Robin does output AC and Dc at the same time. My mate owns his own marine and small engine business and he has had a Robin for years. He said that it is one of the only few gennies that will output both at the same time. He has also never had a problem with burning out diodes before and simply connects his gennie up directly to his battery whilst it is still connected to the car.
This is what I cant understand, he has a marine battery which is both a deep cycle and cranking battery for his main and a deep cycle for his second battery connected up through a redarc switch. He runs his gennie for around 6-8 hours a day and doesn't have a problem with his battery getting too low. I run mine the same and seem to still have a flat battery the next day.

FollowupID: 338307

Follow Up By: Wok - Tuesday, Oct 05, 2004 at 15:24

Tuesday, Oct 05, 2004 at 15:24

Tony,

You mention a low-voltage cut out switch.....do you have any models/part numbers/circuits?
I am looking for an adjustable one (12v...12.2V)

Thanks

FollowupID: 338373

Follow Up By: tonysmc - Wednesday, Oct 06, 2004 at 14:02

Wednesday, Oct 06, 2004 at 14:02

Giday Wok, About the low-voltage cutout
I have a custom made with I have used for about 15 years now, however I am not sure whats on the market now, as I have had no need for another. I just move it from car to car. (It works a bit like a dual battery isolator)
Having said that, I have made up a few for friends using the one from Dick Smith. Just had a look at the web site part number k-3124,$30. It comes made up or in kit form. In kit form is better as you can make a few adjustments. The instructions are easy to follow and you just need a soldering iron.
This next bit will make sense once you have the kit, don't be scared off. ( I'm doing this from memory, so I hope this is right) Change R1 to 56K, R2 to 120k, R6 to 68k and RV1 to 100k. You then adjust the trimpot (RV1) to set it at the voltage you want it to drop out at. Of course you will need a multi meter to do this. I will catch up with one of the people I did it for in the next couple of weeks, so I will check it out but I'm sure this the right combination

FollowupID: 338502

Reply By: troy35 - Tuesday, Oct 05, 2004 at 12:38

Tuesday, Oct 05, 2004 at 12:38

Well my local auto elec just confirmed why I had no luck keeping my battery charged whilst my mates batterys had no problems. It was because my battery was a Calcium battery which he said takes longer to re-charge than a normal cranking battery (up to four days on a bench). As to why my alternator has stopped charging I am still waiting for him to have a look and get back to me but he did say the standard Nissan alternators have weak diodes and are known to fail.

AnswerID: 78910