Lost with dual battery systems and all the options available

Hi Will,

There are people on here who will be able to help you out much more than me but a few observations/questions from your post:

What sort of fridge are you running? Is it a 3 way? Only ask as I make 808W to me is 67amp hours which is very high for a compressor type fridge.

The Piranha and Redarc look to me to be different products. The Piranha is a dual batter isolator that can accepted regulated solar input (up to 100W) and the Redarc is DC-DC charger with a built in MPPT solar regulator. To compare apples to apples you need to look at Redarc BCDC1240 v CTEK D250s dual.

I am only starting out on a similar route as you and will watching for replies with interest.

Cheers
Tate

Will T2

Probably you need to zero in maor accurately so people can reply with their experiences and advice.

1. Your fridge. what voltage does it run on and what type is it? Does it draw 808 watts while running? Not sure what you mean by "808wh and is on 24hrs. Any more fridge info available?

2. A laptop will suck 75 watts from/through the 240v charger but doesn't do that when charged and normally running. Those figures appear to be flat battery, full current flow type figures. It can't be flat all the time. Do you use a DC/DC convertor fro that purpose.

3. Lights, 30 watts is ia about 2.5 amps continuous while running and so is usually stated as 2.5 x 4 hrs = 10AH Anything in AH is then able to easily be calculated for normal charging requirements and use from batteries.
4. Phone charger and other devices 100W! that equals 8 and a bit amps of use which if run for an hour is, say, 8.5 AH

I can understand the lights and the laptop and phone but the fridge has me stumped at present until we know more.

After that is under control and understood then any charging and battery requirements can be estimated, guesstimated or even worked out.

Cheers
Ross M

I have just installed an AGM battery and a Ctek 250s dual charger.
From my research, once you establish the ah required you need to charge the batteries correctly.
The better deepcycle batteries are AGM, they are more expensive but less costly if treated correctly. They don't like engine bay heat and like to be kept charged.
AGM need higher voltage than most generators produce hence the need for a DC to DC charger that protects the crank battery too. The Ctek and any other quality charger will have 5 stages of charge. The final stage is a float charge.

The Ctek limits the charge to 20 amps which may be a disadvantage in the early stages of charge as the alternator can have more amps available. To utilise the additional amps available they produce a Smart Pass module.

You need to establish how much is used from the battery and how much you have put back so you don't drive the battery critically low. Low level discharge reduces productive cycles achievable. The devices to provide reliable info are about $400, using a volt meter only provides ballpark figure. It takes some hours of driving to recharge the batteries fully.

To keep the battery 'full' I use a 250w solar panel that goes through the charger.

There will be many other good chargers look for 5 stages, start battery protection, battery temperature sensing and sufficient capacity to charge the batteries. Use heavy cable, quality swaged terminals and fuses to prevent voltage drop so the charger can work at full capacity.

I have no association with Ctek, just my choice. Hope it goes well for you.

Barry

I prefer voltage sensing relays rather than dcdc converters. In my opinion a 20A charger is too slow for 240Ah of battery. Connect it straight to the alternator and let it charge as fast as the alternator can pump the current. AGM batteries are typically ok to charge at around 0.5C. They have very low internal resistance.

AGM do not require a higher voltage to charge. They do require a higher voltage to charge quickly as they approach 100% charge. So in the final few % of charge a dcdc converter is faster. Direct off the alternator is faster the rest of the time, during the bulk charge stage of the charge. For short runs, for example driving from camp site A to camp site B, or idling to charge your batteries, you are better off direct off the alternator.

I think your calcs are worse case. On average your laptop should use less than 75W, phone charger etc on average less than 100W.

All the products mentioned will do the job. Decide what sort of isolator, what sort of battery, then find a good price.

Will,

I think I can see how you've done your maths now. Your result of 110.7 is actually Amp-hours per day. That's fine, we can work with that.

However, since you have a 240V fridge running off an inverter you need to allow for the inefficiency of the inverter. I don't know what yours is, but lets work on 80% efficiency. So your fridge will need 100/80, = 1.25 times the placarded consumption, = 1.25 x 808, = 1010 Watt-hours.

So now your total watt hours per day = 1010 plus 300 plus 120 plus 100, = 1530 Watt-hours. Divide by 12 to give amp-hours, = 127.5 Amp-hours per day.

That's a lot, but you say you want to be conservative, so we'll run with it.

You've said you're considering 2 x 120AH Thumper batteries, = 240AH battery capacity. I'd say with your daily consumption that 240AH would be the minimum you should consider because the best balance of daily use vs total economic life of the batteries is to only take them down 50%. In round figures, ONE day without charge will get you to that point. Two days without charge will flatten your batteries.

Mate, you're going to have quite a job getting adequate charge into those batteries to both charge them and offset your use and also to maximise your unpowered camping.

I agree with outbackjoe - a 20 amp dc-dc charger is unlikely to be enough unless you plan to spend all your time driving or idling the vehicle.

There is merit in Joe's suggestion that you use the higher output of the alternator to bulk-charge your batteries to about 80%, then finish them off with a dc-dc smart charger. Others may disagree, but I don't think there's any way you can do this on the cheap with your projected useage and those batteries.

Sooo...think about this....

A Ctek D250S coupled with a Ctek Smartpass. As I understand it, this system will use the high output of the alternator (Smartpass maximum is 80 Amps) up to a certain point, then finish off the charge with the 20 Amp multi-stage D250S. The system is good for battery capacities up to 800 AH, so you could increase your battery capacity if you wanted to in the future. Here is a link to the manual. Page 11 shows the configuration which I think you need, and a bit of text to support the idea.

To maximise the benefit of this system, you NEED BIG CABLE from alternator all the way to the D250S/Smartpass, and from there to the van battery. If it's 6 metres from car alternator to the Ctek installation in your van, you will need two runs (a positive and a negative) of 2 gauge (= 2 AWG, = 2B&S). That's big stuff and will be expensive. Assuming your Ctek gear is about a metre from the battery, you will need 6 gauge for both +ve and -ve. I used this calculator to arrive at those figures. I used 13.8Volts, 80 amps, 5% loss between car and van, 6m x 2 for a cable run, = 40ft round figures. Between Ctek and battery, 1m cable x 2=6ft, 2% loss, 80 amps, 13.8 volts.

I think you will need a 40 to 50 amp multi stage mains charger for when you need to run a generator. A good quality one with a high power factor (0.9 or so) could possibly run off a 1kVa genny, though you might need bigger. More research needed. The large mains charger will reduce the running time of the genny and minimise annoyance to other campers. If you're not thinking of a genny, just hooking up to mains, then a smaller, cheaper, less efficient mains charger could suffice.

If you're thinking about solar to extend your time off-grid without running your car or generator, then you're going to need a fair bit of that too, plus a decent regulator. The Ctek D250S can take a solar input which is nice and convenient, but I don't know what its max solar input is. I suspect it will not be enough in the above scenario, so a separate solar regulator may be needed. A subject for another post if you're interested.

Cheers