Hodge's Dual Battery System

[threedogs]

I have the simple Redarc solenoid unit on the firewall running a Trojan 115 AH battery ,
these are wet cell and the same battery they use out on the shipping bouys in the Bay.
You might be better off running a designated wire and thermo couple to power your fridge,
using a merit or better still a 50A Anderson or the twist lock Engel/baintech plug and socket.

[cjb]

My 2 cents what about the SBI12D. If you run your main down it can jump it.

[megatexture]

Ive got a ctek, same sort of thing... regarding jump starts just run a wire from pos to pos and have an isolator switch under the bonnet ..or use jumper leads .if anyones getting a flat enough that they need a switch on the dash to link batteries for convenience then they some bugs to work out.

11.6 volt is half charge of a battery any lower than that and you will do damage to any battery agm's are just able to discharge that little more but it will still be doing damage

[Cuppa]

I have stayed quiet on the dual battery set up discussions lately.
I will make my points in this post & leave it at that, hopefully remembering where I did so, so I can refer back to here in future, rather than becoming overly repetitive.

I have trouble accepting the ‘the battery will charge faster via a smart solenoid than it will via a dc to dc charger’ statements.
Whilst they are true to an extent I believe they are statements which need qualifying.

An alternator will ‘bulk charge’ an aux battery (ie. via a solenoid) faster than most DC to DC chargers.
HOWEVER this bulk charging will only take the battery (AGM) up to around 80% charged (lower for wet batteries), unless as some here have found the owner drives for a long time.
ALSO if the owner is treating his/her battery well, they will not have discharged it below 40% to 50% & thus the period of bulk charging will be far less than the battery murderers who lets their battery become discharged to a point of being ‘flat’. (If treated this way no type of charging will help to prolong battery life).

Where the DC to DC chargers come into their own are as ET suggests when batteries of different voltage/chemistry to the crank battery are used, or where voltage drop courtesy of distance from the alternator are issues. HOWEVER where they also help greatly is in two further ways.

1. Charging the battery from around 80% to 100% in the shortest possible time - might not sound a big deal, but if you treat your battery well to ensure it lasts for many years by not discharging it beyond 50%, that extra 20% virtually doubles your ‘useable capacity’. Getting that extra 20% in with a DC to DC charger will be much quicker than direct from the alternator (via a solenoid).

2. Many batteries are killed by undercharging, but many also by overcharging. Lots of folk have a dual battery system where effectively the aux battery is unused most of it’s life. Every time the engine runs, the alternator is trying to charge it by throwing 14.something volts into it. A DC to DC charger is a bit smarter & ‘floats’ the battery at a much kinder voltage once full. My one at 13.3v. Others usually at around 13.6v.

Ask all the folk who happily state how well their solenoid set up works how long their batteries last for. Most are pleased to to get 4 years, many get a lot less. I will be very disappointed if I get less than 8 years, & hope for more.

It is possible to set up the aux battery to provide emergency jump starting whilst having a dc to dc charger, I have. It does mean however that the emergency ability bypasses the charger.

Worth dispelling the myth that a solenoid is a charger. It is not. It is just a switch which allows the alternator to charge the battery.

I have found that the 40 amp dc to dc charger I have to charge my bank of 360Ah AGM batteries to be quite adequate for my purposes, (in conjunction with solar if camped without driving for more than a couple of days). Should a 40 amp unit be insufficient for anyone, there are a couple of choices which allow the user to continue to be kind to their batteries. Either install a dc to dc charger, but have a means of bypassing it, allowing bulk charge from the alternator to bring the batteries up to around 80% as quickly as possible & then switch over to dc to dc charging to ’top up’. (I have this ability in case I am ever in the situation where my batteries have been discharged lower than normal & I don’t wish to drive a long distance) OR to use the Ctek DC to DC charger in conjunction with it’s Smart Pass ‘box’, which I believe allows smart charging at up to 80amps. (Even though some alternators are capable of putting out more than 80 amps it is unlikely that the battery capacity used by most 4wd’ers would accept this amount of charge. Allowing an alternator to continuously output close to it’s full capability for more than a short time is also a good way to kill the alternator).

Of course it can also be argued that the expense of fitting DC to DC chargers can be offset by using a simpler solenoid set up & buying replacement batteries more often. The point is that those who want their batteries to run their fridge, lights etc over a full weekend will need to do this & batteries are not cheap. In the long run the DC to Dc setup will work out to be cheaper , & I personally feel better about making my batteries last longer, & conserving resources.

Again I want to be clear that I am not looking to be argumentative or tell others what they should do, just repeating my thoughts in a regular ‘horses for courses’ debate.

[Cuppa]

11.6 volt is half charge of a battery any lower than that and you will do damage to any battery agm's are just able to discharge that little more but it will still be doing damage

From the table below you can see that 11.6v would actually be only around 20% capacity - well into battery damage territory. Half charge is around 12.1v (on a rested battery with no loads of course). My preference is to never to let the voltage drop below 12.2v, but with smaller battery capacity this can be impractical. I would suggest a minimum of 11.9v or higher if long term damage is to be avoided.

600573_SoC_Table.jpg

[Hodge]

Thank you for everyones' input. I might have mentioned it in the earlier posts, I have gone down the DCDC path. I'm in the process of getting everything fitted, with a Redarc 25A charger. In a nutshell, the only advantage of the smart solenoid is he ability to parallel the main and the off-set batteries, for starting purposes. Nothing a set of leads could fix.
Cuppa's write up just about sums everything that I've researched, and clarifies the DCDC is what I need.

[lhurley]

From the table below you can see that 11.6v would actually be only around 20% capacity - well into battery damage territory. Half charge is around 12.1v (on a battery rested battery with no loads of course). My preference is to never to let the voltage drop below 12.2v, but with smaller battery capacity this can be impractical. I would suggest a minimum of 11.9v or higher if long term damage is to be avoided.

600573_SoC_Table.jpg

Thats a really handy diagram. Would be something to print out and have on hand, stuck near your voltage meter or in with your multimeter.

[Cuppa]

Thats a really handy diagram. Would be something to print out and have on hand, stuck near your voltage meter or in with your multimeter.

Yes but do bear in mind that it is only going to be accurate(ish) if the battery is doing no work & has done no work for half an hour or so. Something that never happens with a cycling on/off fridge. It really is only a very rough guide. Switch something on & the voltage might drop below (for example) 12.06v. This does not necessarily mean that the battery has been 50% discharged, because when the ‘something’ is switched off, the voltage may slowly rise back up. So voltage cannot be used as a ‘Battery fuel gauge’. Where it IS useful is in providing the battery owner with comparative data, this becomes useful in regard to developing a familiarity with your system over time, so you have a better idea of whether what you observe is ’normal or to be expected in the circumstances.

The only way to have a ‘Battery fuel gauge’ is to use a battery monitor with a shunt. This counts the amps in & amps out, with the user having set the total capacity. Even then it is not always 100% accurate, because of the way a battery works (chemical reaction), but it is the best there is available. Personally I prefer the ‘familiarity model’ but this takes time & regular use to achieve, not easily done on a weekend here & there.

[mudski]

I have stayed quiet on the dual battery set up discussions lately.
I will make my points in this post & leave it at that, hopefully remembering where I did so, so I can refer back to here in future, rather than becoming overly repetitive.......
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.
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Again I want to be clear that I am not looking to be argumentative or tell others what they should do, just repeating my thoughts in a regular ‘horses for courses’ debate.

Mate can you just repeat what you said.

[Hodge]

Got all the bits and pieces, ready to go. Just wondering one thing. The negative from battery to battery. I can just ground the secondary, but am I right to also run a cable between each battery too ??