Duel battery conundrum.....

[Chris]

Well like I said, the one that the alternator goes to is probably the 'main one'. Toyota would be unlikely to re route the original cabling to add in a battery. Pretty sure it's the driver's side that's original.

 

[MarkW]

DS is direct to Alternatorthen there’s a cable that goes across to PS and from there to Starter.

DS is earthed back to alternator or engine iirc and also to the body. PS is earthed back to starter only from what I can see.

(Mine definately wont start off the passenger side although other people’s will, makes no sense to me!)

 

[Matt Vee]

Awesome thanks both

 

[StarCruiser]

Another unit to look at is a DCDC charger is the Intervolt DCC Pro. Neat unit for under bonnet mounting. I’ve done a write up on mine and sold 10 of them, planning to order some more in if there’s the interest.

 

[Beau]

Any reason why we can't just add a relay in series on the positive cable from one side to the other. And then have the relay triggered via something that is live only when the engine is running. Once off, relay disconnects and isolates both batteries. Or simply, control the relay via switch. Of course, the relay would have to cope with whatever draw you put through it like using it for the starter and alternator amperage. I would think 120 amp should suffice?

Something like this... less than $20 on Amazon.

 

[StarCruiser]

No reason why not Beau, except personally I would go for something far beefier in the relay department. I think Chris has used one of the Durite can type or there’s some decent looking (I think 500A) ones on EBay for not more than about £30 or so. Battery chemistry needs to be the same, lead acid wet cell.

 

[Chris]

Sorry Rich. Note to self. I meant Intervolt. I was trying too hard to be helpful.

Yes there is a reason why not to just use a VSR between the two batteries for the reasons I outlined I guess. The alternator can't supply two different batteries differently. All a split charge does is make two batteries one big averaged battery. A voltage sensing relay is perfect for triggering stuff. I use them, but I'm only going to say this once more, you need something more intelligent wrt charging two different use batteries or one of them at least will suffer. If you use your leisure battery over 3 days in the bush, but don't start your engine once, you now have two different types of battery in two different states of discharge and one charge supply. Fit a proper DC2DC system to look after the second battery. Not doing so has been the death of many an expensive battery. There will always be someone who comes along and says I have a split charge and my batteries have been fine for thirty years. Well better to be born lucky than rich, eh?

 

[Towpack]

I think most important issue here is battery type. If they are all the same, wet cell, GM etc then the alternator will charge them all. Alternators supply a voltage, typically 14.4v and a have a rated capacity in amps. The batteries (and all electrical ancillaries) draw what they need so if you flatten your leisure battery while the cranking battery(s) remain near fully charged then the leisure battery will charge at a higher rate when the engine is running. In essence, all the alternator sees in a battery is a variable resistance which goes up as the state of charge increases so it will draw less current. Its only when you start mixing battery types which may require different charge voltages and/or charge rates or have voltage drop issues due to long cable runs /bad connections between batteries that you will run into charging problems. The 12v supply to the tow socket on my tow bar readily charges the caravan battery on a long run as well as the 2 cranking batteries under the bonnett. JMO

 

[Matt Vee]

Sorry Rich. Note to self. I meant Intervolt. I was trying too hard to be helpful.

Yes there is a reason why not to just use a VSR between the two batteries for the reasons I outlined I guess. The alternator can't supply two different batteries differently. All a split charge does is make two batteries one big averaged battery. A voltage sensing relay is perfect for triggering stuff. I use them, but I'm only going to say this once more, you need something more intelligent wrt charging two different use batteries or one of them at least will suffer. If you use your leisure battery over 3 days in the bush, but don't start your engine once, you now have two different types of battery in two different states of discharge and one charge supply. Fit a proper DC2DC system to look after the second battery. Not doing so has been the death of many an expensive battery. There will always be someone who comes along and says I have a split charge and my batteries have been fine for thirty years. Well better to be born lucky than rich, eh?

Thanks dudes, this is what I was referring to in my original post, thanks for clarifying Chris.

 

[Chris]

Agreed TP. The problem is that the alternator doesn't see battery type. It's not that intelligent. So if you've a cranker that wants 14.5 initially then 13.8 but you Siamese in an AGM that really wants 14.5 most of the time then you end up either over supplying one or under supplying the other. It's this charging regime curve in effect that is crucial to batteries and the alternator cannot supply two different ones at the same time. It's like sticking two things in the oven at once. The cake needs 140 degrees and the Yorkshire Pudding needs 200. You can't have both temperatures at the same time. Personally, I'd bin the cake.

 

[StarCruiser]

It's like sticking two things in the oven at once. The cake needs 140 degrees and the Yorkshire Pudding needs 200. You can't have both temperatures at the same time. Personally, I'd bin the cake.

Good analogy Chris.

If it’s AGM or anything different to the starter battery then a dedicated charger (DCDC charger) is essential. Even more so if the leisure battery is any sort of lithium battery. As I said, not such an issue if the chemistry is the same as standard, and with an intelligent form of linking them, Durite split charge relay for example, and short heavy cables, then most issues are eliminated. This is largely old technology that is fine for flooded lead acid batteries of roughly equal capacity.

Think of the batteries as water tanks with their bases at the same level and the split charge relay as a pipe with a valve between the two and the alternator as a water supply through a ball valve into the starter tank. Virtually drain the leisure battery (tank) then start the vehicle closing the split charge relay (open the valve) and current (water) flows from the starter battery into the leisure at a high rate initially, falling away gradually until they reach the same level. The alternator regulator (ball valve) sees this drop in voltage and starts to charge (full) the starter battery (tank). Both batteries eventually equal and are then topped up by the alternator in tandem at the rate the alternator can deliver. Depending at which point the engine is switched off (namely when the batteries have equalised but the alternator has only partly filled them as a pair), you could have a partly depleted starter battery. This is where the DCDC chargers score because they manage the starter battery by not allowing it to fully drain into the leisure battery. A variable valve in the pipe if you like. They are intelligent and configurable. They also manage the charge rate (as Chris has said) based on the battery they are feeding into.
From this you can see how, in the case of a larger leisure battery, it could mean on a shortish trip, your starter battery could be quite well drained without something intelligent controlling the leisure battery charging.