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Snagger

Twin battery, twin alternator vapour build

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Hi everyone.  I’m toying with the idea of a second alternator instead of a split charger for my second battery, which runs all the accessories and power outlets.  The idea is to have short charging time to ensure full batteries, to reduce the need for much solar charging and also as redundancy for a failed alternator while away (the batteries would be fitted with a direct link through an isolator switch, left open and only closed for flat battery situations).

Can anyone who is up to speed on such systems take a quick look at my sketch and see if it it’d work, and if there are any changes that would make it work better, please?  For reference, the “R” boxes are relays for the auxiliary system to disconnect the accessories when the key is out of the ignition to prevent accidental drain and for the ignition switch to indirectly control the diode pack on the secondary alternator, so that the aux battery provides the power to the exciter coils (making sure the voltage on each side of the charge light matches and keeping the two systems isolated under normal conditions).

CCE8EC9A-7EA4-4471-BE60-6785C7E43C4D.jpeg

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It won’t work as there is no ground connections 😉😉😉😉

In this scenario, would it make more sense to have winch off Batt 2? So therefore have no anticipated high drain conditions off the starter battery.
 

Also, I assume you don’t ever want to connect the batteries (and alternators) in parallel for the winch?

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Connect them in series for the winch. 😈😈

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4 hours ago, SteveG said:

It won’t work as there is no ground connections 😉😉😉😉

In this scenario, would it make more sense to have winch off Batt 2? So therefore have no anticipated high drain conditions off the starter battery.
 

Also, I assume you don’t ever want to connect the batteries (and alternators) in parallel for the winch?

I have that at the moment, but that’s because I have two 82AHr main batteries.  I’m assuming running a leisure battery would be far better for the auxiliary system for camping, especially if I ever fit a heater like a Webasto or Eberspacher, but I don’t think that’d be any good for the winch.

The idea is to also have a direct link between batteries with an isolator to allow the leisure battery to start the engine in case of main battery depletion, probably having to let it charge the main battery for a while first.

I forgot to mention those circles with an X near the alternators are the charge warning lights, but you probably worked that out already.

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Rather than the link, carry a jump lead? 

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my 2 batteries  main use/start batt & winch use only, these are feed by a single 100amp alternator  & charged by LR's split charge relay, each battery has it's own voltmeter, in addition to the split charge I added a isolator switch so I can join both batt positive terminals together (as a internal jump start)

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How much charge do you imagine the batteries need, because if you can manage to pour more than 100A into a pair of 82Ah batteries for any length of time something's wrong and/or on fire.

In fact you may be totally wasting your time as a leisure battery may very well not accept more than ~50A from the alternator in the first place. The C/10 rate is 8.2A, the C/5 rate would be 41A so much beyond that is probably not in the battery's happy place.

I mean, if you want to go to the trouble of installing a spare alternator rather than just carrying it (or leaving it at home) by all means... but a single alternator and a cheap split-charge is likely all you need.

Given you've got a TDi, which famously doesn't require electricity to run, why lug a spare alternator around?

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Lead acid batteries don't take more than 25A without being in pain, a twin alternator setup is a waste of time and money.

 

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All interesting replies, and all are consistent in that I’m overthinking things again.  
 

I like the idea of the redundancy of two alternators, but having a spare in a box on long trips is probably more sensible.  I have a 65A alternator right now and a cheap Durite split charger (more of a heavy duty relay, really) that is controlled by the alternator’s charge light/diode pack terminal.  It works well enough, but I was thinking that having two split systems would prevent the main battery from being drained by a depleted auxiliary when running the engine at low rpm with many other electrical demands on and would also allow far more rapid charging so that a short drive would replenish both batteries (evidently not correct from David and John’s responses).

So, leaving as is and perhaps using a 100A alternator (keeping the 65 as a spare) is more prudent, you think?  It saves me from moving the PAS pump and Kenlowe Hotstart from their shared mounting atop the timing case, and saves me having to find a way to attach the19J/Defender 200 PAS and alternator bracket to the Discovery engine (the two longitudinal bolts are fine, but there is no part of the timing case to take the big stabilising lateral bolt).

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I was looking at this set up, but I've decided to just up-rate the alternator to a 100 amp unit.

I found a 65 amp unit was fine for charging purposes 99.9% of the time. But that .01 % of the time, the poor thing was having to work too damn hard

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This is what I've got on the ambulance - bit smarter than standard split-charge, but cheap and fit & forget.

https://www.furneauxriddall.com/acatalog/12-24V-100A-Split-Charge-Relay-SA_BS100.html

I used a ~95Ah AGM Start/Stop battery (Varta Silver Dynamic) on the recommendation of TSD.

The 2nd battery won't drain through the split charge if it's actually working correctly - if the battery voltage drops below what the alternator can keep up with, the battery light would come on & the split-charge should drop out.

Your main battery is only discharged by starting, and that's replenished in a few minutes driving - the rest of the time I don't see how a 200TDi Series can be presenting much of a load on the system even at night with the heater and lights and wipers going... that's maybe 20-30A tops? The reason stuff like P38's have 100A+ alternators is they can have electric windows, air con, heated seats, heated screen, air suspension compressor, ABS pump, zoned heating, etc. all running.

I still don't know why you'd need to carry a spare alternator, if you're out in the sticks using the fridge a solar panel may be a lighter and more useful thing to have wired in - it'll be holding the fridge up when the sun shines, topping up the aux battery and can be used to top up the main battery in a pinch, and your TDi doesn't need any electricity at all to get you home. Why have a 5kg kump of metal rolling round in the boot or adding complexity to your engine bay?

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I have the Woods Auto version of that

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My 109 has a lot of extras, John - heated seats, screens, mirrors, the extra lighting, but no compressors, ECUs or pumps to continuously run.  So, not P38 power levels,  it more than a typical SIII or maybe even TD5 Defender in winter.  But, yes, those items should in theory only be on when the engine is running.  It’d be good to be able to run them and charge the batteries at the same time, though, especially since a fridge and a fuel based heater will be fitted in the future.  Solar makes sense, but given that Northern European sun levels aren’t fantastic even in summer, I don’t want to be dependent on it.  A small charger for a hookup would be more effective when stopping at camp sites, but solar would be useful for wild camping or stops where there are no mains.  I’ll certainly need more than I have now, though.  I like the look of that split charger; I think it’s smarter than mine, and protecting the main battery is important.  Looks a simple and cheap alternative to what I had in mind.

As for redundancy, even just carrying a spare, I’m one of those people who attracts that sort of problem, so I always like to have backups.

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Have you added up your power draws for main & leisure side?

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Did you buy 2 identical batteries new? In which case, just parallel them up. 

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6 hours ago, FridgeFreezer said:

This is what I've got on the ambulance - bit smarter than standard split-charge, but cheap and fit & forget.

https://www.furneauxriddall.com/acatalog/12-24V-100A-Split-Charge-Relay-SA_BS100.html

Are there VSR’s with a manual override ? So if you needed to start with both, you could. Or do you have to wire a switch in parallel ?

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3 hours ago, Anderzander said:

Are there VSR’s with a manual override ? So if you needed to start with both, you could. Or do you have to wire a switch in parallel ?

I believe that one you can connect one of the wires to something to "jump start" or disconnect the ground wire to stop it actuating. For a one-off jump-start a spanner across the terminals does the job though.

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9 hours ago, FridgeFreezer said:

This is what I've got on the ambulance - bit smarter than standard split-charge, but cheap and fit & forget.

https://www.furneauxriddall.com/acatalog/12-24V-100A-Split-Charge-Relay-SA_BS100.html

 

That looks like what I need.

Bookmarked.

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14 hours ago, FridgeFreezer said:

Have you added up your power draws for main & leisure side?

No I haven’t.  I think an 82AHr main battery is sufficient for the job running all the SIII electrics with halogen headlamps and a pair of spots with main beam, plus the Defender rear wash/wipe, and electric cooling fan, neither of which is a routine consumer.  The winch is an insurance policy rather than something that is planned on being used, so if it does ever have to get used, I can run the engine to recharge the main battery afterwards.

The leisure system runs the seat, screen and mirror heaters, the stereo, CB, roof spot lights, camp area lighting (LED), power outlets and will later run a fridge and heater, so that’ll get a fairly deep discharge when camped, hence my intention to replace the secondary normal battery with a deep cycle battery and move the winch feed from the auxiliary to the main.  It’s also why I don’t want a simple parallel system like Daan suggests, and why I have nagging concerns over it dragging down the main battery when my basic split charger is connecting.  It’s the ability to charge both batteries fully and quickly that I wanted, so I was considering a 65a alternator for both systems (nothing huge, but the 65a tyres appear not to have all the cooling vents for carp to get in like the 100+ types, are cheap and yet have enough output to charge both batteries if one should have a problem).  I suppose the twin systems are intended for winching competitions, not overlanding, though.

I like the idea of that relay, John.  It seems a more practical and frugal solution than what I had in mind.

 

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3 hours ago, Snagger said:

I think an 82AHr main battery is sufficient for the job running all the SIII electrics with halogen headlamps and...

Things that run when the vehicle is running are not* loading the battery at all - as long as the alternator is up to it you could run a 10Ah battery and see no difference.

*= Should not be, because the alternator should be covering all the loads.

3 hours ago, Snagger said:

The leisure system runs the seat, screen and mirror heaters, the stereo, CB, roof spot lights, camp area lighting (LED), power outlets and will later run a fridge and heater, so that’ll get a fairly deep discharge when camped

You don't know that - add up your damn power draws before fretting about it and making assumptions!

Your leisure battery is running pretty much the same set of things as mine and unless I'm camped for days without running the engine it's no problem at all.

3 hours ago, Snagger said:

It’s the ability to charge both batteries fully and quickly that I wanted, so I was considering a 65a alternator for both systems (nothing huge, but the 65a tyres appear not to have all the cooling vents for carp to get in like the 100+ types, are cheap and yet have enough output to charge both batteries if one should have a problem).

As I said earlier, you don't even know that your batteries would accept more than 65A of charge anyway - no point fitting a 100A alternator if your system can never pull more than 50A out of it. Our "house" battery is an AGM starter battery and even that rarely manages to draw more than 50A for any length of time.

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10 minutes ago, FridgeFreezer said:

You don't know that - add up your damn power draws before fretting about it and making assumptions!

Indeed. My fridge takes about 1.5A on average. Overnight that's still only ~15Ah. Not negligible, but also not an issue unless you're staying in the same spot for days on end.

An electric heater will draw a whole lot more though.

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1 hour ago, elbekko said:

An electric heater will draw a whole lot more though.

I believe it's a Webasto/Eberspacher, so a brief kick at startup and then ~25W while running.

You won't run any electric heater worth having from a 12v car battery :lol:

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Oh, yeah, in that case also peanuts.

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You have a 65a alternator without any shortages, then, John?  

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1 hour ago, Snagger said:

You have a 65a alternator without any shortages, then, John?  

No, mainly because the 4.6 comes with a 100A or 120A alternator as standard... but I do have current clamps on both batteries, monitoring in realtime. Here's a typical startup showing the starter current, the alternator coming up, and then the split-charge kicking in - you can see that you're unlikely to break the 65A mark for very long:

 

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