[Guide] DC-DC Charger (for leisure battery) -- How I Done It --

[Dellmassive]

a short snipit from post#1 on this thread:


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What size DC-DC charger do i need?

This is a good question, and the answer is very similar to the "what size solar do i need?" . . . Its not just about charging your battery . . . . its also about charging you battery WHILE running your loads. So you need to account for your worst case scenario - which will be a low AUX battery while remote camping while running all your loads - at nighttime - in the rain - and its snowing etc etc ..

Joking aside i believe that the sizing of a DC-DC should account for the above, whats the point in getting in that situation, then having to switch off all your lights and fridges etc . . . just to give the battery as much charge as possible while running the engine for 20mins at nights . . . . . nope thats not right.

So take your AUX battery size say 100Ah and go with 10-20% which is a recommended charge rate for SLA/AGM etc - thats 10-20A charger .

Then measure all your loads, either with a current clamp meter, or add up all your Watts or any other way get an idea of what your worst case scenario is . . . an example may be 6amp, 10amp, 15amp, 20amp . . . whatever. Lets take 10Amp for thius example.

Add the Battery charge rate of 10-20A plus the Loads draw rate of 10A give you a figure of 20-30A.

Now you know that a 30A DC-DC is needed to fully charge your AUX battery while drawing a Load.

If you are a POWER USER like me then the sky is the limit . . . . i have 1000-1500W inverters that will draw 100A easy frona SLA/AGM battery which causes large volt SAG and depleates the battery rapidly . . . im my case i use DC-DC to offset the current draw from the battery . . . . So ill draw say 40A from the DC-DC charger, then the rest from the Aux battery. - doubling up DC-DC chargers in parrallel is also possible so 2x 40A DC-DC chargers will give 80A then only 20A will be drawn from the battery. (remember the cables on the system will need to handle the 80A current)

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[albertramsbottom]

I am slightly confused by reading this massive thread, The most powerful Dc to DC chargers on the market are around 50-70amp but all modern alternators will pump out 100-200amp, does this mean they are useless? My alternator is 100amp smart thingy and I want two 110amp leisure batteries, with a fridge/freezer, TV, lights and 1000w invertor (pure sign wave)??

I want the 50amp Renogy MTTP one but dont want to blow it up with my alternator

 

[Dellmassive]

@albertramsbottom

as you say most modern smart alternators are 100-200A.

you wont blow them up drawing 50A via a DC-DC charger. (50A Renogy or Redarc or Abelmail)(30A Victron)(20A ctek,Renogy)

but you could blow them up if you tried to:

pull above its rated max - ie 200A from a 180A T6 uprated alternator

or

pull high amps for an extended duration - ie 110A from a 140A T6 alternator for a long time.

( i done this years ago in an Astra van on a 3hr drive from London to Bristol. . . pumping out the bassline from a massive ICE install at night in summer . . so AC and lights on. . . lost electrical power and van dead . . towed to a garage. next day the garage said they had never seen a melted + Blued Alternator and couldnt explain it. - i knew why drawing 120A (1600w RMS Ice install on 2x L7 Kicker SoloBaric`s) for a long time - the alternator got super hot and cooked its self)



i think the standard T6 alternator is 140A and the bigger version is 180A - you can get the uprated one when you spec the factory leisure battery option.


++++++++++++

2x 110ah AGMs will take around 20A each to charge if they low.

a 1000w pure sine inverter will try to pull 100A, so it will draw 50A from the dc-dc (in engine running with a 50A dc-dc) and 50A out the batterys.

fridge/freezer, TV, lights - all all small loads and will draw under 10A

++++++++++++

a 50A dc-dc charger is no joke.

nor is a 1000W inverter . . .

you will be dealing with some very high currents - correct fusing and cable gauge will be critical or you will end up with melted fuse holders or even cable catching fire.


+++

 

[JBR80]

I am slightly confused by reading this massive thread, The most powerful Dc to DC chargers on the market are around 50-70amp but all modern alternators will pump out 100-200amp, does this mean they are useless? My alternator is 100amp smart thingy and I want two 110amp leisure batteries, with a fridge/freezer, TV, lights and 1000w invertor (pure sign wave)??

I want the 50amp Renogy MTTP one but dont want to blow it up with my alternator

From my recent research, this should work for you. The renogy has a max charge rate if 50 amps, which should would translate to 0.23 of your 220ah capacity. The alternator current being that much greater than the charger is not a problem.

 

[JBR80]

From my recent research, this should work for you. The renogy has a max charge rate if 50 amps, which should would translate to 0.23 of your 220ah capacity. The alternator current being that much greater than the charger is not a problem.

Started typing before I saw @Dellmassive's much more thorough response

 

[albertramsbottom]

Thanks everyone, the only reason for the invertor is for the TV and laptops because I thought that these needed more careful management. I have two of the battery boxes with the ciggy lighter sockets and 4 USB thingy's. My Fridge runs off 12V/24V or 240 if I have a hook-up TBH a 300w invertor would do, I cant think of anything bar my laptop that I would need it for.

Sorry forgot to say its the DC charger not the alternator that I was worried about blowing up.

 

[albertramsbottom]

There is one other bit I dont understand with the

Renogy 50A DC-DC On-board Battery Charger with MPPT​

It Is that with the YouTube vids they always say that this is the positive side for the solar and a positive you can attach that to your alternator or you starter battery, Which should I do? What is the benefit of attaching it directly to your alternator? I suppose this goes back to my original question 100amp alternator connected to a 50amp DC charger?

 

[t0mb0]

There is one other bit I dont understand with the

Renogy 50A DC-DC On-board Battery Charger with MPPT​

It Is that with the YouTube vids they always say that this is the positive side for the solar and a positive you can attach that to your alternator or you starter battery, Which should I do? What is the benefit of attaching it directly to your alternator? I suppose this goes back to my original question 100amp alternator connected to a 50amp DC charger?

The alternator is wired to the starter battery, you just need a convenient place to pick it up - usually this is most easily done at the battery terminals.

When an alternator says 100amp, that's the maximum it can give. It's not going to force 100amps into whatever is connected to it, if it did then every circuit in your vehicle would be toast... It just means it's more than adequate to supply your 50amp DC-DC.

 

[albertramsbottom]

Ok I have decided for now to go with one of these with one cheap flooded battery and a 50amp DC Renogy and then upgrade to two batteries later and a small 300w inventor for the laptop, TV. But now comes the wiring part, I need it to be good with two batteries, so needs to be hefty and I am a bit confused as to where I should fuse this lot and how big the fuses, boxes need to be? Long wires lost more energy, I would think that the wires need to be 3 metres to get to my trolling battery box at back of van. And would the Renogy be best under the bonnet (not lot of space), of in the van somewhere. I am certain there are some great wiring diagrams on here but most are biblically complicated

 

[t0mb0]

Ok I have decided for now to go with one of these with one cheap flooded battery and a 50amp DC Renogy and then upgrade to two batteries later and a small 300w inventor for the laptop, TV. But now comes the wiring part, I need it to be good with two batteries, so needs to be hefty and I am a bit confused as to where I should fuse this lot and how big the fuses, boxes need to be? Long wires lost more energy, I would think that the wires need to be 3 metres to get to my trolling battery box at back of van. And would the Renogy be best under the bonnet (not lot of space), of in the van somewhere. I am certain there are some great wiring diagrams on here but most are biblically complicated

A cheap flooded battery might involve some more maintenance than a more modern sealed design like an AGM, especially if you have the renogy potentially pumping 50A into it.

 

[JBR80]

Although I couldn't find a conclusive/objective limit to the charge rate of a wet cell battery, many sources indicate that a 0.5C charge rate is too high. Personally I would go to 0.25C as an upper limit.

 

[Scaff]

I'm new here so don't know if this is right place to ask this but it seems like this thread is full of leisure battery knowledge.

I'm after some advise on if this setup sounds ok or if I'd need to upgrade or change anything?

I currently have the factory fitted aux battery setup.

I plan to install the Renogy 50amp dc charger with the factory cables that are already under the passenger seat using the midi fuses as described in the op. (I would of went for the premade plug and play wiring from travelvolts but by the time they add there £15 postage and £10 tax it works out at £60 which I think is a bit nuts!)

I wont be using the solar option (although down the line I would like to add an easily removable solar unit just for longer camping trips)

I then want to upgrade the original aux battery to a 100ah lithium.

This is the list of things I'm planning to eventually run. (obviously everything will be fused separately using midi fuses from midi fusebox)

lighting ( say 10 led spotlights)
15" tv and usb powered mini computer
1000w rms sound system (mainly used while driving)
12v alpicool fridge/cooler (45w/h)
12v air compressor (23amp)

1500W power inverter
will run 1200w pressure washer off this for cleaning bikes
e-bike battery charger (2.5a)
makita battery charger
occasional laptop charging

Look forward to hearing back and sorry if this isn't the place to ask this

 

[LhHutch]

@Dellmassive and others in the know (as I’m not in the know and new) - firstly, thanks for all this brilliant info. I’m currently trying to spec a new vehicle, I’m looking to add solar so leaning to Renology 50A DC-DC and the plug and play cable kit that patches into the relay socket.
If anyone could help suggest which aux battery option would I want to choose in the VW configuration tool?

1. Just AGM with cut-off relay
2. AGM with cut-off relay and battery monitoring

and if I choose option 2 with monitoring, I’m forced to also choose one of the sub options in the grab below - which is needed?

Thanks!

 

[t0mb0]

@Dellmassive and others in the know (as I’m not in the know and new) - firstly, thanks for all this brilliant info. I’m currently trying to spec a new vehicle, I’m looking to add solar so leaning to Renology 50A DC-DC and the plug and play cable kit that patches into the relay socket.
If anyone could help suggest which aux battery option would I want to choose in the VW configuration tool?

1. Just AGM with cut-off relay
2. AGM with cut-off relay and battery monitoring

and if I choose option 2 with monitoring, I’m forced to also choose one of the sub options in the grab below - which is needed?
View attachment 122632
Thanks!

You don’t need the monitoring. Arguably a 50A DC-DC might be a bit over-powered for the factory (is it still 75Ah AGM?) leisure battery but maybe the max current is a configurable option.

 

[LhHutch]

You don’t need the monitoring. Arguably a 50A DC-DC might be a bit over-powered for the factory (is it still 75Ah AGM?) leisure battery but maybe the max current is a configurable option.

Thanks, my understanding is that it’s 25A + 25A if using MPPT for solar input too, plus I would be changing out for 110AH.

 

[t0mb0]

You don’t need the monitoring. Arguably a 50A DC-DC might be a bit over-powered for the factory (is it still 75Ah AGM?) leisure battery but maybe the max current is a configurable option.

Also, if you’re planning to install a DC-DC straight away anyway be aware that the factory aux battery isn’t necessarily the most cost effective route. You could undoubtedly buy a bigger battery yourself for less money - the slight downside being you’d need to install yourself including possibly running the supply and ignition live cables. Some vans seem to have the supply cable hanging around under the seat even without the aux battery option.

 

[t0mb0]

Thanks, my understanding is that it’s 25A + 25A if using MPPT for solar input too, plus I would be changing out for 110AH.

Oh I see. If you're swapping the battery straight out too though, then that aux battery option is a seriously expensive way of running some cables.

 

[LhHutch]

Also, if you’re planning to install a DC-DC straight away anyway be aware that the factory aux battery isn’t necessarily the most cost effective route. You could undoubtedly buy a bigger battery yourself for less money - the slight downside being you’d need to install yourself including possibly running the supply and ignition live cables. Some vans seem to have the supply cable hanging around under the seat even without the aux battery option.

My preferred option would be to not mess with factory install and just take all loads from the aux battery. Warranty issues and all on a brand new van. I note you said ‘some‘ vans have the cabling there anyway but I’d rather know mine definitely has them

 

[t0mb0]

A further random point - check the renogy can cope with whatever panel you're going for. Combined DC-DC/MPPT devices often have more restricted voltage inputs than dedicated MPPT boxes.

 

[Dellmassive]

As above,
I chose option one.

Didnt have the option for monitoring at the time.... id imagine its like the cali panel you get in the top cubby hole.