Voltage Sensing Relay (VSR) question

[Stepenberries]

Hi all,

I'm looking to wire up a second battery in the tub of my D22. I currently have a 100ah AGM battery in a battery box in the tub and want to install a VSR so it also charges whilst driving.

One question I have is, what happens when both the starter and aux battery are fully charged after several hours of driving? Will the VSR know this and cut off from overcharging if i continue to drive? If not, is there any additional things I can add without having to manually shut off the VSR?

I know a DCDC charger will solve all of this, however, trying to see if I can get away with just VSR at this stage.

Cheers

 

[landmannnn]

In simple terms, the alternator voltage regulator prevents overcharging, regardless of the number of batteries.

On a non smart alternator system such as the D22, you will be perfectly fine with a vsr as an AGM battery will be happy with 14.8 volts

There are many fancy solutions to make it better, such as those which only start charging the second battery when the starter is charged.

 

[Scott52]

A typical 12V battery has 6 x 2 volt cells internally connected in series. When the VSR closes you have two lots of 6 x 2 volt cells connected in parallel which is essentially one battery.

As LM says the voltage regulator exists to prevent over charging and we don't have smart alternators so the simple VSR system is fine. That's what I have on my D40 to connect my 2 x 45 Ahr deep cycle AGM auxillaries ... so I have three batteries in parallel when the VSR closes.

In theory the second battery won't reach full charge but I've not had any issues in five years. Periodically I'll attach the 240V mains smart charger to the auxillary batteries to ensure they get to 100%.

 

[Stepenberries]

Thanks both.

So from this, my understanding is that in my 2012 D22, I can install a VSR to charge my AGM battery in the tub, and when stopped, can then plug in my solar panel with regulator to top the battery up/keep it charged?

 

[Old.Tony]

In theory the second battery won't reach full charge

Absolutely.

There will be a small amount of loss of voltage to the rear battery purely because of the length of the wire. Longer wires (and lower voltage) mean more loss, thicker wires (and higher voltage) reduce loss. I use 8Ga (about 21 sq mm) cable to reduce this loss which ends up being 0.3V over 6 metres of cable. In the greater scheme of things, that's not going to harm a battery or stop it from reaching full charge - it might take a little longer, that's all.

The drop down to the fridge in my caravan is more significant - over half a volt (6m of car, another 7m to the fridge) but since that's a purely resistive load (not something like an inverter, or VSR etc) it hardly matters.

 

[Old.Tony]

Thanks both.

So from this, my understanding is that in my 2012 D22, I can install a VSR to charge my AGM battery in the tub, and when stopped, can then plug in my solar panel with regulator to top the battery up/keep it charged?

Yes. If you've got decent cables going to the rear, it should manage it without trouble.

Important note: it's very important to be mindful of the maximum charge rate of the battery you're charging. Starter batteries are designed to be charged at potentially high rates (your alternator can pump 90 amps at full pelt). Lead acid deep cycle batteries need a little more tenderness, and in general goes like this (where "C" is the capacity of the battery in amp hours):

Gel batteries: C divided by 10
AGM: C divided by 5 (unless spiral wound - Optima - can be charged at C/2)
Flooded*: C divided by 4

* Flooded batteries come in two primary types: starter and deep cycle, with different construction inside. They cannot be interchanged nor treated in the same way.

I distinctly only talk about lead acid batteries (which is what most people buy). There's a "new" type of battery that's entering the market - Lithium Iron Phosphate or LiFePO4 - that is not only different, but markedly better - but much more expensive. These batteries hold a constant voltage throughout their discharge cycle, and unlike lead acid batteries which should never be discharged below 50% of capacity (some deep cycle models can go down to 20%), LiFePO4 can be completely discharged. The difference is remarkable: you buy one of each battery, 100Ah in both. You'll pay 3x for the LiFePO4 version, but it's half the weight and gives you twice the amount of power without suffering damage.

We're adding some LiFePO4 batteries to our caravan at some point (possibly this year, have to see how we can wriggle dollars around) and will be able to provide real usage figures once we've started (we're changing the fridge in our caravan from a 3-way to a Bushman compressor fridge).

 

[OldManBeard]

There's a "new" type of battery that's entering the market - Lithium Iron Phosphate or LiFePO4 - that is not only different, but markedly better - but much more expensive. These batteries hold a constant voltage throughout their discharge cycle, and unlike lead acid batteries which should never be discharged below 50% of capacity (some deep cycle models can go down to 20%), LiFePO4 can be completely discharged. The difference is remarkable: you buy one of each battery, 100Ah in both. You'll pay 3x for the LiFePO4 version, but it's half the weight and gives you twice the amount of power without suffering damage.

The bloke across the road from us replaced the batteries in his new caravan with LiFePO4. He now has vastly greater capacity and can draw much higher currents (and for longer) and saved a lot of weight, all while using the same amount of space. If only I'd had that option when I got the batteries for my Navara.

 

[Horatius]

Hi all,

I'm looking to wire up a second battery in the tub of my D22. I currently have a 100ah AGM battery in a battery box in the tub and want to install a VSR so it also charges whilst driving.

One question I have is, what happens when both the starter and aux battery are fully charged after several hours of driving? Will the VSR know this and cut off from overcharging if i continue to drive? If not, is there any additional things I can add without having to manually shut off the VSR?

I know a DCDC charger will solve all of this, however, trying to see if I can get away with just VSR at this stage.

Cheers

When a battery is full (or as full as it is going to get) at alternator voltage, it won't accept any more charge, so there's nothing to worry about. As others have said the alternator alone will never fully charge it, especially with voltage drop to the tub so it's worth putting a battery charger on it occasionally. Or get one of tho$e dc/dc thingies.

Having a low voltage cut out on the battery output is cheap enough to do and not a bad idea either, to stop it getting really flat when using it. They don't like that.

 

[KevinE]

Our current set-up is that we pulled the 2nd cranking battery out from under the bonnet & securely insulated the positive cable. We then bought a dual battery system from the 4WD superstore, inclusive of a VSR. We bought a lead acid deep cycle battery (100AH) & put it where the 2nd cranking battery used to be, with the VSR between the 2 batteries. We ran quite heavy cabling back under the car & up into the cabin from the 2nd battery, with a circuit breaker under the bonnet. We have a 3rd battery (105AH AGM) in a battery box in a foot well in the back seat. This battery is connected to the 2nd battery under the bonnet by Anderson plugs & has a Jaycar DC/DC charger feeding it. We have an Anderson plug dual adaptor that I made up, between the cables coming into the cabin & the DC/DC charger. We have 2 fridges on the back seat; one gets fed by the 2nd battery under the bonnet & the other gets fed by the battery box being charged by the DC/DC charger. We have a solar panel set up to top it up if we're not driving. I probably haven't explained it well, but hopefully it helps.

 

[Stepenberries]

Great. Appreciate all the discussion on this, it seems like the VSR route is a good, low cost option at the moment.

I've looked at DCDC chargers however for the amount of camping I do and cost I'm not sure it's worth it for me.

In terms of the placement of the VSR. Would it be okay to place the VSR closer to the second battery (AGM, in the tub)? There is limited realestate in the engine bay..

 

[Old.Tony]

If the cable between the fuse (that is close to and connects to the battery) and the VSR is large enough to avoid dramatic voltage drop, you could put the VSR in the back.

The trick is that when you place a load on the cable, its resistance increases as it heats up. Resistance kills voltage. A thin cable, when loaded up (the load being the battery you're trying to charge!) will have a pronounced voltage drop over its length. If the voltage at the VSR drops below its threshold, it will turn off - not what you want!

Be very careful selecting electrical cable. If you walk into Supercheap/Repco/Jaycar they'll sell you "automotive cable" (which just means flexible core cable, unlike some household stuff which is really stiff but isn't designed to be moved constantly once it's installed). This automotive stuff is usually rated at its peak current - not its sustained current.

For example, I asked for 60A cabling to be installed in my Navara to handle the electric brakes. They did. The cores are about 6 sqmm thick - and it will handle 60A max (thankfully it only has to handle about 12-13A peak). However, the 8Ga twin-core that I added to run power to the rear is rated at 56A continuous, and its cores are 21 sq mm thick.

There's a HUGE difference in these two cables when using them. Another example from personal experience: in my previous caravan, we changed over the fridge from a 90L Electrolux 3-Way (non-T-rated) to a Waeco CR140 compressor-based fridge. There was already 6 sq mm cable installed (about 7m long), but when the fridge's compressor came on (drawing just 7A), there was a voltage drop of over 1 volt, which triggered the battery protection circuit in the fridge turning off the compressor. 30 seconds later the fridge tried again ... this killed the battery overnight. I changed to this 8Ga stuff (for both positive and negative, you MUST do both) and the drop was just 0.3V and the fridge lasted just fine.

So, heavy gauge cable and a fuse right near the battery and your VSR should be fine up the back.

Or maybe I could have just posted "yes" without the explanation and examples, but then someone else reading it might not have learned anything new!

 

[MANNING]

..maybe I could have just posted "yes" without the explanation and examples, but then someone else reading it might not have learned anything new!

For this, You are a deadset legend my friend

 

[OldManBeard]

In terms of the placement of the VSR. Would it be okay to place the VSR closer to the second battery (AGM, in the tub)?

I have mine installed under the tray, with only about 250mm of cable between it and the batteries (I have 2 x deep cycle). As Old.Tony indicated above, the key to making this work properly is to ensure you use large cables. This simply cannot be overemphasized.

A comparison can be drawn with jumper leads. Most commercially available jumper leads are remarkably light and will quickly heat up even when used on easy to start petrol engines, let alone a stubborn diesel. Compare them to what is used by roadside assist vehicles (NRMA, RACV, etc.). They use heavy-gauge welding cables. Those are required to, for all practical purposes, eliminate voltage drop over the much longer cables and therefore allow them to supply the full voltage to the vehicle being started, giving it the best chance of succeeding.

 

[Old.Tony]

For this, You are a deadset legend my friend

I really do appreciate the sentiments I just get concerned now and again about the amount of words that I post. I like to explain things so that they're understandable at most skill levels, but there's always a thought in my mind that I tend to offer too much!

 

[OldManBeard]

I really do appreciate the sentiments I just get concerned now and again about the amount of words that I post. I like to explain things so that they're understandable at most skill levels, but there's always a thought in my mind that I tend to offer too much!

Just keep in mind that too little might possibly miss the mark, whereas too much is easily ignored.

 

[Old.Tony]

Just keep in mind that too little might possibly miss the mark, whereas too much is easily ignored.

That's true, but with too much I also have the liberty of saying "Well, I bloody told ya!"

 

[KevinE]

Great. Appreciate all the discussion on this, it seems like the VSR route is a good, low cost option at the moment.

I've looked at DCDC chargers however for the amount of camping I do and cost I'm not sure it's worth it for me.

In terms of the placement of the VSR. Would it be okay to place the VSR closer to the second battery (AGM, in the tub)? There is limited realestate in the engine bay..

I'd keep the VSR as close to the battery that the cabling is running off as possible. You will get voltage drop, no matter how thick the cabling you run is. If you put the VSR in the tub, it will shut off prematurely (I've tried it)

 

[OldManBeard]

You will get voltage drop, no matter how thick the cabling you run is.

While technically true, when that voltage drop is in the order of just a few millivolts, which is less than the error reading of the kind of multimeter available to most people, it can be considered to not exist.

 

[Horatius]

I don't think it matters where you have the vsr. As long as it disconnects the starting battery at <12.7v (or whatever it's set at) it's doing what it is supposed to. Not a bad idea to have fuse near the starter battery though.

 

[KevinE]

While technically true, when that voltage drop is in the order of just a few millivolts, which is less than the error reading of the kind of multimeter available to most people, it can be considered to not exist.

Objectively; I ran the same cabling from under the bonnet into the tub on both scenarios.

1. Having the VSR in the tub, caused the VSR to continually cut out just a few minutes after the ignition was switched off (making the power connection useless)

2. Having the VSR under the bonnet, no such issue. The battery ran down overnight with a fridge connected, as it should. Still plenty of juice in it in the morning.

The VSR simply doesn't see the voltage in the tub that it did at the battery.

I've run three different types of VSR & the results vary from one to the other. But none were brilliant.