How to charge auxiliary battery in F150.

Here's the story guys and gals. I have an F150 that has an auxiliary battery in the bed, actually 2 auxiliary batteries (2 100 AH AGM batteries). Anyway, I need to be able to charge these batteries from the alternator of the truck. The truck has either a 155 or 200 amp alternator (I'm getting conflicting information). I don't want to spend a ton of money on this but I want it to work. Speed of charge should not be an issue because I will drive the truck on a regular basis and the batteries will only see occasional heavy use, a winch, and somewhat regular use for lighting. Please don't tell me to get solar because that's not going to happen right now for several reason. My questions are: 1: What size charging device should I be using? 2: Should I be using a battery isolator or a Voltage Sensitive Relay(VSR). I saw what looked like a pretty nice VSR made by Sterling Power that is 210 amps. Does anyone have any experience with Sterling Power. BTW I need to protect my starting (chassis) battery from going dead at all costs. Thanks in advance.

If you just want to charge them then the victron dc to dc charger is probably the most reliable option.

Something like the Orion-Tr Smart DC-DC Charger Isolated model.

Crowz said:

If you just want to charge them then the victron dc to dc charger is probably the most reliable option.

Something like the Orion-Tr Smart DC-DC Charger Isolated model.

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I have the 20A version in my Toyota truck with 1 x 12v 100Ah LFP battery. I think the 30A version is the answer for 2x AGM batteries.

I use a 120 amp battery isolator. Got it at an RV dealer. Ran 8 awg marine wire to the back with breakers at both ends.. Remember with the truck off the battery in the back is an energy source. Still working since 2010.

Crowz said:

If you just want to charge them then the victron dc to dc charger is probably the most reliable option.

Something like the Orion-Tr Smart DC-DC Charger Isolated model.

Click to expand...

From what I can see the DC to DC charger or an isolator doesn't prioritize anything it simply charges 2 batteries from one source. A VSR prioritizes the starting battery over the aux bank which is something I very much want. Is this true or am I missing something?

Johnny Lightning said:

From what I can see the DC to DC charger or an isolator doesn't prioritize anything it simply charges 2 batteries from one source. A VSR prioritizes the starting battery over the aux bank which is something I very much want. Is this true or am I missing something?

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Umm your alternator can put out 100amps or more. That dc to dc charger can use and charge the aux batteries at 30 amps or less depending on the model you go with. 70% or more of the power going to the starter battery its a pretty good priority going to the starter battery wouldn't you say

Crowz said:

Umm your alternator can put out 100amps or more. That dc to dc charger can use and charge the aux batteries at 30 amps or less depending on the model you go with. 70% or more of the power going to the starter battery its a pretty good priority going to the starter battery wouldn't you say

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Let me make sure I've got this right. A 30 amp dc to dc charger will only pull 30 amps from the alternator leaving about 125 amp to charge the house battery assuming I have 155 amp alternator. Will it also isolate the batteries when the alternator is not supplying power? I think it will but just confirming. If so that would be a good solution.

Johnny Lightning said:

Let me make sure I've got this right. A 30 amp dc to dc charger will only pull 30 amps from the alternator leaving about 125 amp to charge the house battery assuming I have 155 amp alternator. Will it also isolate the batteries when the alternator is not supplying power? I think it will but just confirming. If so that would be a good solution.

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Yep.

Crowz said:

Yep.

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Thanks!!!

The DC-DC will only charge the aux battery if the alternator is producing power and the voltage is over the setpoint. You do not want
the aux battery draining your starter battery while the engine is off. Also current cannot flow from aux battery to the starter battery i.e. to prevent your starter motor from drawing power via the relatively thin wires. If you want your aux battery to be a backup to the starter battery you need to install a contactor type relay with heavier cables.

Well folks I was looking at the spec for the DC-DC chargers and it looks like there is a constant draw on the system of some sort. The Renogy model has a standby draw of .4 amps and the Victron something less but still a draw. This won't work. I need -0- draw if the engine is off as there will be no solar involved to supplement. This tends to lead me back to the VSR solution as it is has a -0- draw if the engine is not running. But I may be mistaken that's why I'm asking questions.

The one I linked uses 1ma. Now to put this in context a normal modern car usages is at least 80ma continuous drain in sleep mode.
So were looking at 81ma idle consumption on the starter battery vs 80ma. Your not EVER going to notice this. If the battery runs down it won't be from the DC to DC victron isolated charger it will be from normal drain by the vehicle itself. If this vehicle is so old it has a mechanical fuel pump and a carburetor then it might matter but if its fuel injected you will never know the dc to dc charger is there.

I agree with Crowz BUT i also think the cheap and easy solution is simply to use a VSR (which are $20 and honestly deserve to be, they are usually 2x 70a relays soldered to a board with some voltage sensing circuitry, it is 'a fancy relay'). The AGMs are already compatible with the stock alternator's charging behavior as far as voltage setpoints, etc, so a VSR is by far the cheapest and simplest to install solution in this case. It will prevent your 'aux' batteries from ever draining your starter battery.

Vigo said:

I agree with Crowz BUT i also think the cheap and easy solution is simply to use a VSR (which are $20 and honestly deserve to be, they are usually 2x 70a relays soldered to a board with some voltage sensing circuitry, it is 'a fancy relay'). The AGMs are already compatible with the stock alternator's charging behavior as far as voltage setpoints, etc, so a VSR is by far the cheapest and simplest to install solution in this case. It will prevent your 'aux' batteries from ever draining your starter battery.

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Agreed but, is it a smart alternator? If so then a programmable DC to DC converter would be a better choice.

Well I just started researching the smart alternator thing. At this point it looks like it's a European car thing but I really don't know. As to the age it's a 2014 so yes it has fuel injection so I guess I'm in the 80 mA draw category. I do know that if I let my truck sit for 30 days the battery goes dead. Thankfully the folks at the airport parking lot had a jump box.

Johnny Lightning said:

Well I just started researching the smart alternator thing. At this point it looks like it's a European car thing but I really don't know. As to the age it's a 2014 so yes it has fuel injection so I guess I'm in the 80 mA draw category. I do know that if I let my truck sit for 30 days the battery goes dead. Thankfully the folks at the airport parking lot had a jump box.

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Sounds about normal with modern vehicles now. So the victron would be foolproof drain wise since it pulls almost nothing in comparison of the normal power drain.

The nice thing about the victron is it can safely charge lithium house bank.

However, for basic AGM bank, all ya really need is an isolator.
Something to let the alternator charge the bank when it needs it, but isolate the starting battery when the truck is off.