DC to DC Charger - Ignition Switch Alternative?

OrganicSower · Nov 21, 2022

As part of my overall system, I'm adding a DC to DC charger to charge my RV's lithiums while driving.

My tow vehicle is a Ram 2500 diesel with the upgraded 220A alternator. Will be running 0/2 cable with an Anderson plug for the RV mounted charger. Lithium battery bank at 900Ah.

So here's my scenario:

Nearly all DC to DC chargers want to read the ignition state of my truck to prevent drawing current when it's off. For example, I will be using the Renogy 60A unit, which takes that approach. However, my concern is that doesn't protect the alternator when the truck is idling or in other situations with low available current that could strain the alternator when the charger is trying to pull a full load.

Rather than reading the ignition state, why not put a Victron BatteryProtect between the starter battery and the DC to DC charger? The wiring is much simplier, but more importantly the BatteryProtect can be set to turn off current flow from the lead acid starter battery when it drops below an appropriate voltage. This would protect the starter battery from being drained while also insuring that the DC to DC charger is only drawing current when the starter battery is in a high charge state.

I then would connect the ignition state wire sensor from the DC to DC charger to the main positive lead from the BatteryProtect, meaning the charger will turn on when the BatteryProtect is sending current and automatically turn off otherwise.

Am I off base in my approach or missing something?

Finally, what BatteryProtect voltage cut off setting would be appropriate coming from the lead acid battery? Cut off current if it drops below a full charge state of 12.6V or something less?

meetyg · Nov 21, 2022

I would think that you can achieve the same thing with a cheap adjustable voltage relay.
It will still take some current, but only a few mA (as I suppose the Victron does also).

Like this one:

https://www.aliexpress.com/item/4000602255417.html

It has a few working modes. In your case you can set it to cutoff below a certain voltage and return above a certain voltage (two different voltages, one for cutoff one for return).

As for cutoff voltage, it depends on your alternator. It could put out something like 14v even when idling and maybe 15v when at a higher RPM. I think it will be tricky to control just by alternator voltage.

See this explanation by Victron for their Orion Tr Smart DC-DC converter. I suppose it can help you figure out what voltages are appropriate according to your alternator type:

AndyRonLI · Mar 21, 2023

So for my trailer install, I have tapped into the running light circuit and feed that to the ignition on port of my Renogy 20A DC/DC converter.
System only runs with parking lights on. Not the best solution but gives control from cockpit, next step is to control from uplifter switches.

Vigo · Mar 23, 2023

I think the idea is perfectly sound assuming that the current on the 12v side is comfortably within the limits of the device.

I would think that you can achieve the same thing with a cheap adjustable voltage relay.
It will still take some current, but only a few mA (as I suppose the Victron does also).

You can buy adjustable voltage window switches cheaper than the 'battery protect'. For example, any PWM SCC (which can be ~$10) has that on its load ports. The issue is that then has to control a larger current switching device. While you can DIY it all for cheap, you can't really DIY it much cheaper than it already is if you are buying non-Victron stuff.

For example, in the RV world they just generally call this device a VSR (voltage sensitive relay) and i have bought one for as little as $13 on Amazon. Here is one for $16: VSR

If you open that thing up all it is is 2x 70a 12v relays soldered to a circuit board that has some voltage sensing stuff on it.. In other words, it SHOULD be cheap!!

It will be more subject to unreliability from the quality of connections you make on the bottom (heat buildup), than unreliable because it's $16. The only way you could seriously improve the reliability of such a device, in my opinion, would be to use a solid state relay instead of mechanical points. An 'SSR' also sometimes referred to as optocoupler (although that's really only part of what an SSR is) has no points to get damaged from the arcing of inrush currents when it switches. So you could buy a 100a SSR and a $10 pwm SCC (which would make your voltage setpoints adjustable!) and build the equivalent device in totally solid state form for <$40, which i think is what the Victron piece is (and has some other cool functions). If you don't care about the Victron name or it being solid state you could just buy any generic VSR off amazon for <$20 and run it until it breaks, or more likely.. doesn't.

mikefitz · Mar 23, 2023

A 220 amp alternator wont be worried by a 60 amp DC to DC charger At idle engine speed the alternator output capability will exceed 120 amps.
Most DC to DC chargers designed for battery to battery charging have built in voltage detection that Inhibit charge if the starter battery volts fall too low.
It seems you are making the system unnecessarily complex.

Vigo · Mar 24, 2023

Most DC to DC chargers designed for battery to battery charging have built in voltage detection that Inhibit charge if the starter battery volts fall too low

Not Renogy, apparently? I read the manual and all i found was this:

No mention of it being adjustable, either. I know a lot of high current 12v devices have very low 'low voltage cutoff' points because, as 12v devices, they are prone to being installed by people who know nothing about electricity and suffer huge voltage drop from poor wiring on top of the normal sag of the source battery's voltage from the load. I don't have much opinion about that, would be nice if it was adjustable on everything but then it would be highly prone to operator error from installers who didn't know much.

But i think given the above spec, it's valid to implement some external controls in this case. The manual says ~90a fuse for the input, so i would just stick a $13-16 140a-rated VSR inline with it, and call it done. The non-adjustable voltage setpoints of 12.8 and 13.3 are acceptable for the task at hand, plus they have a 'manual control' wire on them as well if you wanted to manage them with some other device. I controlled mine with load ports of an SCC for a while.

mikefitz · Mar 24, 2023

Well I did say 'most DC to DC chargers'. Since the Renogy is only active with an 'engine running' or 'alternator charging' signal, its very unlikely the voltage at the starter battery will drop to a low value, as the alternator will be the power source.
It's not clear how the Renogy will act if the activation signal, D+, occurs before the power from the VSR relay is switched.

pollenface · Mar 24, 2023

Look up "140a voltage sensing relay or VSR" on ebay or amazon, they're quite cheap. I use those in many 12v projects.

Connect @ 13.3v
Disconnect @ 12.8v

Vigo · Mar 24, 2023

Well I did say 'most DC to DC chargers'. Since the Renogy is only active with an 'engine running' or 'alternator charging' signal, its very unlikely the voltage at the starter battery will drop to a low value, as the alternator will be the power source.

I mostly agree with you. But as an auto technician i may have a distorted worldview because for the most part, when i am working on a vehicle's electrical system it's because it has already done something unexpected and unplanned for. Newer vehicles are smart enough to start selectively shedding loads to keep the vehicle alive as long as possible when the charging system fails. But if you just use 'switched ignition power' to enable this device which draws ~90a and your charging system fails, you will have a dead vehicle in a single-digit number of minutes. A VSR turning off at 12.8 would disable the DC-DC almost instantaneously if the charging system failed, possibly giving you 30+ minutes of drive time (considering it's a diesel truck with 2 batteries) before it stalls out.

Of course, if you have a way to feed a 900ah lithium pack back to the starter battery system at a rate of ~30a or so, you could probably drive for an entire day even if the charging system DID fail.

But if you let the diesel engine stall by letting the starter battery system go to 8v, you would have to be able to actually recharge the 2 starter batteries @30a for.. an hour or more, or be able to flow 400+ amps instantaneous into that system, to actually restart the engine. So... much better to not let it stall! A $16 VSR is a LOT cheaper than the set of jumper cables you'd need to restart the diesel with your lithium bank!

By extension, i also support the idea of using a DC-DC that doesn't need this kind of safety feature to be added externally..

S Davis · Mar 25, 2023

I set up a bypass on my 50amp Redarc so I can feed 120 amps back to the starter batteries from my two 280ah LifeP04 if needed.

mikefitz said:
A 220 amp alternator wont be worried by a 60 amp DC to DC charger At idle engine speed the alternator output capability will exceed 120 amps.
Most DC to DC chargers designed for battery to battery charging have built in voltage detection that Inhibit charge if the starter battery volts fall too low.
It seems you are making the system unnecessarily complex.

I have run a 50 amp Redarc dc/dc every day for almost two years with no issues on my Chevy 2500HD with the 220amp alternator. I charge two 12v 280ah LifeP04. I have a large charge line and very little voltage drop, it draws about 52amps from the alternator and outputs about 49amps.

BrettR · May 18, 2023

Tagging onto this thread instead of starting another.

I'm installing a Renogy 60 amp DCDC charger for a truck camper setup. I have a HD Ram pickup with a diesel engine and 220 amp alternator. My camper has 3 x 100ah LiFePo batteries.

This thread has got me thinking about my install and I need some help. I plan to use my AUX switch to connect the D+ wire to the ignition, so the charger will be powered on only when I push the button. Based on what I'm reading here, that could be a problem because the Renogy isn't voltage sensing.

1. So I should add VSR to prevent battery drain if by chance the alternator fails and is no longer charging the batteries? Is there any other reason?

2. I'm new to this, so be gentle. But, I'm wondering about my 7-pin connector, which is also pushing a charge to the batteries when connected. Is there a problem having both means of charging going at the same time from my starter battery to my house battery? Both methods feed into a fused Lynx bus bar.

3. Where should I connect the negative/ground wire from the DCDC charger to the starter side? Directly to the battery or to the chassis or engine block. My truck has a High Amperage Power Point that is attached to the + battery terminal. It is rated at 300 amps. I was planning to connect to this with a terminal fuse and then ground to the chassis. But the manufacturer recommends anything greater than 50a be grounded to the engine block. Is it best to go directly to the battery with +/- wires from the DCDC charger?

Thanks for any help you can provide!

pollenface · May 19, 2023

1. I use a 200amp relay that piggy backs an ignition associated circuit. I solar charge the main starter battery and need to keep it isolated when engine is off so my dc-dc charger isn't trying to click on/off when the starter voltage is above 13.3v

2. Not sure

3. I've been thinking about doing this, the steel of the vehicle body is not the best electrical conductor. That said, it works the way it is, albeit with some voltage drop

kjanke351 · May 19, 2023

BrettR said:
Tagging onto this thread instead of starting another.

I'm installing a Renogy 60 amp DCDC charger for a truck camper setup. I have a HD Ram pickup with a diesel engine and 220 amp alternator. My camper has 3 x 100ah LiFePo batteries.

This thread has got me thinking about my install and I need some help. I plan to use my AUX switch to connect the D+ wire to the ignition, so the charger will be powered on only when I push the button. Based on what I'm reading here, that could be a problem because the Renogy isn't voltage sensing.

1. So I should add VSR to prevent battery drain if by chance the alternator fails and is no longer charging the batteries? Is there any other reason?

2. I'm new to this, so be gentle. But, I'm wondering about my 7-pin connector, which is also pushing a charge to the batteries when connected. Is there a problem having both means of charging going at the same time from my starter battery to my house battery? Both methods feed into a fused Lynx bus bar.

3. Where should I connect the negative/ground wire from the DCDC charger to the starter side? Directly to the battery or to the chassis or engine block. My truck has a High Amperage Power Point that is attached to the + battery terminal. It is rated at 300 amps. I was planning to connect to this with a terminal fuse and then ground to the chassis. But the manufacturer recommends anything greater than 50a be grounded to the engine block. Is it best to go directly to the battery with +/- wires from the DCDC charger?

Thanks for any help you can provide!

I just read up on a thread on this same topic. I cannot remember where i found it, but they are on here. But here are my answers for your questions below.

1. It seems to be a bit over kill, but with the renogy DCDC, you could just put something on the "D+" wire that triggers it to start charging.

2. This would contradict the purpose of the DCDC charger on your house battery if you run your 7 pin hot wire to the same house battery as your DCDC charger. The DCDC actually bumps up the voltage from your truck so it can properly charge your lithium. If you left the 7 pin wire hooked to the trailer battery, you may burn something up? Because that would be like hooking your starter battery to both sides of your DCDC charger.

3. If you are going to take the time to run a large wire from the battery to the DCDC, run 2 so you can run a ground of the same size with it directly to the DCDC from your starter battery. If you dont have room for that ground cable on your battery, follow the ground cable from your battery to the frame and stack it on top of that end of it.

Also, where did you plan on mounting the DCDC charger? I am doing my camper this weekend and plan on mounting it on the trailer, close to the lithium battery.

Edit: Here is the thread on charging a lithium battery on your RV from your tow vehicle. there are wire diagrams and everything. Very informative!

Charging Lithium from TV while driving

A very informative article on charging Lithiums from Tow Vehicle https://drivinvibin.com/2023/02/03/rv-lithium-battery-system/

diysolarforum.com

fratermus · May 19, 2023

OrganicSower said:
doesn't protect the alternator when the truck is idling or in other situations with low available current that could strain the alternator when the charger is trying to pull a full load.

The canonical answer AFAIK is external regulation with temperature sensing (Balmar, wakespeed, etc).

A poor man's version (OFF/ON) might be a NC temperature switch affixed to the alt case nearest the diodes. It would cut the D+ circuit at a given alternator case temp. Or a NO temp switch that triggers the Renogy's low current mode.

seanrmz · May 20, 2023

I have the victron dc/dc charger and in the settings we can set a minimum value to start pulling amps to protect the tow battery.

Vigo · Jun 1, 2023

My truck has a High Amperage Power Point that is attached to the + battery terminal. It is rated at 300 amps. I was planning to connect to this with a terminal fuse and then ground to the chassis. But the manufacturer recommends anything greater than 50a be grounded to the engine block. Is it best to go directly to the battery with +/- wires from the DCDC charger?

So the vehicle’s 12v chassis ground system doesnt carry any really high current circuits and the conductors are too small for it. The engine itself has a VERY large (like around 2/0 or bigger for a diesel 1ton) ground wire because the starter motor grounds through the engine block to that ground wire so it is sized for the starter. The ground conductor from the body or frame (usually both when they are separate ie not unibody) are sized much smaller.

In my opinion it is ok to use the chassis grounding system to carry high current all the way from the rear of the truck, but if you choose to do so you should either A: verify that the engine’s large ground cable connects to the frame on its way to battery negative (they often do) or B: Install your own sufficiently large ground conductor from the frame to the battery negative.

I would not try to pass high current through the bed or cab or other body sections simply because you will have to upgrade more conductors along the way (they each have their own ground straps) and there may even be corrosion issues. So id stick to the frame rails if you want to do that. But its always fine to just run a dedicated ground conductor all the way under the truck up to the starter battery too, just more copper money.

S Davis · Jun 1, 2023

I ran a 1/0 DLO positive and negative, positive on power stud and negative to main negative under hood, with all the electronics in new vehicles the chances of interference are high. Low current might be fine.

justinm001 · Jun 1, 2023

A 220a alternator should be fine pulling 60amps additional amps at idle especially if you have 2 starter batteries. If it has any issues it'll just pull from the starter batteries. You'll get a light if low voltage. They do much more with smaller engines and smaller alternators in RVs. My old rig was a 5.9 Cummins with a 135amp and many just plug a BIM like from battleborn that just connects/disconnects every 20 minutes so it pulls as much as it can.

Snow plow truck pumps use like 200amps and they go up and down all day and all night plowing parking lots.

I'd be more worried about alternator overheating but even then you should be fine. I think the non upgraded is 180 so you already have 40amps and as long as you don't have a ton of extra 12v stuff you have plenty of room.

If you want to spend money best thing is to get a ws500. I have a dedicated alternator for my lithium and it'll pull all 300 amps to charge then ramp down when it gets hot. Wakespeed integrates to victrons Cerbo so I can see it all while driving.

0truck0 · Jun 7, 2023

So, you are on solar forum so I also assume you have solar that does most of the charging? The alternator is for those times when you need more.
I had a similar truck. Here is what I did. I just used a manual switch to turn on/off the connection from truck to camper battery or DC to DC charger. I think your concern is you want something that does not require remembering to turn it off when at idle or other low alternator situations. However, I used a toggle switch to control a large solenoid that connected the positive battery/alternator power to the trailer wires, and also used a big Anderson plug, not the 7-pin option. The power came from the starter motor positive connection since that is closer to the camper and is run from battery to starter with big wire already.
Most of the time I never needed alternator bc the solar did its job, but on rainy days or whatever reason sometimes I would connect to the alternator when driving. I powered the toggle switch from the ignition (usually the radio fuse) so when the truck is off the toggle is off too. I also kept a paper list like a airline pilot to read through so I do not forget things when starting out or ending the day, helps to not forget important things ever...
As stated above, your power comes from the alternator, not the battery, so the charger will be seeing the voltage that the alternator is putting out, not the SOC of the lead acid battery. The battery holds charge from the alternator, but alternator will often put out higher volts than the battery is.
In other vehicles the danger is that an under-powered alternator can be worked too hard and burn out, ask me how I know from running a DC fridge option on a 130 amp alternator all day when I was younger....
Do not use those plastic relay switches for power, use a solenoid like "starter solenoid". The plastic plug-in relays are great for lights and small projects but you need a big boy solenoid.

justinm001 · Jun 7, 2023

0truck0 said:
So, you are on solar forum so I also assume you have solar that does most of the charging? The alternator is for those times when you need more.
I had a similar truck. Here is what I did. I just used a manual switch to turn on/off the connection from truck to camper battery or DC to DC charger. I think your concern is you want something that does not require remembering to turn it off when at idle or other low alternator situations. However, I used a toggle switch to control a large solenoid that connected the positive battery/alternator power to the trailer wires, and also used a big Anderson plug, not the 7-pin option. The power came from the starter motor positive connection since that is closer to the camper and is run from battery to starter with big wire already.
Most of the time I never needed alternator bc the solar did its job, but on rainy days or whatever reason sometimes I would connect to the alternator when driving. I powered the toggle switch from the ignition (usually the radio fuse) so when the truck is off the toggle is off too. I also kept a paper list like a airline pilot to read through so I do not forget things when starting out or ending the day, helps to not forget important things ever...
As stated above, your power comes from the alternator, not the battery, so the charger will be seeing the voltage that the alternator is putting out, not the SOC of the lead acid battery. The battery holds charge from the alternator, but alternator will often put out higher volts than the battery is.
In other vehicles the danger is that an under-powered alternator can be worked too hard and burn out, ask me how I know from running a DC fridge option on a 130 amp alternator all day when I was younger....
Do not use those plastic relay switches for power, use a solenoid like "starter solenoid". The plastic plug-in relays are great for lights and small projects but you need a big boy solenoid.

Having the alternator allows me to be certain I pull up to camp with 100% battery regardless of weather outside. This is especially nice because we typically drive at night and since we're in an RV we use the AC/heat while driving along with other things.

My coach has a rabbit switch which changes my idle from 700 to 1100rpm I'm assuming it's for the bus air to stay cool but also helps boost the alternator.

DC to DC Charger - Ignition Switch Alternative?

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