Charging from a tow vehicle

[AaronD]

Perhaps this should be in the Trailer Sailors subforum, but electrical questions are often best addressed here in @Maine Sail's forum.

I'm thinking about how to charge our boat's battery bank from a tow vehicle. We have a small compressor cooler / freezer that keeps our ice packs (and ice cream) cold. No problem on a short tow, but on a long trip - or, as happened last year, an unexpected multiday delay - we could arrive at a launch with empty batteries.

Q1: Is something like the Victron Orion battery-to-battery charger appropriate for this application?

Running power directly from the truck to the boat bank seems bad because:

1. The voltage drop over a long cable might reduce the voltage below that needed to actually charge. 40' of 10 AWG (round-trip of 80') drops .96v. (40' seems like a lot, but routed from the front of the truck, through the chassis to the hitch, along the trailer and up to the cockpit, it's probably pretty close)
2. If we did get adequate charge voltage to the boat batteries, we'd have no overcharge protection

If I understand correctly, the B-to-B chargers use a DC-DC converter (switching power supply perhaps?) to raise the input voltage (e.g. 12.6v from a source battery) to an appropriate charge voltage (e.g. 14.3v absorb for our Lifelines). Do I have that straight?
(And of course they apply appropriate 3-stage BULK / ABSORB / FLOAT charging).

So, if I installed a DC-DC charger:
Q2: Can the 18A Victron (their smallest model) be programmed to a lower output current? If not, is there a reliable alternate brand that can be?

The easy solution is just to pull power from the auxiliary 12v line on the 7-pin trailer plug. But that aux line won't have 18A available, and would blow a fuse in the truck (my AGMs would happily take 18A of current in BULK - when discharged to ~60%, they'll take the full 40A from our shore-power charger). I don't really need an 18A charge rate - 4-8A would be sufficient (it only matters on a long tow anyway, in which case we have plenty of time to charge at a lower rate).

Q3: Perhaps a crazy question: a theoretical alternate approach would be to install a 500w inverter in the truck and run 110v from there to the boat's shore-power input. Is this a good or terrible option?

The 40A shore-power charger would max out at ~4-5A input (@110v), so any decent extension cord would be sufficient (no need to route 10 AWG boat cable). A good 500w pure-sine-wave inverter is about the same price as the DC-DC charger. And I'm sure 110v would see some other usage in the truck if we had it available (whereas the DC-DC charger is single-use-only). Running AC power from truck to trailer seems just a little crazy, but it would theoretically work (yes, cable routing to avoid chafe would be really important!)

 

[dlochner]

My first thought is about charging the truck battery. Do you have a heavy duty alternator and battery? If not you'll kill the truck battery while charging the boat.

Have you considered a solar panel? Or just plugging into a nearby outlet with a long extension cord and using a good charger?

 

[AaronD]

My first thought is about charging the truck battery. Do you have a heavy duty alternator and battery? If not you'll kill the truck battery while charging the boat.

Have you considered a solar panel? Or just plugging into a nearby outlet with a long extension cord and using a good charger?

Yes to heavy-duty alternator + dual batteries on the truck. And we're only asking ~4-8A, so not a lot different than plugging a 75w computer charger into the lighter plug. Another reason I'd like to limit the current - I think the alternator would be happier at a steady 6A than an instant 18A (if Victron or another reliable manufacturer made a 6A B-to-B, I'd grab that).

And yes, we have a 40A Sterling Procharge shore-power charger; so we can always start a trip with full batteries. And solar while on the water - but not at 55 MPH.

My wife is dreaming of some cross-country exploring, which might mean splashing after multiple days on the road and away from 110 power. And...importantly, the 11-year-old crew (who's also the most hesitant about sailing adventures) deserves to have her ice cream cold.

 

[dlochner]

How big is the boat's battery? How much does the cooler draw? How often will the cooler run?

 

[JoeWhite]

10 gauge wire should be large enough to charge the battery. Your tow vehicle alternator has a regulator and will not self destruction. Simply parallel the boat battery to the tow vehicle battery. Put a switch in this line so that when you are overnighting at a hotel, the freezer does not drain the tow vehicle battery. You have not mentioned make and model if tow vehicle.

 

[AaronD]

How big is the boat's battery? How much does the cooler draw? How often will the cooler run?

Boat batteries - 210 Ah when new (2x 105 Ah Lifeline AGMs)

Cooler draws just under 4A; duty cycle ~40% in the summer, for a total of ~40 Ah / ~500 Wh per day (I'm wrapping it in 2" of foam-board insulation, so maybe that will drop some this year). That's why I estimated that 4-8A would be plenty to keep up over an 8-9 hour towing day; that's why it seemed reasonable to ask about limiting the charge amperage.

10 gauge wire should be large enough to charge the battery. Your tow vehicle alternator has a regulator and will not self destruction. Simply parallel the boat battery to the tow vehicle battery. Put a switch in this line so that when you are overnighting at a hotel, the freezer does not drain the tow vehicle battery. You have not mentioned make and model if tow vehicle.

Tow vehicle is a 2001 GMC Sierra 2500HD. I believe the alternator is 105A, but I'm not certain on that (I know a 145A is available, but I think 105A is stock, and I don't know that this one has been upgraded. The truck is new to us, and I haven't looked at that yet).

The boat AGMs have pretty low internal resistance, and will charge at .4C or higher. If, as you suggested, I simply parallel the two battery banks, when I turn on said switch (with 14+v at the source), won't we draw 40+A and immediately blow the fuse?

 

[JoeWhite]

Aaron, I have the very same truck. The output voltage of the alternator is about 15 v. These trucks are designed and expected to tow camper trailers and their lighting and batteries can be connected to the the alternator for charging purposes, and they can be connected to your boat batteries aswell. When your batteries are fully charged they need about 5 amps or less to float. This is when you connect the 2 batteries together. Even if your boat batteries were discharged somewhat, your truck battery will supply the current to charge them until you start your engine, then all batteries will be charging.
Your freezer is designed to stop working when the battery voltage drops to 10.5 volts, this is to not destroy the battery. On my boat I have 51 Amp alternator and it charges 2 4D batteries that are paralleled as house battery and a 3rd starter battery. They all get charged with no problem. The maximum charging current I read on battery monitor was 14 AMPS.

 

[dlochner]

The amount of current actually drawn by charging the batteries will be dependent upon the actual load at the moment and the State of Charge. Although the .2C rate for your batteries is 40 amps, unless the batteries are discharged fairly deeply, they will not accept that charge level for very long or if at all.

So a typical scenario, the batteries are close to fully charged, the engine running and a small float charge is being sent to the boat's battery. The cooler kicks on the alternator now provides 4 amps to meet the drain, the battery SOC is not affected because no power is being drawn from the battery. The cooler kicks off, the alternator shifts to float. Rinse and repeat. If the cooler kicks on while at a rest stop, not a big deal. In the worst case scenario, its drawing a couple of ah, easily and quickly made up.

The tough one will be the first start in the morning. The cooler will have been on batteries for say 12 hours for a loss of around 25 ah (4*12*.4). The batteries will draw about as much as the alternator will produce for a few minutes before the charge acceptance rate begins to tail off to an absorption and then float.

I'd be inclined to just properly fuse the wire and run a 10ga or 8ga cable to the boat and let it rip. By using the truck chases for the negative you can shorten the return run by a the length of the truck. Try a temporary connection to see what the actual voltage drop is.

Not sure how you have the solar panel set up, however, I would look for a way to have connected even while driving. Perhaps securing it in the cockpit somehow. This will help at the beginning and end of the day and during rest stops.

 

[AaronD]

A couple quick additions (maybe more to come when I have time)

The amount of current actually drawn by charging the batteries will be dependent upon the actual load at the moment and the State of Charge. Although the .2C rate for your batteries is 40 amps, unless the batteries are discharged fairly deeply, they will not accept that charge level for very long or if at all.

Yes; I've watched 40A from our shore-power charger (when I had purposely run the batteries down some for testing; to ~60% if I recall correctly). No, it doesn't last long, but AGMs will take considerably more than flooded cells (and LiPO4's much more than that).

Not sure how you have the solar panel set up, however, I would look for a way to have connected even while driving. Perhaps securing it in the cockpit somehow. This will help at the beginning and end of the day and during rest stops.

Solar panels are semi-flexible; usually deployed on the bimini with velcro. I can lay them out in the cockpit at rest, but at 55 MPH, I think they'd take flight with almost any mounting system.

 

[kloudie1]

A small inverter generator powered by gasoline would be a pretty good solution. Use it to power your normal battery charger. Carry it to keep fridge cold when cruising.

 

[Benny17441]

Check with RVers, they have that DC to DC charging down pat. The complaint I hear is that the charging currents are low failing high expectations. The power limitations lie with the size of the wires used by the vehicle and the 7 pin connector.

 

[JoeWhite]

dlochner explained well and comete.
Thanks for the explanation.

 

[jviss]

Your tow vehicle alternator has a regulator and will not self destruction.

The regulator is not going to protect the alternator from overheating, it simply regulates the output voltage. Put a low enough resistance load on it and it will indeed self destruct.

 

[SycloneDriver]

I had house bank of 3 AGM group 34s in my 5th wheel. I never had a problem connecting it to my truck through the 7 pin. I would try that 1st and see how it works, it's cheap and easy.

 

[AlexB]

Perhaps this should be in the Trailer Sailors subforum, but electrical questions are often best addressed here in @Maine Sail's forum.

I'm thinking about how to charge our boat's battery bank from a tow vehicle. We have a small compressor cooler / freezer that keeps our ice packs (and ice cream) cold. No problem on a short tow, but on a long trip - or, as happened last year, an unexpected multiday delay - we could arrive at a launch with empty batteries.

Q1: Is something like the Victron Orion battery-to-battery charger appropriate for this application?

Running power directly from the truck to the boat bank seems bad because:

1. The voltage drop over a long cable might reduce the voltage below that needed to actually charge. 40' of 10 AWG (round-trip of 80') drops .96v. (40' seems like a lot, but routed from the front of the truck, through the chassis to the hitch, along the trailer and up to the cockpit, it's probably pretty close)
2. If we did get adequate charge voltage to the boat batteries, we'd have no overcharge protection

If I understand correctly, the B-to-B chargers use a DC-DC converter (switching power supply perhaps?) to raise the input voltage (e.g. 12.6v from a source battery) to an appropriate charge voltage (e.g. 14.3v absorb for our Lifelines). Do I have that straight?
(And of course they apply appropriate 3-stage BULK / ABSORB / FLOAT charging).

So, if I installed a DC-DC charger:
Q2: Can the 18A Victron (their smallest model) be programmed to a lower output current? If not, is there a reliable alternate brand that can be?

The easy solution is just to pull power from the auxiliary 12v line on the 7-pin trailer plug. But that aux line won't have 18A available, and would blow a fuse in the truck (my AGMs would happily take 18A of current in BULK - when discharged to ~60%, they'll take the full 40A from our shore-power charger). I don't really need an 18A charge rate - 4-8A would be sufficient (it only matters on a long tow anyway, in which case we have plenty of time to charge at a lower rate).

Q3: Perhaps a crazy question: a theoretical alternate approach would be to install a 500w inverter in the truck and run 110v from there to the boat's shore-power input. Is this a good or terrible option?

The 40A shore-power charger would max out at ~4-5A input (@110v), so any decent extension cord would be sufficient (no need to route 10 AWG boat cable). A good 500w pure-sine-wave inverter is about the same price as the DC-DC charger. And I'm sure 110v would see some other usage in the truck if we had it available (whereas the DC-DC charger is single-use-only). Running AC power from truck to trailer seems just a little crazy, but it would theoretically work (yes, cable routing to avoid chafe would be really important!)

I would try wind AC generator with external regulator.

 

[mermike]

I do a fair amount of these installations with RV's, and you should not use the trailer wiring or 7-pin plug to power the DC-DC charger. A safer, more reliable solution is to run wire from the vehicle battery to the the bumper and from the RV hitch to the DC-DC charger and RV battery. Use an Anderson Powerpole connector between truck and RV.

 

[AlanSD]

The Victron Smart Orions do not have a setting for reduced current, it would always want to put out the 17A, and pull a little more on the input. There is a 9A standard (non-smart) Orion that would need to be switched on and off manually. But the draw would be appropriate, and it will accept down to 8V input to make up for voltage drop. The output is adjustable, so you could set it for ~14V and top up your batteries without needing to go to float, since it would only be for the time you are driving. 50' (round-trip) of 10 AWG cable would give ~4% voltage drop, so you'd have 13.45V off a 14.0V alternator.

Or, Redarc has a 12V DC-DC charger that is similar in function to the Victron, though without Bluetooth. It's designed for trailers - model BCDC1212T.

While the 120VAC would work, I'd be concerned with the safety aspect of it. In any event, you'd need an inverter capable of 600VA minimum for the fairly efficient 40A charger, which will still have some losses. Since inverters are not usually happy at constant full load, make that an 800VA.

 

[AaronD]

I do a fair amount of these installations with RV's, and you should not use the trailer wiring or 7-pin plug to power the DC-DC charger. A safer, more reliable solution is to run wire from the vehicle battery to the the bumper and from the RV hitch to the DC-DC charger and RV battery. Use an Anderson Powerpole connector between truck and RV.

Thanks. A DC-DC charger definitely seems to be the consensus in the RV world. And thanks for reminding me of Powerpole connectors. They're well made and reliable - and I think I might even have a couple in my electrical box. Hopefully I can still locate my Powerpole crimp dies.

 

[Tedd]

...you should not use the trailer wiring or 7-pin plug to power the DC-DC charger.

Is that because of the potential current draw?

 

[AaronD]

The Victron Smart Orions do not have a setting for reduced current, it would always want to put out the 17A, and pull a little more on the input. There is a 9A standard (non-smart) Orion that would need to be switched on and off manually. But the draw would be appropriate, and it will accept down to 8V input to make up for voltage drop. The output is adjustable, so you could set it for ~14V and top up your batteries without needing to go to float, since it would only be for the time you are driving. 50' (round-trip) of 10 AWG cable would give ~4% voltage drop, so you'd have 13.45V off a 14.0V alternator.

Or, Redarc has a 12V DC-DC charger that is similar in function to the Victron, though without Bluetooth. It's designed for trailers - model BCDC1212T.

While the 120VAC would work, I'd be concerned with the safety aspect of it. In any event, you'd need an inverter capable of 600VA minimum for the fairly efficient 40A charger, which will still have some losses. Since inverters are not usually happy at constant full load, make that an 800VA.

Thanks - those are all helpful suggestions - I'll consider the Redarc and the 9A Orion. I hadn't even thought about a non-smart charger, but it might be fine for this application; I should always have a pretty good idea of the boat bank's state of charge. And it's fine (and expected) to sit at absorb voltage for a few hours, and it's easy enough to switch it off midway through a long tow.

I think I'm too paranoid to use the 120VAC solution. My electrical knowledge says it would work in theory; my lack of electrical knowledge can't predict all the possible failure modes; whatever those failure modes might be, they make me nervous.